GB2156573A

GB2156573A - Process and apparatus for separating ceramic nuclear fuels from metallic carriers

Info

Publication number: GB2156573A
Application number: GB08506273A
Authority: GB
Inventors: Rudiger Koch
Original assignee: Kernforschungsanlage Juelich GmbH
Current assignee: Forschungszentrum Juelich GmbH
Priority date: 1984-03-16
Filing date: 1985-03-11
Publication date: 1985-10-09
Also published as: JPS60211393A; GB8506273D0; GB2156573B; DE3409707C2; FR2561432A1; FR2561432B1; DE3409707A1

Abstract

Ceramic nuclear fuel adhering to metallic carrier structures, especially inside portions of rod- shaped cans after nuclear burn-up, is removed by flushing with high pressure liquid, preferably while submerged. This gives a highly dust-free operation and avoids contamination of the fuel with carrier body material. Apparatus for this purpose has a can portion holder (3) and lances (10) carrying high pressure jet nozzles (8). By means of the lances (10) the nozzles (8) are progressively advanced into the can portions (2) as the fuel is flushed out. <IMAGE>

Description

SPECIFICATION Process and apparatus for separating ceramic nuclear fuels from metallic carriers The invention relates to a process for separating ceramic nuclear fuels from metallic carrier bodies or parts of such bodies in which the nuclear fuel. is adhering, especially after burnup of the nuclear fuel in nuclear reactors. The carrier bodies may in particular be metallic cladding.

A requirement to detach nuclear fuel from carrier parts arises particularly before the reprocessing of the nuclear fuel in reprocessing plants. There are employed as carrier parts, for example, cans consisting of Zircalloy, into which the ceramic fuels are introduced. In some cases the cans are encased in ceramic cladding materials. The nuclear fuel must be separated from the said carrier parts before reprocessing principally in order that the fuel does not become contaminated by metallic residues of the carrier parts during its reprocessing.

It is known to cut the fuel elements into sections mechanically or, for example, by means of a water jet (see German Offenlegungsschrift 3007876) before the fuel is chemically dissolved in dissolvers. After cutting up, the fuel element sections obtained are introduced into the dissolver and the fuel is dissolved out of the residual carrier body parts in boiling nitric acid. This process involves relatively low expenditure. However, on the whole long reaching times must be expected because the chemical dissolution is very timeconsuming, which in turn is due to the fact that the exposed areas to be attacked by the nitric acid are only those areas created by cutting open the fuel elements. When the nuclear fuels are dissolved in nitric acid, the metallic can portions consisting of Zircalloy are substantially preserved.

However, difficulties arise in dissolving nuclear fuel which contains thorium. For dissolving such nuclear fuel, fluoride ions are added to the nitric acid. In consequence the metallic carrier body portions are attacked. The metal which thus enters into solution is troublesome in the subsequent chemical reprocessing steps.

In mechanically separating metallic carrier bodies and ceramic nuclear fuels, it has hitherto not been possible to effect a completely satisfactory separation of the metallic portions and the ceramic material. This is especially the case when the cans or carrier bodies undergo damage such as deformation, swelling, cracking or the like during burn-up of the nuclear fuel. The separating units are subject to considerable wear and the expenditure on equipment is considerable. The formation of radioactive dust or liberation of gaseous radioactive products causes further problems in connection with the retention of radioactive substances and with decontamination. Also, for example, in the processing of carrier bodies consisting of Zircalloy it is necessary to avoid the formation of fine metal particles, because this entails a danger of fire and explosion.For the same reason, high temperatures must also be avoided in the processing of the fuel elements.

The invention seeks to provide a process for separating ceramic nuclear fuel from carrier bodies, wherein it is possible to recover the nuclear fuel without contamination by carrier body material and in a few processing steps.

The process is intended to be readily operable without formation of dust. The invention also seeks to provide a process which can be carried out in a flexible manner to accommodate variable dimensions of the fuel elements and in the vent of encountering damaged elements.

In a first aspect this invention provides a process for separating ceramic nuclear fuel from metallic carrier bodies or parts thereof to which the nuclear fuel adheres, characterised in that the ceramic nuclear fuel is flushed from the carrier body or part thereof by means of a high-pressure liquid jet.

Thus the ceramic nuclear fuel is flushed away from the carrier body by means of a liquid jet which is under high pressure. The liquid jet is directed on to the nuclear fuel and it is found that with a sufficiently high pressure of the liquid the nuclear fuel adhering to the carrier body is completely removed. The action of the liquid jet is not dependent on its exact position. Hence the liquid jet is able to find the interface between the ceramic nuclear fuel and the metallic carrier body material, so that a satisfactory detachment of the nuclear fuel is achieved even when the fuel rod has swollen and the carrier body parts have become deformed. The metallic carrier bodies or carrier body parts are preserved and are removed after the nuclear fuel has been washed away. Only the nuclear fuel fragments enter the chemical dissolver.

