RU2780492C1 - Reactor and method for ensuring safety of the reactor in case of core melting - Google Patents

Abstract

FIELD: nuclear technology.

SUBSTANCE: invention relates to a nuclear reactor (1) with pressurised water. The fluid of the primary circuit (100) enters the vessel (10) of the reactor (1) through the inlet (13) and exits through the outlet (14), passing through the entire core (30). S_input therein constitutes a cross section of one inlet (13) of the first circuit (100) up to the vessel (10), a second circuit (200) fluid-insulated from the primary circuit (100) wherein the fluid of the second water-based circuit circulates, containing at least one steam generator (210) and configured to absorb the heat of the primary circuit (100) and convert the heat into steam in the steam generator (210). Reactor (1) also includes a safety system containing a safety apparatus containing at least one explosive apparatus (216) configured to explode, destructing the insulation between the second circuit (200) and the primary circuit (100) so as to form a passage allowing the flow of the fluid of the second circuit inside the primary circuit (100).

EFFECT: prevented indentation of the reactor vessel along the periphery of the downcomer due to the focusing effect in case of an accidental core melting.

11 cl, 13 dwg

Description

FIELD OF TECHNOLOGY TO WHICH THE INVENTION RELATES

The invention relates to the field of safety of nuclear installations and more specifically to the management of serious accidents in nuclear pressurized water reactors (PWR). Even more specifically, the present invention is applicable to the management of accidents involving the formation of a core molten pool at the bottom of the vessel due to the occurrence of a serious emergency in the context of a strategy for maintaining or retaining the core melt in the vessel.

BACKGROUND OF THE INVENTION

A PWR type nuclear facility is illustrated in Figures 1 and 2.

In general, such a plant comprises a containment 600 within which a reactor 1 is placed. The reactor 1 comprises a housing 10 which, together with the lid 20, forms a hermetic envelope. This sealed envelope contains the fuel assemblies of the core 30. In Figure 2, for clarity, the core is represented by an area with dotted lines.

The housing 10 further comprises at least one so-called cold fluid inlet 13 connected to the first circuit 100 and at least one so-called hot fluid outlet 14 also connected to the first circuit 100. In the first circuit 100 pressurized water circulates.

Thus, during normal operation of the reactor 1, the first circuit 10 provides for the transfer of heat from the core 30 towards the second circuit 200, within which water also circulates.

The second circuit 200 comprises one and preferably several steam generators 210. The heat exchange between the first circuit 100 and the second circuits 200 takes place inside the steam generators 210. In the example illustrated in Figure 1, the loop 214 of the first circuit 100 is illustrated in the steam generator 210.

The simplified diagrams in Figures 1 and 2 illustrate other absolutely classic elements of the primary circuit 100 and the core 30 of the reactor 1, such as: control rods 40, a hydraulic pump 102 at the inlet to the housing 10 to circulate water in the primary circuit 100, a pressure compensator 110 on exit from the housing 10, the containment 600 and structures forming the threshold 601 and the walls 602, limiting the shaft 603 of the housing.

Still during normal operation of the reactor 1, as it passes into the steam generator 210, a portion of the water in the secondary loop 200 vaporizes and reaches a power conversion device such as a turbine 220 movably driven by depressurization of the steam. Subsequently, the mechanical energy output from the turbines 220 is converted into electrical energy by the generators 500 and transformers 510 before being transported to the places of consumption.

At the outlet of turbines 220, the fluid is condensed in condenser 230 before being re-injected into steam generators 210 by pump 240.

In order to condense the steam produced from the turbines 220, the condenser 230 is cooled by an open loop 300 fed by a water source 310 such as a river. This circuit 300 also contains lift pumps 320, 330 and a cooling tower 340.

As clearly seen in the diagram in Figure 1, the water in the primary loop 100 is designed to take the calories generated by the reactor 1 in order to pass them to the second loop 200 to be converted into electricity by a power conversion system such as a turbo generator. During normal operation, the core 30 is critical and the thermal power produced is removed by the primary circuit 100. Core criticality control is guaranteed by the position of the control rods 40 in the core 30, as well as the content of water-soluble boron in the primary circuit water.

Various emergency scenarios, such as a large-hole loss of primary coolant accident, combined with aggravating factors, such as the failure of classified emergency systems, can lead to a major emergency.

Further, it follows that the core 30 enters a melt state to form a core melt. The core melt is a partial or complete set of molten fuel assemblies of the interior 10 of the body in a molten state. The core melt flows under the action of gravity to the bottom 12 of the housing 10 and forms a bath there. Therefore, the core melt contains all or only part of the stock of molten fuel assemblies containing all solid fission products and the residual thermal power associated with them.

Figures 3A-3C illustrate in a very schematic way a severe PWR vessel failure with core molten pool formation and then vessel bursting.

Figure 3A illustrates the disintegration and partial melting of the core 30, the formation of the core melt 70 and the appearance of the pool of melt 70 of the core at the bottom 12 of the housing 10. In a known way, when the pool 71 of the melt 70 of the core is formed, a surface layer 72 of liquid metal appears on the free surface baths 71. Part of the thermal power of the bath 71 of the melt 70 of the active zone is transferred to this layer 72.

Figure 3B illustrates in a very schematic way the appearance of this metal layer 72 (whose thickness is deliberately exaggerated for clarity). The region indicated by reference numeral A illustrates the start of the metal layer 72 piercing the housing 10. This bulging of the housing 10 is due to a phenomenon commonly referred to as the "focusing effect". The metal layer 72 is a good heat conductor and absorbs most of the thermal energy of the core molten pool 71 . The focusing effect corresponds to the situation in which the metal layer 72 transfers part of its heat by conduction along the small surface of the side walls 11 of the body 10. This thermal power, focusing on a small surface, can lead to the walls 11 of the body 10 being punched.

Figure 3C illustrates the completion of the shearing of the body 10. The melt 70 of the core is then poured into the shaft 603 of the body. Moreover, in direct contact between the core melt and the cooling water contained in the shaft 603, explosions called steam explosions (abrupt depressurization of water and expansion of steam) occur and hydrogen is generated. These consequences are unacceptable due to the risk of destruction of the third protective barrier formed by the shell 600 of the reactor. The risk of bursting of the housing in the situation of the wet shaft 603 of the housing 10 must be eliminated. In addition, uncontrolled spreading of the core melt 70, a highly radioactive material having a very high thermal power, must be avoided. The spread of the core melt 70 at the level of the threshold 601 creates the risk of forcing the latter and passing down to potential areas of groundwater.

There are two modes of core melt control in the event of a severe accident in a PWR depending on the developers of the PWER nuclear boiler. The first set of solutions, referred to as "out-of-vessel containment", is to allow the core melt 70 to push through the casing 10 when primary pressure is released and flow into an area dedicated to distributing the core melt and controlling its cooling. This type of solution has the disadvantage that an imposing platform called a recuperator is required to receive and distribute the core melt, which greatly burdens the structure under construction and increases the costs of the reactor shell infrastructure. Another serious shortcoming relates to the demonstration of the integrity of the third protective barrier formed by the reactor shell in order to avoid environmental pollution, since the protective barrier formed by the first circuit was destroyed.

The second set of solutions, referred to as "containment within the housing" or abbreviated as IVR, consists in the use of systems to contain the core melt 70 within the housing 10 by avoiding the latter being forced through.

In this series of solutions, one strategy seeks to retain the core melt 70 in the housing 10 and to extract its residual power through the wall of the housing 10 by external cooling, in particular by installing a circuit with (natural or forced) convection after wetting the shaft 602 of the housing 10.

This type of solution is described, for example, in document WO2009/053322. In this document, a pump located at the bottom of the housing allows increased forced convection of the water located in the shaft 603 of the housing in contact with the outer wall of the housing 10.

A natural circulation of water can also be established in contact with the body, then evaporating and condensing again in the upper section of the shell. This type of solution is proposed to control the removal of residual power in the event of a major accident at the Westinghouse™ AP1000 reactor. This cooling means is completely passive, unlike the solution described in WO2009/053322.

Moreover, depending on the assumptions considered regarding the thickness of the metal layer 72 at the point of focusing effect, there is still the possibility of not achieving sufficient cooling characteristics to avoid shell bursting. For this reason, in general, even in the context of IVR management in a severe accident, complementary devices are described to eliminate the risk associated with hydrogen, and a study to limit the consequences of a steam explosion with the provision of an additional means to cope with the assumption of the worst case of hull bursting.

