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EP-4607537-B1 - FUEL ASSEMBLY FASTENING MECHANISM, METHOD FOR OPERATING A FUEL ASSEMBLY FASTENING MECHANISM AND FUEL ASSEMBLY

EP4607537B1EP 4607537 B1EP4607537 B1EP 4607537B1EP-4607537-B1

Inventors

  • STROHMER, Franz Xaver
  • GROSKA, Judit
  • BRESSEL, Lukas

Dates

Publication Date
20260513
Application Date
20240220

Claims (16)

  1. A fuel assembly fastening mechanism (24) for use in a nuclear reactor (10) comprising a core barrel (12) with a bottom core plate (18), the bottom core plate (18) having a lattice structure with a plurality of open cells (20), including a fuel assembly (22) comprising a fuel assembly header (28) and a fuel assembly footer (30) arranged at opposite ends of the fuel assembly (22) along a fuel assembly main axis, the fuel assembly footer (30) having a footer housing (55) and comprising a holding lever (66) having a nose (74), the holding lever (66) being rotatable between an unlocked position and a locked position, a securing wedge (56) arranged within the fuel assembly footer (30), characterised in that the securing wedge (56) being movable between an upper position and a lower position, wherein the holding lever (66) is in the unlocked position when the securing wedge (56) is in one position chosen from the upper position and the lower position of the securing wedge (56), and the holding lever (66) is in the locked position when the securing wedge (56) is in the other position chosen from the upper position and the lower position of the securing wedge (56), a nozzle (26) for mounting the fuel assembly (22) in one of the open cells (20) of the bottom core plate (18), the nozzle (26) comprising a nozzle housing (34) engaging the footer housing (55) along the fuel assembly main axis, the nozzle housing (34) comprising a nozzle window (36), wherein the nose (74) of the holding lever (66) is received by the nozzle window (36) in the locked position of the holding lever (66) such that a contact surface (108) between the nose (74) of the holding lever (66) and the nozzle window (36) is arranged perpendicular to the fuel assembly main axis.
  2. The fuel assembly fastening mechanism (24) of claim 1, wherein the holding lever (66) comprises an opening (75) in which a rotation pin (76) is inserted, the rotation pin (76) defining a rotation axis of the holding lever (66).
  3. The fuel assembly fastening mechanism (24) of claim 1 or 2, wherein the footer housing (55) has a footer window (54) associated to the holding lever (66), the footer window (54) being aligned with the nozzle window (36), and wherein the nose (74) of the holding lever (66) is received by the nozzle window (36) and by the footer window (54) in the locked position of the holding lever (66).
  4. The fuel assembly fastening mechanism (24) of claim 3, wherein the nose (74) of the holding lever (66) has a first sloping surface (100) and the footer window (54) has a second sloping (102) surface being in contact with the first sloping surface (100) in the locked position of the holding lever (66).
  5. The fuel assembly fastening mechanism (24) of any of the preceding claims, wherein the fuel assembly footer (30) comprises more than one holding lever (66) and the nozzle housing (34) comprises more than one nozzle window (36), wherein each of the holding levers (66) is associated to one of the nozzle windows (36).
  6. The fuel assembly fastening mechanism (24) of any of the preceding claims, wherein the fuel assembly header (28) comprises an operation handle (84) and the fuel assembly (22) comprises a connection rod (82) connecting the operation handle (84) with the securing wedge (56).
  7. The fuel assembly fastening mechanism (24) of claim 6, wherein the operation handle (84) engages a groove (86) of the fuel assembly header (28), the groove (86) extending from an upper rest section (88) to a lower rest section (92), the operating handle (84) being movable from the upper rest section (88) to the lower rest section (92).
  8. The fuel assembly fastening mechanism (24) of any of the preceding claims, wherein the securing wedge (56) comprises a protruding portion (58) contacting the holding lever (66).
  9. The fuel assembly fastening mechanism (24) of any of the preceding claims, wherein the nozzle housing (34) engages a footer groove (78) in the footer housing (55).
  10. The fuel assembly fastening mechanism (24) of any of the preceding claims, wherein the fuel assembly footer (30) comprises an alignment pin (52) which is received by an alignment opening (50) of the nozzle (26).
  11. The fuel assembly fastening mechanism (24) of any of the preceding claims, wherein the nozzle (26) has a thread (40) for connecting the nozzle (26) with the open cell (20) of the bottom core plate (18).
  12. The fuel assembly fastening mechanism (24) of any of the preceding claims, wherein the nozzle (26) has a meshed orifice (44) at a lower end (42) of the nozzle (26).
  13. The fuel assembly fastening mechanism (24) of any of the preceding claims, wherein the securing wedge (56) is movable to a safety removal position, and wherein the securing wedge (56) is arranged within a debris basket (110) of the nozzle (26) in the safety removal position.
  14. The fuel assembly fastening mechanism (24) of any of the preceding claims, wherein the fuel fastening assembly (24) further comprises a safety unlocking device (112) comprising an unlocking cylinder (116) being movable from a first position to a second position, wherein the unlocking cylinder (116) is configured to push against the nose (74) of the holding lever (66) in the second position.
  15. A method of operating a fuel assembly fastening mechanism (24) according to any of the preceding claims, the method comprising the following steps: - Mounting the nozzle (26) in one of the open cells (20) of the bottom core plate (18); - Inserting the fuel assembly (22) in the core barrel (12), while the holding lever (66) is in its unlocked position, by lowering the fuel assembly (22) onto the nozzle (26) such that the nozzle housing (34) engages the footer housing; - Moving the securing wedge (56) from its upper position to its lower position or from its lower position to its upper position, thereby rotating the holding lever (66) to its locked position.
  16. A fuel assembly (22) for a fuel assembly fastening mechanism (24) according to any of the preceding claims 1 to 14, the fuel assembly (22) comprising a fuel assembly header (28) and a fuel assembly footer (30) arranged at opposite ends of the fuel assembly (22) along a fuel assembly main axis, the fuel assembly footer (30) having a footer housing (55) and comprising a holding lever (66) with a nose (74), the holding lever (66) being rotatable between an unlocked position and a locked position, a securing wedge (56) arranged within the fuel assembly footer (30), the securing wedge (56) being movable between an upper position and a lower position, wherein the holding lever (66) is in the unlocked position when the securing wedge (56) is in one position chosen from the upper position and the lower position of the securing wedge (56), and the holding lever (66) is in the locked position when the securing wedge (56) is in the other position chosen from the upper position and the lower position of the securing wedge (56).

