NUCLEAR REACTOR CORE SUB-ASSEMBLIES

Abstract: A nuclear reactor core sub-assembly is provided with a removable lifting head (12)/absorber pin (20) assembly. The absorber pins (20) are connected to the lifting head (12) in such a way that they can be disconnected, inverted and reconnected the other way round, either collectively as a unit or individually. This allows the same absorber pins to be used at least twice over in different sub-assemblies since only the absorber material closest to the core tends to become depleted during irradiation.

Description: This invention relates to nuclear reactor core sub-assemblies and, in particular, to the neutron shielding sections which may be incorporated in the upper ends of such sub-assemblies. The neutron shielding section may comprise an array of spaced, generally parallel, axially extending tubular pins containing pellets of a suitable neutron absorbing material such as boron carbide. Over the life of a typical fuel sub-assembly, it is found the neutron absorbing capacity of the neutron shielding section is only partly depleted, primarily over a short axial section nearest to the fuel-bearing section of the sub-assembly. In British Patent Specification No 2159319, a sub-assembly for a fast nuclear reactor is described in which inter alia an upper lifting end of the sub-assembly is removable from an intermediate fuel bearing section and can be reconnected to a new intermediate section. A neutron shielding section is associated with this upper lifting end and is reconnected with the upper lifting end to the new intermediate section. Thus the neutron shielding section may have several periods of use and re-use before finally being discharged for storage or reprocessing. It is an object of the present invention to provide a nuclear sub-assembly capable of a more efficient use of its neutron shielding section than the sub-assembly of the afore-mentioned British Patent Specification No 2159319. According to the present invention there is provided a nuclear reactor core sub-assembly comprising an elongate tubular wrapper, the wrapper having a removable lifting head at one end, a mounting member at the opposite end, and enclosing a fuel-bearing section intermediate its ends, and a neutron shielding section between the fuel-bearing section and the lifting head, the neutron shielding section comprising an array of spaced, generally parallel, axially extending tubular pins enclosing neutron-absorbing material, the absorbing pins being provided either individually or collectively with attachment means at opposite ends of the pins for selectively suspending the pins from the lifting head in each of two orientations such that the pins when in one orientation are inverted with respect to the other orientation whereby the pins are disconnectible from the lifting head and are reconnectible to the lifting head in inverted relation to their orientation prior to disconnection. In this way, the lifting head and the associated neutron shielding section can be removed from a spent fuel-sub assembly, the shielding section inverted, and the lifting head and inverted shielded section re-assembled with an unirradiated wrapper containing unirradiated fuel thereby making more efficient use of the absorbing material, which is costly to manufacture, enrich and fabricate. The attachment means may be so arranged that disconnection, inversion and reconnection of the absorbing pins can be effected individually. Alternatively, the attachment means may be arranged so that the entire array of absorbing pins or sub-arrays thereof can, as a unit, be disconnected, inverted and reconnected to the lifting head. The lifting head may be mechanically connected to the wrapper such that on disconnection of the mechanical connection the lifting head and the array of the absorbing pins are removable as a unit from the wrapper. The wrapper and the lifting head may be of generally polygonal section such that the lifting head fits into the wrapper with the vertices of the lifting head in registry with the vertices of the wrapper, and the mechanical connection may comprise pads releasably secured to the lifting head and received within openings or windows of the wrapper in such a way that the pads can be disconnected from the