The IVR strategy described in document FR2763168 or EP0907187 is to provide a core melt extractor at the bottom of the casing. The core melt flows under the action of gravity from the core to this recuperator. Moreover, gravity water injection is provided by an additional tank connected to the body shaft.

This solution has the disadvantage that it requires an additional system formed by the recuperator, which burdens and substantially increases the size of the reactor vessel. In addition, the means for injecting water into the housing, formed by a reservoir directly connected to the primary circuit, is only an auxiliary injection means, complementary to those already available to deal with accidents of various sizes. The residual pressure in the primary circuit, which prevails in the event of a serious accident, requires a significant height for the gravity of this tank, which greatly burdens the structure under construction.

Consequently, all solutions that have been proposed to control the cooling of the core melt have disadvantages. There is a need to develop a solution that limits and, if possible, eliminates at least some of these shortcomings. The present invention pursues this goal.

Another object of the present invention is to reduce and, if possible, eliminate the risk of shell bulging due to the focusing effect.

SUMMARY OF THE INVENTION

In order to achieve at least one of these objects, the present invention provides a method of securing a nuclear pressurized water reactor in the event of at least partial melting of the reactor core to form a core molten pool, wherein the reactor comprises, during the operating phase, at least:

- a first circuit in which the water-based primary fluid is intended to be circulated, which is designed so that the primary fluid penetrates into the reactor vessel and crosses the core contained within the vessel in order to remove the heat generated by the core,

- a second circuit in which the water-based second circuit fluid is intended to circulate, which is fluidly isolated from the first circuit, contains at least one steam generator and is designed to absorb heat from the first circuit and convert it at least partially , into steam in the steam generator.

The method comprises at least the following step, in which, in response to detection of an event characterizing the melting, at least partially, of the reactor core with the formation of a core molten pool at the bottom of the vessel and with the formation of a liquid metal layer on the surface of the core molten pool:

- destroying the fluid insulation by means of an explosion to install the second circuit in fluid communication with the first circuit so that the second circuit fluid follows the first circuit to flow inside the housing along the specified liquid metal layer of the core molten pool.

Thus, the pressurized secondary circuit fluid, typically water at saturation pressure and temperature, contained in the secondary circuit, and in particular in the steam generator(s), flows to the bottom of the housing over the metal layer.

Therefore, this injection of the coolant is carried out in a passive manner, since the water in the secondary circuit is at a higher pressure than the water in the primary circuit.

Typically, the pressure of the water contained in the steam generator is in the range of 6 to 7 MPa, while the pressure of the primary circuit water is usually lower than 2 MPa when a serious emergency occurs.

In a very short time, the water of the secondary circuit begins to come into contact with the remelting layer of liquid metal, causing a sharp decrease in the heat flux at the point of occurrence of punching due to the focusing effect. All or part of the water from the secondary circuit evaporates upon contact with the molten metal layer and the core molten pool, flowing down the reactor pressure vessel from the cold leg side, preferably in a volume commonly referred to as the downcomer.

Water from the secondary circuit flows for a period sufficient to cool the molten metal layer for at least the entire period during which this molten metal layer has a thickness that is small enough to create a risk of shell bursting. Thus, this flow can be considered as controlled. However, the flow velocity through the liquid metal layer should not be very high.

One way to control this flow rate and this duration of injection of water from the secondary circuit into the housing is to accurately calibrate the section of the open hole(s) between the second circuit and the first circuit.

As a non-limiting example, the flow rate of the secondary water entering the housing corresponds to a hole with a diameter of about 20 mm, and more generally between 10 and 30 mm.

All of the water contained in one or more steam generators (GVs) then gradually flows into the reactor vessel. The usual value of the flow rate of the liquid entering the first circuit from the second circuit is about 5 kg/s. As a non-limiting example, this flow rate is more generally in the range between 2 kg/s and 10 kg/s. This value is significantly lower than that of all emergency injections intended for core wetting. Thus, the method according to the invention provides for a long duration of transfer of water from the GV towards the area of potential shell bursting due to the focusing effect, as well as a significant restriction of the steam generated as a result of the interaction of water and liquid metal, which limits the overpressure in the primary circuit.

Also, as a non-limiting example, the duration of injection of water from the second circuit into the housing is in the range of three hours, and more generally between 30 minutes and 5 hours. For comparison, in a solution that consists of wetting the core by pouring water from the secondary circuit into the vessel, the duration of the injection will be from about one minute to several minutes.

Depending on the emergency scenarios, the available amount of water per steam generator varies between 29 tons and 70 tons per steam generator for a reactor having a capacity of 1300 MW (French Palier N4 type).

With regard to the time scale of a major accident, and more specifically, regarding the time scale of core melt and core melt formation, the time during which water from the secondary circuit flows through the core molten bath is short when the method according to the invention is carried out.

However, in the context of the development of the present invention, it has been found that this duration is sufficient to quite significantly reduce, and even eliminate, the risk of shell bulging due to the focusing effect.

Moreover, it has been noted that the period of time during which the focusing effect can lead to bulging of the housing is relatively limited. It has been found that the risk of bursting due to the focusing effect corresponds to the time interval when the core molten pool has a liquid metal layer on the surface with a relatively small thickness, typically a few centimeters. In this configuration, most of the thermal power received from the core molten bath is transferred to this thin metal coating, this power is then transferred on contact with the inner wall of the case and causes the latter to gradually burst.

Over time, internal elements of the body continue to enter the core molten pool, which gradually melt, and the layer of liquid metal on the surface thickens due to an increase in the supply of molten metal. The contact surface between the layer of liquid metal and the inner wall of the housing is increased. The thermal power transmitted by the latter is then distributed over a greater thickness of the liquid metal layer, and the body punching power in this case is less concentrated. In this case, the risk of punching the body is reduced.

As soon as the entire internal supply of metal has melted, the thickness of the surface metal layer in this case becomes such that the thermal power at the periphery and in contact with the body is no longer sufficient to punch through the body. In general, at this level, the external cooling of the case is sufficiently effective to dissipate the thermal power received from the bath. The punching of the body in this case finally stops.

For example, if 3 MW are transferred to a metal layer with a thickness of 10 cm and a periphery of 12 m without any possibility of heat removal by the upper surface, then in this case there will be a heat flux applied to the body of 3/1.2 = 2.5 MW / m ² . Such a flow cannot be diverted by conventional external cooling when the housing is immersed in a wetted housing shaft. The excess thermal power transferred to the body is then converted into the heat of fusion of the body itself, and the body is pressed through from the inside. As soon as the liquid metal layer reaches a double or triple thickness, the flow as a result of this is much reduced, and external cooling in this case becomes sufficient to stop the hull punching stroke, since all the thermal power received from the bath is diverted to external water without causing metal melting. corps.

When the liquid water of the secondary circuit comes into contact with the layer of liquid metal from the cold branches of the primary circuit, preferably at the place where the shell is pressed, in other words, at the periphery of the downcomer, which will be described in detail below, every kilogram of water per second that evaporates causes the liquid layer to cool. at about 2-3 MW. With a flow rate of about 5 kg/s over a period of about 3 hours, the cooling of the liquid metal layer corresponds to the removal of a thermal power of about 10-15 MW for three hours, which drastically reduces and even prevents the case from bursting.

In addition, once the core molten pool reaches a predetermined height, the liquid metal layer on the surface comes into contact with the core baseplate illustrated in Figures 2 and 3A at 17. The temperature of the molten metal superheated by the thermal power of the core molten pool then decreases sharply due to significant melting of the lower section of the core base plate. Moreover, the melting temperature of the metal is much lower than that which can be reached by the layer of liquid metal, thus causing the effect of focusing.

Thus, in the context of the development of the present invention, it has been noted that it is possible to effectively deal with the bulging of the housing due to the focusing effect by injecting some water at a relatively low flow rate into the interior of the housing. The volume of water contained in the steam generators is in this case large enough to effectively cool the liquid metal layer during the entire period during which the liquid metal layer has a thickness that is small enough for the focusing effect phenomenon to cause the shell to burst.

