WO2001050479A1

WO2001050479A1 - Method and installation for storing irradiated products, especially irradiated nuclear fuel assemblies

Info

Publication number: WO2001050479A1
Application number: PCT/FR2000/003510
Authority: WO
Inventors: Gérard ALETON; Philippe Moreau
Original assignee: Framatome Anp
Priority date: 1999-12-29
Filing date: 2000-12-13
Publication date: 2001-07-12
Also published as: FR2803427A1; FR2803427B1

Abstract

At least one irradiated product, such as a fuel assembly (1), is introduced into a metallic container (4) delimiting a chamber whose cross section is adapted to accommodate the cross section of a (single) product such as a fuel assembly (1). A neutral gas is introduced into the chamber of the container (4) and the chamber is sealed in tightly in such a way that the neutral gas cannot escape. The container (4) with the fuel assembly (1) is placed in a channel (10) of a storage facility, the container (4) and the fuel assembly (1) being positioned horizontally. Cooling air (25) which is in contact with the outer surface of the channel is circulated (10). Contamination inside the channel (10) can be monitored continually in a space (31) surrounding the container (4).

Description

The invention relates to a method and a device for dry storage in a ventilated room, of irradiated products and in particular of irradiated nuclear fuel assemblies, .

In certain types of nuclear reactor, for example nuclear reactors cooled by light water (PWR, BWR), fuel assemblies are used which are introduced into the nuclear reactor in adjacent positions, to constitute the reactor core. Such fuel assemblies comprise nuclear fuel elements of elongated shape, for example of circular cross section which are assembled together in arrangements such that their longitudinal axes are parallel, to constitute bundles of fuel elements or rods. The fuel rods are themselves constituted by cladding tubes filled with oxide pellets such as uranium oxide enriched in fissile uranium. In the case of nuclear reactors cooled by pressurized water (PWR), the assemblies and the fuel rods have a length which corresponds to the height of the nuclear reactor core, this length generally being of the order of 4 m.

In the case of nuclear reactors of the CANDU type cooled by heavy water, the nuclear fuel elements constituted by cladding tubes filled with natural uranium oxides have a much shorter length, of the order of 500 mm , and are assembled in the form of bundles in which the fuel elements are all mutually parallel, these assemblies being called clusters. Subsequently, the term "nuclear fuel assembly" will denote both the fuel assemblies constituted by bundles of long rods which are arranged in the reactor core, along the entire height of the core, in an arrangement vertical, than the assemblies or clusters of shorter length which are placed in the core of the nuclear reactor, in a horizontal arrangement. In certain types of nuclear reactors, for example RBMK reactors, large fuel assemblies are used. pig, for example 10 meters, which comprise, along their length, two active parts constituted by fuel rods arranged in the form of bundles and joining or handling parts of the fuel assembly. One of the problems which confronts the operator of nuclear power plants relates to the storage of irradiated assemblies which are extracted from the core of the nuclear reactor, after a certain period of operation of the reactor. It is in fact generally necessary to carry out a long-term storage of nuclear fuel assemblies, after they have left the core and after temporary storage in a swimming pool in the fuel building of the nuclear power station, so as to decrease their level of activity up to a level enabling them to be transported to an installation such as a repair, reprocessing, storage or long-term storage installation. A first type of storage is achieved by placing the fuel assemblies immersed in racks in a water pool ^{'deactivation} and resting on the bottom of the pool. This type of storage requires the construction of costly installations comprising in particular a swimming pool and means for handling fuel assemblies. In addition, the operating costs of such facilities are also high.

There is a second type of storage of fuel assemblies known as storage or dry storage which consists in placing the fuel assemblies inside a ventilated storage room, or silo, in which the fuel assemblies are immersed. in the air.

In both cases, the fuel assemblies are generally placed in a vertical position, that is to say with their longitudinal axis in a vertical direction.

In this case, it is necessary to provide high-rise installations for handling and storing the fuel assemblies.

An inherent problem in storing used nuclear fuel assemblies, in addition to the criticality problem, is related to disposal residual energy released by fuel assemblies in the form of heat. In the case of wet storage, the residual energy is absorbed by the water in the storage pool and in the case of dry storage, the residual energy of the fuel assemblies in the form of heat is evacuated by the ventilation air in the storage room.

