WO2009095767A1

WO2009095767A1 - Cooling system for evacuation of decay heat of irradiated fuel elements of a nuclear reactor

Info

Publication number: WO2009095767A1
Application number: PCT/IB2009/000145
Authority: WO
Inventors: Luciano Cinotti
Original assignee: Del Nova Vis SRL
Current assignee: Del Nova Vis SRL
Priority date: 2008-01-31
Filing date: 2009-01-29
Publication date: 2009-08-06
Anticipated expiration: 2010-07-31
Also published as: ITMI20080154A1

Abstract

The present invention relates to a cooling system (1) for evacuation of decay heat of irradiated fuel elements of a nuclear reactor. The system (1) substantially consists of heat conductive elements (4), preferably shaped as plates, of high conductivity material and of a handling device (51) to move the elements (4) with respect to the fuel element (3) to be cooled, which is of the type formed by a plurality of elementary structures (7) made of fuel material one beside the other, for instance bars. Each element (4) has a first portion (4a), which in use may be inserted into the fuel element between the elementary structure to receive heat therefrom and transfer heat by conduction to a second portion (4b), which in use remains outside the fuel element and is cooled by a cooling gas.

Description

COOLING SYSTEM FOR EVACUATION OF DECAY HEAT OF IRRADIATED FUEL ELEMENTS OF A NUCLEAR REACTOR

TECHNICAL FIELD The present invention relates to a cooling system for evacuation of decay heat of irradiated fuel elements of a nuclear reactor.

In particular, the system according to the invention serves to cool the fuel elements after the latter have been removed from the core of the reactor and precisely during handling and/or during stocking while waiting for reprocessing. BACKGROUND ART Evacuation of decay heat is one of the main problems in handling irradiated fuel elements of nuclear power plants, in the time period between the removal from the core and the entry into reprocessing plants and therefore during the steps of moving and stocking the fuel elements. In general, the decay power is too high to be evacuated by irradiation or natural air or gas circulation. Consequently, the fuel elements are either cooled for a long-enough time before their extraction from the reactor, although the load factor of the reactor is thereby penalised, or they are transferred immersed in a liquid refrigerant, with the obvious complication of the fuel transfer devices and stocking systems . Once the fuel elements are extracted from the reactor, they are generally left to decay in water pools also characterised by structural and functional complexity, related to the water cooling and processing systems and to the criticality prevention systems .

DISCLOSURE OF INVENTION It is an object of the present invention to provide a system that overcomes the highlighted drawbacks of the known solutions, and that in particular is easily applicable to the fuel element, allows an efficient evacuation of the heat to air or to another gas for environments in which air is not permissible, and has structural and safety advantages with respect to the known solutions .

The present invention therefore relates to a cooling system for evacuation of decay heat of irradiated fuel elements of a nuclear reactor according to appended claim 1 and, as regards its auxiliary features and plant configurations, according to the dependent claims .

BRIEF DESCRIPTION OF THE DRAWINGS The invention is disclosed in the following non- limitative embodiments, with reference to the accompanying drawings, in which:

- Figure 1 is a perspective view, with parts partially sectioned and/or removed for clarity, of a cooling system according to the invention, shown while it is coupled to a fuel element to be cooled;

- Figure 2 is a partial perspective view of the system of Figure 1, shown in an open operational configuration and in the absence of the fuel element;

- Figure 3 is a partial horizontal section view of the system of Figure 1, shown in a closed operational configuration around the fuel element; - Figure 4 is a perspective view, with parts partially sectioned and/or removed for clarity, of a pantograph mechanism actuator device of the system of Figure 1;

Figure 5 is a perspective view, with parts partially sectioned and/or removed for clarity, of a variation of the system of Figure 1, in particular having a linear guide mechanism actuator device;

- Figures 6 and 7 are horizontal section views of the system of the invention incorporating further variations and respectively shown in an open operational configuration and in a closed operational configuration. BEST MODE FOR CARRYING OUT THE INVENTION In Figures 1-3, numeral 1 indicates, as a whole, a cooling system for evacuation of decay heat of irradiated fuel elements of a nuclear reactor (known and not shown) ; in particular, system 1 cooperates with a fuel element 3 to cool fuel element 3 after the latter has been removed from the core of the reactor, during the handling and/or the stocking of fuel element 3 while waiting for its reprocessing.

Figure 1 shows the situation in which fuel element 3, extracted from the core of the reactor by means of a dedicated lifting machine, has been taken in a predetermined coupling position with respect to structures 2.

