DEVICE FOR LIFTING OR LOWERING A BUNDLE OF CORE FUELS FROM AND INTO A TANK OF A NUCLEAR FACILITY

Description

[0001] The present invention relates to a device for raising or lowering a nuclear fuel assembly into a pool of a nuclear facility, or "descender".

[0002] The invention also relates to a method for checking the leaktightness of a nuclear fuel assembly using such a device, and a renovation method for obtaining this device from an existing device.

[0003] A nuclear reactor comprises a vessel in which a plurality of nuclear fuel assemblies are arranged, together forming the core of the nuclear reactor. The nuclear reactor is generally arranged in a reactor shaft filled with water when the reactor is shut down, the shaft communicating with at least one storage pool also filled with water for carrying out maintenance operations. The storage pool comprises various systems for moving, testing, repairing or storing the nuclear fuel assemblies.

[0004] The pool includes a lowering device, i.e. a device adapted to lower new nuclear fuel assemblies from the surface of the storage pool to the bottom of the storage pool. The lowering device also allows a nuclear fuel assembly to be raised close to the surface for intervention. The lowering device generally includes a nacelle adapted to receive a nuclear fuel assembly, and to travel along two vertical rails. The nacelle is perforated to allow the heat released by the nuclear fuel assembly to be evacuated.

[0005] Each nuclear fuel assembly consists of a bundle of nuclear fuel rods, each nuclear fuel rod comprising a tubular cladding closed at both ends and containing nuclear fuel. During operation, a coolant circulates through the core, along the nuclear fuel assemblies, and particularly along their nuclear fuel rods. The coolant maintains the core at an operating temperature and also acts as a moderator for the nuclear reaction.

[0006] During operation, a nuclear fuel rod in a nuclear fuel assembly may develop a leak, for example due to perforation or cracking of the rod cladding. Such a leak may allow fission products resulting from the nuclear reaction to escape. These fission products mix with the coolant and may be deposited on the components of the nuclear reactor.

[0007] The leaktightness of the rods of nuclear fuel assemblies loaded into a nuclear reactor is monitored by regular measurements of the radioactivity of the coolant. Measurements of gas and iodine component activities can detect leaktightness and are used to estimate the number of affected rods, their burnup rate, their location in the core and the size of the defect(s). However, these measurements do not allow determining which nuclear fuel assembly contains a fuel rod with a leaktightness. This determination is made by individually monitoring potentially affected irradiated nuclear fuel assemblies when the reactor is shut down.

[0008] To detect a possible leak in an irradiated nuclear fuel assembly, it is known to carry out a leak test by penetrant testing (" sipping » Bleeding involves causing a relative increase in the internal pressure of the nuclear fuel rods of a nuclear fuel assembly compared to the external pressure, or an increase in the volume of fluids inside the nuclear fuel rods of a nuclear fuel assembly. As the pressures inside and outside the fuel rod tend to naturally balance, there is a transfer of fission products from the inside of the rod to the outside. The relative increase in internal pressure or the volume of fluids inside the fuel rods is achieved, for example, by causing an increase in the temperature of the nuclear fuel assembly or a decrease in the external pressure.

[0009] Penetrant leak testing can be performed in a fixed penetrant cell in the storage pool. However, in older nuclear power plants, these fixed penetrant cells are also old, and their maintenance and continued operation are expensive.

[0010] As an alternative, mobile penetrant cells are sometimes used, which are specially installed in the pool before nuclear reactor maintenance operations and then removed after maintenance operations are completed. However, the installation and removal of mobile penetrant cells is expensive.

[0011] Document JP S57 12397 A discloses a device substantially corresponding to the preamble of claim 1.

[0012] One aim of the invention is to reduce the cost of leak testing of nuclear fuel assemblies, while maintaining the reliability of the tests.

[0013] For this purpose, the invention relates to a device according to claim 1.

[0014] According to particular embodiments, the device comprises one or more of the features corresponding to claims 2 to 5.

[0015] The invention also relates to a leak testing method according to claim 6.