In the case of vertical storage, an annular space for the circulation of cooling air is generally provided around a vertical housing for receiving the fuel, open at its upper end. Such installations are complex and do not guarantee perfect insulation between the cooling air and possible leaks of radioactive gases from the stored fuel.

There has also been proposed, in US Pat. No. 4,780,269, a dry storage system for irradiated nuclear fuel assemblies in which concrete storage modules are used, each intended to receive a metal envelope containing irradiated fuel assemblies which are extracted from the core of a nuclear reactor, introduced inside the metal envelope and transported by a transfer unit, to the storage modules. The storage module includes a ventilation system allowing air to circulate in contact with the external surface of the metal casing containing the fuel assemblies, without the cooling air being able to come into contact with the fuel assemblies. The metal casing is arranged in the module in such a way that the fuel assemblies are in the horizontal position during their storage.

One of the drawbacks of this system is that the cooling air circulating in the module, generally by natural circulation, and discharged into the atmosphere is only separated from the fuel assemblies by a single wall formed by the metal casing . In the case of the rupture of a sheath of a fuel element of one of the assemblies, radioactive products spread inside the metallic envelope and are not separated from the cooling air circuit. escaping the atmosphere only through the wall of the envelope. In addition, the metal casing contains a plurality of fuel assemblies which are placed juxtaposed. The evacuation of the heat given off by the fuel assemblies by transfer of this heat to the wall of the metal casing and consequently the cooling of the fuel assemblies are therefore not carried out under the best conditions. The temperature inside the envelope containing the fuel assemblies is generally close to 400 ° C.

In addition, in the case of a leak on an element of a fuel assembly, there is no means making it possible to identify the fuel assembly having a leak.

DE 39 16 359 also discloses an installation for the storage of irradiated combustible materials arranged inside cases which are placed in a horizontal arrangement on support rails of storage racks and connected by elements. articulated, to be introduced into these racks or extracts, in a grouped manner. The cases are cooled by an air circulation, in contact with their external surface, so that the irradiated products are only separated from the cooling air by the wall of the cases. Due to the great diversity of shape and size of the nuclear fuel assemblies used in the various reactors in service, it is also very difficult to design a completely versatile storage process. It may also be necessary to carry out the storage of activated waste which comes in a wide variety of shapes and sizes. Such activated waste can however be packaged, for example in the form of cylinders.

The object of the invention is therefore to propose a method for storing irradiated products and in particular nuclear fuel assemblies, of elongated shape and having a substantially uniform cross section in their longitudinal direction, in a storage room comprising at at least one storage location for at least one irradiated product of elongated shape, in a substantially horizontal position, cooled by an air circulation inside the storage room, this process making it possible to make the storage conditions for fuel assemblies safer and encourage cooling and leak detection in these assemblies.

For this purpose: - at least one irradiated product of elongated shape is introduced into a metal case delimiting a housing extending longitudinally and having a cross section adapted to accommodate the cross section of a single irradiated product of elongated shape,

a neutral gas is introduced into the housing of the case containing the at least one irradiated product and the housing of the case is closed in a sealed manner to neutral gas,

the case containing the irradiated product is introduced into a storage channel having the shape of a tube whose axis is substantially horizontal, in the axial direction of the channel, through an end opening of the channel opening out through a wall of biological insulation in an airtight separate loading area of an internal part of the storage room enclosing the storage channel., and

- cooling air is circulated in the internal part of the storage room, in contact with an external surface of the channel - in which the case is housed.

In order to clearly understand the invention, we will describe by way of example, with reference to the attached figures, the implementation of the process of the invention, in the case of the storage of fuel assemblies discharged from a CANDU reactor. Figure 1 is a perspective view of a fuel bundle of a CANDU reactor.

Figure 2 is an axial sectional view of a storage case for irradiated clusters of the CANDU reactor.

Figure 3 is a schematic sectional view through a vertical plane of part of a local storage of spent fuel bundles, according to the method of the invention.

FIG. 4 is a front view along 4 of FIG. 3. Figure 5 is a cross-sectional view of a channel of a storage installation for implementing the method according to the invention, in which is placed a case containing irradiated fuel bundles. Figure 6 is a plan view of a portion of the spent fuel storage facility.

FIG. 7 is a view in longitudinal section along 7-7 of FIG. 6.