System 1 comprises a plurality of heat conductive elements 4 made of a suitable material (for instance metal) having a high thermal conductivity, and a handling device 51 to displace elements 4 with respect to fuel element 3 to be cooled, which has previously been extracted from the core of the reactor in a known manner, for instance by means of a dedicated lifting machine, known and not shown.

Fuel element 3 is of the common type which extends longitudinally along an axis A (in use, vertical) and is formed by a plurality of elementary structures 7 made of fuel material one beside the other; elementary structures 7 may have different shapes such as plates or, as in the shown example, bars, or other. In any case, elementary structures 7 are organised in adjacent series or layers 6 separated from one another by substantially parallel and vertical interstices 5 or free gaps .

Fuel element 3 is of the "open" type, thereby intending that fuel element 3 has no side closing box or side containment walls which enclose elementary structures 7; interstices 5 therefore communicate with and are accessible from outside fuel element 3.

Fuel element 3 normally also has spacing elements 9 arranged so as to be vertically spaced from one another, for instance spacing grids or wires, connected to elementary structures 7 to avoid vibrations. Opposite longitudinal ends of fuel element 3 are provided with respective box-like support structures 14, 15, respectively an upper and a lower one, which surround the active part of fuel element 3.

Each element 4 is essentially configured as a substantially flat plate and comprises two portions 4a, 4b connected to one another; portion 4a is an insertion portion which is at least partially insertable in use in fuel element 3 between elementary structures 7 and specifically in an interstice 5 laterally delimited by two layers 6 of elementary structures 7, to receive heat from elementary structures 7 and transfer heat by conduction to portion 4b, which is a conduction portion and in use remains external to fuel element 3 and is cooled by a cooling gas circulating in system 1, for instance by natural circulation.

In particular, system 1 comprises a pair of displaceable comb structures 2 opposite and facing one another, provided with respective sets 52 of opposite elements 4. Elements 4 of each set 52 are arranged like a comb on respective structure 2 and extend from transversal end wall 10 of structure 2 and face one another substantially parallel. Portion 4a of each element 4 is configured and dimensioned so as to be insertable in an interstice 5. In particular, portion 4a has: a thickness such as to be inserted in an interstice 5 between elementary structures 7, preferably with a slight side clearance; a (horizontal) length such as to penetrate interstice 5 up to about half the length of fuel element 3;

- a (vertical) height slightly smaller than the height of elementary structures 7, so as to be inserted between the end support structures 14, 15.

If fuel element 3 has spacing elements 9, portion 4a has notches 8 substantially horizontal in a position corresponding to spacing elements 9 and configured so as to accommodate respective spacing elements 9.

While portion 4a is thin and has a thickness smaller or substantially equivalent to the width of interstices 5 to allow the introduction of portion 4a between elementary structures 7, portion 4b may be thicker and, preferably, portion 4b is thicker than portion 4a, in order to improve heat transmission by conduction within element 4. In order to facilitate the insertion of portions 4a in interstices 5 even in the case of deformations of elementary structures 7, portions 4a may even be segmented vertically (Figure 2), i.e. be formed by a plurality of segments 49 separated by horizontal cuts, so as to increase the flexibility of portions 4a.

Portions 4b, also parallel to one another, are separated by vertical gaps defining, in each structure 2, ventilation channels 11 for the circulation of cooling gas delimited by portions 4b and by wall 10.

The two outermost portions 4b of each, set 52 are connected to respective side partitions 16 which in use remain laterally outside fuel element 3. Structures 2 are fitted at opposite longitudinal ends thereof with positioning members 12, for instance internally borne by partitions 16 and provided with front wedge lead-in portions 13; positioning members 12 cooperate with fuel element 3 and specifically with support structures 14, 15 to guide the horizontal displacement of structures 2 with respect to fuel element 3 , so that portions 4a are perfectly facing and aligned to interstices 5 and therefore the introduction of conductive elements 4 in fuel element 3 is ensured. Structures 2 are also provided with hooking members 17 reciprocally cooperating to hook structures 2 integral to one another. For instance, hooking members 17 comprise tilting ratchet gears 18 which frontally extend from respective longitudinal ends of partitions 16 of a first structure 2 to hook respective pins 19 borne by partitions 16 of opposite structure 2; ratchet gears 18 are loaded by springs 20 and are optionally connected two by two with connecting rods 21.

Handling device 51 comprises (figure 4) a containment structure 31 and a mechanism 54 borne by containment structure 31 and cooperating with structures 2 so as to selectively approach and move structures 2 away from one another and with respect to fuel element 3, so as to close structures 2 and therefore respective sets 52 of elements 4 on fuel element 3.

