CN1266706C - Oxide materials for nuclear reactor fusion liner wells - Google Patents

Abstract

An oxide material for a nuclear reactor melt liner well, comprising a coolant and oxidant for cooling _the melt liner and oxidizing the most active component therein, and further comprising a target additive, the target additive consisting of at least one oxide selected from _Gd2O3 , _{Eu2O3 and Sm2O3} , the content of the target additive preferably being at most 4wt%, more preferably 0.1-4wt%. _The coolant and oxidant preferably comprises _Fe2O3 and/ _{or Fe3O4 and Al2O3 ,} wherein the content of _Fe2O3 and/or _Fe3O4 is 46-80wt%, _and the content of _Al2O3 is 16-50wt%. The material of the present invention may also comprise _SiO2 , _the content of which is at most 4wt%, preferably _1-4wt %.

Description

Oxide materials for nuclear reactor fusion liner wells

technical field

The present invention relates to the atomic energy industry and in particular to so-called sacrificial materials, i.e. the materials of nuclear reactor melt lining wells, used to locate the molten corium of closed water-cooled nuclear reactors in the event of a hypothetical accident. When such an accident occurs, the material interacts with the high-temperature molten lining of the nuclear reactor, causing the melt to stay (locate) in the trap and cool it down, creating a subcritical state and preventing a self-sustaining chain The occurrence of fission reactions, i.e. preventing the transition of nuclear reactions in the melt to supercritical mode. Thereby, the sacrificial material itself gradually dissolves through complex physicochemical processes and no longer exists in its original form.

Background technique

After the large-scale accidents at Chernobyl Nuclear Power Plant 4 and the TMI nuclear power plant in the United States, as well as other accidents at nuclear plants, it is particularly important to develop sacrificial materials in devices that allow the formation of molten linings to be located during nuclear power plant accidents . Currently, the development of the nuclear power industry relies in many cases on reliable systems for locating the molten lining of nuclear reactors and the fabrication of effective sacrificial materials for nuclear reactors.

The sacrificial material is fundamentally a new type of material, and there is not much research and development related to it, and since it is impossible to directly carry out a full-scale experiment, it can only be based on a system engineering method using theoretical calculations and model experiments.

It is well known that nuclear reactor melt liners comprise two phases: a metallic phase (lighter) and an oxide phase (heavier). In order to effectively reduce the temperature of superheated metal components in the melt, steel can be used as a coolant. However, steel cannot affect the oxide portion of the melt, whereas the main emissions are located in the oxide and can undergo a nuclear chain reaction. Moreover, this coolant cannot oxidize the zirconium dissolved in the oxide portion of the melt and partly enters the metal portion, causing unoxidized zirconium to react with water vapor to generate hydrogen gas, which burns and explodes.

It is known to use oxide materials, for nuclear reactor melting liner wells, containing silicon oxide or aluminum oxide as coolant and oxidant (see: RU2165106, International Classification Number G21C9/016, 12/10, 4th of 2001 October 10). These oxides mix with the uranium dioxide present in large quantities in the melt, reducing its concentration and thus the possibility of nuclear reactions in the melt reaching supercritical mode; and due to their rather high heat capacity, these oxides cool the melt and positioned in the well. In addition to using this material, zirconium can be oxidized by silica and alumina, to a greater extent by silica than alumina.

In order to oxidize zirconium more efficiently, another oxide material was proposed for nuclear reactor melt lining wells (see: Markus Nie. Application of sacrificial concrete for the retention and conditioning of molten corium in the EPR core melt retention concept. OECDWorshop on Ex-Vessel Debris Coolability, Karlsruhe, Germany, 15-18 November 1999). In this material, a mixture including iron oxide, silicon dioxide, and aluminum oxide and oxides of boron, calcium, magnesium, and chromium is used as the coolant and oxidizer. This material contains about 22-45% iron oxide, about 25% silicon dioxide and about 2% aluminum oxide.

Known sacrificial materials dilute the nuclear reactor melt lining, helping to limit the multiplication factor of neutrons produced by the nuclear reaction process below a value at which the nuclear reaction in the melt reaches a supercritical mode, which is not allowed. However, the issue of nuclear safety remains very important by reliably maintaining subcritical conditions through nuclear reactions in semi-melts.