It is preferable for the nuclear fuel to be flushed away from the metallic carrier body or carrier body portions while submerged below the surface of a liquid bath. In this case, the breaking up and washing out of the ceramic material by the liquid jet is brought about by cavitation in which implosion of steam bubbles in the liquid jet causes high-pressure and high-frequency surges. This high dynamic loading acts on the brittle ceramic material but has scarcely any effect on the relatively ductile metallic cladding or carrier material.

Both when used in a gaseous atmosphere and when submerged within the liquid medium, the effectiveness of the liquid jet is determined by its cross-section and the pressure of the liquid. Depending upon the nature and adhesion of the nuclear fuel, a minimum nozzle diameter and a minimum pressure are necessary for detaching the ceramic material from the carrier body or the carrier body portions. The minimum diameter and the minimum pressure can be determined empirically.

In a second aspect this invention provides apparatus for carrying out the process. This apparatus comprises means to hold the metallic carrier bodies or parts thereof such that the internal spaces thereof in which the ceramic nuclear fuel adheres is accessible at least one liquid-jet nozzle with duct means for pressurised liquid supply thereto, and means to move the liquid-jet nozzle into the said internal space of a carrier body or part thereof.

The apparatus may in particular be constructed to handle rod-shaped fuel elements in which the ceramic nuclear fuel is embedded in cans. In this case the means to hold carrier bodies or parts thereof will be a holder shaped to hold the metallic cans, or portions thereof obtained by cutting the cans. The apparatus' one or more liquid-jet nozzles will act to flush the fuel out of the cans or can portions.

To push the nozzle(s) into the internal space of a carrier body or part thereof, particularly when this is a can or a portion of a can, each nozzle is preferably mounted on a lance.

Then, during the flushing out of the ceramic material the lance can be progressively advanced into the cans along the cavity created by the flushing out of material. There may be one or a plurality of nozzles mounted on a lance.

The lance may be rotatably mounted, or at least one liquid-jet nozzle may be rotatably mounted on the lance. Preferably the nozzle is disposed in an eccentric position relative to the axis of rotation, whereby a more intensive flushing is achieved over the whole crosssection of the nuclear fuel and the can.

The invention will be further explained by the following description of an embodiment.

This is given by way of example only. The embodiment is diagrammatically illustrated in the drawings, in which: Figure 1 illustrates a flushing installation for separating ceramic nuclear fuel from carrier body parts, and Figure 2 is a fragmentary sectional view of the flushing installation according to Fig. 1, along the section line ll-ll.

The cans of rod-shaped fuel elements are cut into portions. As illustrated in the drawings the resulting can portions 2 (i.e. carrier body portions of rod-shaped fuel elements) are processed while submerged in a tank 1. The can portions 2 are filled with ceramic nuclear fuel. The ceramic material adheres at least partially to the can, especially after burn-up of the nuclear fuel. Therefore, for testing the apparatus of the illustrated embodiment, ceramic simulation material was fixed in a portion of a Zircalloy can by means of a two-component adhesive. This adhesive created an adhesion between the ceramic material and the can portion which substantially exceeded the degree of adhesion which actually occurs after burning of the fuel elements in a nuclear reactor.

The can portions 2 are introduced into the immersion tank 1 by means of a remotely operable device. There is suitable for this purpose, for example, a rotary magazine 3, illustrated in side elevation in Fig. 2, which can be turned about its (horizontal) axis in the direction indicated by arrow 4. The can portions are deposited onto the periphery of the rotary magazine 3 from a funnel-shaped store 5, and then secured thereon. In the embodiment illustrated, the rotary magazine is moved stepwise by means of a belt or chain 6 passing over guide rollers 7. The rotary magazine 3 thus brings the can portions 2 into the range of liquid-jet nozzles 8. These are disposed in a treatment zone 9 below the liquid surface in the tank 1. In the embodiment shown, the liquid-jet nozzles 8 are fixed on two lances 10.These are mounted such that they can be pushed into a can portion 2, from both ends of the latter.

As a variation of the embodiments shown, a plurality of liquid-jet nozzles may be mounted on each of the lances. The liquid-jet nozzles on the lances or the lances themselves may be rotatable. In addition, the liquid-jet nozzles may be disposed eccentrically in relation to the axis of rotation. Notably, this can provide that a jet which is directed, forwardly from the lance is carried around the axis of rotation.