Handling device 51 displaces structures 2 transversally to fuel element 3 and so as to bring elements 4 of two sets 52 within fuel element 3 on opposite sides of fuel element 3.

In particular, handling device 51 displaces structures 2 from an open operational position shown in Figures 1 and 2, in which structures 2 are spaced one from the other and elements 4 are outside fuel element 3, to a closed operational position, shown in Figure 3, in which structures 2 are one close to the other and portions 4a of elements 4 penetrate within fuel element 3 in interstices 5. In the closed operational configuration, system 1 removes heat from fuel element 3 : heat is transferred from elementary structures 7 to portions 4a both by conduction-convection of the cooling gas circulating in system 1, and by irradiation, and is transferred to portions 4b and therefore outside fuel element 3 by conduction along elements 4. The circulation of cooling gas along channels 11 cools portions 4b.

Height and section area of channels 11 may be designed so as to allow the operation of system 1 in natural circulation of the cooling gas, caused by the difference in temperature between the (warmer) gas inside channels 11 and the (cooler) gas outside the channels . In particular, portions 4b may be provided with respective extension portions 45 which extend upwards so as to increase both the surface of heat removal and the length of channels 11 (and therefore the radiator effect) .

Alternatively or in addition to natural circulation, system 1 may be associated to means for the forced circulation of the cooling gas, for instance by means of a blower (known and not shown) . Two handling devices 51 which may be used in system 1 are hereinafter disclosed by mere way of example. It is understood that system 1 may include other different actuator devices also depending on the specific application. In the embodiment shown in Figures 1-4, handling device 51 comprises a pantograph mechanism 54. Handling device 51 intervenes once fuel element 3 has been taken to the correct elevation by means of the dedicated lifting machine. The correct vertical positioning of fuel element 3 with respect to structures 2 is obtained for instance by means of control devices 47 (known and indicated only diagrammatically in Figures 1 and 4) which detect the position of fuel element 3 and operate on the lifting machine of fuel element 3. Laser or photoelectric cell devices may for instance be used, mounted on containment structure 31 to intercept fuel element 3 when the latter reaches a predetermined position with respect to containment structure 31 and to signal the lifting machine to stop. The vertical positioning may also be achieved, as an alternative or redundantly, by an electromechanical device, again mounted on containment structure 31 and operated by fuel element 3 upon the desired level being reached by means of a mechanical system for instance consisting of leverages and cams.

During the first step of driving handling device 51, structures 2 are positioned with respect to element 3 with interstices 5 facing and aligned to elements 4 and spacing elements 9 facing and aligned to notches 8. Subsequently, handling device 51 completes the stroke by inserting elements 4 in interstices 5.

Handling device 51 comprises a carriage 26 vertically slidable on containment structure 31, and two specular pairs of pantographs 22 which operate on structures 2 in a synchronised manner; each structure 2 is driven by a pair of pantographs 22 arranged side by side on opposite sides of structure 2, so as to distribute the stresses in a balanced manner.

Carriage 26 comprises a base 29, provided with an opening through which fuel element 3 is insertable, and two uprights 28 which engage respective vertical guides 30 borne by respective opposite walls of containment structure 31. All of pantographs 22 are mechanically connected to the same common carriage 26 which synchronises the displacement of pantographs 22. Pantographs 22, which are side by side on opposite sides of each structure 2, are connected at the back by a common closing frame 27.

Each pantograph 22 comprises two pairs of substantially parallel rods 23, 24 and two connecting rods 25. Rods 23, 24 are hinged by means of respective opposite ends to carriage 26 and to one of frames 27; connecting rod 25 is hinged on opposite ends to a side wall of containment structure 31 and to the central position of rod 23. Handling device 51 is releasably connected to structures 2 and therefore to elements 4 so that elements 4 may be released from handling device 51 once portions 4a are inserted in fuel element 3.

In the example shown, structures 2 are connected to handling device 51 and specifically to frames 27 by means of releasable connection members 55 which completely constrain structures 2 to frames 27 horizontally, although only downwards vertically. In particular, structures 2 are suspended to frames 27, for instance by means of hooks 32 which extend from walls 10 and engage respective hooking seats 33 formed on frames 27.

Structures 2 are horizontally drawn by frames 27, which are driven by an actuator assembly 34 (known and not disclosed in detail for simplicity) , mounted on containment structure 31 and which directly operates on carriage 26 leading it to vertically translate. Actuator assembly 34 is fitted with sensors and interblocks required to allow the safety operation of structures 2 and is appropriately positioned and shielded against radiation.