The basic object of the present invention is to provide oxide materials for use in nuclear reactor melt liner wells to more reliably maintain the melt in a subcritical mode, thereby ensuring nuclear safety. Its composition can be well mixed with the uranium oxide in the molten lining of the nuclear reactor, and completely oxidize the zirconium present in the melt, reduce the generation of volatile substances in the melt, and obtain a higher rate of reaction with the melt, and a lower Reaction onset temperature, low crystallization onset temperature, and stability of oxide sacrificial materials. This provides reliable positioning of the melt, makes it less likely that the nuclear reaction will reach supercritical mode, prevents combustion and explosion of hydrogen in the container, and reduces release of radionuclides to the environment.

Contents of the invention

Considering the above basic purpose, an oxide material for nuclear reactor melt lining is proposed, including cooling and an oxidizing agent for cooling the melt lining and oxidizing the most active components therein. According to the present invention, said material also contains a target additive consisting of at least one oxide selected from Gd ₂ O ₃ , Eu ₂ O ₃ and Sm ₂ O ₃ .

The introduction of these oxides in the sacrificial material can absorb neutrons in a wide range of energy spectrum lines to ensure more reliable nuclear safety of the molten lining, that is, by effectively absorbing radioactive isotopes of uranium and other radioactive elements in the melt Neutrons emitted by nuclear decay allow the molten lining of nuclear reactors to be more reliably maintained at subcritical conditions. During the solidification of the melt, under conditions where the local hazard of the system (supercritical state) cannot be achieved, i.e., the neutron multiplication factor is kept at such a level that the uranium-containing oxide part of the melt is guaranteed to be in a subcritical state, to Gd in the form of Gd ₂ O ₃ , Eu in the form of Eu ₂ O ₃ , and Sm in the form of Sm ₂ O ₃ co-crystallize with uranium dioxide and most fissile isotopes.

The target additive content can be as high as 4 wt%. When Gd ₂ O ₃ is used, its content is preferably 0.1 to 0.4 wt%. When Eu ₂ O ₃ and/or Sm ₂ O ₃ are used, their content is preferably 1˜4 wt % because their ability to absorb neutrons is lower than that of Gd ₂ O ₃ .

A preferred cooling and oxidizing agent for _the oxide material of the molten liner well of _a nuclear reactor includes 46-80wt% _Fe2O3 and/or _Fe3O4 and 16-50wt% _{Al2O3 .} This oxidizing and cooling agent composition provides good oxidation of zirconium in the melt due to iron oxide and efficient cooling of the melt due to the high heat capacity of alumina. At the same time, the oxides of iron and aluminum have good mixing properties with the oxides of uranium in the molten lining without causing delamination of oxide phases (segregation), so that the melt is well diluted and still more reliable maintain a subcritical state. Also, oxides of iron and aluminum react to form a solid solution whose crystallization temperature is lower than that of alumina. This promotes the reaction between the molten liner and the sacrificial material, resulting in good dilution of the melt and keeping it in a subcritical state.

The oxidizing and cooling agent for the sacrificial material of the present invention may also contain silicon dioxide, the content of which can be as high as 4wt%, preferably 1-4wt%. However, silica is poorly miscible with uranium dioxide in the melt, but such a small amount can be dissolved in the oxide part of the melt without stratification, thereby lowering the onset temperature of crystallization somewhat, which is helpful Helps to dilute uranium dioxide and keep it subcritical. Also, adding such a small amount of silicon dioxide increases the strength of the sacrificial material by 40-50%, because a compound of SiO ₂ and Al ₂ O ₃ (mullite) is formed.

Detailed ways

The sintered oxide sacrificial material according to the invention is obtained by a secondary calcination method, resulting in a dimensionally stable brick. In the initial stage, the raw materials are prepared in _the mixing ratios required subsequently, then the raw materials representing the cooling and oxidizer base materials are mixed and _dry vibratory milled, and the target additives ( _Gd2O3 , _Eu2O3 and/or Sm ₂ O ₃ ) is subjected to dry vibratory grinding alone, and powders with a particle size of no more than 63 mm are finally obtained. Grinding was stopped when a particle size of 63 mm was reached. The target additive oxide powder was mixed with a part of the base material in a ratio of 1/10 to 1/5 (the target additive was not used in the comparative example). The resulting mixture is mixed well before adding to the rest of the base ingredients. After mixing again, the fine powder of the target additive is evenly distributed in the powder containing the base material. Then use 5% polyvinyl alcohol aqueous solution as a burnable binder, press into bricks, and calcinate at 1280-1300° C. for 2 hours. Thereafter the bricks were crushed, ground, graded, mixed with a temporary binder (5% polyvinyl alcohol in water) and pressed. Finally, it is calcined at 1320° C. for 6 hours in air.