In the illustrated embodiment, the lances 10 serve as means to advance the nozzles into the interior of a can portion 2, and at the same time as water supply tubes to the nozzles. The lances are connected to highpressure water supply ducts 11. The highpressure ducts 11 are fed by an electrically driven liquid pump 1 2. Situated below the immersion tank 1 is a receptacle 1 3 for collecting the material flushed out of the carrier body portions with the flushing liquid.The flushed-out material and the flushing liquid flow through a central aperture 1 4 in the base of the immersion tank and into the collecting receptacle 1 3. The washing liquid is recirculated to the pump 1 2 along an overflow pipe 1 5 from the collecting receptacle 1 3. The overflow from the receptacle 1 3 is provided with a screen which retains the ceramic fragments in the collecting receptacle. Within the receptacle 13, the retained fragments slide with liquid residues to an outlet 16, from which they are pumped into the chemical dissolver (not illustrated in the drawings) for the nuclear fuel.

In the illustrated embodiment, the liquid-jet nozzles were fed with pressurized water and had a diameter of 1.8 mm. The water pressure was 750 bar. The ceramic material contained in can portions having a length of 200 mm was flushed out by the cavitating water jet from a nozzle distance of about 50 mm within about 30 seconds. If the diameter of the liquid-jet nozzles or the water pressure is reduced, the flushing times are increased under otherwise unchanged conditions. Thus, when the diameter of the liquid-jet nozzles was reduced to 1.6 mm, or the water pressure was reduced to 650 bar, the processing time was lengthened to substantially double that in the case of the aforesaid illustrated embodiment, under otherwise equal conditions. The ceramic material obtained by in this processing displayed particle sizes varying between 1 mm and 5 mm. There were no residues of material in the cans after the processing of the simulated fuel element fragments with underwater jet nozzles.

The acid employed in the dissolver may also be used as the working liquid and medium. In this case it is necessary to choose appropriate materials for the flushing installation and the liquid-jet nozzles, which must be so constructed as to be acid-resistant. Moreover, this possibility arises only in the case of UO2 and Pu02 fuels, for the reasons mentioned at the beginning.

Claims

1. Process for separating ceramic nuclear fuel from metallic carrier bodies or parts thereof to which the nuclear fuel adheres, characterised in that the ceramic nuclear fuel is flushed from the carrier body or part thereof by means of a high-pressure liquid jet.

2. Process according to claim 1, in which the carrier body or carrier body part is immersed in liquid together with the nuclear fuel adhering thereto and the nuclear fuel is flushed off by the high-pressure liquid jet while submerged in the liquid.

3. Apparatus for carrying out the process of claim 1 or claim 2 comprising means to hold the metallic carrier bodies or parts thereof such that the internal spaces thereof in which the ceramic nuclear fuel adheres is accessible at least one liquid-jet nozzle with duct means for pressurized liquid supply thereto, and means to move the liquid-jet nozzle into the said internal space of a carrier body or part thereof.

4. Apparatus according to claim 3 wherein the means to hold carrier bodies or parts thereof is shaped to hold carrier bodies or parts which are rod-shaped cans or rodshaped portions thereof, and the means to move the liquid jet nozzle are constructed for pushing the nozzle into the interior of the can or portion thereof.

5. Apparatus for carrying out the process according to claim 1 or claim 2, comprising a holder for metallic carrier bodies or parts of such carrier bodies in whose internal space, accessible from the outside, the ceramic nuclear fuel adheres, and comprising at least one liquid-jet nozzle adapted to be connected to a pressure duct, characterised in that the liquid-jet nozzle for the removal of the nuclear fuel is arranged to be pushed into the interior of the carrier body or carrier body part.

6. Apparatus according to any one of claims 3 to 5 wherein one or more liquid-jet nozzles are mounted on a lance.

7. Apparatus according to claim 6, wherein at least one liquid-jet nozzle on the lance or the lance itself is rotatably mounted.

8. Apparatus according to claim 7 wherein the liquid-jet nozzle is disposed eccentrically in relation to the axis of rotation.

9. Apparatus according to claim 8 wherein the nozzle is disposed such that the jet from it extends forwardly from the lance and by reason of the eccentric position of the nozzle is carried around the said axis of rotation.

10. Process for separating ceramic nuclear fuel from metallic carrier bodies or parts thereof, substantially as herein described with reference to the drawings.

11. Apparatus for separating ceramic nuclear fuel from metallic carrier bodies or parts thereof, substantially as herein described with reference to the drawings.