When approached, structures 2 reciprocally hook to one another by means of pairs of ratchet gears 18 which operate in virtue of the relative motion of structures

2. Springs 20 serve a safety function against the accidental unhooking of ratchet gears 18.

Once structures 2 are brought in the closed position, they remain fixed to fuel element 3 as elements 4 lowerIy rest on lower support structure 15 of fuel element 3. Connection members 55 do not constrain structures 2 to handling device 51 vertically and upwards, while elements 4, being interposed between support structures 14, 15, vertically constrain structures 2 to fuel element 3 in both directions. This configuration allows lifting of fuel element 3 with structures 2 hooked (although without handling device 51) . The upward drawing of fuel element 3, driven by means of the dedicated lifting machine, releases hooks 32 from hooking seats 33 and allows detachment of structures 2 from respective frames 27. In fact, portions 4a abut on lower support structure 15 of fuel element 3, during the lifting step, thus regaining the clearance that allowed the introduction of portions 4a in fuel element. Structures 2, in the closed operational configuration, remain integral to fuel element 3 in each subsequent handling step. Pantograph mechanism 54 intended to couple structures 2 to fuel element 3 is also used for the opposite uncoupling operation, for which ratchet gears

18 are previously opened, for instance by the use of (known) remote manipulators .

In the embodiment of Figure 5, in which details similar or identical to those already disclosed are indicated by the same numerals, handling device 51 comprises a linear guide mechanism 54. Figure 5 (as well as Figure 1) show the situation in which fuel element 3 has been taken in the predetermined coupling position with respect to structures 2, controlled by means of control devices 47. In this variation, structures 2 are also releasably suspended to respective frames 27 by means of releasable connection members 55 (for instance hooks 32 and hooking seats 33) identical to those previously disclosed.

Frames 27 are provided with sliding blocks 35 which slide along horizontal guides 36 fixed to containment structure 31.

Also in this case, the simultaneous displacement of structures 2, which are drawn horizontally by respective frames 27 they are hooked to, is obtained by means of a

(known) actuator assembly 34 mounted on containment structure 31, which drives frames 27 in a synchronised manner by means, for instance, of cinematic chains. In the non-limitative example of Figure 5, actuator assembly 34 comprises a pair of actuators 57 operating on respective frames 27 by means of rack/pinion couplings 37; actuators 57 are electrically or mechanically synchronised and fitted with sensors and interblocks required to allow safety driving of structures 2. The hooking modes of structures 2 to fuel elements 3 are the same as those disclosed previously.

Figures 6 and 7, in which details similar or identical to those already disclosed are indicated by the same numerals, depict further modifications which may be singularly or cumulatively be made to system 1.

System 1 comprises a preheating system 58, obtained for instance by means of heating wires 38, serving the function of preheating elements 4 before their insertion into fuel element 3, in order to avoid excessive thermal shocks of elementary structures 7 and/or, in case of fuel elements 3 which have operated in liquid metal, avoid the risk of elements 4 sticking to possible layers of liquid metal adhering to elementary structures 7. Possible freezing after insertion of conductive elements 4 into fuel element 3 is instead advantageous to avoid further dripping during transportation of fuel element 3. Heating wires 38, to be disabled upon successful insertion, are mounted on conductive elements 4 in non critical areas for the normal mechanical operation, for instance they are arranged vertically along conductive elements 4 where portions 4a are joint to portions 4b.

Structures 2 are also provided with respective transversal auxiliary partitions 39, substantially orthogonal to elements 4 and άisplaceable with respect to elements 4. Partitions 39 are vertically supported by- portions 4a and have longitudinal slits 40, in which portions 4a are slidingly inserted, and catch elements 43, which frontalIy extend from partitions 39 to rest against support structures 14, 15 of fuel element 3. Partitions 39 are displaceable with respect to elements 4, during handling of structures 2, between two stop positions (respectively shown in Figures 6 and 7) ; partitions 39 are held in the stop positions by means for example of springs 41 which engage respective seats 42, 42' formed on partitions 16. Partitions 39 serve the function of delimiting, together with partitions 16, a vertical duct 56 having a substantially box-like configuration around fuel element 3, for the possible, preferably forced, circulation, of a cooling gas which preheats fuel element 3 before insertion of elements 4.

Portions 4b of conductive elements 4 are laterally spaced from one another with a greater step than portions 4a and are connected to portions 4a by means of respective fanned out connection portions 44, so as to increase both the heat removal surface and the section of channels 11 (and therefore the radiator effect) .

Portions 4b are provided with transversal fins 46, which may in particular extend between adjacent portions 4b and form an alveolar structure together with portions 4b, in order to promote the radiator effect.