lifting head and removed through the openings or windows to allow withdrawal of the lifting head from the wrapper. To promote further understanding of the invention, reference is now made to the accompanying drawings, in which: Figure 1 is a side view of a fuel sub-assembly for use in the core of a liquid metal cooled fast neutron reactor; Figure 2 is a longitudinal section, across the corners, of the top portion of one embodiment of a sub-assembly according to the invention; Figure 3 is a view in the direction 3 in Figure 2; Figure 3A is a fragmentary detail view in transverse section; Figure 4 is a sectional view in the direction 4-4 in Figure 2; Figure 5 is longitudinal section, across the corners, of the top portion of a second embodiment of a sub-assembly according to the invention; Figure 6 is a sectional view in the direction 6-6 in Figure 5; Figure 7 is a fragmentary view of part of the lifting head, from the direction 7 in Figure 6, showing the slot of a bayonet-type coupling with the neutron shielding section, the wrapper being omitted; and Figure 8 is a section view in the direction 8-8 in Figure 7. Referring to Figure 1, the sub-assembly shown is intended for a liquid sodium cooled fast neutron reactor and comprises a tubular wrapper 10 of hexagonal cross-section having at one end a lifting head 12 (see Figures 2 and 5) and, at the other end, a mounting member 14 in the form of a spike for engagement in the diagrid of a pool-type fast neutron reactor so that, in use, the sub-assembly is mounted with its longitudinal axis disposed generally vertically. Intermediate its ends, the wrapper 10 encloses a fuel-bearing section (not shown) in the form of a hexagonal array of spaced, generally parallel, axially extending pins containing nuclear fuel material which may be in pellet form. The sub-assemblies are mounted side-by-side on the reactor diagrid and are spaced apart by spacing pads 16 which may be disposed at different levels according to the particular method of core constraint employed. Heat is extracted from the fuel-bearing sections of the sub-assemblies by primary liquid sodium coolant from a cold pool region of the reactor which enters via holes in the spike 14, flows upwardly past the fuel pins and emerges from the top ends of the assemblies into a hot pool region of the reactor. The coolant is then returned to the cold pool via intermediate heat exchangers in which heat transfer takes place between the primary coolant and a secondary coolant which may also be liquid sodium. Above the fuel-bearing section, the sub-assembly incorporates a shielding section which, in both of the embodiments to be described, comprises a hexagonal array of pins 20 each containing pellets of a suitable neutron-absorbing material such as boron carbide or possibly a fertile material. The wrapper 10 and the lifting head 12 may be composed of a ferritic steel. Ferritic steels cannot be successfully bonded together by welding and, in order to achieve secure connection between the wrapper and the lifting head, mechanical jointing is employed. Referring to Figures 2, 5 and 3A, the mechanical jointing is effected by means of pads 22 which fit into recesses 24 machined into the corners of the hexagonal outer periphery of the lifting head 12 and are secured to the lifting head by screw-threaded fasteners comprising headed bolts 26, having non-circular shank portions 28 fitted into complementary holes in the base of the recess 24, and nuts 30 accommodated in sockets 32. The pads 22 are designed to be accessible through slots or windows in the corners of the wrapper so that they can be assembled to, and disconnected from, the lifting head externally of the wrapper. Once assembled as shown in Figures 2 and 5 the pads 22 render the lifting head axially captive with the wrapper. However, when it is desired to dismantle the sub-assembly in preparation for reprocessing the fuel, the lifting head can be readily freed by releasing the nuts 30 and withdrawing the pads. The lifting head may then be re-used simply by re-assembling it to a new wrapper containing unirradiated fuel. As shown in the illustrated embodiments, the pads 22 project beyond the outer profile of the wrapper and serve the dual purpose of connecting together the lifting head and