GB2073476A

GB2073476A - Dismantling irradiated fuel elements

Info

Publication number: GB2073476A
Application number: GB8107967A
Authority: GB
Original assignee: Kraftwerk Union AG
Current assignee: Kraftwerk Union AG
Priority date: 1980-03-26
Filing date: 1981-03-13
Publication date: 1981-10-14
Also published as: JPS56151395A; DE3011760A1; FR2479537A1

Abstract

At least partially spent or irradiated nuclear reactor fuel elements or rods are dismantled by at least partially hydrogenating the casings, for example the zirconium alloy casings, of the elements or rods and then pounding or breaking them up and separating the fragments of the casing from the irradiated fuel mixed therewith. The method eliminates the need for a cutting or, shear device to cut up the elements or rods.

Description

SPECIFICATION A method of dismantling at least partially spent nuclear reactor fuel elements or rods This invention relates to a method of dismantling at least partially spent or irradiated nuclear reactor fuel elements or rods.

The present invention is more particularly, but not exclusively, concerned with the dismantling of irradiated fuel rods clad in zirconium alloy during the course of reprocessing the nuclear fuel material.

Zirconium encased fuel rods are normally used in light water reactors. After irradiation in a nuclear reactor, the fuel rods are reprocessed after intermediate storage hastaken place, so that a large part of their radioactivity has decayed. This re-processing begins with mechanical dismantling the fuel elements and/or fuel rods, followed by chemically disolving the irradiated nuclear fuel and the fission products contained therein, and the chemical recovery of dissolved valuable material.

The mechanical disassembly of fuel rods and/or fuel elements is carried out with so-called bundle cutters or shears. The service life of these bundle shears is limited naturally, such that their susceptibility to malfunction often limits the throughput for the whole reprocessing cycle.

What is required is a method of dismantling irradiated fuel elements or rods before the actual re-processing, which manages without the sensitive bundle shears and in which phenomena of wear play practically no part.

According to the present invention there is provided a method of dismantling an at least partially spent nuclear reactor fuel rod having fuel material in a casing, or an element comprising a plurality of such rods, the method comprising the steps of a) at least partially hydrogenating the casing of the fuel rod(s); b) pounding and/or otherwise breaking up the fuel rod(s); and c) separating the casing of the or each fuel rod from the spent fuel material previously contained therein.

Preferably the method is that wherein: before step (a), the or each fuel rod is stored in an hermetically sealed oven; during step (a), the or each fuel rod is heated to a temperature in the range from 300 to 400"C in the oven; and after step (a), the or each fuel rod is cooled. The or each fuel rod- is preferably hydrogenated in the oven by means of a hydrogen atmosphere, which can be maintained at a superatmospheric pressure to accelerate the process, in the oven.

The casings of the or each fuel rod is preferably completely hydrogenated.

When the method is carried out on fuel elements or rods encased in zirconium alloy casings, stepq (c) preferably comprises separating zirconium hydrides and other metallic components from the spent fuel.

Step (b) is conveniently effected in a pounding or breaking machine.

Step (c) can be effected by chemically dissolving the spent fuel material. Alternatively, when the method is carried out on a fuel rod or rods having a casing of a different density from the density of the spent fuel material, step (c) can be effected by utilising the difference in densities between the casing material and the spent fuel material, e.g. by flotation.

After step (c), the following additional step (d) is preferably carried out: (d) dehydrogenating the material of the casing(s) and any metallic scrap present.

When this step (d) is applied to zirconium casings, they are preferably dehydrogenated to airconium or zirconium dioxide powder, which can be compacted.

After step (d), the following additional step (e) is preferably carried out, in order to reduce the hydrogen consumption: (e) recovering the hydrogen produced by dehydrogenation of the casings and using the recovered hydrogen for step (a).

Step (b) can comprise pulverising the fuel rod(s).

The present invention also provides a reprocessing method for at least partially spent nuclear reactor fuel rods or elements, which reprocessing method includes a method of dismantling at least partially spent nuclear reactor fuel rods or elements as just defined.

For a better understanding of the present invention and to show more clearly how it may be carried into effect, a description will now be given of the method according to the present invention as applied to zirconium clad fuel rods.

The dismantling method of the present invention uses the otherwise undesirable characteristic of zirconium, that it is hydrogenated by absorption of hydrogen, and thus becomes brittle. This phenomenon, which must be most carefully avoided in the production of fuel elements and rods and during reactor operation, is now utilised for disassembly.

Preferably, the method ensures that a complete hydrogenation of the zirconium constructional components of the fuel elements, i.e. the jacket-encasing tubes and/or spacing grids, is effected. Before this hydrogenation step, head and foot pieces of the fuel rods, which usually consist of rust-proof steel or a chrome-nickel alloy, must be removed in the known manner. Then the fuel rods are piaced or stored in an oven, which is impervious to gas, i.e. hermetically sealed, and then heated in the oven to 300 to 400"C.

