DEVICE FOR LIFTING OR LOWERING A BUNDLE OF CORE FUELS IN A PIT 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 allowing maintenance operations to be carried out. The storage pool comprises various systems for moving, testing, 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 in particular 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 defects 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 defect. 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 upgrade 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. Mobile penetrant cells are designed to withstand incidents, particularly in the event of an earthquake. However, the installation and removal of mobile penetrant cells is expensive.

[0011] Document US 2019 / 362862 describes a device according 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 characteristics corresponding to claims 2 to 6, taken in isolation or in all technically possible combinations.

[0015] The invention also relates to an assembly according to claim 7 or, according to a particular embodiment, according to claim 8.

[0016] The invention also relates to a leak testing method according to claim 9.

[0017] The invention also relates to a renovation method according to claim 10.

[0018] 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.

[0019] In reference to the figure 1 , an assembly 5 is described comprising a device 7 according to the invention, and advantageously a removable heating unit 9, for example located below the device 7.

[0020] According to variants not shown, the heating unit 9 is permanently fixed to the device 7, or is absent.

[0021] The device 7 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).

[0022] 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.

[0023] 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.

[0024] 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.

[0025] The device 7 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 7 also makes it possible to raise the nuclear fuel assembly 12 to the surface.

[0026] The device 7 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 40, the nacelle 22 being mounted movably on the guidance system 20 in translation in the direction V.

[0027] In the example, the device 7 advantageously comprises a cover 26, for example rotatably mounted on the nacelle 22 around a transverse axis T, and a member 28 for actuating the cover 26.

[0028] The device 7 comprises a sampling system 62 for taking at least one water sample 68 from the housing 24, with the aim of detecting fission products from the rods 18.

[0029] Advantageously, the device 7 also comprises a recovery and analysis system 30 adapted to recover and analyze any sweat gases 31 from the fuel assembly 12.

[0030] Thanks to the heating unit 9 or to the residual thermal power of the fuel assembly if this proves sufficient, and to the sampling system 62, the device 7 makes it possible to carry out a sweating test by heating the nuclear fuel assembly 12.

[0031] Alternatively, the penetrant test is performed by raising the device 7 so as to lower the hydrostatic pressure around the nuclear fuel assembly 12.

[0032] The cover 26 and the recovery and analysis system 30 allow analysis of any sweat gases 31.

[0033] The transverse axis T is for example substantially perpendicular to the direction V.

[0034] The guidance system 20 comprises, for example, two vertical rails (not shown).

[0035] The nacelle 22 comprises an upper end 32 defining a passage 33 adapted to allow loading of the nuclear fuel assembly 12 into the housing 24 from above, a side casing 34 adapted to surround the nuclear fuel assembly 12 around the direction V, and a lower end 36 configured to serve as a fixing support for the heating unit 9. The nacelle 22 advantageously comprises a temperature sensor 38 located in the housing 24.

[0036] According to a variant not shown, the nacelle 22 comprises several temperature sensors located in the housing 24 and similar to the sensor 38.

[0037] The heating unit 9 is adapted to heat the water 40 located in the housing 24. The heating unit comprises, for example, one or more electrical resistors (not shown).

[0038] Water 40 comes for example from body of water 16.

[0039] Water 40, once heated, has for example a temperature 20°C to 90°C higher than that of body of water 16.

[0040] The lower end 36 is advantageously adapted to be crossed by water coming from the body of water 16, while serving as a support for the nuclear fuel assembly 12. In the example, feet 42 of the nuclear fuel assembly 12 rest on the lower end 36. In the absence of the heating unit 9, or when the heating unit 9 is not in operation, the lower end 36 is advantageously adapted to allow external water (a portion of the body of water 16) to enter the housing 24 to cool the fuel assembly 12.

[0041] The lower end 36 is advantageously configured to allow easy assembly and then disassembly of the heating unit 9, while the nacelle 22 is immersed in the water 16 of the swimming pool 14. The heating unit 9 can for example be put in place and removed by means of a pole tool when the nacelle 22 is raised to the extreme upper position without assembly.

[0042] The lateral envelope 34 is configured to form a sheath 44 confining the water 40 in the housing 24, from the lower end 36 towards the upper end 32, and for channeling any sweating gases 31 towards the upper end 32. The lateral envelope 34 laterally delimits the housing 24 around the direction V, and is watertight.

[0043] The lateral envelope 34 advantageously comprises a thermal insulator 46 arranged between the water 16 contained in the pool 14 and the water 40 contained in the sheath 44. By thermal insulator, we mean here a material whose thermal conductivity is for example less than 0.04 Wm -1< .K -1< .

[0044] For example, the side casing 34 comprises a double wall 48 made of stainless steel, and an air layer 50 located in the double wall 48, the air of the air layer 50 here forming the thermal insulator 46.

[0045] The lateral envelope 34 advantageously has a prismatic shape in the direction V, for example with a square or hexagonal base, depending on the type of the nuclear fuel assembly 12.

[0046] The housing 24 is furthermore delimited in the direction V by the lower end 36 and the upper end 32.

