GB2197112A

GB2197112A - Fuel sub-assembly

Info

Publication number: GB2197112A
Application number: GB08626239A
Authority: GB
Inventors: Robert Jolly
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1986-11-03
Filing date: 1986-11-03
Publication date: 1988-05-11
Anticipated expiration: 2006-11-03
Also published as: FR2606200A1; GB8626239D0; GB2197112B; FR2606200B1

Abstract

A nuclear fuel sub-assembly includes a hexagonal bundle of parallel, spaced apart fuel pins 12 coupled at one end to an end-holding grid comprising a number of transverse spaced apart rails 20 to each of which is connected a series of pin-receiving cells which render the pins axially captive with the rails. The series of cells are defined by a pair of metal strips 22 each of which has a series of pocket formations 28 such that when the pocket formations are in registry they define cylindrical shaped cells provided with internal projections 32 which engage annular recesses 16 in the end caps 14 of the fuel pins 12 to effect axial constraint of the pins 12. <IMAGE>

Description

SPECIFICATION Nuclear fuel element sub-assemblies This invention relates to fuel sub-assemblies for liquid cooled nuclear reactors.

In one kind of liquid cooled nuclear reactor for example, a fast neutron reactor, the nuclear fuel assembly comprises a multiplicity of upstanding parallel fuel pins between which liquid metal coolant flows upwardly. To facilitate insertion and withdrawal of fuel to and from the fuel assembly the fuel pins are grouped in side-by-side sub-assemblies each comprising a bundle of pins enclosed by a tubular wrapper. The pins are spaced apart by cellular grids at intervals along the length of the bundle and are supported by end abutment with a lower grid within the wrapper.

The pins are free to move longitudinally within a limit of clearance afforded by an upper grid within the wrapper. Although the fuel pins must be free to extend longitudinally under irradiation growth and linear thermal expansion, it is desirable for the reactivity stability of the core that the pins be held captive against the levitating forces of the reactor coolant flow. However, the fuel pins must be readily demountable by remote handling means and a captivating means for the pins, affording this facility and at the same time being sufficiently compact that it does not interfere seriously with flow of the coolant through the bundle, is not readily available.

According to the present invention there is provided a nuclear fuel sub-assembly including a bundle of generally parallel, spaced apart fuel pins arranged in a polygonal array and coupled at one end of the bundle of transverse spaced apart rails to each of which is connected a series of pin-receiving cells which render the pins axially captive with the rails.

The axial restraint provided by the cells may be effected by interfitting formations on end caps of the fuel pins and on the cells. In one embodiment of the invention, the pin end caps are formed with annular recesses and the cells have internal projections which engage in the recesses.

The cells are preferably defined by metal pressings; for example, each series of cells may be defined by a pair of metal strips secured to opposite faces of a respective rail, each strip being formed with a series of pocket formations and the strips being arranged with each of their pocket formations in registry with respective pocket formations of the other strip so that each pair of registering pocket formations form a cell for reception of a fuel pin end cap.

The pocket formations may be of generally semi-cylindrical shape to form cells of generally cylindrical shape and the pocket formations may be separated by flat sections so that when a pair of strips are assembled faceto-face they may be secured together, for example by spot welding the flat sections of the strips together.

The pocket formations may each include a pressed out portion which projects into the corresponding cell for engagement with the annular recess of the corresponding pin end cap.

The strip may also incorporate, as extensions of the pocket formations, tapering formations each of which at one end merges with a respective pocket formation and tapers in a downward direction to assist smooth flow of coolant past the pin-receiving cells.

To promote further understanding, the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a perspective view of part of a liquid metal cooled fast neutron reactor fuel sub-assembly with certain components shown in exploded form; Figure 2 is a view illustrating an alternative rail-locking arrangement to that shown in Fig.

1.

The fuel sub-assembly illustrated in Fig. 1 is intended for use in a sodium-cooled fast neutron reactor of the type in which the reactor core is made up of an array of sub-assemblies each of which comprises a hexagonal tubular wrapper forming a coolant flow channel and enclosing a bundle of fuel pins which may contain and/or fertile nuclear material. The bundle may typically comprise 325, 271, 217 or 169 pins depending on the pin diameters employed and the pins may be spaced transversely apart either by helical wires extending around the individual fuel pins (wirewrapped design) or by cellular, honeycomb, grids spaced longitudinally of the wrapper (gridded design). The present invention is applicable to both the wire-wrapped and gridded designs.At the lower end of the fuel subassembly, the wrapper may terminate in a spike by means of which the sub-assembly can be mounted in a support tube of the reactor diagrid. The spike may be removable to allow access to the fuel pin bundle when the sub-assembly is to be dismantled in preparation for reprocessing of the fuel following irradiation to the desired burn-up.

The fuel pins in the bundle are organised in a hexagonal array and may therefore be considered to comprise a succession of layers of pins. Fig. 1 for the sake of clarity iilustrates two only of the fuel pin layers 10 and only the lower end portions of the fuel pins 12.

Each pin 12 terminates in an end cap 14 formed with annular recess 16. To restrain the pins against the levitation forces exerted by coolant flow in the direction A, the pins are all connected to a fixed hollow block 18 which is located within the wrapper and is accessible upon removal of the spike (not shown).

Each layer of pins 10 in the hexagonal array is connected to the block via a rail 20 and a pair of joggled end cap attachment strips 22.