In a particularly preferred manner, the invention does not require any additional components in the reactor, such as a recuperator, or additional systems in the vessel shaft, such as a forced convection means.

Moreover, the invention does not require any additional tanks located at a significant height. However, taking advantage of the pressure of the second circuit, the invention provides for a very rapid injection of water.

However, it is necessary to provide for cooling the outer wall of the housing in the configuration of holding the core melt inside the housing and cooling by wetting the housing shaft.

Therefore, the proposed solution involves passive operation, in other words, without any pump for pouring the coolant onto the core melt bath, improving the strategy for controlling the core melt inside the case by combating the effects of the focusing effect, which can compromise the success of blowout prevention. reactor vessel.

Moreover, the invention does not provide for additional components or taps directly on the reactor pressure vessel, since this may lead to a decrease in the reliability of the safety of the reactor.

Thus, the invention makes it possible to significantly improve the safety of a pressurized water reactor in the event of a core melt to form a core molten pool.

The invention also relates to a pressurized water nuclear reactor comprising at least:

- a housing containing the reactor core, which contains at least one inlet and at least one outlet,

- a first loop, at least one first end of which is connected to the inlet of the vessel and at least one end of which is connected to the outlet of the vessel so that the primary fluid, preferably water-based, circulating in the primary loop penetrates into the reactor vessel through the specified inlet and exits through the specified outlet, while passing through the entire core in order to take away the heat produced by the core,

- a second circuit fluid-isolated from the first circuit, in which the water-based second circuit fluid is intended to circulate, containing at least one steam generator and configured to absorb the heat of the first circuit and convert it, at least partially, into steam in the steam generator.

The reactor contains a safety system that contains a safety device containing at least one explosive device designed to explode in order to destroy the fluid insulation between the second circuit and the first circuit so as to form at least one and preferably only one passage allowing the secondary fluid present in the at least one steam generator to flow within the primary circuit.

Preferably, the explosive device is designed to explode in the event of at least partial melting of the core to form a core molten pool at the bottom of the housing and to form a liquid metal layer on the surface of the core molten pool.

Thus, the explosion of the explosive device allows the secondary fluid contained in at least one steam generator to flow in the housing while having previously passed through the primary circuit.

More specifically, when a core molten pool is formed and a liquid metal layer appears on the surface of the pool, the pressurized secondary fluid contained in the steam generator is poured into the primary circuit, preferably in the cold leg(s), and then into the housing. This fluid cools the liquid metal layer, which reduces the phenomenon of the focusing effect. In this way, bulging of the housing is avoided.

S _hole is the minimum section of the specified passage, which allows you to inject inside the first circuit fluid medium of the second circuit, present in at least one steam generator.

According to non-limiting example S, _{the holes} are less than 20 cm ² (10 ^-2 meters).

According to one example, the S _holes are greater than 2 cm ² and preferably the S _holes are greater than 3 cm ² . According to one example, the S of the _hole is in the range between 2 cm ² and 20 cm ² .

According to one example, the S of the _hole is in the range between 0.2 cm ² and 20 cm ² , preferably 0.8 cm ² and 20 cm ² , and even more preferably between 2 cm ² and 7 cm ² .

The cross section of the inlet opening on the side of the cold branch of the reactor pressure vessel is much larger than the cross section of the hole formed between the second circuit and the first circuit.

Typically, according to a non-limiting example, S _holes < 0.05 * S _inlet , preferably S _holes < 0.01 * S _inlet , preferably S _holes < 0.005 * S _inlet , and preferably S _holes < 0.001 * S _inlet , S _inlet is the minimum cross section of the passage primary circuit fluid in the primary circuit and up to the casing.

If the primary circuit contains several inlets in the shell, which happens when there are several steam generators, then the section S _inlet is the sum of the sections of the inlets of the primary circuit up to the shell.

Typically, the flow area of the fluid medium of the first circuit from the side of the cold branch is in the range of 6000 cm ² .

Typically, when the S _inlet has a circular section, the S _inlet has a diameter ranging between 800 and 900 mm (10 ^-3 meters).

Thus, the cross section through which the secondary fluid flows into the first loop is much smaller than the cross section through which the primary fluid normally flows in the housing. This ratio of the cross sections S of the _hole and the S _inlet makes it possible to inject into the primary circuit, and therefore into the housing, a flow of fluid from the second circuit for a relatively long period of time. More specifically, for a period of time that is sufficient to cover the duration during which the thickness of the liquid metal layer is small enough to push against the housing wall due to the focusing effect.

These features make it possible to reduce and, if possible, to eliminate the risk of the case being punched through due to the focusing effect.

BRIEF DESCRIPTION OF THE DRAWINGS

Aims, objects, as well as features and advantages of the invention will be better understood from a detailed description of the embodiments of the latter, which are illustrated in the following attached drawings, in which:

Figure 1 schematically shows a PWR-type nuclear installation.

Figure 2 schematically illustrates a vertical section of a PWR reactor vessel in its vessel shaft in an operational condition without a major emergency.

Figures 3A-3C schematically illustrate different phases of a severe accident leading to partial or complete core melt, formation of a core molten pool and then pressure vessel in the reactor illustrated in Figure 2 due to the focusing effect.

Figure 4 schematically illustrates an embodiment of the invention with cooling of the core molten bath by injecting water from the second circuit through the first circuit down to the bottom of the vessel.

Figures 5A and 5B schematically illustrate a PWR-type nuclear facility incorporating a safety system according to a first embodiment of the invention. Figure 5B illustrates, on an enlarged view, the hydraulic connections between the casing and the steam generator.

Figures 6A-6C are detailed views of a non-limiting example corresponding to the first embodiment, more generally shown in Figures 5A and 5B, allowing secondary water to be injected inside the steam generator into the primary circuit.

Figure 7 is a diagram illustrating the operation of the invention in perspective view of a solution implementing the embodiment illustrated in Figures 5A and 5B.

Figure 8 illustrates in a very schematic way a section of the housing and the parameters that allow fuses to be located along the generatrix of the housing.

The drawings are presented as examples and do not limit the invention. They represent block diagrams intended to facilitate understanding of the invention and do not necessarily correspond to the scope of practical applications. In particular, the relative dimensions of the various components of the installation, in particular, the relative sizes of the constituent elements of the reactors and their pipes, the liquid metal layer and the various elements of the installation do not correspond to reality.

DETAILED DESCRIPTION

Before starting a detailed overview of the embodiments of the invention, it should be recalled that the invention according to its first aspect contains, as far as possible, in particular the optional features described below, which can be used in combination or alternatively.

According to one example, said detection of the formation of a liquid metal layer on the surface of the core molten pool is performed by at least one fuse located on the wall of the housing, which is configured to melt when the liquid metal layer reaches it.

According to one example, at least one fuse has a fuse melting temperature greater than or equal to the temperature threshold Tf, with Tf ≥ 400°C, preferably Tf ≥ 500°C and preferably Tf = 600°C.

According to one example, the reactor contains several fuses distributed according to at least one generatrix of the body wall, so that two adjacent fuses form a cut of the body, while the _cut volumes V of the cuts are identical.

According to one example, the installation of the second circuit in communication with the first circuit is started according to the detection, for example, by at least one fuse, that the temperature of the inner wall of the housing exceeds the temperature threshold Tf, while Tf exceeds 400°C and preferably Tf exceeds 500°C.

According to one example, the height profile of the liquid metal layer in the housing is determined by means of fuses located on the inner wall of the housing and preferably located according to at least two generators of this wall. The point in time at which the second loop is established in fluid communication with the first loop is determined according to this profile.

Preferably, a series of fuses is located on the inner surface of the housing wall. Preferably, fuses are located on the generatrix of the bottom of the housing and the side wall of the housing. The course of the rise in the level of the core melt, as well as the beginning of the destruction of the body by a layer of liquid metal on its surface, is detected by the sequential deactivation of the fuses of this generatrix. Starting from a certain height of the core melt, activation of the communication by fluid is started.

According to one example, the reactor contains at least one fuse on the vessel wall. The fuse is designed such that when the liquid metal layer reaches the fuse, it causes it to melt. For example, the melting temperature of the fuse is greater than or equal to the temperature threshold Tf, with Tf ≥ 350°C, preferably Tf ≥ 450°C and preferably Tf = 600°C.