[0016] The invention also relates to a renovation method according to claim 7.

[0017] The invention will be better understood on reading the following description, given solely by way of example and with reference to the appended drawings, in which: there figure 1 is a schematic side view of part of a pool of a nuclear installation comprising a device according to the invention; and the figure 2 is a view analogous to that of the figure 1 , the swimming pool comprising a state-of-the-art descender which can be the subject of a renovation process according to the invention, in order to transform it into a device according to the invention.

[0018] In reference to the figure 1 , a device 10 according to the invention is described.

[0019] The device 10 is adapted for raising or lowering a nuclear fuel assembly 12 into a pool 14 of a nuclear installation (of which only the pool 14 is partially shown).

[0020] The pool 14 is filled with a body of water 16. By "water" is meant here an aqueous solution which may include other constituents desired in the water of the pool 14, or undesired, for example pollutants from the nuclear fuel present in the nuclear fuel assembly 12 or in another assembly.

[0021] The nuclear fuel assembly 12 comprises, for example, a plurality of rods 18 extending in a substantially vertical direction V, and containing nuclear fuel. The nuclear fuel assembly 12 is, for example, new, i.e., not having already been in service in a reactor (not shown) of the nuclear installation, or irradiated, i.e., having already been in service in a reactor and removed from this reactor for maintenance or storage.

[0022] The fuel assembly 12 is for example taller than it is wide, and typically extends several meters vertically. The nuclear fuel assembly 12 has for example a square or hexagonal section perpendicular to the direction V.

[0023] The device 10 can be described as a “descender”, because it is adapted to lower the nuclear fuel assembly 12 from the surface of the body of water 16 to the bottom of the pool 14. The device 10 also makes it possible to raise the nuclear fuel assembly 12 to the surface.

[0024] The device 10 comprises a guidance system 20 fixed to the pool 14, and a nacelle 22 defining a housing 24 for receiving the nuclear fuel assembly 12 and water 26 around the nuclear fuel assembly 12, the nacelle 22 being mounted movably on the guidance system 20 in translation in the direction V. The device 10 comprises a cover 28, and a member 29 for actuating the cover.

[0025] The device 10 comprises a sampling system 30 for taking at least one water sample 32 from the housing 24, for the purpose of detecting fission products from the rods 18. The device 10 comprises a depressurization system 34.

[0026] The device 10 advantageously comprises an air injection system 35.

[0027] Advantageously, the device 10 also comprises a recovery and analysis system 36.

[0028] Thanks to the sampling system 30 and the depressurization system 34, the device 10 makes it possible to carry out a sweating test by vacuuming the nuclear fuel assembly 12. The cover 28 and the recovery and analysis system 36 allow an analysis of possible sweating gases 38.

[0029] The guidance system 20 comprises, for example, two vertical rails (not shown).

[0030] Water 26 comes for example from body of water 16.

[0031] The nacelle 22 comprises an upper end 40 defining a passage 42 adapted to allow loading of the nuclear fuel assembly 12 into the housing 24 from above. The nacelle 22 is advantageously configured to channel any sweat gases 38 towards the upper end 40.

[0032] The cover 28 is movable relative to the nacelle 22 between an active position ( figure 1 ) and a rest position (not shown, but deduced from the active position by moving the cover away from the nacelle). The cover 28 is for example rotatably mounted on the nacelle 22.

[0033] In the active position, the cover closes the passage 42. The nacelle 22 and the cover 28 are then sealed against the water 16 of the pool 14, and the housing 24 is adapted to contain a gaseous sky 44, for example obtained by injection of air by the air injection system 35 into the housing 24.

[0034] In the rest position, the cover 28 leaves the passage 42 free to allow the loading of the nuclear fuel assembly 12 into the housing 24.

[0035] The member 29 is adapted to move the cover 28 from the active position to the rest position, and vice versa. The member 29 comprises, for example, a rod 46 mounted on the cover 28.