In use, the oxide sacrificial material of the present invention is placed in a well, for example below a nuclear reactor, preferably together with a metal sacrificial material. When an accident occurs and the melt penetrates the nuclear reactor wall, the molten lining, which reaches a temperature of 2,700°C, flows down the well and reacts with the sacrificial material. In this case, the sacrificial material is melted and mixed with the molten liner, the melt is first cooled to about 2000°C to prevent it from melting the walls of the well, thereby immobilizing the melt; secondly, the uranium dioxide in the melt is diluted, This keeps the nuclear reaction in the melt subcritical. In addition, the sacrificial material oxidizes the zirconium in the molten liner, reducing the amount of hydrogen released by the reaction of zirconium with water.

In the process of cooling the molten liner of nuclear reactors, the oxides of Gd, Eu and Sm are located in the oxide part of the active region containing a large amount of fission isotopes. They co-crystallize with uranium dioxide, which has similar physicochemical properties. During the decay process of radioactive isotopes of uranium and other radioactive elements and fission products, neutrons are emitted; when neutrons are captured by nuclei of other fissionable materials, they can induce chain nuclear reactions. The ratio of the number of neutrons produced in a later step of decay to the number of neutrons produced in an earlier step is expressed as the neutron multiplication factor and is used to characterize the mode of a nuclear reaction. If this factor is greater than 1, the nuclear reaction is in supercritical mode, where the number of neutrons increases like an avalanche, causing an explosion. For a nuclear reaction in subcritical mode, ie without an increase in the number of neutrons, the aforementioned K-factor is less than 1. Gadolinium oxide (Gd ₂ O ₃ ), europium oxide (Eu ₂ O ₃ ), or samarium oxide (Sm ₂ O ₃ ) present in oxide sacrificial materials whose metallic nuclei absorb neutrons because these nuclei The spectrum has a large neutron absorption effective cross-section, thereby preventing these neutrons from being captured by the uranium nucleus, reducing the number of neutrons produced in the next step of decay, that is, reducing the neutron multiplication factor, and preventing the chain The nuclear reaction reaches supercritical mode.

Although the introduction of Gd ₂ O ₃ , Eu ₂ O ₃ or Sm ₂ O ₃ in the basic oxide sacrificial material composition yields the same technical results, the use of Gd ₂ O ₃ is preferred because its preferred content is 0.1 to 0.4 wt %, which is 10 times smaller than the preferred content of Eu ₂ O ₃ and Sm ₂ O ₃ of 1-4 wt%. The content of Eu ₂ O ₃ and Sm ₂ O ₃ must be greater than that of Gd ₂ O ₃ because of their low neutron absorption capacity. However, _{although Gd2O3} is more neutron absorbing, it is more _advantageous to use _Eu2O3 and _Sm2O3 because they are much _{cheaper than Gd2O3} .

The determination of the lower limit concentration of Gd ₂ O ₃ , Eu ₂ O ₃ or Sm ₂ O ₃ is based on the fact that the neutron multiplication factor must be reliably provided to ensure that the uranium-containing oxide part of the melt is in a subcritical state. The determination _of the upper _limit concentration _of these oxides is based on economical considerations, namely the high cost of _Gd2O3 , _Eu2O3 and _Sm2O3 ; Cool the melt and oxidize the zirconium.

When the melt reaches the sacrificial material, the sacrificial material melts and cools the melt. When using _the material with the preferred composition, that is, the cooling and oxidizing agent contains 46-80wt% _Fe2O3 and/or _Fe3O4 and 16-50wt% _Al2O3 , in the melt and this _{oxide sacrificial} At the front of the reaction of a large amount of iron oxide contained in the material, due to the strong oxidation of zirconium in the melt, an exothermic high-speed exothermic reaction occurs, so that the melt remains in the liquid phase for a period of time, which is conducive to its uniformity with the sacrificial material Mixed and effectively diluted by sacrificial material. The high activity of the reaction allows rapid formation of a homogeneous melt of the sacrificial material and the molten liner, and rapid cooling of the melt due to the large amount of heat absorbed by the walls of the well from the liquid in the well. The exotherm at the interface between the sacrificial material and the molten lining prevents crystallization of the melt at the interface, thereby preventing the reaction between the sacrificial material and the molten lining from transitioning from a high velocity liquid phase reaction to a low velocity solid phase reaction. This also prevents unreacted sacrificial material from forming a solid phase shell near the walls of the well, reducing heat dissipation from the walls and thus slowing the cooling rate.

