GB1597189A

GB1597189A - Nuclear reactor fuel sub-assemblies

Info

Publication number: GB1597189A
Application number: GB20581/78A
Authority: GB
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1978-05-18
Filing date: 1978-05-18
Publication date: 1981-09-03

Description

(54) NUCLEAR REACTOR FUEL SUB-ASSEMBLIES (71) We, UNITED KINGDOM ATOMIC ENERGY AUTHORITY, London, a British Authority, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed. to be particularly described in and by the following statement: This invention relates to fuel sub-assemblies for liquid metal cooled fast breeder nuclear reactors.

A fuel sub-assembly for a liquid metal cooled fast breeder nuclear reactor comprises a bundle of spaced fuel pins within a tubular wrapper or sleeve through which liquid metal coolant can be flowed in heat exchange with the fuel pins. The pins may be spaced by a series of cellular grids attached to the wrapper each pin occupying a discrete cell of each grid and the wrapper has a spiked extension at one end (hereinafter termed the lower end) whereby the sub-assembly can be located in a support structure and arranged with other sub-assemblies to upstand in sideby-side array to form a fuel assembly. The upper end of the wrapper is extended by a tubular neutron shield of massive steel. In Patent No 1.478.985 there is disclosed a nuclear reactor fuel element sub-assembly comprising a bundle of spaced parallel fuel pins within a tubular wrapper, the pins having peripherally enlarged heads at one end portion and the heads being captive between a fixed end abutment grid and a fixed cellular grid. the one end portion of each pin being formed with coolant flow passages. In the disclosed fuel sub-assembly one end of each fuel pin is held captive between a fixed end abutment grid and a fixed cellular hold down grid so that axial displacement of the pins is prevented without restricting the pins ability to expand longitudinally. After irradiation of a fast breeder sub-assembly it is necessary to reprocess it to recover unused fissile material and to separate newly formed plutonium from the residues but because of irradiation embrittlement of the pins dismantling of the subassembly is difficult. Prior to the present invention the technique used for dismantling a fuel sub-assembly has been to remove the lower end spike extension and end abutment grid from the wrapper and then carefully to withdraw the fuel pins one by one for reprocessing but as the operation must be carried out by remote control because of radioactivity it is a time consuming and expensive process.

One object of the present invention is to provide a fuel sub-assembly for a liquid metal cooled fast breeder nuclear reactor of such construction that it will facilitate dismantling for reprocessing purposes and another object is to provide an improved method of dismantling a fuel sub-assembly.

According to the invention a fuel subassembly for a liquid metal cooled fast breeder nuclear reactor comprises a bundle of fuel pins within a tubular wrapper and spaced apart by a series of compliant cellular bracing grids (as defined hereinafter) each pin occupying a discrete cell of each grid, the wrapper having a lower end spike extension and an upper end tubular neutron shield extension and wherein the pins are each retained at one end by a cellular hold down grid (as defined hereinafter) which is secured to an end region of the tubular wrapper.

Conveniently, the pins may be retained at their lower ends by a cellular hold down grid which is secured to the lower end region of the tubular wrapper.

By "compliant cellular bracing grid" is meant a grid of honeycomb construction wherein each cell has resiliently yieldable contact pads for slidably guiding a fuel pin, the pads being arranged in a first set which normally brace the pin co-axially within its cell and a second set which, though normally clear of the pins, provides a second brace in the event of excessive displacement of the pads of the first set. By "cellular hold down grid" is meant a grid having means for retainably engaging ends of the pins thereby to prevent withdrawal of the pins from the cells of the grid during normal operation of the nuclear reactor. In a preferred construction of sub-assembly according to the invention the lower end of each pin has a radial projection for abutting the underside of the hold down grid.

A fuel sub-assembly according to the invention can be easily dismantled by severing the end regions of the wrapper immediately below the tubular neutron shield and immediately above the spike extension followed by severing of the wrapper immediately above the hold down grid so that the hold down grid with a fragment of the wrapper and the complete bundle of fuel pins can be withdrawn from the remnants of the wrapper. The compliant nature of the grids facilitates withdrawal of the pins from the series of bracing grids and thereby reduces fracture of the embrittled pins.

The invention will also reside in a method of dismantling a fuel sub-assembly comprising a bundle of fuel pins within a tubular wrapper and spaced apart by a series of compliant cellular bracing grids (as defined hereinbefore). the wrapper having a lower end spike extension and an upper end tubular neutron shield extension and wherein the pins are retained at their lower ends by a cellular hold down grid (as defined hereinbefore) which is secured to the lower end region of the tubular wrapper, the method comprising the steps of severing the upper and lower end regions of the wrapper to separate the tubular neutron shield and spike extensions from the wrapper, severing the wrapper immediately above the cellular hold down grid and withdrawing the combination of hold down grid and bundle of fuel pins from the combination of wrapper and series of bracing grids.

