ELECTROCHEMICAL DISSOLUTION OF NUCLEAR FUEL PINS

Abstract: Electrochemical dissolution, especially of nuclear fuel pins, is performed using a substrate container in the form of a perforate stepped truncated pyramid. The polarity of the electrodes may be reversed during an initial stage of the process, to activate the fuel pins.

Description: ELECTROCHEMICAL DISSOLUTION OF NUCLEAR FUEL PINS The present invention concerns electrochemical dissolution. More specifically, it relates to the reprocessing of irradiated nuclear fuel and particularly to the dissolution of nuclear fuel pins. Nuclear fuel pins consist of pellets of fissile material, e.g. U0 2 , contained in a cladding which is normally a zirconium alloy sold under the trade mark Zircaloy. A cluster of pins form a fuel assembly. Commercial reprocessing of irradiated (spent) nuclear fuel uses the Purex process, which involves chopping up the pins of an assembly prior to dissolution of the fissile material in nitric acid. The pins must be chopped up to expose the pellets to nitric acid because the bulk zirconium alloy is resistant to attack by nitric acid, as is an oxide skin which irradiated zirconium alloy possesses. The chopping up of the pins is undesirable because it requires mechanical apparatus which is subject to serious wear and therefore relatively frequent repair; it will be appreciated that there are difficulties in repairing machinery which processes radioactive material. In a period between the 1950's and 1970 considerable experimental work was carried out on the electrochemical dissolution ("ECD") of complete (unchopped) fuel pins. A review of the development of ECD up to 1970 can be found in V P Caracciolo and J H Owen, pp 81-118, Progress in Nuclear Energy, Series III, Vol 4, Pergamon Press 1970. The principle of ECD is that a fuel pin is placed in nitric acid and a potential difference is applied between the fuel cladding and the nitric acid surrounding it. If this potential is large enough then the inert nature of the cladding is overcome and it becomes reactive to the nitric acid. Caracciolo and Owen describe that the fuel pins are placed in wedge-shaped baskets whose sides taper towards each other towards the bottom of the basket. Depending on the basket design, the pins may be oriented horizontally (i.e. with a horizontal longitudinal axis) or vertically in the basket. The basket is placed in nitric acid with an anode and a cathode externally contiguous with opposed sides of the basket. A direct electric current is then passed between the electrodes and, in doing so, it passes through the pins in the basket which are thus dissolved. In such a design the current passes through only relatively short lengths of cladding. As a pin dissolves, it moves down the basket and, due to the shape of the basket, remains close to each electrode. In this respect, Caracciolo and Owen explain that: "The ideal liquid contact cell is one in which the dissolving fuel remains close to the electrodes at all times without touching them. " We have found that the techniques described by Caracciolo and Owen are not effective at dissolving Zircaloy cladding, which is the very cladding used in practice, and consume considerable energy. The present invention, therefore, is based on the problem of providing ECD processes which at least offer an improvement over prior art ECD processes for dissolving metals and their oxides. In one aspect, the invention provides a process for the electrochemical dissolution of a metallic material having a passive oxide film, which process is characterised in that, during an initial stage of the process, the polarity of the electrodes is reversed. The metallic material may be a zirconium alloy, for example the alloy sold under the trade mark Zircaloy. and is preferably nuclear fuel cladding. A particular process is a process for the electrochemical dissolution of an irradiated nuclear fuel pin having a zirconium alloy cladding during an initial stage of which the polarity of the electrodes is reversed. In another aspect, there is provided an ECD process characterised by the use of rectified alternating current (which may be half wave or full wave rectified) or biased alternating current. Also provided by the invention is a container for use in holding materials to be subjected to ECD, characterised in that it comprises an internal wall area which is stepped in a direction to converge with an opposing wall area. In some embodiments, the container is rectangular, for example square, in transverse cross section and the four side walls of the container are stepped. Such containers are in one preferred embodiment for the electrochemical dissolution of nuclear fuel assemblies. The invention includes a method for dissolving a metal containing product by ECD, comprising passing an electric current through the product when placed in a container of the invention which is an electrolyte. A yet further aspect of the invention is a method for electrochemical dissolution of a nuclear fuel assembly, wherein the assembly is placed in a container capable of: a) supporting the initial assembly with its outermost row of pins spaced from the electrodes, b) after a predetermined length of the cladding of a predetermined number of outer pin rows has been dissolved, supporting the resultant assembly with its outermost layer of undissolved pins spaced from the electrodes, and c) repeating support function (b) until all the pins of the assembly are dissolved. Preferably, each said outermost layers of pins are spaced from the electrodes with a predetermined minimum distance therebetween which is the same in each case. Such a container is itself included in the invention and may comprise a plurality of steps, each step to support outer pins of