According to one example, the reactor contains several fuses distributed according to at least one generatrix of the vessel wall. The fuses are located along the generatrix so that if the volume of the liquid metal layer increases at a constant rate, then the time intervals separating the time moments at which the liquid metal layer reaches two successive fuses of the generatrix remain constant.

According to one example, the secondary fluid flows inside the housing over the liquid metal layer for at least the entire period during which the liquid metal layer has a thickness e ₇₂ that is small enough to at least partially pierce the inner wall of the housing.

According to one example, the secondary fluid flows inside the housing over the liquid metal layer for at least thirty minutes and preferably for at least one hour and preferably for at least two hours.

According to one example, the water flow of the second circuit passes in the first circuit through the entire passage, whose minimum cross section S of the _hole is in the range between 0.2 cm ² (0.2 * 10 ^-4 ) and 20 cm ² and preferably between 0.8 cm ² and 7 cm ² . If the minimum _hole section S of this passage is circular, then its diameter is in the range between 5 and 50 mm and preferably between 10 and 30 mm. Usually this diameter is about 20 mm.

According to one example, the secondary fluid flows within the housing (10) at a flow rate below 10 kg/s (103 g/s) and preferably below 7 kg/s.

According to one example, the reactor contains an inner shell located inside the vessel, surrounding the core and forming with the inner wall of the vessel an annular volume, called the downcomer, which is designed so that during normal operation of the reactor:

- the inlet leads out of the shell and into the downcomer so that the primary fluid coming from the inlet is directed to the bottom of the casing,

- the outlet leads inside the shell so that the primary fluid present in the core can exit the reactor through the outlet.

The reactor is designed so that when the explosive device forms at least one fluid isolation passage between the second circuit and the first circuit, the secondary circuit fluid contained in the steam generator in this case flows along the bottom of the vessel, while previously passing through the specified housing inlet and then through the downcomer chamber.

This embodiment contributes to the flow of water of the second circuit along the inner walls of the housing. This makes it possible to cool the liquid metal layer even more efficiently than in the case when the fluid medium of the second circuit penetrates into the body from the outlet of the latter or from the side of the hot leg and, therefore, without passing through the downcomer chamber. Moreover, due to the runoff from the side of the downcomer, the evaporation of the water of the second circuit and, in connection with this, the cooling of the layer of liquid metal occurs in the area where the place of punching of the housing is located. To achieve this embodiment, the connection on the secondary side must be made on the side of the cold branch of the steam generator, in other words, on the side of the heat exchanger, where the temperature of the primary circuit corresponds to the cold return flow of the primary circuit (cold water box).

According to one example, the reactor contains at least one fuse located on the wall of the housing, which is designed so that when the liquid metal layer reaches the fuse, it causes it to melt, while the melting temperature of the fuse exceeds or equals the temperature threshold Tf, when this Tf ≥ 400°C, preferably Tf ≥ 500°C and preferably Tf=600°C.

According to one example, the reactor contains several fuses distributed according to at least one generatrix of the body wall, the fuses are distributed along the generatrix so that two adjacent fuses according to this generatrix determine the body cut, the _cut volumes V of the cuts are identical.

According to one example, the steam generator comprises a shell surrounding the secondary fluid and the primary fluid and the surrounding insulation fluidly isolating the fluids of the second and primary circuits, while the safety system is designed to eliminate the isolation between the fluids of the second and primary circuits. inside the shell of the steam generator, thereby forming said passage. The removal of this insulation corresponds to a hole with a limited and controlled diameter (typically 20 mm in diameter).

According to one example, the steam generator comprises tubes within which the primary circuit fluid circulates, while the secondary circuit fluid is in contact with the outer wall of the tubes, while the security system includes an explosive device containing at least one explosive plug placed inside at least at least one of said tubes, and a launcher, wherein the explosive plug is configured to explode when it is actuated by the launcher so as to separate at least the tube in which it is placed, thereby forming said passage. Prior to exploding, the plug forms, at least in part and preferably by itself, said fluid isolation between the second and first circuits.

According to another example, the steam generator comprises:

- the first section in which the primary fluid circulates,

- the second section in which the fluid medium of the secondary circuit circulates,

- a plate with tubes insulating the first and second sections in terms of fluid, containing at least one tube, continuing, at least partially, in the second section and inside which the fluid of the first circuit circulates, while the fluid of the second circuit present in the second section, is in contact with the outer wall of the tube,

while the security system contains:

- at least one communication channel between the first and second sections,

- an explosive device containing at least:

- an explosive plug placed inside said channel and preventing the circulation of fluid in said channel;

- starting device.

Explosive plug is designed to explode when it is actuated by the launcher, so as to at least partially collapse to allow circulation through the specified channel, thereby forming a passage between the first and second sections. Prior to exploding, the plug forms, at least in part and preferably by itself, said fluid isolation between the second and first circuits.

According to one example, the trigger is configured to be activated from outside the steam generator.

According to one example, the starting device comprises at least one wire crossing the wall of the steam generator shell.

According to a non-limiting possibility, the launcher comprises:

- a magnetic contactor located inside the shell of the steam generator and connected by a wire to an explosive plug, and

- a magnetic exciter located outside the shell of the steam generator, designed to interact with the magnetic contactor so that when the magnetic exciter is activated, it activates the magnetic contactor, which starts the explosion of the explosive plug.

According to one example, the safety device is configured such that the secondary fluid contained in the steam generator flows in the casing at a flow rate between 4 and 5 kg/s for a steam generator pressure in the range of 6.8 MPa.

Thus, even with a limited amount of cooling fluid, the phenomenon of the focusing effect can be slowed down and bursting of the housing can be avoided.

The terms "substantially", "about", "within the range" mean taking into account manufacturing and/or measurement tolerances and may in particular correspond to "within 10%".

In the following description, the normal operation of the reactor 1 or plant refers to the operating phase in the absence of accidents or major accidents. An accident such as a loss of primary coolant, a large rupture accident or a very large rupture accident does not correspond to the normal operating phase of reactor 1.

The invention will now be described in detail with reference to Figures 4-8.

Figure 4 illustrates a reactor 1, for example, of the same type as the reactor described with reference to Figures 2-3A.

All features described with reference to Figures 2-3A apply to the embodiments illustrated in Figure 4. In this Figure 4, core 30 is melted or partially melted. A pool 71 of core melt 70 has formed at the bottom 12 of housing 10. A layer 72 of liquid metal has formed or is about to form on the surface of bath 71.

Before this layer 72 of liquid metal begins to pierce the housing 10, or shortly after the start of this partial piercing, a cooling fluid is provided to be poured onto the bottom 12 of the housing 10 and therefore over this layer 72.

This cooling fluid comes from the inlets 13 and/or outlets 14 of the primary circuit 100.

As will be explained in detail below, this cooling fluid consists of water from the second circuit 200, which flows in the first circuit 100.

It should be noted that, traditionally, in the event of a serious accident, depressurization of the primary circuit 100 is triggered. This can be done by opening a specific valve located, for example, at the top of the pressure compensator 110. This primary circuit depressurization may be triggered upon reaching a threshold temperature, such as the node spigot temperature, which will reach 650°C or more. This depressurization of the primary circuit 100 causes the latter to have a pressure lower than the pressure of the second circuit 200. Typically, the pressure in the depressurized primary circuit 100 is less than 2 MPa. Due to the pressure difference between the first 100 and second 200 circuits, the communication of these circuits 100, 200 causes a rapid injection of fluid from the second circuit 200 into the first circuit 100.

According to a particularly preferred embodiment, the fluid 800 of the second loop 200 is poured into the first loop 100 and reaches the interior of the housing 10 through the inlet 13. Preferably, the reactor 1 comprises an inner shell 15 located inside the housing 10, surrounding the core 30 and forming, together with the inner wall 11 of the housing 10 an annular volume, commonly referred to as the downcomer 16 (descending section). This inner shell 15 is designed so that during normal operation of the reactor 1 (in other words, in the absence of a major accident, for example):

- the inlet 13 leads outside the shell 15 and into the descent chamber 16 so that the cold fluid coming from the inlet 13 is directed to the bottom 12 of the housing 10,

- the outlet 14 leads inside the shell 15 so that the hot fluid present in the core 30 can exit the reactor 1 through the outlet 14.