Constructions of fuel sub-assemblies embodying the invention for use in a fast breeder liquid metal-cooled nuclear reactor are described by way of example with reference to the accompanying drawings wherein Figure 1 is a diagrammatic sectional view of a typical fuel sub-assembly, Figure 2 is a fragmentary sectional view of one construction according to the invention, Figure 3 is a fragmentary cross-sectional view on line 111III of Figure 2, Figure 4 is a fragmentary sectional view of an alternative construction of fuel sub-assembly.

Figure 5 is a diagrammatic view of an alternative feature for a fuel sub-assembly generally similar to those shown in Figures 2 and 4, Figure 6 is an isometric view of a discrete cell of a compliant cellular grid, and Figure 7 is a diagrammatic sectional view of a liquid metal-cooled fast breeder nuclear reactor.

A fuel sub-assembly as shown in Figure 1 for a liquid metal-cooled fast breeder nuclear reactor comprises a central fuel section 1, a lower end locating section 2 and an upper end neutron shielding section 3. The fuel section I comprises a bundle of 325 elongate fuel pins 4 enclosed within a tubular wrapper 5 of hexagonal cross-section and are spaced apart by a series of compliant cellular bracing grids 6 each pin occupying a discrete cell of each grid. The cellular grids 6 are welded to the wrapper 5 and the fuel pins 4 are supported within the wrapper at their lower ends by an abutment grid 7. The fuel pins 4 are retained at their lower ends by a cellular hold down grid 7a which is secured to the lower end region of the tubular wrapper 5.

The lower end locating section 2 comprises a spike extension 8 for engaging sockets in a fuel assembly support structure 9 and has apertures 10 through which coolant can flow from within the diagrid. The upper section 3 is of massive steel and comprises a tubular member 11 having an extension 12 with an internal lip 13 for engagement by lifting means.

Referring now to Figures 2 and 3, each fuel pin comprises a stack of fuel pellets sheathed by a stainless steel tube 14 which is sealably closed by each end by plugs 15 and each pin has a lower end overcap 16 which engages a cell of the hold down grid 7a. The overcap 16 is centrally positioned within the cell by 3 equally angularly spaced pads 17 so that there is a clear passage about the overcap for upward flow of liquid metal coolant. The overcap carried a hook which provides a radial projection 18 lying intermediate the hold down grid 7a and the abutment grid 7.

During transportation of the fuel subassembly, the fuel pins are supported at their lower ends by the abutment grid 7 and during normal operation of the nuclear reactor, the pins are restrained against levitation by upward flow of coolant by abutment of the hooks 18 with the underside of the hold down grid 7a.

The compliant cellular bracing grids 6 are of the kind disclosed in UK Patent Application GB 2011150 A each of which has a right hexagonal boundary and comprises 325 unit cells arranged in honeycomb pattern, the cells being generally of hexagonal shape as shown in Figure 6. Each unit cell is formed from strip material and three sides have a rectangular window 33 whilst the three alternating sides each have an elongate embossment or dimple 34 the dimples forming a guide pad for a fuel pin. There is a pair of smaller embossments 35 disposed one beyond each end of the elongate embossments and which provide additional bracing pads or backstops for a fuel pin in the event that bowing of the pin occurs during irradiation of the fuel sub-assembly. Each pair of abutting sides of the cells are secured together by a pair of spot welds disposed in opposed end regions of the sides. A compliant grid of this nature enables the pins to be slidably guided with the minimum of bearing loading whilst the backstop pads 35 provide adequate bearing loading to resist irradiation bowing of the pins.

For reprocessing purposes after irradiation of the fuel sub-assembly, the wrapper is separated from the lower end locating section 2 and the upper end neutron shielding section 3 by severing the upper and lower end regions of the wrapper, then the wrapper is severed again immediately above the hold down grid 7a. The entire fuel pin bundle and hold down grid combination can then be withdrawn from the wrapper 5 and cellular grids 6 combination in one operation, the withdrawal operation being made possible by the compliant nature of the cellular grids 6.

In the alternative construction shown in Figure 4, the radial projection 18 takes the form of an eye 19 whilst in the construction shown in Figure 5, the radial projection 18 is a flange generally of triangular form. In each of the constructions, the radial projection is of a form which offers little restriction to the coolant flow passages through the hold down grid 7a.

The described fuel sub-assemblies are used in a liquid metal-cooled fast breeder reacton of the pool kind shown in Figure 7. The construction comprises a fuel assembly 21 submerged in a pool 22 of liquid sodium coolant in a primary vessel 23. The primary vessel is suspended from the roof of the containment vault 24 and there is provided a plurality of coolant pumps 25 and heat exchangers 26 only one of each of pump and heat exchangers being shown. The fuel assembly 21 mounted on a support member 27 is housed with the heat exchangers- in a core tank 28 whilst the pumps 25 which deliver coolant to the support member are disposed outside of the core tank. Control rods 29 and instrumentation 30 penetrate the roof of the vault.

