GB2220602A

GB2220602A - Method of, and apparatus for, cutting

Info

Publication number: GB2220602A
Application number: GB8904954A
Authority: GB
Inventors: Thomas David Hodgson; D J Baker
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1988-03-10
Filing date: 1989-03-03
Publication date: 1990-01-17
Anticipated expiration: 2009-03-03
Also published as: GB2220602B; GB8805752D0; GB8904954D0

Abstract

An object, such as a bundle of fuel pins 12, is cut at an intermediate position along its length by immersing it up to that position in an inert liquid 72, and covering the inert liquid with a thin layer of dissolvent liquid 70 of lower density. The rate of dissolution may be enhanced electrochemically, by making the object 12 an anode and arranging a cathode 42, e.g. a basket to retain the cutpieces near it in the dissolvent liquid. The dissolvent liquid may be continuously recycled, the level being maintained by an overflow or weir 53. <IMAGE>

Description

Method of Cutting This invention relates to a method for cutting an object at an intermediate position along its length, and also to an apparatus for performing such a cutting operation.

In the reprocessing of irradiated fuel from nuclear reactors, which fuel is in the form of fuel pins enclosed in a cladding material, several of which clad fuel pins form a fuel sub-assembly, it is known to cut the fuel pins into lengths so that a dissolvent can then be used to dissolve the fuel via the cut ends. The cut pieces may be of length about 50 mm. It is known to perform this cutting process by shearing, either of a complete sub-assembly or of individual fuel pins, although this requires the use of massive mechanical shearing machines which must operate in a radioactive environment and which generate fuel and cladding fines. Another mechanical technique is described in EP 0 106 633.

According to the present invention there is provided a method for cutting an object at an intermediate position along its length, the method comprising the operations of partially immersing the object in an inert liquid, and covering the inert liquid with a thin layer of a dissolvent liquid which is of lower density than the inert liquid, is immiscible with the inert liquid, and which causes dissolution of the portion of the object exposed to it.

By the term inert is meant that the inert liquid does not react chemically or physically with the object or with the dissolvent. The inert liquid might for example be a fluorocarbon such as perfluoroheptane or perfluorooctane.

The dissolvent liquid obviously depends upon the material to be dissolved; for example if the object is of zircaloy, the dissolvent liquid might be a nitric acid/hydrochloric acid mixture, whereas if the object is of stainless steel the dissolvent liquid might be nitric acid. The layer of the dissolvent might be between 5 mm and 50 mm thick, depending on the thickness of cut required, but the preferred thickness is between 10 mm and 20 mm.

The dissolution may proceed sufficiently far to cut completely through the object, or may be terminated after cutting only partly through the object. The method preferably also includes the operation of circulating the dissolvent liquid continuously during the cutting process to remove the products of the dissolution, and to control any temperature rise of the dissolvent liquid. It may also include the operation of electrochemically enhancing the rate of dissolution. For example the object might be connected to the positive terminal of a dc electrical supply to be an anode, with there being another electrode connected to the negative terminal to be a cathode, dipped into the dissolvent liquid adjacent to or around the object.

Where a plurality of spaced apart cuts along the length of the object are required, the object may be gradually lowered into the inert liquid stepwise, pausing for long enough for a desired degree of dissolution to occur whenever the surface of the inert liquid is at a position on the object at which a cut is required.

Alternatively the depth of the inert liquid might be gradually and stepwise increased while holding the object stationary.

This method is particularly advantageous in the cutting up of fuel pins of irradiated nuclear reactor fuel as no heavy machinery is required, the fuel sub-assemblies do not have to he dismantled and can be loaded to the dissolver with the cladding containment intact. the claddinq is cut cleanly without distortion, no small nieces of cladding are produced, and only a small proportion of the claddina need be dissolved. The cut-up pieces of clad fuel pin can then he transferred to a conventional fuel dissolver apparatus in which the fuel is dissolved to leave only cladding hulls undissolved; or alternativelv the inert liauid might be replaced by a fuel dissolvent liauid, so no transfer of the cut-up pieces is necessary.

The invention also provides an apparatus for performing the method.

The invention will now be further described hv way af example only, and with reference to the accompanyinq drawinqs, in which: Figure 1 shows a sectional view in a vertical plane through a cutting apparatus; Figure 2 shows a sectional view on the line II-II of Figure 1, to a larger scale; and Fiqure 3 shows a sectional view of Dart of the apparatus of Figure 1, on the line Ill-Ill of Figure 2.