the assembly during a respective one of the support function phases and the steps for successive support phases being arranged successively down the container. Preferred containers further comprise electrodes which include an anode/cathode pair above each step for passing current through a fuel assembly during the step's respective support function phase, the members of each said pair being located generally in a common vertical plane and preferably one above the other. In preferred embodiments the containers of or used in the invention include a cathode and an anode located such that current flow between them in use is parallel to the orientation of the pins, i.e. to the direction of movement of the pins. The present invention is further described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic vertical cross section through a substrate container of the invention; and Figure 2 is a diagrammatic top view of the substrate container of Figure 1 which for clarity omits the second and subsequent steps of the container; and Figure 3 is a diagrammatic representation of apparatus used in the Examples. Figure 1, therefore, illustrates a substrate container useful in the ECD of, in particular, nuclear fuel assemblies. The container has an internal wall area which is stepped as at step 4 to converge with an opposing wall area. In this case, the substrate container has opposed wall areas which are stepped in this way; indeed, the embodiment shown is rectangular in horizontal cross section and each of the four side walls is stepped. More particularly, the substrate container comprises a perforate basket, for example a mesh basket, which is suitably made of a valve metal, for example niobium or tantalum, which because of its valve properties does not itself function as an electrode. Alternatively, a non-metal basket may be used; suitable non-metals include polydivinylbenzene. The illustrated basket has uppermost vertical side walls 1 arranged in a rectangle and each connected at its bottom to an uppermost inward step 4. The steps 4 are arranged in a continuous or discontinuous rectangle to form a shelf in the form of a rectangular frame. Each step 4 is connected at its inner edge to a side wall 1' of a rectangle of smaller dimensions than the preceding rectangle formed by the uppermost side walls 1, each of which side walls 1' is connected in turn at its bottom to an inward step 4' whose width (i.e. separation between inner and outer edges) is the same as that of uppermost steps 4. This stepped pattern is repeated a number of times until the base 5 of the container is reached. The illustrated container, therefore, is a perforate stepped truncated pyramid, which is in inverted orientation in use. Within the basket, at the level of the uppermost 1 and subsequent 1', 1" and 1'" side walls, there are disposed cathodes 2 and anodes 3. The cathodes can be made of materials such as, for example, titanium, platinum, niobium or titanium coated with platinum group oxides. The anodes can be made only of platinum or a substrate, eg titanium, coated with platinum or a platinum group oxide (i.e. Ru, Ir or Ta oxides). However, if polarity is to be reversed during the ECD process, then individual electrodes must be able to operate as both anodes and cathodes, i.e. have a surface of platinum or a platinum group oxide. The members of each cathode/anode pair are juxtaposed to a common side wall. It will be seen that the members of a cathode/anode pair are positioned generally in the same vertical plane and, specifically, one above the other (longitudinally of the container) rather than in horizontally opposed relationship. Alternatively the electrodes may be placed externally of the basket; in either case, the electrodes are insulated from the basket, for example by the use of alumina insulators. The electrodes 2. 3 may be an integral part of the container when completely manufactured or they be removable therefrom, e.g. in modular fashion. In use, the basket and electrodes are placed in a vessel containing nitric acid and typically provided with agitation means, for example an air sparge, to cause circulation of the nitric acid. A fuel assembly is placed with a number of rows of fuel pins in upright orientation resting on the horizontal shelf formed by the uppermost steps 4. The pitch of the pins varies between different designs of fuel assembly which may be expected to be dissolved in the same apparatus. The distance between outermost pins and electrodes will therefore vary in dependence upon the particular fuel design. Nonetheless, a minimum electrode to outermost pin distance will be maintained in practice due to the need to prevent physical contact between pin and electrode. The number of rows of pins supported by a shelf of any basket will depend upon the particular fuel assembly design and will often be from 1 to 4 rows of pins. The electrodes are connected to a current source. The potential to be applied across an electrode pair is determined by the desired effectiveness of the process and the resistivity of the electrolyte. Suitably the potential is from 5 to 50V, although we have found potentials of from about 20 to about 25V to be particularly satisfactory; current densities of from 2 to 6 A/cm 2 have been achieved with such potentials. The electrochemical dissolution is preferably but not necessarily conducted using an initial period of changing polarity and/or using rectified alternating current or biased alternating current, as described below. In any event, the electric current passes through the zirconium alloy cladding of the pins and causes them to dissolve. The dissolution of the pins commences with the outer pins, i.e. those closer to the electrodes. The core of pins not supported on the shelf formed