Thus, during normal operation of the reactor 1, the cold fluid of the primary circuit 100 enters the reactor 1 through the inlet 13; descends by gravity into the descent chamber 16 to reach the bottom 12 of the housing, rises inside the shell 15, while passing through a perforated plate, commonly referred to as the base plate 17; crosses the core 30 to remove fission heat and exits the reactor 1 through the outlet 14.

In the context of an embodiment of the present invention, the cooling fluid obtained from the second loop 200 and which penetrates 801 into the housing 10 along the loop 100, therefore also descends along the wall 11 of the housing 10 and reaches the liquid metal layer 72. Therefore, this safety coolant follows the natural path of the water in the reactor 1. This coolant comes into contact with the surface of the liquid metal layer 72. More specifically, the cooling fluid reaches the liquid metal layer 72 at the most critical location, i. at the boundary between the latter and the wall 11 of the housing 10. Therefore, the cooling fluid provides a function of cooling around the entire periphery where the liquid metal layer 72 can push the inner wall 11 of the housing 10 due to the focusing effect. Therefore, the cooling fluid from the descent chamber 16 offers a particularly effective solution for reducing the risk of the housing being punched through due to the focusing effect.

Also preferably, this mode of contact of liquid water with the remelting metal layer 72 is carried out by flowing from the inner wall of the housing 10, which is much softer than a strong injection of water into the core molten pool. Strong injection of water into the core molten bath can cause steam shock, which is detrimental to the integrity of the reactor pressure vessel.

In Figure 4, the cooling fluid is represented as a body 802 distributed over the free surface of the metal layer 72. Naturally, when the latter is not yet sufficiently cooled, the cooling fluid evaporates upon contact with the metal layer 72.

It should be noted that it is highly preferable to keep the pressure relief valves of the primary loop 100 open in order to vent steam produced by contacting the cooling fluid obtained from the second loop 200 with the layer 72 of liquid metal. Moreover, the depressurization of the primary circuit 100 facilitates the injection of cooling fluid from the secondary circuit into the housing 10.

Thus, this cooling fluid allows the layer 72 of liquid metal to be cooled when the latter has a thickness e ₇₂ that is thin enough to concentrate the thermal power of the bath 72 on a too reduced surface so that it can push through the inner wall 11 of the body 10.

This cooling fluid is poured in until the thickness e ₇₂ of the liquid metal layer 72 becomes large enough to transfer the thermal power of this layer 72 over a larger surface, and therefore the power per surface area is low enough to prevent the internal walls 11 of the body 10.

As illustrated in Figure 4, cooling of the outer wall of the housing 10 is also provided. water is injected or poured between housing 10 and shaft 603. This cooling is usually sufficient in the event of an IVR (hold inside housing) type of accident, but is naturally not sufficient if a focusing effect occurs.

Moreover, cooling from the outside of the housing by wetting the shaft 603 of the housing 10 allows, for example, to extract 1 MW per square meter (1 MW/m ² ). In a focusing effect situation, this cooling is no longer sufficient, since it is necessary to be able to extract 1.5 MW/m ² and even 2 MW/m ² in the area in which the focusing effect forms a punching of the housing 10.

According to a non-limiting example, to wet the shaft 603 of the housing 10, you can use the water contained in the reservoir, for example, in the pool for loading fuel. This tank can be used in the reactor building or outside the latter. Preferably, at least one section of this reservoir should be located high enough relative to the body 603 to allow flow to flow into the latter under the action of gravity. Most often, at least one section of this tank should be located above the cover or casing 20 of the reactor 1.

According to one embodiment, the flow rate of the cooling fluid obtained from the second circuit 200 is not controlled. In contrast, the simulation of this flow rate can be easily calculated if the pressure of the second loop 200 and optionally the first _loop 100 is known. cooling time. Calculations show that a very limited portion of this total supply of steam generators 210 is sufficient to sufficiently cool the remelting metal layer 72 and avoid the casing 10 being punctured, while the metal layer 72 is sufficiently thickened on the surface.

Typically, the safety device is configured such that the bore section S of the _passageway (s) between the second circuit 200 and the first circuit 100 allows water from the second circuit to enter the housing 10 at a flow rate of less than 10 kg/s and preferably less than 7 kg/s. Typically, this flow rate is in the range between 4 and 5 kg/s for an initial pressure (in other words, before opening the passage(s) towards the primary circuit 100) in the steam generator (GV) in the range of 6.8 MPa.

This makes it possible to have sufficient cooling of the liquid metal layer for a duration sufficient to avoid bursting of the housing.

According to a non-limiting example, in order to control this cooling time, it is possible to provide for an accurate calibration of the section S of the _hole .

According to one example, S _holes are less than 20 cm ² (10 ^-2 meters). Preferably, the S _holes are greater than 2 cm ² . According to one example, the S of the _hole is in the range between 2 cm ² and 20 cm ² . Preferably it is in the range between 2 cm ² and 7 cm ² .

The S _inlet is the minimum flow area between the primary loop 100 and the inlet 13 of the housing 10. It usually therefore consists of the minimum section for the passage of the primary fluid during normal operation of the reactor. For example, S _input corresponds to the section of the inlet 13 in the body. This section is illustrated in Figure 8. If there are multiple inlets of the primary circuit 100 in the body 10, for example, as illustrated in Figure 7, then the S _inlet is the sum of all the inlets of the body 10.

The _slit S is the cross section of the passage, or the sum of the cross sections of the passages when there are multiple passages, establishing the secondary fluid present in at least one steam generator 210 in fluid communication with the primary circuit 100.

The cross section of the inlet opening on the side of the cold branch of the reactor pressure vessel is much larger than the cross section of the hole formed between the second circuit and the first circuit. Typically, according to a non-limiting example, S _holes < 0.05 * S _inlet and preferably S _holes < 0.01 * S _inlet and preferably S _holes < 0.005 * S _inlet .

Typically, the flow area of the fluid medium of the first circuit from the side of the cold branch is in the range of 6000 cm ² .

Typically, the pressure of the water contained in the steam generator 210 is in the range of 6-7 MPa. In turn, the primary circuit 100 is depressurized. Moreover, devices for opening valves at the level of pressurizer 110 are actuated to depressurize the primary circuit 100 in the event of a serious accident. This makes it possible to avoid release of core fission products under pressure in the event of a case rupture. Moreover, this depressurization of the primary circuit allows the injection of secondary fluid into the interior of the housing 30 to be facilitated.

In most scenarios leading to a serious accident, the secondary circuits are closed and isolated, on the one hand, by closing the steam injection lines leading to the turbine, and, on the other hand, by exhaust valves to the atmosphere.

According to one example, the triggering of the safety device is performed by the operator. In order to determine the point in time at which the fluid of the second loop 200 should be poured into the first loop 100, it is preferable to be able to estimate the height of the molten pool of the core and preferably the profile of change of this height.

To this end, one or more fuses 900 can be provided located on the wall of the housing 10. They are designed to melt when a threshold temperature Tf is applied to these fuses 900. Typically, this temperature Tf is reached when the core melt forms in the vessel 10 and comes into contact with the fuses 900. When the temperature inside the vessel corresponds to the normal operation of the reactor, the fuses 900 do not melt. According to one example Tf > 400°C, preferably Tf ≥ 500°C, preferably Tf ≥ 600°C.

When the fuse 900 melts, it prevents the passage of an electrical signal. In this regard, the resistance of the circuit integrated into this fuse is infinite.

The fuse contains a cartridge made of electrically conductive material and a sheath that is electrically insulating. In this way, a short circuit between the metal housing and the conductive cartridge is avoided.

For example, the cartridge is made of a metal such as aluminum, whose melting point is close to 600°C, or of antimony. For example, the insulating sheath is made of ceramic.

For example, a fuse forms a cable having two ends connected to a safety device and a bend located between the two ends. The bend corresponds to the lowest point of the fuse. Thus, when a fuse changes from a conductive configuration in which current flows in the cartridge from one end to the other (resistance R1), to a non-conductive configuration in which current no longer flows in the cartridge from one end to the other (resistance R2 > R1, preferably infinite R2), which means that the core molten bath has melted the bend. Thus, it was concluded that the height of the free surface of the molten pool of the core corresponds to the height of the bend of the fuse 900 relative to the bottom of the housing 10.

Using a fuse turns out to be much more stable and reliable than using temperature sensors.