the wrapper and acting as spacing pads (see upper pads 16 in Figure 1). In this case, as shown in Figure 3A, the pads 22 are provided with lateral projections 33 which overlie the wrapper walls and hence act as clamps. However, in an alternative embodiment (not shown) where spacing pads are not required adjacent the top end of the sub-assembly, the pads 22 may be designed so that, when secured to the lifting head, their outer surfaces are substantially flush with the outer surfaces of the wrapper walls, ie so that the pads 22 in this instance do not project beyond the hexagonal profile of the wrapper wall. Referring now to the embodiment of Fgures 2-4, the neutron absorbing pins 20 are designed to be removable with the lifting head 12 as a unit and also invertable individually relative to the head 12. Each pin 20 terminates at both ends in substantially identical screw-threaded stems 34 formed with necks 36. The lifting head 12 has a base 38 comprising a series of spaced bars 40 formed with internally threaded bushes 42 into which the stems 34 of the pins 20 can be screwed, the arrangement being such that, when the stem 34 is fully screwed home, the neck 36 is located above the base 38 - this applies whichever way round the pin is assembled to the base 38. Fingers 44 of reticulated configuration are inserted through apertures 46 in the sides of the lifting head (prior to assembly of the latter to the wrapper) so that the sides 48 of the fingers 44 enter the recesses afforded by the necks 36 thereby securely locking the pins 20 to the lifting head. During dismantling of the sub-assembly, the lifting head and the pins 20, may, on disconnection of the pads 22, be removed as a unit from the wrapper and reinserted into a fresh wrapper. Prior to re-insertion into the fresh wrapper, the pins 20 may be disconnected from the lifting head, inverted and reconnected, this being possible by virtue of the substantial identity of the stems 34 at the opposite ends of the pins. However, such inversion may not be necessary each time the lifting head is re-used since depletion of the absorbing material after one period of irradiation may not be so extensive as to require this. Thus, for example, inversion of the absorbing pins 20 may not be necessary until the lifting head/shielding section is coming up for re-use in a third period or irradiation. Referring now to Figures 5-8, this embodiment differs from that of Figures 2-4 in that the shielding section can be inverted as a unit with respect to the lifting head. In this case, the pins 20 are connected at each end, via peened over stems 50, to hexagonal grids 52 of substantially identically configuration. Each grid 52 comprises hexagonal perimeter 53 and a series of parallel bars 54, the perimeter and bars being formed with bushes 56 for reception of the stems 50 and location of the pins 20 in a hexagonal, spaced apart, array. At each vortex of the hexagonal perimeter 53 each grid is provided with a radial projection 58 for engagement, as a bayonet-type fitting, with slots 60 (see Figures 7 and 8) located at the vertices of the lifting head. The slots 60 are L-shaped and have an entry portion 62 which is offset from the vertex and a lateral portion 64 into which the projection 58 fits when the grid is correctly engaged with the lifting head. When correct engagement is obtained, the projection 58 is aligned with the corresponding vertex of the lifting head. In this condition, the lifting head/absorbing pin assembly can be inserted into the wrapper and the projections 58 are then trapped by the vertices of the wrapper thus preventing disengagement of the absorbing pin assembly from the lifting head. The lifting head may therefore be secured in position by means of the pads 22. When the irradiated sub-assembly is subsequently removed from the reactor, the lifting head/absorbing pin assembly can, following disconnection of the pads 22, be removed as a unit. The absorbing pin array may then be separated from the lifting head, refitted the other way round; and the assembly can then be inserted into a fresh wrapper. If desired, instead of the entire array of absorbing pins being invertable as a unit, the arrangement may be such that groups or sub-arrays of the pins are each invertable as a unit.