Figure 8 is a schematic view of part of the storage facility for spent fuel bundles.

In FIG. 1, a fuel bundle 1 of a CANDU reactor has been represented which is constituted by fuel elements 2 of elongated cylindrical shape maintained by spacer grids such as 3, in the form of a bundle in which the fuel elements 2 are all parallel to each other. Each of the fuel elements 2 is constituted by a tube made of a zirconium alloy such as Zircaloy 4 filled with pellets of natural uranium. The fuel elements 2 are kept in the bundle so that there remains a certain space between the fuel elements, over the entire length of these elements. The fuel elements 2 and the cluster 1 generally have a length close to 500 mm and the cluster has an outside diameter of the order of 100 mm.

A first characteristic step of the process according to the invention consists in placing the fuel bundles (or assemblies) which have been discharged from the nuclear reactor in cases which are filled with neutral gas and hermetically sealed.

In the case of very long fuel assemblies, it may be necessary to cut off the fuel assembly and then to introduce the longitudinal sections corresponding to the active parts in cases. In the case of activated waste, a preliminary conditioning of the waste is carried out in order to obtain products of standard shape, for example cylindrical. These operations and packaging are generally carried out on the site of the nuclear power station, near the building of the fuel in a loading room comprising a case loading cell in communication with a fuel storage pool of the power station and a sealed case closing cell.

In Figure 2, there is shown a case 4 which can be used for the storage of fuel bundles 1 discharged from a CANDU nuclear reactor.

The case 4, which is made of metal and preferably of stainless steel, has a generally tubular shape and can be constituted for example by a central tube 4a closed at one of its ends by a bottom 4b in the shape of a warhead welded to the end of this tube 4a. At the end of the tube 4a opposite the end closed by the bottom 4b, is fixed by welding an annular part 4c having a conical outer surface machined internally to form a groove 5 for receiving a cover 6 for closing the case. Each of the tubular cases 4 delimits an internal housing of cylindrical shape and axis 7 intended to receive irradiated fuel bundles.

The cases 4 and their closing cover 6 are delivered to the loading room located on the site of the nuclear power station in sufficient number to receive the charge of fuel extracted from the heart of one or more reactors of the nuclear power station and deposited in the fuel building swimming pool.

To carry out the loading of a case, the case is generally placed in an arrangement such that the axis 7 of the housing is horizontal and one engages in the housing which is closed at one of its ends by the bottom 4b and open at its second end, a first spent fuel bundle 1 (shown in dotted lines in FIG. 2), in the direction of the axis 7, to the bottom of the housing. Then engages successively and one by one other clusters 1, so as to fill the housing of the case 4. After filling, the cover 6 is put in place and welded by a peripheral weld 6 ', inside of the groove 5 of the open end part of the case 4. The cover 6 has a central opening and, fixed along this opening by a sleeve 8 ', a tube 8 constituting a nozzle through which one can carry out an evacuation of the air contained in the housing of the case 4, then the filling of the housing of the case 4 containing the fuel bundles 1 with a neutral gas such as argon, nitrogen or helium. According to a technique well known in the manufacture of fuel elements, the case is closed in a completely gas-tight manner, after filling with neutral gas, by melting and welding the tube 8.

The case can be loaded in a loading room, as indicated above. The case is placed in the welding cell of the room, in a horizontal arrangement so that the open end of the case in which the fuel assemblies are engaged opens into the loading cell through an opening passing through a partition wall between the load cell and the weld cell. The fuel assemblies are extracted from the fuel pool and dried in the loading cell before being introduced one by one into the case. The case filled with fuel assemblies is then moved towards the interior of the welding cell in order to carry out the filling with neutral gas and the sealed closure of the case.

In the case of loading and storing fuel bundles of a CANDU reactor, for example cases of more than 3.5 m in length are used, in each of which seven fuel bundles are placed, the one after the other, in the axial direction 7 of the housing of the case 4. The inside diameter of the tubular part of the case 4 constituting the diameter of the housing is slightly greater than the outside diameter of the fuel bundles, a diametral clearance being provided between the fuel bundles and the wall of the housing, to allow an introduction of the fuel bundles without risk of jamming.

Of course, the geometrical and dimensional characteristics and the method of loading the cases are adapted to the type of fuel assembly or, more generally, to the irradiated product which is stored.