Preferably, the fuse is located on the inner wall 11 of the housing 10. This can improve the reliability of detecting the occurrence of the focusing effect. Moreover, by placing a fuse on the outer wall of the housing 10, the temperature measurement will essentially depend on the boiling point of the water in contact with the housing wall, which does not effectively detect the rise of the core molten pool, as well as the formation of the focusing effect layer.

Preferably, the safety device comprises a number of fuses 900 located along at least one generatrix of the inner wall 11 of the housing 10. Preferably, the fuses are located along at least two generators. Thus, if the core melt occurs along the generatrix, the fuses located in the upper section can be reached and not characterize the gradual rise of the core melt pool at the bottom of the case.

Preferably, for each generatrix of a row of fuses 900 is located on the generatrix of the hemispherical section, forming the bottom 12 of the housing 10, the last and other section of the fuses is located on the side wall of the housing 10.

These fuses make it possible to determine the point in time at which the molten pool 71 of the core begins to form, as well as the point in time at which water from the second circuit should be injected into the first circuit 100.

For example, depending on the height profile of the molten pool of the core, which is estimated according to the signals sent by the fuses, it is possible to choose by simulation the most appropriate time to start the injection of water from the second to the first circuit 100.

The profile of this temperature change also makes it possible to detect the rise in the level of the core melt in the body 10. This profile also makes it possible to detect the beginning of the last layer 72 of liquid metal being forced through.

For example, fuses 900 of the same generatrix or fuses 900 of two different generators having different melting points can be provided. To this end, different materials can be provided for the sheath and/or fuse holder. It is preferable to provide a device that can be completely dismantled and replaced, for example during a ten year inspection, in order to be able to have a set that is essential to safety, whose service life does not exceed 10 years of reactor operation.

According to a particularly preferred example, it is desirable to arrange the devices for detecting the height of the core molten pool so that the rate of rise of the core molten pool is determined by the fuses. This allows more precise control of the point in time at which the focusing effect may occur and the point in time at which the fluid of the steam generators 210 is to be poured into the first circuit 100. Thus, this arrangement of the fuses 900 is made such that the _slice volumes V of the cartridge slices located between two series or adjacent fuses 900 are constant.

More specifically, two adjacent fuses, according to the vertical distribution, form a section of the body. This casing section is defined on the one hand by the inner wall 11 of the casing 10 and, on the other hand, by two vertical planes, each of which passes through one of these adjacent fuses 900. At least some, preferably all, volumes of the casing sections are identical. . Preferably adjacent fuses according to the vertical distribution are located according to the same curvature, preferably forming a generatrix of the housing 10.

Figure 8 shows in a very schematic way a portion of the housing 10 as well as the fuses 900 arranged along the generatrix G. The fuses 900 form cuts with substantially equal _cut volumes V .

Thus, if the rate of formation of the core molten pool (and therefore the rate of formation of the core molten pool) is constant, fuses of the same generatrix are reached by the core molten pool with identical time intervals between two successive fuses of this generatrix.

With reference to Figure 8, an example of calculating the position of the fuses will now be described.

In this example, fusible detection devices are placed along 2-4 generatrices evenly distributed around the axis of the hemispherical bottom of the housing 10. Thus, for the number of generators equal to 2, 3 and 4 respectively, these generators will be separated at an angle of 180°, 120° and 90° respectively.

It is believed that the bottom of the housing 10 is formed by a portion of a sphere having a diameter of 4 meters.

It is desirable to locate devices for detecting the height of the core molten pool in accordance with 3, 6, 9, 12 and 15 m ^{3 of} the spilled molten core. This makes it possible to determine the rise profile of the molten pool of the core. With a clear understanding of the change in the rate of rise of the core molten bath, the operator (or automatic safety device) can determine what is the most appropriate time to start the water injection of the steam generators 210 into the primary circuit 100.

The total filling of the hemispherical bottom is 16.75 m ³ .

Figure 8 shows the following parameters:

- R = radius of the hemispherical section of the body,

- h = height of the fuse in relation to the bottom of the housing,

- L _arcs = length along the casing wall between the bottom of the casing and the position of the fuse,

- r = distance between the axis of the cylinder of the cylindrical section of the housing 10 (namely, the axis passing through the center of the sphere and which is perpendicular to the plane P).

The plane P corresponds to the junction between the hemispherical section of the housing and the side walls of the housing, continuing along the cylinder.

The volume V of _{the lid} of the hemispherical portion can be calculated according to the following equation:

The position of the fuse can be determined according to the L value _{of the arc} calculated according to the following equation:

If it is desired to have five fuses on each generatrix, the fuses can be arranged as follows in order to match the rate of rise of the core molten pool and the melting of these fuses:

- fuse No. 1: height h = 0.738 m; L _arc \u003d 1.776 m;

- fuse No. 2: height h = 1.079 m; L _arc \u003d 2.185 m;

- fuse No. 3: height h = 1.362 m; L _arcs = 2.492 m;

- fuse No. 4: height h = 1.617 m; L _arc = 2.756 m;

- fuse No. 5: height h = 1.860 m; L _arcs = 3.002 m.

Alternatively, the communication of the second loop 200 with the first loop 100 can be controlled on a schedule basis starting from the time when the formation of the core molten pool 71 is detected by the fuses 900.

By simulation, it can be determined much in advance that there is a duration D1 between the start of the formation of the core molten pool 71 and the start of the formation of the layer 72 of liquid metal at the point of occurrence of the bursting of the casing 10. Naturally, this duration varies from reactor to reactor. For some reactors this duration D1 is about one hour. The operator must activate the safety device at time t1, whereby t1 = t0 + (D1 - k1), k1 is the safety factor to ensure that the secondary water is poured onto the liquid metal layer 72 early enough to avoid significant weakening of the casing 10 and preferably to avoid the start of bursting of the body. For example, k1 is between -5 minutes and 15 minutes.

By simulation it is also possible to determine the duration D2 between the start of the formation of the core molten pool 71 and the time t2 at which the liquid metal layer 72 has a thickness e ₇₂ that is large enough to make it impossible for the casing 10 to be punched through.

The flow area of the passage(s) between the second circuit 200 and the first circuit 100 is sized to allow pouring between times t1 and t2 at a flow rate Qmin sufficient to cool the liquid metal layer 72.

As indicated above, it is very preferable that the fluid 800 obtained from the second circuit and which is poured onto the remelting metal layer 72 descends by flowing down the wall 11 of the housing 10. However, alternatively or in combination with this embodiment, it can be provided that this fluid 800 reaches the interior of housing 10 by entering the latter through outlet 14.

Figures 5A-7 illustrate various embodiments that allow fluid from the second loop 200 to be injected into the housing 10 through the first loop 100.

The solution to all these embodiments is to install the second circuit 200 in communication with the first circuit 100. This communication is carried out thanks to the safety system designed to deliberately destroy the protective barrier that isolates these two circuits 100, 200. It should be recalled that during normal operation, in other words, in the absence of incidents and during the power generation phase of the plant, the first 100 and second 200 circuits are fluidly isolated from each other.

The first embodiment is illustrated in Figures 5A-7.

In this embodiment, the hydraulic connection between the second circuit 200 and the first circuit 100 consists of a hydraulic connection internal to the steam generators 210.

The steam generator 210 forms a shell that surrounds the fluid of the second circuit 200 and simultaneously surrounds at least one pipe 214 through which the fluid of the first circuit 100 circulates. Thus, the fluid of the second circuit 200 comes into contact with the outer wall of the pipe 214.

According to the first solution, the security system includes a device that allows at least one hole to be formed between the inside of said pipe 214 and the secondary fluid 200 present in the shell of the steam generator 210.

Thus, the hole forms said passage allowing water of the second circuit to be injected into the first circuit 100.

This solution is schematically illustrated in Figure 5B. It will be described in detail in the form of some embodiments with reference to Figures 6A-7.

Figure 6A illustrates the lower portion of the steam generators 210. This steam generator 210 includes a shell 260 with a generally cylindrical shape. This sheath 260 defines an upper portion 211 having openings 211b, 211b' in communication with the second circuit 200 and a lower portion 212 having openings 212b, 212b' in communication with the first circuit 100. The upper 211 and lower 212 portions are separated by a tube plate 213 . The lower surface of the plate 213 with tubes defines with the lower section 212 the volume forming the water chamber. Preferably, the water chamber has a hemispherical shape. It is divided into two sections 212a, 212a' by a partition wall 2121.