To make the insertion of portions 4a into interstices 5 easier even in case of deformations of elementary structures 7, portions 4a are frontally tapered and/or have respective thinned end portions 48.

For the same purpose, opposite portions 4a may be configured as a horizontal guillotine: each portion 4a has an inclined front edge defining a tip which, during insertion of portions 4a into fuel element 3, penetrates first into fuel element 3 near one of support structures 14, 15 or one of spacing elements 9, where the geometrical correctness of interstice 5 is ensured; portion 4a then gradually inserts into interstice 5 if necessary spacing elementary structures 7 which are possibly deformed. For a better covering of interstices 5, opposite guillotine-shaped portions 4a will be one with the tip pointing downwards and the other with the tip pointing upwards .

As structures 2 are destined to remain joined to fuel element 3 during the steps of transporting and stocking thereof, in order to avoid criticality risks during stocking, some parts of structures 2 and in particular of elements 4 may be provided with coverings made of neutron-absorbing material .

The advantages of the present invention are apparent from the above. - The system allows to cool an open fuel element even when the latter is immersed in a processing fluid with poor cooling features such as gas .

Due to the possibility of extending the channels the cooling gas flows through, in section, height and surface, solutions with natural circulation cooling may be implemented even with fuel elements having a high decay power. - The system is suitable for cooling fuel elements both during transfer and during stocking.

The system may be integrated in normally employed machines for the transfer of fuel and may also be a portion of duct for the forced circulation of the processing fluid in dedicated channels for the extraction of fuel from the reactor. In particular, the system may advantageously be coupled, to take over the function of cooling the fuel element outside the reactor, to a gas forced circulation cooling system such as that disclosed in patent application N. MI2007A001856 which may operate only as long as the fuel element remains hydraulically connected to the reactor.

The system may easily be removed, when decay power is reduced in the course of time and the volumes required for the stocking of fuel are to be reduced.

The comb structures, in some parts constitutive, may be provided with neutron absorbing materials to avoid criticality risks in case of accumulation of many fuel elements in stocking. It should be understood that further modifications and variations which do not depart from the scope of the appended claims may be made to the system disclosed and shown herein.

Claims

1. A cooling system (1) for evacuation of decay heat of irradiated fuel elements of a nuclear reactor, comprising a plurality of heat conductive elements (4) and a handling device (51) to move the elements (4) with respect to a fuel element (3) to be cooled formed by side-by-side elementary structures (7) of fuel material; each element (4) having a first portion (4a) , which in use is insertable into the fuel element between the elementary structures to receive heat therefrom and transfer heat by conduction to a second portion (4b) of the element (4) , which in use remains outside the fuel element (3) and is cooled by a cooling gas.

2. The system according to claim 1, wherein the handling device (51) moves the elements (4) substantially transversally with respect to the fuel element (3 ) .

3. The system according to claim 1 or 2, wherein the elements (4) are substantially shaped as plates.

4. The system according to one of the preceding claims, wherein the first portion (4a), intended for the introduction into the fuel element (3), is thinner than the second portion (4b) , intended to remain outside the fuel element (3) .

5. The system according to one of the preceding claims, wherein the second portions (4b) delimit ventilation channels (11) for the circulation of cooling gas .

6. The system according to one of the preceding claims, wherein the cooling gas circulates in natural circulation.

7. The system according to one of the preceding claims, comprising two displaceable comb structures (2) facing one another and provided with respective sets (52) of opposite elements (4) .

8. The system according to claim 7, wherein the handling device (51) is connected to the structures (2) so as to selectively approach and move away structures (2) one to/from the other and with respect to the fuel element (3 ) .

9. The system according to claim 7 or 8, wherein the handling device (51) moves the structures (2) so as to bring the elements (4) of the two sets (52) within the fuel element (3) on opposite sides of the fuel elements .

10. The system according to one of the preceding claims, wherein the elements (4) are releasably connected to the handling device (51) , so as to be released from the handling device (51) once the first portions (4a) of the elements (4) are inserted in the fuel element (3) .

11. The system according to one of the preceding claims, wherein the handling device (51) comprises a pantograph mechanism (54) .

12. The system according to one of the preceding claims, wherein the handling device (51) comprises a linear guide mechanism (54) .

13. The system according to one of the preceding claims, wherein the structures (2) are provided with partitions (16, 39) defining, in use, a vertical duct (56) having a substantially box-like configuration around the fuel element (3), for the preferably forced circulation of cooling gas .

14. The system according to one of the preceding claims, comprising a preheating system (58) for preheating the elements (4) before their insertion into the fuel element (3).