Due to the strong oxidation of zirconium, the reaction of zirconium with water vapor in the container and the reaction with water cooling the melt is avoided, and hydrogen gas is generated and accumulated to cause combustion and explosion.

Since the oxides of aluminum and iron dilute the oxides of uranium in the melt very well, it is more reliable to keep the chain reaction of the oxides of uranium at subcritical conditions. Despite the high alumina content, the melting point of the system, including the oxide material in the well and the melting liner, does not rise when high melting alumina is added, which helps the system remain liquid for a longer period of time, thus improving And accelerate the melt dilution and more reliably maintain the subcritical state. This is because the oxides of iron and aluminum form a compound (solid solution) whose melting point is significantly lower than that of pure aluminum oxide.

Preferably, the content of iron and aluminum in the composition of the sacrificial material of the present invention is balanced, so as to form a high-speed reaction between the well oxide material and the melt; the acceleration of melt dilution also helps to maintain subcritical conditions.

If the content of _Fe2O3 and/or _Fe3O4 in the material is less than _46wt %, the reaction rate of the well oxide material with the _melt will not be very high, reducing the reliability of the melt to maintain subcritical conditions . Also, the zirconium cannot be fully oxidized due to the lack of oxygen, resulting in the formation of hydrogen and other harmful gaseous and volatile products. If the _Fe2O3 and/or _Fe3O4 content in _the material exceeds ₈₀ wt%, the overall reaction effect will be exothermic, leading to the formation of substantial and impermissible gaseous and volatile products.

If the _Al2O3 content is less than 16 wt%, the _exothermic reaction cannot be offset by the endothermic effect produced by heating the oxide material of the trap, and the resulting exothermic effect will self-heat the entire melt positioning system. If the _Al2O3 content exceeds _50wt %, the liquidus temperature of the melt increases, and correspondingly the rate at which the melt is diluted by the oxide material of the trap decreases, the reliability of maintaining subcritical conditions decreases, and due to the _{Fe2O 3} and/or Fe ₃ O ₄ is insufficient to prevent zirconium from being fully oxidized, resulting in unoxidized zirconium contacting with water vapor to generate hydrogen gas, which significantly increases the possibility of hydrogen combustion and explosion.

As mentioned above, adding a small amount of SiO ₂ to the material of the present invention can significantly increase its strength, and at the same time improve the dilution of uranium oxide in the melt, thereby helping to maintain the nuclear reaction in a subcritical mode.

If the _SiO2 content exceeds 4 wt%, the release of gaseous products increases, and the possibility of melt stratification due to the segregation process also increases, thereby reducing the dilution of uranium oxides in the melt. If the SiO ₂ content is less than 1 wt%, the strength of the sacrificial material will not be significantly increased.

Therefore, the oxide sacrificial material of the present invention is applied to the melting liner well of a nuclear reactor. When the melt enters the well, the nuclear reaction can be reliably maintained in the subcritical mode, while the melt can be effectively cooled, and the melt can be reliably positioned in the well. Prevent hydrogen gas from accumulating in sufficient quantities to cause fire and explosion.

Through model experiments and thermodynamic calculations, the ability of the material of the present invention to effectively locate the molten lining of a nuclear reactor was evaluated.

The model experiment carried out on the experimental equipment is to realize the high-frequency induction melting process in a cold crucible. According to the program specially developed for this experiment, the following values are determined: The temperature of the melt and the liquidus temperature of the sacrificial material mixture. A sample of the sacrificial material of the present invention was placed at the bottom of the cold crucible. The system is isolated from the induction coil by a water-cooled shield. A cover is fitted to the sealed quartz shell of the furnace above the cold crucible, with openings for a pyrometer to measure the depth of the molten pool and observe the surface of the melt. Previously, non-radioactive reactor fuel was used to prepare the molten liner by induction melting in a cold crucible, bringing the melt into contact with the sacrificial material mass by moving the mass to the area in contact with the melt. The crucible containing the melt is placed on a table equipped to move the crucible vertically relative to the induction coil and shield. The velocity of the reaction front is measured by timing the moment the melt touches the hot end of a thermocouple attached to the sacrificial material.