The rails 20 are engaged with keyhole-shaped grooves 24 formed in the upper face of the block 18, the rails 20 having dumbell-shaped edge features 26 enabling the rails to be inserted endwise-on into, and subsequently withdrawn from, the grooves 24 with a sliding action. Once fully inserted into the grooves 24, each rail 20 is axially captive with the block 18 and projects substantially perpendicularly to the upper face of the block 18.

The pins 12 are rendered axially captive with the rails 20 by the attachment strips 22 which each comprise a series of semi-cylindrical pockets 28 separated by flat sections 30 so that when two strips 22 are placed in face-to-face relation with their pockets 28 in register, each pair of registering pockets define a generally cylindrical cell for reception of a respective fuel pin end cap. Each pocket 28 has an indent 32 extending into the cell for engagement with the groove 16 in the respective pin thereby rendering the pins axially captive with the attachment strips 22 when the latter are assembled together in face-to-face relation.

The strips 22 also include tabs 34 which, when the strips are assembled face-to-face are spaced apart by a distance substantially equal to the thickness of the rails 20 so as to embrace each rail 20 between the two sets of tabs 34 of the associated attachment strips.

The tabs 34 are anchored to the respective rail for example by spot welding, the spot weld positions being indicated by numeral 36 for the attachment strip shown at the top of Fig. 1. As well as being secured to the rail 20, each pair of strips 22 may also be secured together, for example by spot welds 37 made between the flat sections 30 in the vicinity of the upper edges of the strips 22.

To afford smooth flow surfaces for the coolant, the tabs 34 are formed with generally semi-conical sections 38 which merge, at their larger diameter ends, with the pockets 28.

During assembly the pairs of strips 22 are initially secured in face-to-face relation with the respective rail 20. The pins are then inserted into the cells defined by the pockets 28 so that each pin is engaged by reception of the indents 32 in the recess 16. At this stage, each pair of strips 22 extend in cantilever fashion from the rail 20 and can then be secured together, eg by spot welding, at the flat sections 30. The assembly of pins with strips 22 and rail 20 may subsequently be coupled to the block 18 by sliding the rails 20 into the grooves 24.

The strips 22 may conveniently be formed as pressings in long lengths and the lengths required for a particular layer of pins in the hexagonal array can then simply be cut from the long length. The strip thickness is such that, when the flat sections 30 are in face-toface relation, their combined thickness does not exceed, and is conveniently less than, the thickness of the corresponding rail thereby avoiding any obstruction to coolant flow at the junction between the upper edge of the rail and the lower edges of the flat sections 30.

Fig. 2 illustrates an alternative form of rail 40 which is received edgewise-on in complementary-shaped grooves in the block 18 and is locked in position by a locking pin 42 passing through aligned holes 44 in the rails 40 and bores (not shown) in the block 18.

It will be seen that dismantling of a fuelassembly can be effected by pulling the block 18 lengthwise of the wrapper after the spike has been removed, thereby pulling the pin bundle with the block. Subsequently the pins can be released from the block by effecting relative sliding between the rails 20 and the block 18 or by releasing the locking pin 42 in the case of Fig. 2.

Claims

1. A nuclear fuel sub-assembly including a bundle of generally parallel, spaced apart fuel pins arranged in a polygonal array and coupled at one end of the bundle to an end holding grid structure comprising a number of transverse spaced apart rails to each of which is connected a series of pin-receiving cells which render the pins axially captive with the rails.

2. A nuclear fuel sub-assembly as claimed in Claim 1 in which interfitting formations are provided on end caps of the pins and on the cells to effect the axial restraint provided by the cells.

3. A nuclear fuel sub-assembly as claimed in Claim 2 in which the interfitting formations comprise annular recesses formed in the end caps of the fuel pins and internal projections on the cells which engage in the recesses.

4. A nuclear fuel sub-assembly as claimed in Claim 1, 2 or 3 in which the cells are defined by metal pressings.

5. A nuclear fuel sub-assembly as claimed in Claim 4 in which each series of cells is defined by a pair of metal strips secured to opposite faces of a respective rail, each strip being formed with a series of pocket formations and the strips being arranged with each of their pocket formations in registry with respective pocket formations of the other strip so that each pair of registering pocket formations form a cell for reception of a fuel pin end cap.

6. A nuclear fuel sub-assembly as claimed in Claim 5 in which the pocket formations are of generally semi-cylindrical shape so that the cells formed by registering pocket formations are of generally cylindrical shape.

7. A nuclear fuel sub-assembly as claimed in Claim 5 or 6 in which the pocket formations are separated by flat sections so that when a pair of strips are assembled face-toface they can be secured together at the flat sections.

8. A nuclear fuel sub-assembly as claimed in Claim 5, 6 or 7 in which the pocket formations each include a pressed out portion which projects into the corresponding cell for engagement with the annular recess of the corresponding pin end cap.

9. A nuclear fuel sub-assembly as claimed in any one of Claims 5 to 8 in which each strip incorporates as extensions of the pocket formations, tapering formations each of which at one end merges with a respective pocket formation and tapers in a downward direction to assist smooth flow of coolant past the pinreceiving cells.

10. A nuclear fuel sub-assembly as claimed in Claim 1 substantially as hereinbefore described with reference to, and as shown in, Fig. 1 or Fig. 1 when modified as shown in Fig. 2 of the accompanying drawings.