[0036] The recovery and analysis system 36 advantageously comprises at least one gas intake 46 adapted to sample a fraction of the gaseous atmosphere 44, a β radioactivity detector 48, a γ radioactivity detector 50, and a pipe 52 adapted to conduct the fraction sampled from the gas intake 46 to the analyzers 48, 50. The recovery and analysis system 36 comprises, for example, a pump 54 to set the sampled gases in motion.

[0037] Advantageously, the recovery and analysis system 36 is adapted to reinject said fraction of the gaseous air 44 into the housing 24, for example via a pipe 56.

[0038] The analyzers 48, 50 are adapted to determine whether the sampled fraction contains sweat gases 38.

[0039] The sampling system 30 advantageously comprises at least one sampling socket 58, and a pipe 60 adapted to conduct the water samples to the water sample 32. The sampling system 30 advantageously comprises a pump 61 and a pipe 61A.

[0040] The water sample(s) 32 are for example analyzed in the laboratory, to detect the presence of one or more fission products resulting from the bleeding of the nuclear fuel assembly 12. Advantageously, a single water sample is taken at the end of the leak test.

[0041] In addition, fission products are, for example, continuously searched for in the water taken from sampling point 58 during the bleeding of fuel assembly 12.

[0042] The depressurization system 34 comprises, for example, a vacuum pump 62 and a pipe 64 connecting the gaseous airspace 44 to the vacuum pump.

[0043] The operation of the device 10 follows from its structure and will now be briefly described. This operation illustrates a method for testing the leaktightness of the nuclear fuel assembly 12 by penetrant testing according to the invention.

[0044] It should first be noted that the device 10 can operate as a conventional descender.

[0045] Indeed, the cover 28 being in the rest position, it is possible to load the nuclear fuel assembly 12 into the housing 24 via the passage 42 located in the upper end 40. The device 10 then makes it possible to lower or raise the nuclear fuel assembly 12 into the pool 14 in the direction V.

[0046] The nuclear fuel assembly 12 is optionally unloaded from the device 10 via the passage 42, for example to be stored in the pool 14.

[0047] If it is desired to carry out a leak test of the nuclear fuel assembly 12, it is left in the housing 24 or it is loaded into the housing 24. The cover 28 is placed in the active position. The gaseous canopy is formed in the housing 24 which has become leaktight by means of the air injection device 35. The filling of the cover 28 with air is advantageously carried out until air escapes between the cover 28 and the nacelle 22, the cover and the nacelle not being connected to each other in a leaktight manner during the air injection.

[0048] Then, the depressurization system 34 extracts a portion of the gaseous air 44 from the housing 24 and lowers the pressure of the gaseous air.

[0049] This drop in ambient pressure in the housing 24 triggers bleeding of the rod(s) 18 having a sealing defect.

[0050] The sampling system 60 provides the water sample 32, advantageously at the end of the test. The water sample 32 is analyzed to detect the presence of one or more fission products resulting from the bleeding of the nuclear fuel assembly 12.

[0051] As a possible addition, water is taken from the housing 24 at the sampling point 58, advantageously continuously, and fission products possibly present in the water taken are searched for continuously or at the end of the sweating cycle.

[0052] For example, the liquid sample is degassed and an analysis of the beta and gamma radioactivity of the extracted gas is carried out. In order to concentrate the fission products, the liquid and gas samples are advantageously reinjected at the bottom of the nacelle 22 via line 61A.

[0053] Eventually, the sweat gases 38 rise into the nacelle 22 which channels them, then pass through the upper end 40, and are collected in the gaseous ceiling 44.

[0054] Then, as a possible addition, at least a fraction of the gaseous sky 44 is advantageously recovered and analyzed by the recovery and analysis system 36, which makes it possible to identify sweat gases by their radioactivity.

[0055] If no fission products are detected, the tested nuclear fuel assembly 12 is declared leak-proof. If, on the contrary, fission products are detected using the sampling system 30, the nuclear fuel assembly 12 is declared leak-proof. Analysis of the radioactivity of the gaseous atmosphere 44 advantageously provides additional information.