For example, in the case of a nuclear reactor cooled by pressurized light water, the fuel assemblies which are shaped general parallelepiped have a square section having a side of a length close to 20 cm and a length of about 4 m. In this case, it is preferable to provide cases allowing each to receive a single fuel assembly. Advantageously, the case can have a housing having a square section whose side is very slightly greater than the side of the maximum section of the fuel assembly and whose total length between the bottom and the closure cover is greater than the length of the fuel assembly.

In the case of the storage of cylinders resulting from the packaging of activated waste, provision is made for cylindrical tubular cases in which a cylinder or several cylinders can be accommodated one after the other in the axial direction in the case.

In all cases, the cross section of the housing of the case will be provided so as to contain the cross section of a single fuel assembly or more generally of a single irradiated product. Inside the case, the fuel elements such as rods of an irradiated fuel assembly are placed in the position they occupy in the fuel assembly in service and immersed in an atmosphere of neutral gas. It is possible to fill the case with a neutral gas at a pressure substantially equal to atmospheric pressure or at a slightly higher pressure. It is also possible to pressurize the internal housing of the case by introducing a neutral gas under higher pressure, so as to promote heat exchanges between the fuel rods and the wall of the case 4 and to keep the fuel rods below a temperature threshold defined according to the type of fuel. For example, in the case of CANDU fuel, the temperature must remain below 168 ° C.

Irradiated fuels indeed have a residual power which is released in the form of heat, over time, when the activity of the fuel decreases. During the storage of the fuel, the wall of the cases containing fuel assemblies, to dissipate the energy released in the form of heat by the spent fuel.

As can be seen in FIGS. 3, 4 and 5, the fuel assemblies 1 are stored by introducing the cases 4, each inside a channel 10 of circular section and horizontal axis, of a storage facility.

The storage installation has been shown diagrammatically in FIGS. 6 and 7.

During their storage inside the channels 10, the fuel assemblies which are placed in the cases so that their longitudinal axis parallel to the fuel elements is in the axial direction of the housing of the case are therefore placed with their horizontal longitudinal axis.

The channels 10 of the storage installation consist of stainless steel tubes whose internal diameter is greater than the maximum external diameter of the cases 4 closed in leaktight manner at one of their ends by a bottom 10a. The axes of the channels can be slightly inclined downward, towards their end closed by the bottom 10a, so as to avoid the exit of the cases in the event of an earthquake. The length of the tubes 10 in their axial direction 9 which is the direction of loading and unloading of the cases is greater than the overall length of the cases 4.

As can be seen in FIG. 6, the storage installation, generally designated by the reference 11, comprises a storage room 12 constituted by a concrete building which can be very large, in the cases where it is necessary to provide for the storage of a very large number of fuel assemblies.

As can be seen in Figures 6 and 7, inside the storage room 12, concrete walls delimit a proper storage area 13, itself separated into successive modules such as 13a and 13b , a gallery 14 for handling cases 4 and a maintenance gallery 15. Inside each of the storage modules such as 13a and 13b, are arranged two rows such as 16a, 16'a or 16b, 16'b of storage racks 17 each comprising a set of storage channels 10 parallel to each other, as shown in Figures 3 and 4. The two rows of racks such as 16a and 16'a of a module such as 13a are separated by an aisle 18 in which a case loading machine 19 runs in the longitudinal direction 18 'of aisle 18, the two rows 16a and 16'a of racks 17 and aisle 18 constituting a span of storage racks contained in the storage module 13a. At one end of the aisle 18, the concrete wall of the module 13a has an opening 20 'bringing the end of the aisle 18 into communication with a lower part of the handling gallery 14 of the cases 4, l 'opening 20' can be closed by a door 20 so as to completely isolate the module 13a. The specialized workshop 22 or local case for loading cases 4 with irradiated fuel assemblies is generally located near the fuel building of the nuclear reactor on which the fuel assemblies have been unloaded. As indicated above, the loading room 22 may include a loading cell communicating with the fuel storage pool of the reactor and a case welding cell. After sealing welding, the cases are transferred from the welding cell of the room 22, in a transport container having walls ensuring biological protection. A means of transport such as a truck 23 makes it possible to transport the containers containing the cases loaded with spent fuel assemblies between the loading room 22 and the storage building 12 which can be separated by a considerable distance, for example several kilometers.

