GB2163888A

GB2163888A - Fission gas plenum chamber for nuclear fuel element sub-assembly

Info

Publication number: GB2163888A
Application number: GB08421925A
Authority: GB
Inventors: Kenneth Macgregor Swanson; John Alan Dodd
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1984-08-30
Filing date: 1984-08-30
Publication date: 1986-03-05
Also published as: GB8421925D0; GB2163888B

Abstract

The fuel pins (12) of a nuclear reactor fuel sub-assembly are connected to a common plenum vessel (14) provided with a valve (36) accessible from the open top of the sub-assembly wrapper (100 to permit release of fission gases from the fuel pins). <IMAGE>

Description

SPECIFICATION Fission gas plenum chamber for nuclear fuel element sub-assembly This invention relates to the storage of fission gases in nuclear fuel element sub-assemblies of the type which comprise a plurality of elongate fuel enclosures (e.g. fuel pins) mounted in generally parallel spaced relation within an external shroud (known as a wrapper) which, inter alia, isolates flow of coolant to the sub-assembly. The invention has particular application to liquid metal cooled fast breeder nuclear reactors in which fission gas liberation tends to be problematic in imposing limitations on the degree of burn-up that can be attained.

It is well-known to provide a plenum space or spaces within each fuel enclosure for accommodating the liberated fission gases. However, the plenum volumes provided hitherto have proved inadequate in terms of enabling burn-up of the order of 15% and higher.

Proposals have been made in connection with gas cooled reactors to link the plenum spaces of the fuel enclosures together by means of manifolds. Such proposals are disclosed in US-A-3231476, US-A3432388, GB-A-1475174 and GB-A-1317589. The last three patents are concerned with venting arrangements for the fission gases whereas US-A-3231476 discloses storage of the fission gases in spaces provided in the jacket and tie rods of the fuel element construction.

Fast reactor fuel element sub-assemblies incorporate breeder material sections in addition to fissile material and, in one sub-assembly design used in practice, the breeder material is enclosed within a plurality of enclosures (so-called mixer-breeder pins) located above the fuel enclosures.

The object of the present invention is to provide an improved sub-assembly with improved fission gas storage.

According to the present invention there is provided a nuclear fuel element sub-assembly of the type initially referred to in which at least some of the fuel enclosures also contain breeder material and in which the interiors of at least some and preferably all of the fuel enclosures are connected to a common plenum vessel separate from the fuel enclosures and accommodated within the shroud.

Preferably the fuel enclosures are connected to the common plenum vessel via one or more manifolds.

In practice, the plenum vessel will be sized according to the degree of burn-up desired. As the vessel is accommodated within the shroud, it will lie in the path of coolant flow; preferably therefore the vessel is located adjacent the upper end of the shroud and an outlet port or ports may be provided in the wall or wall of the wrapper in the vicinity of the vessel to allow egress of at least part of the coolant flow whereby the coolant flow is at least partially channelled externally of the shroud and by-passes the plenum vessel.

In the gap between the outer periphery of the vessel and the inner periphery of the shroud, fins may be arranged to effect mixing of the coolant that flows through the gap. The fins may extend generally helically about the plenum vessel and may be provided on the external surface or surfaces of the latter.

Preferably the top of the shroud is open and the plenum vessel is located within the shroud so as to be accessible from the open top. Where it is desired to release stored fission gases from the plenum vessel, this may be achieved by accessing the plenum vessel via the open top of the shroud. It is envisaged that the plenum vessel may incorporate a valved port for this purpose, the valve being operable through the top end of the shroud while the sub-assembly remains in the core of the reactor, for example by means of a gas collection attachment provided on the charge machine for loading and withdrawing the sub-assemblies into and from the core.

To promote further understanding of the invention, one example will now be described with reference to the accompanying drawing, the sole Figure of which is a diagrammatic longitudinalsectional view of part of a fuel element subassembly for a sodium cooled fast breeder reactor.

The sub-assembly shown comprises a tubular shroud or wrapper 10 of known design which terminates at its lower end in a spike which fixes into the bottom core plate or diagrid of the reactor and through which sodium coolant enters for flow along the length of the shroud. At its upper end, the wrapper terminates in a generally cylindrical endpiece provided with lifting lugs. Between its ends the wrapper is of generally hexagonal cross-section.