Description

The invention relates to a fuel assembly fastening mechanism for use in a nuclear reactor, a method for operating a fuel assembly fastening mechanism and a fuel assembly for a fuel assembly fastening mechanism. A nuclear reactor comprises a plurality of fuel assemblies arranged within a core of the nuclear reactor. The fuel assemblies need to be fixed within the core to ensure a correct alignment and allow for a precisely controllable handling of the fuel assemblies. Typically, a nuclear reactor comprises a bottom core plate and an upper core plate which can be used for fixation of the fuel assemblies. However, fuel assemblies can only be fixed either at the bottom core plate or the upper core plate to avoid mechanical stress on the fuel assemblies caused by thermal grow or neutron-induced grow. Common nuclear reactor designs use a coolant which is pumped from the bottom core plate in the direction of the upper core plate. Any fixation solution relying on fixation on the lower core plate and/or on the upper core plate therefore needs to work against forces generated from the coolant flow. Dependent on the type of coolant used in the nuclear reactor, the fuel assemblies can be buoyant within the coolant, e.g. in molten metal-cooled nuclear reactors like lead-cooled nuclear reactors. Therefore, it is not sufficient to rely on gravity alone as fixation force for fuel assemblies in the bottom core plate in such reactors. One solution is the addition of ballast weights on the fuel assemblies made of a material with a specific weight higher than the specific weight of the coolant. However, fuel assemblies with such ballast weights are difficult to handle and might cause problems on the overall structure of the fuel assemblies during operation or in abnormal operating conditions like earthquakes. Even if coolants are used in which the fuel assemblies are not buoyant, it is still desirable to provide some kind of additional fixation mechanism for the fuel assembly in the bottom core plate to counteract coolant flow and to ensure that the fuel assembly is properly held in place. Further, it is desirable that each fuel assembly within the nuclear reactor can be handled individually, i.e. can be placed and removed in and from the core of the nuclear reactor, respectively, without interfering with other fuel assemblies placed in the core. Especially in nuclear reactors which are used for generation of desired radionuclides, e.g. isotopes for medical applications or isotopes generated by transmutation, it is desirable to be able to handle the so-called "breeding" fuel assemblies individually to optimize their time spent within the nuclear reactor, and without interrupting the operation of the nuclear reactor. CN 106 531 232 B discloses a fuel assembly for integrated refueling to be used in nuclear reactors which are cooled by a lead-based alloy. The fuel assembly comprises a plurality of individual fuel elements which are fixed to each other by an upper support grid. The upper support grid is connected to a fixing rod whose lower end is fixed in a bottom plate of the nuclear reactor. However, it is not possible to operate each of the fuel elements individually because they share the same holding structure. WO 2022 / 231461 A1 and WO 2023 / 009023 A1 describe fuel assemblies which are stabilized vertically based on friction caused by a spring-loaded mechanism. The friction force counteracts the buoyancy of the fuel assembly in the coolant, thereby holding the fuel assembly in position. The use of spring-loaded elements to ensure fixation of the fuel assembly does not meet safety requirements, as the metal of the spring-loaded elements will experience embrittlement due to the high neutron flux in the core of the nuclear reactor (also called "neutron embrittlement"). Therefore, the force which is necessary to remove the fuel assemblies to overcome the tension force of the spring-loaded element is expected to change during the operation time of the fuel assembly. This will increase the risk of the fuel assembly becoming stuck in the core of the nuclear reactor or that a breakage of the spring-loaded element will occur, at least after several core cycles. A long-term reliable operation of the nuclear reactor can therefore not be assured. From CN 109 767 850 A, a fuel assembly lower locking device is known comprising a spring-loaded cylinder with a holding pin and a wedge to secure the position of the fuel assembly during operation. The wedge is used to unlock the pin before removing the fuel assembly. For the removal, a dedicated unlocking action is necessary without which removing the fuel assembly is not possible. However, the holding pin experiences mechanical stress during insertion and removal of the fuel assembly, as it will be pressed by the holding spring of the spring-loaded cylinder against an inner side of a bottom core plate, resulting in strong friction forces acting on the holding pin. Further, during removal,