Referring to Figure 1 there is shown an apparatus 1G for cutting a sub-assembly 12 of stainless-steel clad fuel pins containing uranium oxide pellets, from a fast reactor, and which can subsequently be used for dissolving the oxide to leave stainless steel hulls. The apparatus 10 comprises a stainless steel tubular vessel 11 defined by an upright, square cross-section dissolution limb 14 of width 250 mm, an upright services limb 16, a sloping cross-lim 18 connecting points towards the unper ends of the limb 14 and the limb 16, and a sloping cross-limb 20 connecting the base of the dissolution limb 14 with a point just above the base of the services limb 16. The portion of the services limb 16 below the cross-limb 20 forms a sump 22.The upper ends of the dissolution limb 14 and the services limb 16 are closed by covers 24 and 26 respectively, the former defining an upright tubular port 25, and gas extract ducts 28 communicate with the limbs 14 and 16 just below the covers 24 and 26. The extract ducts 28 lead to a condenser (not shown) for any liquid which evaporates from the vessel 11. An overflow port 30 is provided near the upper end of the dissolution limb 14 but below the level of the extract duct 28. The services limb 16 is provided with a cooling jacket 32 to which cold water may be supplied during operation, while the dissolution limb 14 is provided with a jacket 34 to which either steam or cold water may be supplied during operation, the arrows indicating the flow direction for cold water.

The central portion of the dissolution limb 14 is provided with a close-fitting, removable, stainless steel protective sleeve 38 with a coarse stainless steel mesh base. The fuel sub-assembly 12 is supported by a niobium grab mechanism 40 within the dissolution limb 14 and is surrounded by a stainless steel or niobium mesh basket 42 which is separately connected to the grab 40. The grab 40 is at the bottom end of a niobium extension tube 44 which extends upwardly through the tubular port 25, and has at its top end a lifting mushroom 46 for a hoist (not shown).

The basket 42 is electrically isolated from the sub-assembly 12, and the tube 44 incorporates electrical connections such that the basket 42 and the sub-assembly 12 can be connected to an electrical circuit 48 (shown diagrammatically), such that the sub-assembly 12 is an anode, and the basket 42 a cathode.

The cover 24 supports two tubes 50 and 52 which extend downwardly about a third the height of the dissolution limb 14, and provide respectively an inlet and an outlet for a cutting liquid; as shown in Figure 2 these tubes 50 and 52 are near opposite corners of the limb 14, between the basket 42 and the sleeve 38. As shown in Figure 3, the outlet tube 52 ends a short distance below the lower end of the inlet tube 50 and defines at its lower end a J-tube 53 which acts as a weir.The cover 26 supports four tubes: an inlet tube 54 for a support fluid, which ends near the top of the limb 16; an outlet tube 56 for the support fluid, which extends downwardly to end within a filter 58 in the sump 22; an inlet tube 60 for a fuel dissolving acid, which also extends downwardly into the filter 58; and an outlet tube 62 for the fuel dissolving acid, which extends downwardly to the bottom of the sump 22, outside the filter 58.

In operation of the apparatus 10 for cutting up the sub-assembly 12 the overflow port 30 is closed, and gas is extracted through the ducts 28 to ensure that the gas flow through the tubular port 25 is downwards. The support fluid, in this case perfluoroheptane, is supplied through the tube 54 until a level just below the lower end of the tube 52 is reached. The level of the support fluid may be controlled by metering the volume of support fluid supplied. The sub-assembly 12 and the basket 42 are then lowered into the dissolution limb 14 until a desired length of the sub-assembly 12 is immersed in the support fluid.

Cutting acid, in this case 8 molar nitric acid, is supplied through the tube 50 and extracted at the same rate through the tube 52 so creating (as shown in Figure 3) a thin layer about 10 mm thick of cutting acid 70 floating on the surface of the support fluid 72.

Because of the potential difference between the basket 42 and the sub-assembly 12, the latter being an anode, a current which may be between 50 and 1000A flows through the thin acid layer 70, so that the sub-assembly 12 dissolves where it contacts the acid layer 70 and hence is cut through at that level. The progress of the cutting operation can be monitored from the applied voltage, as this increases rapidly towards the end of the operation.

Throughout the cutting process the cutting acid is continuously recycled and cooled by passage through a heat exchanger (not shown); and in addition cold water is passed through the jacket 34 to prevent the support fluid from becoming hot. Any support fluid which does evaporate is returned to the vessel 11 via the tube 54, from the condenser. The cut pieces fall down into the bottom of the basket 42. The sub-assembly 12 and the basket 42 can then be lowered again, and the cutting process repeated at a different position.