Claims: A nuclear reactor core sub-assembly comprising an elongate tubular wrapper (10), the wrapper (10) having a removable lifting head (12) at one end, a mounting member (14) at the opposite end, and enclosing a fuel-bearing section intermediate its ends, and a neutron-shielding section between the fuel-bearing sections and the lifting head (12), the neutron-shielding section comprising an array of spaced, generally parallel, axially extending tubular pins (20) enclosing neutron-absorbing material, characterised by the absorbing pins (20) being provided either individually or collectively with attachment means (34,50) at opposite ends of the pins (20) for selectively suspending the pins (20) from the lifting head (12) in each of two orientations such that the pins (20) when in one orientation are inverted with respect to the other orientation whereby the pins (20) are disconnectible from the lifting head (12) and are reconnectible to the lifting head (12) in inverted relation to their orientation prior to disconnection. A sub-assembly as claimed in Claim 1 in which the attachment means (34) is so arranged that disconnection, inversion and reconnection of the absorbing pins (20) can be effected individually. A sub-assembly as claimed in Claim 1 in which the attachment means (50) is so arranged that the entire array of the absorbing pins (20) can, as a unit, be disconnected, inverted and reconnected to the lifting head (12). A sub-assembly as claimed in Claim 1 in which the attachment means is so arranged that sub-arrays of the absorbing pins (20) can each, as a unit, be disconnected, inverted and reconnected to the lifting head (12). A sub-assembly as claimed in Claim 2 in which each of the absorbing pins (20) is provided with an attachment portion (34,50) at each end, each of the attachment portions (34,50) of each said absorbing pin (20) being adapted for engagement with a grid structure (38,52) associated with the lifting head (12). A sub-assembly as claimed in Claim 3 in which the absorbing pins (20) are engaged at each end thereof with a respective grid structure (52) and both said grid structures (52) are adapted to be releasably engaged with the lifting head. A sub-assembly as claimed in Claim 6 in which the lifting head (12) and each said grid structure (52) are provided with interconnectible bayonet-type fittings (58,60). A sub-assembly as claimed in Claim 6 or 7 including locking means for preventing separation of the lifting head (12) and the grid structure (52) connected thereto. A sub-assembly as claimed in Claim 8 in which the locking means is constituted by the wrapper (10), the arrangement being such that the corners of the wrapper (10) trap and prevent disengagement of interengaging elements (58,60) of the lifting head (12) and the connected grid structure (52). A sub-assembly as claimed in Claim 9 in which the wrapper (10), the lifting head (12) and the grid structures (52) are all of polygonal cross-section and in which the interengaging elements (58,60) comprise L-shaped slots (60) in the vicinity of the vertices of the lifting head (12) and radial projections (58) at the vertices of the grid structures (52), the slots (60) and the projections (58) being adapted to engage as bayonet-type fittings in such a way that, when correctly engaged, the projections (58) are aligned with the vertices of the lifting head (12) and, when the lifting head (12) is inserted into the wrapper (10), are prevented from disengaging from the slots (60) by the vertices of the wrapper (10). A sub-assembly as claimed in any one of the preceding Claims in which the lifting head (12) is mechanically connected to the wrapper (10), and in which on disconnection of the mechanical connection the lifting head (12) and the array of the absorbing pins (20) are removable as a unit from the wrapper (10). A sub-assembly as claimed in Claim 11 in which the lifting head (12) and the wrapper (10) are of polygonal cross-section such that the lifting head (12) fits into the wrapper (10) with the vertices of the lifting head (12) in registry with the vertices of the wrapper (10), the mechanical connection comprising pads (22) releasably secured to the lifting head (12) and received within openings or windows of the wrapper (10) in such a way that the pads (22) can be disconnected from the lifting head (12) and removed through the openings or windows. A sub-assembly as claimed in Claim 12 in which the windows are located at vertices of the wrapper (10). A sub-assembly as claimed in Claim 12 or 13 in which the pads (22) project beyond the external polygonal profile of the wrapper (10) so as to act, in use, as spacing pads for maintaining a minimum spacing between adjacent sub-assemblies of the reactor core. A sub-assembly as claimed in Claim 14 in which the pads (22) have lateral projections (33) which overlie the wrapper (10) and are clampable to the wrapper (10). A sub-assembly as claimed in Claim 12 or 13 in which the externally presented faces of the pads (22) lie within or substantially flush with the external polygonal profile of the wrapper (10).

Inventors: CRABTREE, GARRY CARLISLE, GIDDINS, PAUL GERARD, GILLESPIE, JOHN WOODSIDE
Applicants: BRITISH NUCLEAR FUELS PLC
Application year: 1988
Main classification: G21C3
IPC codes: G21C3