Using validated programs and the IVTANTHERMO database containing data on thermodynamic properties, gas formation was determined experimentally and theoretically as the quantity of gaseous products (gas and vapor) within a system at the melt pool formation temperature where the system consists of molten liner and sacrificial material composition.

Using the same program and the IVTANTHERMO database, the thermal effect was calculated as the difference in the enthalpy of the system, i.e., the heat released when the melt is cooled by the sacrificial material, by thermodynamic calculations, taking into account all the reactions taking place in the system.

In determining the critical parameters of the mixture of the molten liner and the oxide sacrificial material with or without the additive of interest, the propagation factor is calculated using the well-known SAPPHIRE program, which is applied to neutron physics calculations and validated against an extensive experimental database.

The following table lists examples of sacrificial materials tested by the present inventors and containing the target additives of the present invention, the composition and properties of which are shown in the table. Examples 1-17 are sacrificial materials containing target additives in the present invention, and Example 18 is a comparative material without additives.

Table Effect of adding Gd ₂ O ₃ , Eu ₂ O ₃ or Sm ₂ O ₃ to the basic oxide sacrificial material on the subcritical condition of molten liner of nuclear reactor containing uranium Example number Material composition, wt% Proliferation factor (K) _{Fe2O3 _} Fe ₃ O ₄ Al ₂ O ₃ SiO ₂ Gd ₂ O ₃ Eu ₂ O ₃ Sm ₂ O ₃ 1 46.0 - 49.9 4.0 0.1 - - 0.93 2 49.0 - 49.9 1.0 0.1 - - 0.93 3 64.8 - 31.0 4.0 0.2 - - 0.74 4 68.8 - 29.0 1.0 0.2 - - 0.74 5 80.0 - 18.7 1.0 0.3 - - 0.61 6 - 80.0 18.7 1.0 0.4 - - 0.55 7 - 46.0 49.9 4.0 0.1 - - 0.93 8 - 64.8 31.0 4.0 0.2 - - 0.74 9 29.0 20.0 49.9 1.0 0.1 - - 0.93 10 41.0 39.0 18.7 1.0 0.3 - - 0.61 11 30.0 32.6 35.0 2.0 0.4 - - 0.55 12 32.6 30.0 33.4 3.0 - 1.0 - 0.94 13 50.0 - 45.0 3.0 - 2.0 - 0.75 14 - 44.0 49.5 2.5 - 4.0 - 0.57 15 25.0 20.0 50.0 4.0 - - 1.0 0.92 16 78.0 - 16.0 3.0 - - 3.0 0.59 17 - 59 35.5 1.5 - - 4.0 0.53 18 50.0 46.0 4.0 - - - - 1.19

As can be seen from the above table, the oxide sacrificial material containing the target additive of the present invention, compared with the control sample 18, can reliably reduce the neutron multiplication factor; the K value drops to 0.53 to 0.57, indicating that the simulated nuclear reactor The melt that melts the lining has a deep subcritical state, ensuring reliable nuclear safety. _It can also be seen from the data in the table that gadolinia is particularly effective as a targeted additive, exceeding _the results obtained with _Eu2O3 or _Sm2O3 by orders of magnitude.

Industrial applicability

The material of the invention can be used in the wells of the molten lining of nuclear reactors, especially reactors of nuclear power plants and other nuclear power equipment.

Claims (5)

1, a kind of oxide material for molten-core catcher of nuclear reactor comprises for cooling and the oxidant of cooling fusing lining and the most active composition oxidation that will be wherein, it is characterized in that described material also contains subject additives, and described subject additives is by from Gd ₂O ₃, Eu ₂O ₃And Sm ₂O ₃The middle at least a oxide of selecting forms, and the content of described subject additives is 0.1-4 weight %.

2, oxide material as claimed in claim 1 is characterized in that using Gd ₂O ₃As subject additives, its content is 0.1-0.4 weight %.

3, oxide material as claimed in claim 1 is characterized in that using Eu ₂O ₃And/or Sm ₂O ₃As subject additives, its content is 1-4 weight %.

4, oxide material as claimed in claim 1 is characterized in that described cooling and oxidant comprise Fe ₂O ₃And/or Fe ₃O ₄, and Al ₂O ₃Fe wherein ₂O ₃And/or Fe ₃O ₄Content be 46-80 weight %, Al ₂O ₃Content be 16-50 weight %.

5, as any described oxide material in the claim 1 to 4, it is characterized in that this material also comprises SiO ₂, its content is 1-4 weight %.