When the sub-assembly 12 has been cut into the desired number of lengths the electric circuit 48 is disconnected, and the flow of cutting acid through the tubes 50 and 52 terminated. The support fluid is then emptied out of the vessel 11 through the outlet tube 56, any small particles of fuel or cladding being caught by the filter 58 in the sump 22. The vessel 11 is then filled with a fuel dissolving acid, which may again be 8 molar nitric acid, through the inlet tube 60 (and so backwashing the filter 58) until the level is just below that of the overflow port 30, which is now open. Steam is passed through the jacket 34 and cold water through the jacket 32 so that the acid circulates around the limbs 14, 18, 16, 20 of the vessel 11 in the direction of the arrows A. Fuel pellets dissolve in the acid, so leaving just the hulls in the basket 42.It will be appreciated that to enhance the dissolution of fuel from within cut lengths of cladding it is advantageous to cause the pressure in the acid to oscillate, as described for example in GB 2 028 293. When the dissolution process is complete the liquid in the vessel 11, which is at this stage an acidic solution of uranium nitrate and other salts, is extracted through the outlet tube 62 along with any small particles of cladding which have collected in the sump 22.

It will be appreciated that the cutting process may differ in various ways from that described above. For example the basket 42 and sub-assembly 12 might be held at at a fixed position, and the support fluid level increased in steps, at each step a layer of cutting acid being used to cut through the sub-assembly 12. The different layers of cutting acid might be created by raising the outlet tube 52 at each step, or by providing several fixed outlet tubes of different lengths. The time taken for the cutting process depends upon the number of pins to be cut and on the current supplied; typically it requires between about 5 000 and 9 000 coulombs for each pin, so that the time may be reduced by increasing the current, although this also increases the thermal power which must be dissipated. It will be understood also that the cathode may be provided by a component other than the basket 42, for example a conducting shroud might surround the sub-assembly 12; or indeed the limb 14 might be the cathode, preferably with the basket 42 being coated in glass to minimize corrosion of the basket 42.

Claims

1. A method for cuttinq an object at an intermediate position alonq its lenqth, the method comprising the operations of partially iminersing the object in an inert liquid, and covering the inert liquid with a thin layer of a dissolvent liquid which is of lower density than the inert liquid, is immiscible with the inert liquid, and which causes dissolution of the portion of the object exposed to it.

2. A method as claimed in Claim 1 wherein the inert liquid is a fluorocarbon.

3. A method as claimed in Claim 2 wherein the fluorocarbon is perfluoroheptane or perfluorooctane.

4. A method as claimed in any one of Claims 1, 2 or 3 wherein the dissolvent liauid is nitric acid.

5. A method as claimed in anv one of Claims 1, 2 or 3 wherein the dissolvent liquid is a rnixture of nitric acid and hydrochloric acid.

6. A method as claimed in any one of the preceding Claims wherein dissolvent liquid is circulated during the cutting to remove products of dissolution and/or to allow temperature control of the dissolvent liquid.

7. A method as claimed in any one of the preceding Claims wherein dissolution of the portion of the object is enhanced electrochemically.

8. A method as claimed in Claim 7 wherein the object is connected to a positive terminal of a DC electrical supplv and a second electrode is provided, said second electrode being in contact with the dissolvent liquid and connected to a neqative terminal of the DC electrical supply.

9. A method as claimed in any one of the preceding Claims wherein a plurality of spaced apart cuts are made along the length of the object.

10. A method as claimed in Claim 9 wherein the object is lowered stepwise into the inert liquid such that a desired deqree of dissolution occurs at a plurality of positions correspondinq to the positions at which the object is stopped so as to contact the dissolvent liquid.

11. A method as claimed in Claim 9 wherein the depth of inert liauid is raised in stenwise manner such that the layer of dissolvent liquid contacts a plurality of position along the length of the object.

12. A method as claimed in any one of the preceding Claims wherein the object is a nuclear fuel pin.

13. A method as claimed in any one of the preceding claims wherein the object is a fuel sub-assembly, containinq a plurality of nuclear fuel pins, such that a plurality of fuel pins are cut without dismantling the fuel subassembly.

14. Apparatus for use in cutting an object at an intermediate position along its length comprising a vessel for containinq an inert liquid, means for-covering the inert liquid with a thin layer of a dissolvent liquid and means for locating an object to be cut in the vessel such that it may be cut at an intermediate position along its length.

15. Apparatus for cutting an object substantially as hereinbefore described with reference to any one of Figures 1, 2 or 3 of the accompanying drawings.

16. A method for cutting on object substantially as hereinbefore described with reference to any one of figures 1, 2 or 3 of the accompanying drawings.