GB2165688A

GB2165688A - Treating irradiated fuel elements

Info

Publication number: GB2165688A
Application number: GB8519186A
Authority: GB
Inventors: Brian Edwin Meredith; James Shaw Burns
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1984-08-21
Filing date: 1985-07-30
Publication date: 1986-04-16
Also published as: GB8519186D0; FR2569490A1; GB2165688B; DE3529312A1; FR2569490B1

Abstract

Irradiated nuclear fuel elements of the type comprising a fuel pin bundle (5) located and supported in a hexagonal wrapper (4) by grids (6) are dismantled for subsequent reprocessing of the fuel by cutting the casing (4) around its periphery in the region (L2) of maximum swelling of the pins (5), removing the upper part of the casing and the grids associated with that part and pushing the pin bundle and grids through the remaining casing part so that the grids (6) successively stack against one another. The pins (5) are then separated from the stacked grids by for example laser-cutting the pins through non-fuel bearing sections thereof either before or after disengagement of the stacked grids from the remaining casing part. <IMAGE>

Description

SPECIFICATION Dismantling irradiated nuclear fuel elements This invention relates to dismantling of irradiated nuclear fuel elements, and in particular to the dismantling of that kind of nuclear fuel element, employed in a fast breeder nuclear reactor cooled by liquid metal, which has a hexagonal outer casing containing a multiplicity of nuclear fuel pins supported in spaced transverse grids. Such a nuclear fuel element is hereafter referred to as "of the specified kind".

In order to minimise the amount of structural material introduced into the reprocessing of nuclear fuel by means of which the uranium and plutonium in the fuel is extracted for reuse and the fission products are separated for storage, it is prudent in respect of each fuel element to separate the hexagonal casing and the transverse grids from the fuel pins, so that only the fuel pins, cropped into convenient pieces, are employed as feed at the reprocessing plant, the casing and grids being discarded and stored as radioactive waste.

It has already been proposed to cut the casing circumferentially in such a manner as to produce a first part which can be removed, and a second part containing the fuel pins, which can subsequently be removed from the second part. It was also proposed that such grids as were secured to the first part of the casing can be removed with the first part. The proposal gave preference to a central circumferential cut, and the cut was to have been made by cutting tools.

There are practical objections to such proposals in that it is difficult to effect gripping of the fuel pin bundle so as to separate the bundle from the casing portion enclosing the pins sufficiently rapidly to meet the throughput requirements of commercial plant.

It is an object of the present invention to provide a dismantling route and means for effecting such dismantling, which are not subject or are less subject to the drawbacks set forth in the previous paragraph.

According to one aspect of the invention there is provided a method of dismantling an irradiated nuclear fuel element of the specified kind in which a pushing force is applied to the fuel pin bundle in order to effect displacement of the bundle relative to the outer casing.

According to a second aspect of the invention there is provided a method of dismantling an irradiated nuclear fuel element including the steps of cutting the outer casing to separate it into two axial parts at a position intermediate the ends of the fuel pin bundle, and pushing the pin bundle from one end of the casing at least partially through the one casing part which extends from said position towards said one end of the pin bundle.

Preferably the grids are also pushed lengthwise of said one casing part so that they collect as a stack adjacent said position. The stacked grids can be separated from the fuel pins by cutting the latter at non-fuel-bearing parts of the fuel pins either while the stacked grids are still within said one casing part or following displacement of the grids and pin bundle entirely out of said one casing part.

Dismantling can be performed by cutting the outer casing circumferentially at about its mid point and, in particular, in that region where the fuel pins have undergone maximum swelling during irradiation within the reactor core, removing the upper part of the casing together with those grids which are associated therewith, and then holding stationary the remainder of the casing and applying ram means to the fuel pin bundle to push it at least partially out of the said remainder and at the same time effect stacking of those grids which are associated with the said remainder, the latter being removed from the pins by for example cutting the pins. The cutting operation may be performed by laser.

In a preferred application, a ram for pushing as set forth in the previous paragraph has a hexagonal face plate with corner cut-outs to accommodate the support legs of the transverse grids and thereby allow the face plate to slide over said legs, the use for dismantling involving clamping the said remainder of the casing, initially contacting with the face plate the bottom transverse grid of the fuel pin bundle, then operating the ram to push the fuel pin bundle longitudinally in said remainder sequentially breaking away the grids so that they collect together at the end of the fuel pin bundle and together with the first grid form a stack, the stack of grids being located adjacent the lower end of the pin bundle. The assembly of grids and pins may be pushed entirely out of the said remainder, and finally the stack of grids may be removed from the pins by cutting the latter.Alternatively, the assembly may be pushed until non-fuel-bearing sections, eg the plenum spaces, of the pins project beyond the one end of the lower casing portion in which case the pins can be cut at the non-fuel-bearing sections thereof so as to leave the grid stack inside the lower casing portion.

One embodiment, and variations, of the invention will now be described with reference to the accompanying drawings, wherein: Figure 1 is a side view in section of a typical fuel element of the specified kind, and Figure 2 is an enlarged plan view in section on line 11-11 of Figure 1.