[0047] The temperature sensor 38 is adapted to measure the temperature of the water 40 in the housing 24.

[0048] A control system (not shown) of the device 7 is advantageously adapted to feedback on the heating unit 9 in order to control the temperature of the water 40.

[0049] The upper end 32 has, for example, a general shape that widens in the V direction upwards. The upper end 32 is, for example, adapted to serve as a fixing support for equipment 51 carried by the nacelle 22, which will not be detailed here.

[0050] The upper end 32 is advantageously adapted to allow any sweating gases 31 to pass from the housing 24 towards the cover 26.

[0051] In the example, the cover 26 is movable in rotation relative to the nacelle 22 between an active position (shown in the figure 1 ), in which the cover 26 is located above the upper end 32, the cover 26 advantageously delimiting a gas retention pocket 52, and a rest position, in which the cover 26 leaves the passage 33 free to allow the loading of the nuclear fuel assembly 12 into the housing 24. The rest position is not shown, but is deduced from the active position shown on the figure 1 by a rotation around the transverse axis T.

[0052] The pocket 52 advantageously extends to the upper end 32 and is adapted to prevent water 40 from exiting the housing 24 through the upper end 32. The pocket 52 advantageously consists of air and gas from the housing 24.

[0053] The member 28 is adapted to move the cover 26 from the active position to the rest position, and vice versa. The member 28 comprises, for example, a rod 54 mounted on the cover 26.

[0054] According to variants not shown, the cover 26 is mounted differently on the nacelle 22, or is not mounted on the nacelle 22. In the latter case, the member 28 is advantageously configured to bring the cover 26 into the active position, and to remove it.

[0055] The recovery and analysis system 30 advantageously comprises at least one analyzer 56, a gas intake 58 adapted to sample any sweating gases 31 present in the gas pocket 52 formed by the cover 26, and a pipe 60 adapted to conduct the sweating gases 31 sampled from the gas intake 58 to the analyzer 56.

[0056] The analyzer 56 is for example configured to measure β and / or γ radioactivity, in order to determine whether the sampled gases contain sweat gases 31.

[0057] The water sampling system 62 advantageously comprises at least one sampling outlet 64, and a pipe 66 adapted to conduct the water samples to the sample 68.

[0058] The water sample(s) 68 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, for example when a predefined temperature is measured by the temperature sensor 38.

[0059] In addition, fission products are, for example, continuously searched for in the water taken from sampling point 64 during the bleeding of the fuel assembly.

[0060] The operation of the assembly 5 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.

[0061] It should first be noted that the device 7, with or without the heating unit 9, can operate as a conventional descender.

[0062] Indeed, the cover 26 being in the rest position, it is possible to load the nuclear fuel assembly 12 into the housing 24 via the passage 33 in the upper end 32. The device 7 then makes it possible to lower or raise the nuclear fuel assembly 12 into the pool 14 in the direction V.

[0063] The heat released by the nuclear fuel assembly 12 is removed by circulating water in the sheath 44, advantageously by natural convection, i.e. from bottom to top. In this case, the heat released heats the water 40 present in the housing 24 and causes it to rise in the sheath 44. The heated water escapes via the upper end 32 and returns to the pool 14. This also causes water to enter the housing 24 via the lower end 36.

[0064] In a variant not shown, the water 40 is set in motion by forced convection in the sheath 44.

[0065] The nuclear fuel assembly 12 is optionally unloaded from the device 7 via the passage 33, for example to be stored in the pool 14.

[0066] If it is desired to perform a leak test of the nuclear fuel assembly 12, it is left in the housing 24 or is loaded into the housing 24. The cover 26 is placed in the active position. The gas pocket 52 is formed in the cover 26 and extends vertically to the upper end 32. The heating unit 9 is fixed to the lower end 36 of the nacelle 22, if it was not already.

[0067] Then, the heating unit 9 heats the water 40, which circulates in the housing 24. The water 40 is advantageously blocked upwards by the gas pocket 52.

[0068] In the housing 24, the heated water 40 heats the nuclear fuel assembly 12 and triggers bleeding of the rod(s) 18 having a leaktightness defect.

[0069] Alternatively, since the heating unit 9 is not present or not activated, the water 40 is heated by the nuclear fuel assembly 12 itself. In other words, a rise in temperature of the nuclear fuel assembly 12 is achieved in whole or in part thanks to its radioactivity.

[0070] According to yet another variant, instead of or in addition to heating the water 40, the nacelle 22 is raised using the guidance system 20, so as to lower the hydrostatic pressure of the water 40 around the nuclear fuel assembly 12 and cause the nuclear fuel assembly 12 to sweat. For example, the nacelle 22 is raised over a distance greater than 5 meters, for example approximately 6 meters.

[0071] The sampling system 62 provides the water sample 68, advantageously at the end of the test. The water sample 68 is analyzed to detect the presence of one or more fission products resulting from the bleeding of the nuclear fuel assembly 12.

[0072] As a possible addition, water is taken from the housing 24 at the sampling point 64, advantageously continuously, and fission products possibly present in the water taken are continuously searched for.