The loading room could also be located near or even on the site of the storage building. The containers containing the cases are brought into an access part of the handling gallery 14 and are supported by an overhead crane 34 circulating in the longitudinal direction in an upper part of the handling gallery 14 and deposited on the machine. tank- gement 19 in the loading position in the lower part of the gallery 14, in the extension of an aisle 18 whose door 20 has been opened to release the opening 20 ′ for the passage of the loading machine 19. The loading machine 19 which comprises means for transferring the case placed in the container to move it horizontally along the axis of a channel 10 placed in communication with an outlet opening of the container makes it possible to load a channel 10 both by movement in the direction 18 'of the aisle 18, stopping in the loading position and loading maneuver in the axial direction 9 of the channel 10. The loading can be carried out case by case, each of the successive cases arranged in a container being removed by the loading machine or in groups of two or three or more cases whose containers are taken care of by the loading machine 19.

The part of a module such as 13a located between the two rows 16a and 16'a of racks 17, directly above the aisle 18, in which the loading machine 19 circulates, constitutes a loading hall 24 for the channels 10 of the module. Each of the successive modules such as 13a and 13b has its own loading hall, the operations of each of the modules can be carried out completely independently. As can be seen in particular in FIG. 7, the channels 10 of each of the racks 17 of the storage installation are cooled by an air circulation (shown diagrammatically by the arrows 25) in contact with the external surface of the channels.

As can be seen in FIG. 3, the tubes constituting the channels 10 of a storage rack 17 are fixed at their end closed by the bottom 10a in an opening of a first wall of the rack and engaged and fixed by their end opposite open in a second wall 26 constituting the front loading face of the rack 17, the first and second walls ensuring the support of the channels 10 being parallel to the aisle 18 of the loading hall 24. The tubes constituting the channels 10 can be also maintained by an intermediate plate parallel to the first and to the second wall. The wall 26 constituting the loading face of the rack is arranged on the edge of the aisle 18 so that it is accessible to the loading means of the loading machine 19.

As can be seen in FIG. 4, the engagement openings of the open ends of the channels 10 are placed in a regular network in which the openings of the successive rows are offset and staggered.

The open ends of the channels 10 are tightly fixed in the wall constituting the loading face 26.

Atmospheric air circulates, inside the storage building 12, between an inlet opening 27 and outlet gates 28, by natural circulation, the opening 27 being placed facing the direction of the prevailing wind over the site of the storage building and the gills 28 being placed at the top of an ventilation chimney. The natural air circulation in the storage building 12 has been represented by the arrows 25. Inside the storage building, the circulating air is distributed by a manifold 29, at the bottom of each. storage racks 17. The circulating air passes through the rack 17, in the vertical direction and from bottom to top, to exit in an upper manifold 29 communicating with the ventilation chimney. In this way, circulating air comes into contact with the outer surface of the channels 10 of each of the racks 17, in a lower part of the rack 17 separated, in an airtight manner, from the loading hall 24 by the wall 26, and provides cooling of the wall of the channel from the outside. The structure of the storage racks as shown in FIG. 3, for example, is analogous to the structure of a heat exchanger comprising a bundle of tubes fixed at their ends in tubular plates and the outer surface of which is cooled by a cooling gas. In the case of storage of fuel assemblies or fuel bundles of a CANDU reactor whose fuel elements consist of natural uranium oxide, a natural circulation of cooling air sowing in the storage building is sufficient to dissipate the heat produced when the spent fuel is deactivated.

In some cases and in particular in the case of fuel enriched in fissile elements, it may be necessary to use forced circulation inside the storage building.

As can be seen in FIG. 5, the combustible material of the elements 2 is separated from the cooling air circulating in the storage building in contact with the external surface of the channels 10, as shown by the arrows 25, all of which first by the sheath of the fuel elements, then by the wall of the case 4 in which the fuel bundle is arranged and finally by the wall of the channel 10. In fact, the sheaths of the fuel elements cannot be considered as likely to form a real and reliable barrier, insofar as their durability cannot be guaranteed and they are very often deteriorated when the fuel assemblies are stored.

In the event of a rupture of a fuel element sheath or when the fuel element sheaths are deteriorated or nonexistent during the storage of irradiated products, radioactive substances are released inside the housing of the case 4 These radioactive substances are separated from the air 25 circulating in the storage building by a first barrier constituted by the wall of the case 4 and by a second barrier constituted by the wall of the channel 10. The risk of leakage from radioactive substances in the cooling air is therefore very low.