Section 212a has an opening 212b hydraulically connected to the inlet 13 of the body 10. Section 212a' has an opening 212b' hydraulically connected to the outlet 14 of the body 10. Thus, sections 212a and 212b, as well as tubes 214, are part of the first circuit 100 .

Pressurized water at high temperature entering from the core 30 of the nuclear reactor 1 into the outlet 14 of the housing 10 penetrates the water box portion 212a' and then circulates in the tubes 214 of the tube network of the steam generator 210. Moreover, the tube plate 213 holds the tube group 214, one end of which leads to section 212a', and the other end of which leads to section 212a. Typically, these tubes 214 are in an inverted U-shape. These U-tubes are wetted by the secondary water 200 present in the upper section 211 of the steam generator 210.

Thus, pressurized hot water circulates starting from section 212a', first from bottom to top up to the top of the U-bend and then from top to bottom to reach section 212a of the water box. All along this path, the water circulating through the tubes 214 transfers heat to the secondary loop fluid 200 present in the upper portion 211 of the steam generator 210. Once it has reached the water box portion 212a, the water may then exit through the outlet 212b and return back to the inlet. 13 of the housing 10 for reheating the core 30.

It should be noted that each of the sections 212a, 212a' of the water box is provided with a hatch hole 2122 closed with a plug 2123. This hatch hole 2122 has a size that is large enough to allow a human or robot to access the interior of the water box.

In a particularly preferred manner, the safety system comprises a device, also referred to as an element, configured to break the insulation between the first circuit 100 and the second circuit 200 within the steam generator 210.

Typically, this device is an explosive device 216 designed to create an explosion capable of creating a hole that deliberately destroys the hydraulic protective barrier between the first circuit 100 and the second circuit 200. Preferably, this hole is formed in the so-called cold section of the pipe 214, in other words, along move after a bend with an inverted U-shape. This allows the gravity flow of the second loop fluid in the primary loop 100 to be facilitated. gravity of all the secondary water contained in the steam generator 210.

According to the first embodiment, this device, which has the reference position 216 in Figure 6B, contains an explosive plug 216a placed inside the tube 214'. Preferably, this tube 214' is identical to the tubes 214 of the tube plate 213, which greatly simplifies the manufacture of the steam generator 210.

An explosion plug 216a is inserted into the tube 214' above the tube plate 213. When activated, the explosive plug 216a separates at least the tube 214' in which it is placed. The interior of this tube 214' then comes into fluid communication with the water of the secondary circuit 200 present in the upper section 211 of the steam generator 210.

This water is then rapidly poured into the water box and then into the first circuit 100 to reach the vessel 10 and the core 30 of the reactor 1. This water is rapidly directed into the reactor 1 by the pressure prevailing in the steam generator 210. Moreover, it should preferably be recalled that the steam generator 210 is hydraulically isolated very quickly in the event of an accident, thereby trapping the water masses contained in it and keeping them in the pressure prevailing in it.

According to this embodiment, if only one tube 214 is divided, then the section S of the _hole is essentially equal to the section of the tube 214 that was divided, if several tubes 214 are divided, then S of the _hole is essentially equal to the sum of the sections of the divided tubes 214. The section S of the _hole has such size so that the secondary water flows into the housing in a controlled manner, as indicated below.

In the diagram in Figure 6B, the water present in the upper section 211 is poured into the section 212a of the water box. Preferably, the latter is hydraulically connected to the inlet 13 of the housing 10, which allows water to reach the remelting metal layer by descending through the descent chamber 16, thereby providing the above-mentioned advantages.

A hole 215 for sealed passage of the control wires 217b of the explosive device 216 must be formed in the wall of the heat generator 210. Thus, these control wires 217b have one end accessible from the outside of the steam generator 210 to start the explosion. These wires 217b are installed during the installation of the explosive device 216.

According to one embodiment, the security system includes, in addition to the explosive plug 216a, a thermowell, a magnetic connector, and a magnetic exciter for triggering the explosion. The thermowell has a portion that extends within the steam generator 210 up to the blast plug 216a. This section contains a magnetic contactor and a wire connecting the magnetic contactor to the explosive plug 216a. The thermowell has an area accessible from the outside of the steam generator 210. This area holds or is accessible from a magnetic driver configured to activate the contactor. When the contactor is activated, it triggers the explosion of the explosive plug 216a.

In this case, the thermowell is preferably made of a non-magnetic material.

This solution has the advantage of avoiding the passage of the wire through the wall of the steam generator 210, which simplifies the sealing of the system.

Naturally, provision may be made to equip multiple tubes 214' with blaster 216 in order to introduce redundancy and reduce any risk of failure of blast plug 216a. This also makes it possible to increase the total cross-section S of the _hole of the fluid passage of the second circuit 200 inside the first circuit 100.

It is also possible to provide for the integration of several explosive devices 216 in the same tube 214' in order to introduce redundancy and reduce any risk of failure of the explosive plug 216a. To this end, the control wires 216b of the upper tubes 214' themselves must remain accessible during the installation of devices placed at a lower level.

Explosive device 216 can be placed during the manufacture of steam generator 210. Alternatively, this solution can also be applied to steam generators in existing installations. To this end, work is being done to seal the tube 214 or several tubes 214 already in existence with a blast plug 216a.

This embodiment has the advantage that no additional elements are introduced into the steam generator 210. Moreover, the explosive device is integrated into one of the tubes found in conventional steam generators 210.

An example of an explosive device 216 is illustrated in Figure 6C. This figure shows a tube 214' sealed with a blast plug 216a. Explosive plug 216a includes a housing 216c, inside which are placed the main charge 216d and at least one section of the detonator 216e. Detonator 216e is in contact with charge 216d. Preferably, housing 216c is made of or contains an explosive material. One or more wires 216b connect detonator 216e to a firing circuit accessible from outside steam generator 210. Preferably, containment 216f isolates detonator and charge.

Figure 6B illustrates another alternative embodiment of explosive device 216 in the right section.

In this example, a through passage 218 is provided formed in the tube plate 213 and accommodating in communication the secondary water contained in the upper section 211 and the primary water contained in the lower section 212 of the water box, preferably the section 212a connected to the inlet. 13 of the housing 10. This bore 218 is sealed with an explosive device 216. For example, this explosive device 216 may be identical to the previously described explosive plug 216a.

The advantage of this embodiment is that the section S of the _hole is formed by the original section of the channel 218. However, this section of the channel 218 is perfectly controlled. This allows more precise control of the rate and duration of secondary fluid flow within housing 10. Naturally, for this purpose, the explosive load must be large enough to completely remove plug 216 from channel 218.

It should be noted that the same steam generator 210 may contain explosive devices 216 of different types, as illustrated in Figure 6B. Most often, the same steam generator 210 will contain one separate explosive device 216 or several explosive devices 216 of the same type.

Figure 7 illustrates in perspective view the solution of the embodiment of Figures 5A and 5B. This figure shows a reactor core 1 with a vessel 10 whose bottom 12 contains a core molten bath 70 . Also illustrated is a booster 110 for controlling and increasing the pressure of the primary circuit 100. Pipes hydraulically connect the housing 10 to the steam generators 210, of which there are two.

For each steam generator 210, the housing 10 is fed through two inlet pipes. On each of these inlet pipes between the steam generator 210 and the housing 10 is located a pump 101, 102 to circulate the fluid of the primary circuit 100.

These pumps 101, 102 lie on the fluid passage 800 obtained from the secondary circuit 200 when the explosive device 216 was fired.

In this example, pumps 101, 102 are of the submersible rotor type. Moreover, the rotors of these pumps lie vertically below the fluid passage contour. This type of pump avoids the presence of a U-shaped branch in the pipe connecting the steam generator 210 to the inlet 13 of the housing 10. This allows the water obtained from the secondary circuit 200 to be facilitated by gravity down to the cold section of the housing 10. As indicated below, this gravity flow along the downcomer chamber 16 improves the cooling efficiency of the liquid metal layer 72 at the point where the shearing of the body 10 occurs due to the focusing effect. Therefore, the solution illustrated in Figure 7 has a significant advantage.