[0073] Eventually, the sweating gases 31 rise in the sheath 44 which channels them, then pass through the upper end 32, and are then collected in the pocket 52 delimited by the cover 26.

[0074] Then, as a possible addition, at least a portion of the sweating gases 31 is advantageously recovered and analyzed by the recovery and analysis system 30, which allows them to be qualified as sweating gases.

[0075] The temperature sensor 38 advantageously measures the temperature of the water 40 and makes it possible to control the heating unit 9.

[0076] 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 62, the nuclear fuel assembly 12 is declared leak-proof. Analysis of the radioactivity of the penetrant gases 31 advantageously provides additional information.

[0077] Thanks to the characteristics described above, the device 7 is a descender and also allows a penetrant test to be carried out quickly and at low cost. Thanks to the device 7, the pool 14 does not need a conventional fixed or mobile penetrant cell.

[0078] A renovation method according to the invention will now be described.

[0079] For example, it consists of transforming a device 100 shown on the figure 2 , which is a classic descender, in the device 7 shown on the figure 1 .

[0080] The device 100 comprises a guide device 120 similar to the guide device 20, and a nacelle 122 comprising an upper end 132 similar to the upper end 32. The nacelle 122 defines a housing 124 adapted to receive a nuclear fuel assembly 12.

[0081] 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 sealed, unlike the lateral casing 34, nor thermally insulated.

[0082] Further, the nacelle 122 includes a lower end 136 which is not intended to receive any heating unit.

[0083] 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 .

[0084] It then becomes possible to implement an optional heating unit similar to the heating unit 9, a cover similar to the cover 26, and a sampling system (62) similar to the sampling system 62. The device thus renovated makes it possible to easily carry out a leak test by sweating the nuclear fuel assembly 12, by heating, or by lowering the hydrostatic pressure of the water 40.

Claims

1. A device (7) for raising or lowering a nuclear fuel assembly (12) in a pool (14) of a nuclear installation, the device (7) comprising: - a guidance system (20) intended to be fastened to the pool (14), and - a basket (22) defining a housing (24) for receiving the nuclear fuel assembly (12) and water (40) surrounding the nuclear fuel assembly (12), the basket (22) comprising an upper end (32) defining a passage (33) adapted for allowing loading of the nuclear fuel assembly (12) into the housing (24) from above, and a lateral jacket (34) adapted for surrounding the nuclear fuel assembly (12) around a direction (V) intended to be substantially vertical, and - at least one sampling system (62), characterized in that: - the basket (22) is mounted on the guidance system (20) movable in translation according to the direction (V), - the lateral jacket (34) is watertight, and - the sampling system (62) is adapted to provide at least one water sample (68) taken from the housing (24).

2. The device (7) according to claim 1, wherein the lateral jacket (34) comprises a thermal insulator (46) arranged between the water (16) contained in the pool (14) and the heated water (40) contained in the housing (24).

3. The device (7) according to claim 1 or 2, wherein the lateral jacket (34) comprises a stainless steel double wall (48) and an air layer (50) located within the double wall (48).

4. The device (7) according to any one of claims 1 to 3, wherein the lateral jacket (34) is configured to form a sheath (44) for channeling any sipping gases (31) from the nuclear fuel assembly (12) toward the upper end (32), the device (7) comprising a recovery and analysis system (30) adapted for recovering and analyzing the sipping gases (31) coming from the fuel assembly (12).

5. The device (7) according to any one of claims 1 to 4, further comprising a cover (26) movable between an active position, in which the cover (26) is located above the upper end (32), the cover (26) forming a pocket (52) for receiving gases and preventing an outflow of water (40) from the housing (24) via the upper end (32), and a rest position, wherein the cover (26) leaves free said passage (33) to allow said loading of the nuclear fuel assembly (12).

6. The device (7) according to claim 5, wherein the cover (26) is rotatably mounted on the basket (22) between the active position and the rest position.

7. An assembly (5) comprising a device (7) according to any one of claims 1 to 6, and a heating unit (9) adapted for heating the water (40) present in the housing (24).

8. The assembly (5) according to claim 7, wherein a lower end (36) of the basket (22) is configured to allow assembly and disassembly of the heating unit (9) while the basket (22) is immersed in the water (16) of the pool (14).

9. A method for controlling the sealing of a nuclear fuel assembly (12) by sipping, using a device (7) according to any one of claims 1 to 6, or an assembly (5) according to claim 7 or 8, and comprising the following steps: - loading the nuclear fuel assembly (12) into the basket (22), - sipping of the nuclear fuel assembly (12) by heating the water (40) and / or increasing the temperature of the nuclear fuel assembly (12) due to its radioactivity, and / or by raising the basket (22) in translation along the direction (V), and - taking at least one water sample (68) from the housing (24).

10. A renovation method for obtaining a device (7) according to any one of claims 1 to 6, comprising: - replacing a pre-existing perforated basket (122) with the basket (22) of said device (7), and - adding at least one sampling system (62) adapted for providing at least one water sample (68) taken from the housing (24).