In addition, in the event of the wall of one of the fuel storage cases 4 breaking or cracking, in which radioactive substances have spread, this risk of contamination of the storage building can be detected, for example by a detector arranged in the storage channel 10.

When a channel 10 containing a leaking case has been detected, the loading machine is unloaded from the loading machine by remote control, then discharged into the transport castle to a specialized workshop where the case can possibly be repaired. In the case where the channels 10 are not closed and open into the loading hall 14, leak detection and storage monitoring can be carried out from the loading hall.

The various functions of the storage installation according to the invention have been shown in FIG. 8. First of all, the cooling function has been shown diagrammatically by the arrow 25 for air circulation in contact with the external surface. channels 10.

It can be seen in particular that the circulating air 25 coming into contact with the external surface of the channels 10 is separated from the irradiated fuel assemblies 1 by a first barrier constituted by the wall of the case 4 and by a second barrier constituted through the wall of channel 10.

The internal part of the rack 17 of the storage room 12 in which the cooling air circulates in contact with the external surface of the channels 10 is separated from the loading hall 24 by a wall 26 comprising a metal plate 26a and a layer of concrete 26b lining the metal plate 26a on the inside of the wall 26 directed towards the internal space of the rack 17.

The metal plate 26a is traversed by a network of openings in each of which is engaged and fixed by a sealed weld, one end of a tubular channel 10 and thus constitutes an element analogous to a tubular plate of a heat exchanger comprising a bundle of tubes parallel to each other. The internal space of the racks 17 of the storage room 12 is thus separated, in an airtight manner, from the loading halls 24. In addition, the concrete layer 26b of the wall 26 provides biological insulation between the part internal storage room and loading halls 24.

Preferably, the concrete layer 26b is continuous along the entire length of the row of racks 17 bordering an aisle 18, on one side; on the other hand, the metal wall for fixing the channels 10 may consist of discontinuous metal plates. The openings of the channels 10 opening into the loading hall 14 are closed by plugs 33a, in an airtight manner, during the storage of the fuel.

In addition, biological isolation plugs 33b are engaged in the outlet parts of the channels 10 directed towards the hall 24 before the channels are closed by the plugs 33a. The plugs 33a can be metal plugs which are welded to the plate 26a and the plugs 33b can be made of concrete or resin loaded with absorbent products. After carrying out the loading of a case in a channel 10, a biological isolation plug 33b must be put in place in the inlet part of the channel 10, this operation being carried out by the loading machine. In order to unload a channel 10, the leaktight plug 33a for closing the channel must be dismantled and the biological isolation plug 33b removed, which has a terminal attachment portion for a traction tool.

The concrete wall 26b provides a. biological protection and sealed insulation from storage, when the insulating plugs 33a and 33b have been put in place.

In some cases, and in particular when monitoring possible leaks inside the loading hall 24, the ends of the channels directed towards the loading hall 24 can be left open.

The process according to the invention and the corresponding installation therefore allow very long-term storage of spent fuel assemblies to be carried out very safely. The invention is not limited to the embodiment which has been described.

Thus, the cases and the channels can have shapes and dimensions different from those which have been described, these shapes and dimensions being adapted to the shapes and dimensions of the irradiated fuel assemblies which are stored. The invention applies to the storage of a large number of irradiated products of elongated shape and having a substantially uniform cross-section and in particular of fuel assemblies of various types constituted by fuel rods, the combustible material of which nuclear is contained in a tubular sheath. The rods can be grouped in various ways to form assemblies, the number and shape of which may vary depending on the type of reactor in which they are used. The terms "assemblies", "cassettes", "clusters" or "bundles" may be used to refer to them.

Different cases can arise as mentioned below:

In PWR - BWR western water reactors, the rods are grouped to form square section assemblies of a length equal to the height of the reactor. In Russian VVER water reactors, the rods are grouped into assemblies with hexagonal sections, of a length equal to the height of the core,

In CANDU type heavy water reactors, the rods are assembled in cassettes or clusters about 50 cm long with a substantially circular section.