Within the wrapper, a hexagonal array of fuel pins 12 are mounted. The fuel pins 12 may be spaced from one another by means of spacing grids or by means of wires wound helically around the pins. In a conventional sub-assembly design, the fuel pins are filled with annular pellets of oxide fuel material and fertile material arranged so that there is a lower breeder section, an upper breeder section and a section of oxide fuel material between the two breeder sections with top and bottom plenum spaces for the collection of fission gases generated during fuel burn-up. Because of the need to provide the plenum spaces within the pins of a conventional sub-assembly, the lengths of the breeder sections within the pins are necessarily restricted. Additional breeder material can therefore introduced into the sub-assembly by way of so-called mixer-breeder pins located above the fuel pins.

In accordance with the preferred form of the invention, the plenum spaces within the fuel pins are virtually eliminated and storage of fission gases is provided for by a separate plenum vessel 14 which is housed within the wrapper 10 at the location normally occupied by the mixer-breeder pins which may be omitted provided some other forum of mixing arrangement is included. As mentioned, the fuel pins 12 (of which only three are illustrated) are mounted in a hexagonal array and may be considered to be arranged as a series of rows extending parallel to a line joining opposite vertices of the hexagon. Each row of pins is connected to a respective manifold 16 which, in turn, is connected to the vessel 14 via a respective pipe 18. Thus, the vessel 14 acts as a common reservoir for all the fission gases generated within the pins.

The pins 12 may be generally similar in design to the conventional pins described above with the important difference that the plenum spaces are no longer required. Consequently, the pins 12 may be loaded with additional fissile our fertile material. For example, the ommision of the conventional mixerbreeder pins can be compensated for by loading the pins 12 with additional breeder pellets with the advantage that a greater quantity of breeder material is subjected to the higher neutron fluxes prevailing in the vicinity of the fuel pins. Thus, as shown, the fuel pins 12 each contain a lower breeder section 20, an intermediate section 22 of the fuel material, and an upper breeder section 24. The sections 22, 24 are composed of annular pellets so as to provide a central path for fission gas flow to the manifolds 16 and thence to the vessel 14.The lower breeder section 20 however may be composed of solid pellets since there is a reduced requirementto provide a flow path. Reference numeral 26 depicts wire mesh spacers located between the adjacent sections.

As in a conventional sub-assembly, sodium coolant flows upwardly through the wrapper and through the inter-pin spaces. The flow may continue past the vessel 14 which may be provided with fins (not shown) on its outer periphery to aid mixing of the coolant. At least part of the coolant flow may be caused to by-pass passage around the vessel 14. For example, the wrapper may be provided with one or more outlet ports 30 to allow egress of sodium from the wrapper in the vicinity of the vessel 14.

Although the vessel 14 may be sized to accommodate the entire volume of fission gases produced within the target burn-up range, it may be desirable to vent the vessel at an intermediate point during the burn-up range. Such venting may be readily achieved if the vessel 14 is arranged to be accessible through the open top of the wrapper. The vessel may for example be provided with a valved port 34, the valve member 36 of which can be displaced downwardly to open the valve against the biassing action of a spring 38. Operation of the valve may for instance be effected by means of a gas collection attachment provided on the charge machine for loading and unloading the sub-assemblies.

Claims

1. A nuclear fuel element sub-assembly of the type referred to in which at least some of the fuel enclosures also contain breeder material and in which the interiors of at least some and preferably all of the fuel enclosures are connected to a common plenum vessel separate from the fuel enclosures and accommodated within the shroud.

2. Asub-assembly as claimed in Claim 1 in which the fuel enclosures are connected to the common plenum vessel via one or more manifolds.

3. A sub-assembly as claimed in Claim 1 or 2 in which the vessel is located adjacent the upper end of the shroud and an outlet port or ports are provided in the wall or wall of the wrapper in the vicinity of the vessel to allow egress of at least part of the coolant flow whereby the coolant flow is at least partially channelled externally of the shroud and by-passes the plenum vessel.

4. A sub-assembly as claimed in Claim 1,2 or 3 in which in the gap between the outer periphery of the vessel and the inner periphery of the shroud, fins may be arranged to effect mixing of the coolant that flows through the gap.

5. A sub-assembly as claimed in any one of Claims 1-4 in which the top of the shroud is open and the plenum vessel is located within the shroud so as to be accessible from the open top.

6. A sub-assembly as claimed in Claim 5 in which the plenum vessel incorporates a valved port for fission gas release, the valve being operable through the top end of the shroud while the sub-assembly remains in the core of the reactor.

7. A sub-assembly as claimed in any one of Claims 1-6 in which the upper ends of the fuel pins enclose breeder material.

8. A sub-assembly as claimed in any one of Claims 1-7 in which the fuel pins are connected to said common plenum vessel in groups via respective manifolds associated with said groups.

9. A fuel sub-assembly substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.