by the steps 4 descends continuously as the length of the outer pins shortens due to their dissolution. When the outer pins have sufficiently shortened, the outer pins of the descending inner core will be supported by the next step 4'. In the result, the outermost fuel pins are located from the electrodes 2, 3 with a predetermined minimum distance therebetween (in this case, the same predetermined distance as when the assembly rests initially on the uppermost steps 4). The outer pins resting on the shelf are dissolved by electrochemical dissolution and the assembly descends as before. Typically, there are four dissolution stages (i.e. four shelves, including the base 5 of the container) and the dissolution of an assembly takes about 12 hours. Of course, the invention is not restricted as to the number of shelves. A difference between the process operated using the illustrated design and prior art assemblies is that the current flow using the illustrated apparatus is parallel to the direction of movement (and orientation) of the fuel assembly whereas in the prior art processes the current flow is perpendicular to the direction of movement of the dissolving assembly. The result of this is that the invention enables a relatively large anode to cathode gap to be maintained throughout the container whereas the cathode approaches the anode towards the bottom of the container in the wedge-shaped designs of the prior art. With a smaller anode athode gap, as at the bottom of the prior art wedge design, a relatively large proportion of the current will pass wastefully through the solution rather than usefully through the fuel pins. Additionally, the design is such that current flows through relatively long sections of pin; this results in a relatively large change in the pin to solution potential between where current enters and leaves the pins and hence in an increased driving force for the dissolution reaction. If the metal subjected to the ECD process is an irradiated fuel assembly having zirconium alloy cladding, it will initially have a passive oxide layer and resist dissolution. We have found that such passivated zirconium alloy may be activated by reversing the electrode polarity and thus removing the oxide coat. Experiments indicate that there is a linear dependence of the necessary activation time on the thickness of the oxide coat but that the time is insensitive to the frequency of polarity reversals. In many cases, an activation time of between 45 and 180 minutes is suitable, but the activation time is dependent to a large degree on the electrode surface area. Irrespective of whether the metal has an inert oxide layer, we have found that the performance of the process can be improved by replacing the prior art DC power supply with rectified AC or with biased AC. We have shown full or half wave rectified AC and biased AC to be more effective in bringing about dissolution than either DC or straight AC. Exposed fuel obtained by a process of the invention may be dissolved and then subjected to one or more further processes to make a nuclear fuel product, for example fissile material (e.g.UO 2 or MOX powder or gel), pellets, fuel pins or fuel assemblies. More generally, a product obtained using any process or product of the invention may be subjected to one or more processes to make a further product. EXAMPLES Example 1 Effect of polarity reversal frequency and oxide thickness on activation period The equipment used for this work is illustrated in Figure 3 and consisted of a cylindrical dissolution vessel (capacity ~ 15 litres) in which was situated a structure made of polydivinylbenzene which supported two electrodes 2, 3 in nitric acid 6 and held a Zircaloy pin 7 at a set distance from the electrodes 2, 3. The electrodes 2, 3 were both made of titanium coated with platinum group oxides and were connected to an appropriate power supply. Three power supply units were used, one which operated at mains frequency (50Hz), a second unit which gave a rectified a.c. supply, either full wave or half wave, the polarity of which could be altered manually by a control switch and a third which was a controllable d.c. supply unit. A cooling circuit incorporating a heat exchanger 8 was added to the unit to remove excess heat resulting from the ohmic heating of the system due to current passage. Arrows a and b indicate nitric acid circulation into and out of the heat exchanger 8, while arrows c and d indicate the direction of flow of cooling water. The container was filled with 8 mol/litre nitric acid and heated, by applying a known potential across the electrodes, thus causing current to pass through the electrolyte, until it had reached a temperature of 90 °C. The current was then turned off, an oxidised Zircaloy pin inserted into the container and the current restored. Activation of the pin was then conducted with polarity reversal at a predetermined frequency whilst maintaining an operating temperature of 90 °C, until the current being passed through the system was noted to rise and begin fluctuating. This is evidence that dissolution of the pin was occurring. This procedure was repeated with pins coated with different thicknesses of oxides and with different polarity reversal frequencies. Table 1 The above figures indicate that there is a linear dependence of the necessary activation time on the thickness of the oxide coat but that the time is insensitive to the frequency of polarity reversals. Example 2 Effect of waveform on dissolution rate The same equipment was used as in Example 1 and the electrolyte was heated in an identical manner. Once it had reached the required temperature an unoxidised pin was inserted and allowed to dissolve. The dissolution rate was determined by measuring the height of the pin against a fixed reference point at regular intervals. From these measurements a linear dissolution rate was determined. This procedure was repeated using the different power supply units. Table 2