Referring firstly to Figure 1, a nuclear reactor fuel element of the specified kind, designated 1, has a top end 2 for fuel element handling, a bottom end 3 for location with a multiplicity of similar fuel elements in the fuel element support means of the reactor (not shown) and for entrance of iiquid metal coolant to the interior of that and the other fuel elements, a hexagonal outer casing 4 containing a multiplicity of fuel pins 5 (one only of which is shown for the sake of clarity) and a plurality of spaced transverse support and location grids 6 for the pins 5. The top and bottom grids are support grids and are designated 6T and 6R respectively. Each intermediate grid 6 is a spacer and location grid and each grid 6 has legs 61 for angular and axial location.

After irradiation in and discharge from the nuclear reactor, it is necessary to dismantle the fuel element 1. Specifically, and involving remote handling, the outer casing 4 and grids 6 are separated from the fuel pins which are then cropped and fed to the dissolving stage of the reprocessing plant. The separation of casing and grids from the fuel pins will now be described.

Referring to Figure 1, a first transverse laser cut through the hexagonal outer casing 4 and the legs of bottom support grid 6B is made at location L1. A second transverse laser cut through the casing 4 is made at location L2, which is considered to be the approximate position of maximum pin swelling as a result of irradiation. That portion of the casing to the right of location L2 (as seen in Figure 1) is held and the portion of the casing of the fuel element to the left of location L2 is pulled away from the held portion, bringing with it the top end 2, the top support grid 6r, some of the grids 6 and their grid legs 61 but leaving the fuel pins 5 in the held portion.

Next, the held portion of the casing is transversely cut by laser at location L3, this cut also serving to release the bottom support grid 6B and expose the pin ends and the first grid 6 which also happens to be the holddown grid for the fuel pins. A ram having a hexagonal face plate with corner recesses enabling it to slide over grid legs 6' is now brought up to the exposed ends of the fuel pins 5 and to the said first grid 6, the ram being operated to break away this grid and displace it and the pins axially with respect to the lower casing portion. Initially only the pins and the first grid are displaced since the pins will slide relatively freely through the remaining grids owing to the cross-sectional taper of the pins away from the position of maximum radiation-induced swelling.Continued operation of the ram brings the first grid into contact with the next causing the grids 6 to break away from their attachment to the casing 4 one at a time and finally to be stacked and collected at the bottom end of the pin bundle, the cutaway corners of the hexagonal face plate enabling the plate to slide along the grid legs 6'.

The pins and grids are thus pushed out of the casing 4, the latter being removed and sent to radioactive waste storage.

Finally, the stack of grids 6 is removed by transverse laser cutting through the bottom end region of the fuel pins, ie. that part termed the plenum section and not occupied by fuel. The fuel pins 5 are then ready for passing to the cropping stage and subsequently to the dissolving stage of the reprocessing plant.

Alternatively pushing can be discontinued as soon as the plenum sections of the pins begin to emerge from the casing. The fuelled portions of the pins can then be parted off leaving the grids and plenum sections within the casing for disposal to waste storage.

In some circumstances, it may be preferable to avoid breaching the internal integrity of the fuel pins, ie as occurs if they are cut through their plenum sections. Various possibilities exist. Por example, the pins may be withdrawn from the stacked grids by pulling them, one at a time or groups, from the stacked grids or they may be pushed out of engagement with the grids by displacing them relative to the grids by means of push rods either with the grids still within the lower casing portion or after the grids have been rammed out of the lower casing portion.

Claims

1. A method of dismantling an irradiated nuclear fuel element of the specified kind in which a pushing force is applied to the fuel pin bundle in order to effect displacement of the bundle relative to the outer casing.

2. A method of dismantling an irradiated nuclear fuel element including the steps of cutting the outer casing to separate it into two axial parts at a position intermediate the ends of the fuel pin bundle, and pushing the pin bundle from one end of the casing at least partially through the one casing part which extends from said position towards said one end of the pin bundle.

3. A method as claimed in Claim 2 in which the grids are also pushed lengthwise of said one casing part so that they collect as a stack adjacent said position.

4. A method as claimed in Claim 3 in which the pushing step is performed until the pin bundle projects beyond the stacked grids to such an extent that only non-fuel bearing parts of the fuel pins remain engaged within the stacked grids.

5. A method as claimed in Claim 4 in which the fuel pins are subsequently separated from the stacked grids.

6. A method as claimed in Claim 5 in which the separation step is carried out while the stacked grids are within said one casing part.

7. A method as claimed in Claim 6 in which the assembly of fuel pin bundle and stacked grids is pushed out of said one casing part and the separation step is carried out thereafter.

8. A method as claimed in any one of Claims 5 to 7 in which the separation step comprises cutting the pins at non-fuel bearing parts thereof projecting beyond the stacked grids.

9. A method as claimed in Claim 8 in which said non-fuel bearing parts comprise plenum sections of the fuel pins.

10. A method as claimed in any one of Claims 5-8 in which said separation step is carried out without breaching the internal integrity of said fuel pins.

11. A method as claimed in Claim 5, 6 or 7 in which said separation step comprises dis placing the fuel pins relative to the stacked grids until they disengage from the grids.