The RBMK reactors comprise assemblies of very long length (approximately 10 m) comprising two active beams of approximately 3.5 m, eighteen rods each and of approximately circular section. Only the bundles are packaged in cases to store these assemblies.

Various other embodiments of fuel assemblies can be envisaged, including assemblies comprising only one rod.

In all cases, placing a single irradiated product, such as a fuel assembly, in the cross section of the receiving case for the fuel assembly ensures very good cooling of the irradiated product through the wall of the case and of the channel containing the case.

Claims

CLAIMS 1.- Method for storing irradiated products and in particular nuclear fuel assemblies (1), of elongated shape and having a substantially uniform cross section along their longitudinal direction, in a storage room (12) comprising at least one storage location (10) of at least one irradiated product of elongated shape, in a substantially horizontal position, cooled by an air circulation inside the storage room (12), characterized by the fact :

that at least one irradiated product (1) of elongated shape is introduced into a metal case (4) delimiting a housing extending longitudinally and having a cross section adapted to accommodate the cross section of a single irradiated product ( 1) elongated,

that a neutral gas is introduced into the housing of the case containing the at least one irradiated product and the housing of the case (4) is closed so as to be neutral gas-tight,

- Introducing the case (4) containing the irradiated product (1) in a storage channel (10) having the shape of a tube whose axis is substantially horizontal, in the axial direction (7) of the channel (10), by an end opening of the channel (10) opening through a biological insulation wall (26) into a loading area (24) airtightly separated from a part internal of the storage room (12) containing the storage channel (10), and

- That cooling air is circulated in the internal part of the storage room (12), in contact with an external surface of the channel (10) in which the case (4) is housed.

2.- Method according to claim 1, characterized in that one carries out continuously during storage, the detection of possible leaks of radioactive substances through the wall of the cases (4), inside the storage channels (10).

3.- Method according to claim 2, characterized in that, when a leak of radioactive substances has been detected in a channel (10), the case (1) disposed in the channel (10).

4.- Method according to any one of claims 1 to 3, in the case where the irradiated products (1) of elongated shape each consist of at least one section of an irradiated fuel assembly consisting mainly of a bundle of nuclear fuel elements of elongated shape assembled in parallel arrangements with each other, characterized in that there is, in the case (4), a charge having at least one of the following forms: a single fuel assembly, a plurality of fuel assemblies placed one after the other in the longitudinal direction (7) of the housing of the case (4), at least one longitudinal section of fuel assembly.

5.- Method according to claim 4, characterized in that the fuel assemblies (1) are fuel bundles made of natural uranium oxides, discharged from a nuclear reactor moderated by heavy water, in particular of the CANDU type.

6.- Method according to any one of claims 1 to 5, characterized in that the neutral gas introduced into the housing of the case (4) is a gas such as argon, helium, nitrogen .

7. Storage installation for implementing the method according to any one of Claims 1 to 6, characterized in that it comprises, inside the storage room (12):

- at least one rack (17) comprising a plurality of parallel tubular storage channels (10), each comprising a first closed longitudinal end and a second opposite longitudinal end fixed in an opening passing through a wall (26) and comprising an opening loading and extracting a case (4), in the storage channel (10), the rack (17) having a structure similar to that of a heat exchanger with parallel tubes,

- a loading machine (19) for taking up cases (4) and loading and extracting cases in the storage channels (10) of the rack (17), from a loading hall (24 ), and

- means (29, 30) for channeling cooling air to make it circulate in contact with the external surface of the storage channels (10) in an internal part of the rack (17) separated, in a sealed manner air, loading hall (24) through the wall (26) in which the second ends of the storage channels (10) are fixed.

8.- Installation according to claim 8, characterized in that the wall (26) comprises a metal plate (26a) traversed by a network of openings in each of which is fixed, in an airtight manner, the second end a storage channel (10) and a layer of concrete (26b) covering an internal face of the metal plate (26a) directed towards the internal part of the rack (17).

9.- Installation according to claim 8, characterized in that the openings of the second end portions of the storage channels (10) comprise an airtight closure plug (33a).

10.- Installation according to any one of claims 8 and 9, characterized in that a biological isolation plug (33b) is engaged in each of the openings of the second end portions of the storage channels (10) containing a case (4).

11.- Storage installation according to claim 7, characterized in that it further comprises means for monitoring leaks of radioactive substances in each of the storage channels (10).