Claims: CLAIMS 1. An electrochemical dissolution container for use in holding materials during the electrochemical dissolution thereof, characterised in that it comprises an internal wall area which is stepped in a direction to converge with an opposing wall area 2. A container of claim 1 which is rectangular in transverse cross section and of which two opposed sides are stepped. 3. A container of claim 1 of which all the four side walls are stepped. 4. A container of any of claims 1 to 3 which is for use in the electrochemical dissolution of nuclear fuel assemblies. 5. A container of any of claims 1 to 4 which further includes an anode/cathode pair between each step for passing current in use through a product contained in the dissolver. 6. A container of claim 5 wherein the members of each anode/cathode pair are located juxtaposed to the same side wall. 7. A container of claim 5 wherein the members of each anode/cathode pair are spaced apart longitudinally of the container. 8. A container of any of claims 5 to 7, wherein the anodes and the cathodes have a platinum surface. 9. A container of any of claims 1 to 7 wherein the container comprises a metallic or non-metallic mesh. 10. A container of claim 9, wherein the mesh is made of niobium, tantalum or polydivinylbenzene. 11. A fuel assembly container for the electrochemical dissolution of nuclear fuel assemblies which comprises a perforate stepped truncated pyramid. 12. A fuel assembly container for the electrochemical dissolution of nuclear fuel assemblies, which is capable of: a) supporting the initial assembly with its outermost row of pins spaced from the electrodes, b) after a predetermined length of the cladding of a predetermined number of outer pin rows has been dissolved, supporting the resultant assembly with its outermost layer of undissolved pins spaced from the electrodes, and c) repeating support function (b) until all the pins of the assembly are dissolved. 13. A container of claim 12, which comprises a plurality of steps, each step to support outer pins of the assembly during a respective one of the support function phases and the steps for successive support phases being arranged successively down the dissolver. 14. A container of claim 13 which further comprises electrodes which include an anode/cathode pair above each step for passing current through a fuel assembly during the its respective support function phase, the members of each said pair being located one above the other. 15. A method for electrochemical dissolution of a nuclear fuel assembly, wherein the assembly is placed in a dissolver capable of: a) supporting the initial assembly with its outermost row of pins spaced from the electrodes, b) after a predetermined length of the cladding of a predetermined number of outer pin rows has been dissolved, supporting the resultant assembly with its outermost layer of undissolved pins spaced from the electrodes, and c) repeating support function (b) until all the pins of the assembly are dissolved. 16. A method of claim 15 wherein the dissolver further includes the feature(s) recited in claim 13 or claim 14. 17. A method for dissolving a metal-containing product by electrochemical dissolution, comprising passing an electric current through the product when placed in a dissolver of any of claims 1 to 11 which is itself in an electrolyte. 18. A method of claim 17 wherein the product is an irradiated nuclear fuel assembly and the electrolyte is nitric acid. 19. A process for the electrochemical dissolution of an irradiated nuclear fuel pin having a zirconium alloy cladding, which process is characterised in that, during an initial stage of the process, the polarity of the electrodes is reversed. 20. A process of claim 19 wherein there are subjected to electrochemical dissolution a plurality of said pins constituting a fuel assembly. 21. A process for the electrochemical dissolution of an irradiated nuclear fuel pin having a zirconium alloy cladding, which process is characterised by the use of rectified alternating current or biased alternating current. 22. A process of claim 21 wherein the current is half wave rectified alternating current. 23. A process of claim 21 wherein the current is full wave rectified alternating current. 24. A process of any of claims 21 to 23 wherein there are subjected to electrochemical dissolution a plurality of said pins constituting a fuel assembly. 25. A process of claim 24 wherein the cladding of the pin or assembly is a zirconium alloy. 26. A process of claim 25 which further includes the feature(s) recited in one or both of claim 19 or claim 20. 27. A method of any of claims 20, 24, 25 or 26 wherein the nuclear fuel assembly is contained in a dissolver of any of claims 1 to 11 or 12 to 14 which is in nitric acid as electrolyte. 28. A method of any of claims 15, 16, 18, 19, 20 or 24-27 which is performed using a potential of from 5 to 50 volts. 29. A method of any of claims 15 to 21 when dependent on any of those claims which further includes dissolving the nuclear fuel contained in the fuel pin or fuel assembly and subjecting the dissolved fuel to one or more further processes to make a nuclear fuel product.

Inventors: RANCE PETER JONATHAN WATSON, WHILLOCK GUY OLIVER HECTOR, RANCE, PETER, JONATHAN, WATSON, WHILLOCK, GUY, OLIVER, HECTOR
Applicants: BRITISH NUCLEAR FUELS PLC, RANCE PETER JONATHAN WATSON, WHILLOCK GUY OLIVER HECTOR, RANCE, PETER, JONATHAN, WATSON, WHILLOCK, GUY, OLIVER, HECTOR
Application year: 1998
Main classification: C25F5
IPC codes: B65D21, B65D83, C22B34, C25D5, C25D17, G21C19