In operation, relatively cold coolant is drawn from the pool outside the core tank 28 and passed upwardly through the fuel assembly by way of the support member 27 and then flows through the heat exchanger 26 in heat exchange e with a second coolant which flows through steam generating plant disposed outside of the vault. The main coolant is returned from the heat exchanger to the outer region of the pooi.

WHAT WE CLAIM IS: 1. A fuel sub-assembly for a liquid metal cooled fast breeder nuclear reactor comprising a bundle of fuel pins within a tubular wrapper and spaced apart by a series of compliant cellular bracing grids (as defined hereinbefore) each pin occupying a discrete cell of each grid, the wrapper having a lower end spike extension and an upper.end tubular neutron shield extension and wherein the pins are each retained at one end by a cellular hold down grid (as defined hereinbefore) which is secured to an end region of the tubular wrapper.

2. A fuel sub-assembly according to claim 1 wherein the pins are retained at their lower ends by a cellular hold down grid which is secured to the lower end region of the tubular wrapper.

3. A fuel sub-assembly according to claim 2 wherein the lower end of each pin has a radial projection for abutting the underside of the hold down grid.

4. A method of dismantling a fuel subassembly comprising a bundle of fuel pins within a tubular wrapper and spaced apart by a series of compliant cellular bracing grids (as defined hereinbefore), the wrapper having a lower end spike extension and an upper end tubular neutron shield extension and wherein the pins are retained at their lower ends by a cellular hold down grid (as defined hereinbefore) which is secured to the lower end region of the tubular wrapper, the method comprising the steps of severing the upper and lower end regions of the wrapper to separate the tubular neutron shield and spike extension from the wrapper, severing the wrapper immediately above the cellular hold down grid and withdrawing the combination of hold down grid and bundle of fuel pins from the combination of wrapper and series of bracing grids.

5. A fuel sub-assembly for a liquid metal cooled fast breeder nuclear reactor substantially as hereinbefore described with reference to the accompanying drawings.

6. A method of dismantling a fuel subassembly substantially as hereinbefore described.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. beyond each end of the elongate embossments and which provide additional bracing pads or backstops for a fuel pin in the event that bowing of the pin occurs during irradiation of the fuel sub-assembly. Each pair of abutting sides of the cells are secured together by a pair of spot welds disposed in opposed end regions of the sides. A compliant grid of this nature enables the pins to be slidably guided with the minimum of bearing loading whilst the backstop pads 35 provide adequate bearing loading to resist irradiation bowing of the pins. For reprocessing purposes after irradiation of the fuel sub-assembly, the wrapper is separated from the lower end locating section 2 and the upper end neutron shielding section 3 by severing the upper and lower end regions of the wrapper, then the wrapper is severed again immediately above the hold down grid 7a. The entire fuel pin bundle and hold down grid combination can then be withdrawn from the wrapper 5 and cellular grids 6 combination in one operation, the withdrawal operation being made possible by the compliant nature of the cellular grids 6. In the alternative construction shown in Figure 4, the radial projection 18 takes the form of an eye 19 whilst in the construction shown in Figure 5, the radial projection 18 is a flange generally of triangular form. In each of the constructions, the radial projection is of a form which offers little restriction to the coolant flow passages through the hold down grid 7a. The described fuel sub-assemblies are used in a liquid metal-cooled fast breeder reacton of the pool kind shown in Figure 7. The construction comprises a fuel assembly 21 submerged in a pool 22 of liquid sodium coolant in a primary vessel 23. The primary vessel is suspended from the roof of the containment vault 24 and there is provided a plurality of coolant pumps 25 and heat exchangers 26 only one of each of pump and heat exchangers being shown. The fuel assembly 21 mounted on a support member 27 is housed with the heat exchangers- in a core tank 28 whilst the pumps 25 which deliver coolant to the support member are disposed outside of the core tank. Control rods 29 and instrumentation 30 penetrate the roof of the vault. In operation, relatively cold coolant is drawn from the pool outside the core tank 28 and passed upwardly through the fuel assembly by way of the support member 27 and then flows through the heat exchanger 26 in heat exchange e with a second coolant which flows through steam generating plant disposed outside of the vault. The main coolant is returned from the heat exchanger to the outer region of the pooi. WHAT WE CLAIM IS:

1. A fuel sub-assembly for a liquid metal cooled fast breeder nuclear reactor comprising a bundle of fuel pins within a tubular wrapper and spaced apart by a series of compliant cellular bracing grids (as defined hereinbefore) each pin occupying a discrete cell of each grid, the wrapper having a lower end spike extension and an upper.end tubular neutron shield extension and wherein the pins are each retained at one end by a cellular hold down grid (as defined hereinbefore) which is secured to an end region of the tubular wrapper.