This Figure 7 also illustrates atomization in the form of a jet (spray) 803, which occurs when the water of the second circuit is advanced into the first circuit 100 when the protective fluid barrier is destroyed.

It should be noted that the invention is also applicable in the case where the primary circuit pipe 100 has a U-shaped branch between the steam generator 210 and the casing 10.

Under the condition of the destruction of the tube 214 of the steam generator 210 or the punching of a hole in the plate 213 with tubes from the cold side 13 of the primary circuit 100, the presence of a U-shaped branch along the way in front of the pump 101 of the primary engine is unfavorable for the injection of liquid under the action of gravity from the formed hole before pouring into down chamber 16. However, the advancing puncture spray 803 transports the liquid fraction through the return circuit up to down chamber 16. However, the presence of the U-shaped branch prevents the liquid flowing solely by gravity down to bed 72 from cooling. However, the steam generated by the secondary water tends to flow preferably from the cold side up to the downcomer 16 instead of rising through the set of tubes 214 of the steam generator 210 and then lowering and returning back to the casing 10 from the hot side 14, which helps to cool the liquid layer 72 metal at the place where the puncture occurs housing 10 due to the focusing effect. This cooling is not as effective as flowing liquid solely by gravity by running off, since in this case the mixture of steam and spray comes into contact with the entire metal surface present in the downcomer chamber 16. In order to obtain liquid flow solely by gravity, it is necessary wait until the U-shaped branch is completely filled with liquid, so that excess water then begins to pour into the volute of the pump 101 of the primary circuit 100 and then towards the cold branch with the inlet 13 of the housing 10 and finally into the downcomer 16. In connection with Therefore, the geometry of the submersible rotor primary pump without any U-branch, as illustrated in Figure 7 and mentioned above, is preferable for obtaining optimum water injection efficiency as a result of the destruction of the tube 214 of the steam generator 210, since the preference is given to the flow of liquid solely under the action of gravity.

Given the above description, it is clear that the invention provides a reliable and stable solution for significantly improving the safety of a PWR type nuclear reactor, in particular in the event of a loss of primary coolant.

Preferably, and as described above, the point of communication between the first circuit and the second circuit is chosen so that the passage of the water of the second circuit towards the first circuit occurs almost exclusively from the cold branch of the first circuit. Thus, the water from the steam generators will follow the cold branches of the primary circuit before flowing under the action of gravity into the downcomer, where the shell is forced through due to focusing effects.

The invention is not limited to the previously described embodiments and extends to all embodiments covered by the claims.

Naturally, the invention is not limited to the reactor having the design illustrated in Figure 4, and numerous modifications to this reactor can be made without deviating from the scope of protection defined by the claims.

In particular, only one inlet 13 and one outlet 14 are shown in the figures. Preferably, the reactor 1 has multiple inlets and multiple outlets. Preferably, it also contains several steam generators 210.

Claims (33)

1. Nuclear reactor (1) with pressurized water, including at least - housing (10) containing the core (30) of the reactor (1), containing at least one inlet (13) and at least one outlet (14), - the first circuit (100), at least one first end of which is connected to the inlet (13) of the body (10) and at least one end of which is connected to the outlet (14) of the body (10) so that the fluid of the first circuit , circulating in the primary circuit (100), penetrates into the vessel (10) of the reactor (1) through the specified inlet (13) and exits through the specified outlet (14), while passing through the entire core (30), for heat extraction produced by the active zone (30), while the S _inlet is a section of at least one inlet (13) of the first circuit (100) up to the housing (10), - a second circuit (200) isolated from the first circuit (100) in terms of fluid, in which the water-based second circuit fluid is intended for circulation, containing at least one steam generator (210) and configured to absorb the heat of the first circuit (100 ) and converting it, at least partially, into steam in the steam generator (210), characterized in that the reactor (1) includes a safety system containing: - a safety device containing at least one explosive device (216), made with the ability to explode, in order to destroy the fluid insulation between the second circuit (200) and the first circuit (100) so as to form at least one and preferably only one passage allowing the secondary fluid present in the at least one steam generator (210) to flow within the primary circuit (100). 2. Reactor according to claim 1, in which the passage has a cross section S of the _hole in the range between 0.2 cm ² and 20 cm ² , preferably 0.8 cm ² and 20 cm ² and even more preferably between 2 cm ² and 7 cm ² . 3. The reactor according to any of the previous paragraphs, including at least one fuse (900) located on the wall (11) of the housing (10), made so that when the liquid metal layer (72) on the surface of the bath (71) of the melt ( 70) of the active zone formed on the bottom (12) of the housing (10) reaches the fuse, this liquid metal layer (72) causes the fuse to melt, while the melting temperature of the fuse (900) exceeds or equals the temperature threshold Tf, while Tf ≥ 400°C, preferably Tf ≥ 500°C and preferably Tf = 600°C. 4. The reactor according to claim 3, including several fuses (900) distributed according to at least one generatrix (G) of the body wall (10), fuses (900) are distributed along the generatrix (G) so that two adjacent fuses (900) according to this generatrix form a cut of the body (10), the volumes V of the _cut of the cuts are identical. 5. The reactor according to any of the previous paragraphs, including an inner shell (15) located inside the housing (10), surrounding the active zone (30) and forming an annular volume with the inner wall (11) of the housing (10), called the downcomer chamber (16) , which is designed so that during normal operation of the reactor (1): - the inlet (13) leads out of the shell (15) and into the down chamber (16) so that the primary circuit fluid coming from the inlet (13) is directed to the bottom (12) of the housing (10), - the outlet (14) leads inside the shell (15) so that the primary fluid present in the core (30) can exit the reactor (1) through the outlet (14), wherein the reactor is configured such that when the explosive device forms at least one fluid isolation passage between the second circuit (200) and the first circuit (100), the second circuit fluid contained in the steam generator (200) in this In this case, it flows along the bottom (12) of the body (10), while first passing through the specified inlet (13) of the body (10) and then through the down chamber (16). 6. The reactor according to any of the previous claims, in which the steam generator (210) contains a shell (260), the surrounding part of the fluid medium of the second circuit and the part of the fluid of the first circuit and the surrounding insulation, isolating the fluid of the second and primary circuits, while the safety system is designed to eliminate the isolation between the secondary and primary fluids inside the steam generator shell, thereby forming said passage. 7. Reactor according to any of the previous claims, in which the steam generator (210) contains tubes (214), inside which the fluid of the first circuit circulates, while the fluid of the second circuit is in contact with the outer wall of the pipes (214), and the explosive device ( 216) contains: - at least one explosive plug (216a) placed inside at least one (214') of said tubes (214) and - starting device, while the explosive plug (216a) is configured to explode when it is actuated by the launcher, so as to separate at least the tube (214') in which it is placed, thereby forming the specified passage. 8. The reactor according to any one of paragraphs. 1-6, in which the steam generator (210) contains: - the first section (212), in which the primary fluid circulates, - the second section (211), in which the fluid medium of the second circuit circulates, - a plate (213) with tubes (214) insulating the first (212) and second (211) sections in terms of fluid, which contains at least one tube (214) continuing at least partially in the second section (211) and inside which the primary fluid circulates, while the secondary fluid present in the second section (211) is in contact with the outer wall of the tube (214), - while the security system contains: at least one communication channel (218) between the first (212) and second (211) sections, - explosive device (216), containing at least: an explosive plug (216a) placed inside said channel (218) and preventing circulation of fluid in said channel (218), starting device, wherein the explosive plug (216a) is configured to explode when it is actuated by the launcher, so as to break at least partially to allow circulation through said channel (218), thereby forming a passage between the first (212) and the second (211) plots. 9. The reactor according to any one of paragraphs. 7 or 8, in which the trigger is configured to be activated from outside the steam generator (210). 10. Reactor according to claim 8, wherein the starter comprises at least one wire (216b) crossing the shell wall (260) of the steam generator (210). 11. The reactor according to any one of paragraphs. 7-9, in which the starting device contains: - a magnetic contactor located inside the shell (260) of the steam generator (210) and connected by a wire (217b) to an explosive plug (216a), and - a magnetic exciter located outside the shell of the steam generator (210), designed to interact with the magnetic contactor so that when the magnetic exciter is activated, it activates the magnetic contactor, which triggers the explosion of the explosive plug (216a), thereby forming the specified passage .

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