The oven atmosphere is then replaced by gaseous hydrogen. To speed up the hydrogenation process, it is expedient to raise the internal pressure of the oven. After the zirconium has been completely hydrogenated, the fuel elements or rods are cooled.

After the fuel rods have been cooled, they are removed from the oven and fed into a pounding or breaking machine. Since with hydrogenated zircaloy jacket-encasing tubes etc., even the slightest deformation leads to breakage, the distruction and breaking of the fuel rods can then be effected without large amounts of energy being consumed in the pounding and breaking machines used for this step of the method. The fuel rods or elements may be pulverised.

The pounding and breaking machines are expediently sealed off from the exterior to exclude dust, so that no radioactive particles can escape into the atmosphere. Additionally, an inert atmosphere is advantageously maintained in these machine, so that inflamation or combustion of the zirconium hydride dust can be prevented with certainty.

After the fuel elements or rods have been broken up in the pounding or breaking machines, the irradiated fuel in the elements or rods, comprising unused fuel and fission products, is then separated from the metallic scrap of the casings and associated spacer grids etc. The irradiated fuel can then be separated by chemically dissolving it. Alternatively, the irradiated fuel can be separated by utilising the difference in mass or density between the irradiated fuel and the hydrogenated zirconium alloy casing etc., e.g. by flotation etc. The separated spent fuel can then be fed into a chemical process for separating the different fission products etc.

The metallic hydrogenated scrap, on the other hand, is then placed in a vacuum oven and heated to 400"C or higher to dehydrogenate the metallic scrap.

The resultant liberated hydrogen is sucked off. As a result of this dehydrogenation process, the scrap is converted to zirconium dioxide powder which, like nuclear fuel material, is re-processed and re-utilised or can be discarded as refuse. The hydrogen, liberated in this process, is fed back into step (a) of the method, so that the effective consumption of hydrogen bythe method of the present invention is very small.

Since, as stated, the hydrogenated zirconium parts are very brittle, the wear in the pounding and breaking machines is also reduced, such that their susceptibility to malfunction should be small and the requirement for maintenance should not cause any problems.

Claims

1. A method of dismantling an at least partially spent nuclear reactor fuel rod having fuel material in a casing, or an element comprising a plurality of such rods, the method comprising the steps of: a) atleast partially hydrogenating the casing of the fuel rod(s); b) pounding and/or otherwise breaking up the fuel rod(s); and c) separating the casing of the or each fuel rod from the spent fuel material previously contained therein.

2. A method as claimed in claim 1, wherein: before step (a), the or each fuel rod is stored in an hermetically sealed oven; during step (a), the or each fuel rod is heated to a temperature in the range from 300 to 400"C in the oven; and after step (a), the or each fuel rod is cooled.

4. A method as claimed in claim 3, wherein the hydrogen is maintained atsuperatmospheric pressure.

5. A method as claimed in claim 1,2,3 or 4, wherein step (a) comprises completely hydrogenat ing the casing of the or each fuel rod.

6. A method as claimed in claim 1,2,3,4 or 5, when carried out on a fuel rod or fuel rods having a zirconium alloy casing, wherein step (c) comprises separating zirconium hydrides and other metallic components from the spent fuel material.

7. A method as claimed in any preceding claim, wherein step (b) is effected in a pounding and/or breaking machine.

8. A method as claimed in any preceding claim, wherein step (c) is effected by chemically dissolving the spent fuel material.

9. A method as claimed in any one of claims 1 to 7, when carried out on a fuel rod or fuel rods having a casing of different density from the density of the spent fuel material, wherein step (c) is effected by utilising the difference in densities between the casing material and the spent fuel material.

10. A method as claimed in any preceding claim, wherein after step (c), the following step (d) is carried out: (d) dehydrogenating the material of the casing(s) and any metallic scrap present.

11. A method as claimed in claim 10 when appendant to claim 6, wherein in step (d) the zirconium hydride is dehydrogenated to zirconium or zirconium dioxide powder, which can be compacted.

12. A method as claimed in claim 10 or 11, wherein after step (d), the following step (e) is carried out: (e) recovering hydrogen produced by dehydrogenation of the casing material and using the recovered hydrogen for step (a).

13. A method as claimed in any preceding claim, wherein step (b) comprises pulverising the fuel rod(s).

14. A method of dismantling an at least partially spent nuclear reactor fuel rod or an element comprising a plurality of such rods substantially as herein before described.

15. A reprocessing method for at least partially spent nuclear reactor fuel rods or elements which reprocessing method includes a method of dismantling at least partially spent nuclear reactor fuel rods or elements as claimed in any preceding claim.