[0056] Thanks to the characteristics described above, the device 10 is a descender and also allows a penetrant test to be carried out quickly and at low cost. Thanks to the device 10, the pool 14 does not need a conventional fixed or mobile penetrant cell.

[0057] A renovation method according to the invention will now be described.

[0058] For example, it consists of transforming a device 100 shown on the figure 2 , which is a classic descender, in the device 10 shown in the figure 1 .

[0059] The device 100 comprises a guide device 120 similar to the guide device 20, and a nacelle 122 comprising an upper end 14 similar to the upper end 40. The nacelle 122 defines a housing 124 adapted to receive the nuclear fuel assembly 12.

[0060] The nacelle 122 differs from the nacelle 22, in particular because it is perforated to allow circulation of water 16 around the nuclear fuel assembly 12. The nacelle 122 has a lateral casing 134 which is not watertight 16. In order to renovate the device 100, the nacelle 122 is replaced by a nacelle similar to the nacelle 22 shown in the figure 1 , and a sampling system and a depressurization system similar to the sampling system 30 and the depressurization system 34 are added. It then becomes possible to put in a cover similar to the cover 28. The device thus renovated makes it possible to easily carry out a leak test by sweating of the nuclear fuel assembly 12 by lowering the pressure in the housing 24.

Claims

1. A device (10) for raising or lowering a nuclear fuel assembly (12) in a pool (14) of a nuclear facility, the pool being intended to contain water (16), the device (10) comprising: - a gondola (22) defining a housing (24) for accommodating the nuclear fuel assembly (12) and water (26) around the nuclear fuel assembly (12), the gondola (22) comprising an upper end (40) defining a passage (42) suitable for the loading of the nuclear fuel assembly (12) into the housing (24) from above, - a cover (28) movable relative to the gondola (22) between an active position wherein the cover (28) closes said passage (42), wherein the gondola (22) and the cover (28) are watertight with regard to the water (16) of the pool (14), and wherein the housing (24) is suitable for containing a gas blanket (44), and a rest position wherein the cover (28) leaves said passage (33) free for said loading of the nuclear fuel assembly (12), and - at least one sampling system (30) suitable for providing at least one water sample (32) taken from the housing (24), characterized in that: - the device (10) comprises a guide system (20) intended to be attached to the pool (14), - the gondola (22) is mounted apt to move on the guide system (20) in translation along a direction (V) intended to be substantially vertical, and - the device (10) comprises a depressurization system (34) suitable for extracting from the housing (24) a portion of the gas blanket and lowering the pressure of the gas blanket (44) in the housing (24), the cover (28) being in the active position.

2. The device (10) according to claim 1, wherein the depressurization device (34) comprises a vacuum pump (62) and a pipe (64) connecting the vacuum pump (62) to the gas blanket (44).

3. The device (10) according to claim 1 or 2, wherein the gondola (22) is configured to channel sipping gases (38), if any, from the nuclear fuel assembly (12) to the gas blanket (44), the device (10) comprising a recovery and analysis system (36) suitable for recovering and analyzing a fraction of the gas blanket (44).

4. The device (10) according to claim 3, wherein the recovery and analysis system (36) is suitable for re-injecting said portion of the gas blanket (44) into the housing (24).

5. The device (10) according to any of claims 1 to 5, wherein the cover (28) is mounted apt to rotate on the gondola (22) between the active position and the rest position.

6. A method of checking the leak-tightness of a nuclear fuel assembly (12) by sipping, using a device (10) according to any of claims 1 to 5, comprising the following steps: - loading the nuclear fuel assembly (12) into the gondola (22), - moving the cover (28) from the rest position to the active position, - extracting a part of the gas blanket and lowering the pressure of the gas blanket by the depressurization device, and - collecting at least one water sample (32) from the housing (24).

7. A retrofitting method for obtaining a device (10) according to any of claims 1 to 5, comprising: - replacing a pre-existing perforated gondola (122) by the gondola (22) of said device (10), and - adding of the sampling system (30) and of the depressurization system (34).