US3644104A

US3644104A - Process for processing canned irradiated ceramic fuel elements

Info

Publication number: US3644104A
Authority: US
Inventors: Georges Manevy; Georges Matcheret
Original assignee: Individual
Current assignee: Individual
Priority date: 1968-10-23
Filing date: 1968-10-23
Publication date: 1972-02-22
Anticipated expiration: 1989-02-22

Abstract

The decanning step constituting the first step in the reprocessing of ceramic nuclear fuel elements is carried out by cutting the fuel elements in short-length fragments and then crumbling the fuel into a powder which leaves the can fragments by processing the fuel fragments in a ball mill. Prefluoration of the fuel into UF4 and PuF4 may take place in the ball mill.

Description

United States Patent Manevy et a1.

[ Feb. 22, 1972 PROCESS FOR PROCESSING CANNED IRRADIATED CERAMIC FUEL ELEMENTS Inventors: Georges Manevy, 68 Rue Velpeau, 92-Antony; Georges Matcheret, Cite de la Plaine, lmmeuble M 21, Apt 1599, 92- Clamart, both of France Filed: on. 23, 1968 App1.No.: 769,898

US. Cl. ..23/324, 23/344, 23/353, 23/354, 252/301.1, 264/5, 241/24 Int. Cl ..C0lg 56/00 Field of Search ..23/324, 352, 344, 326, 347, 23/354, 355; 264/.5; 252/301.1; 29/225; 241/24,

[ "mm References Cited UNITED STATES PATENTS 3,145,078 8/1964 Strickland et al .,23/324 3,222,124 12/1965 Anderson et al ..23/324 X 3,268,303 8/ 1966 Ramaswami et al. ..23/ 324 3,316,065 4/ 1967 Baertschi et al. "23/324 3,371,133 2/1968 Nishijima et a1 ..264/.5

Primary Examiner-Carl D. Quarforth Assistant Examiner-Stephen J Lechert, Jr. Attorney-Cameron, Kerkarn & Sutton [57] ABSTRACT The decanning step constituting the first step in the reprocessing of ceramic nuclear fuel elements is carried out by cutting the fuel elements in short-length fragments and then crumbling the fuel into a powder which leaves the can fragments by processing the fuel fragments in a ball mill. Prefluoration of the fuel into UP, and PuF may take place in the ball mill.

PROCESS FOR PROCESSING CANNED IRRADIATED CERAMIC FUEL ELEMENTS This invention is directed to a process and apparatus for the treatment of irradiated ceramic fuel elements, wherein said process comprises as a first step the separation of the fuel material from the can.

The first step in the treatment of irradiated fuel elements usually consists in separating the can from the fuel material contained therein. Fuel elements of the type intended for use in fast reactors are usually designed in the form of pellets of sintered ceramic material which is therefore relatively brittle, said pellets being stacked within a stainless steel can of small diameter.

A number of different methods have already been proposed with a view to extracting the material from the can. In particular, a chemical process has been proposed for the destruction of the can under the action of a gaseous mixture of hydrofluoric a'cid'and oxygen at about 600 C. A mechanical method has also been proposed, including cutting the fuel element into short fragments or drilling holes in the can so that the attacking action of fluorination reagents can subsequently take place right through the fuel material.

In practice, none of these solutions is wholly satisfactory. The first method referred to has disadvantages in that it entails the use of reagents which are highly corrosive at 600 C. and gives rise to products resulting from the decanning operation which are liable to interfere with the removal of uranium and plutonium in the fluorination stage. In the second method mentioned above, a complete reaction cannot readily be carried out, especially when the fuel is an oxide (U or U0,- PuO ln fact, the intermediate products employed in the formation of Ul" are in the solid state and prevent the diffusion of the reagents and reaction products at the surface of the pellets; there is formed the intermediate compound U 0 whose specific volume is greater than that of the starting oxide U0 and results in jamming of the pellets within the can.

The object of the invention is to propose a method which satisfies practical requirements more effectively than those which have been proposed heretofore, particularly insofar as it ensures rapid and complete extraction of the fuel from the can and acceleration of the attacking reaction which is carried out either simultaneously or subsequently. To this end, the invention proposes a method of treatment which comprisesafter cutting the fuel element into fragments of small lengthhammering said fragments in a rotary grinding mill of the ball type for a sufficient length of time to result in crumbling of the fuel material and removal of this latter from the can fragments in powdered form.

This process applies in particular to uranium oxides and/or plutonium oxides which are at present the most widely employed fuels in fast reactors. However, the process is also applicable to ceramic fuels consisting of sintered carbides, nitrides or, more generally, sintered ceramic fuels which are sufficiently brittle to be reduced to powder under the impacts of the balls on the fragments.

The invention further consists in other arrangements which can advantageously be employed in conjunction with the preceding but which can also be employed independently and entails the use of a device for carrying out the process referred to above.

A better understanding of the invention will be gained from the following description of one mode of application which is given by way of nonlimitative example, reference being made to the accompanying drawings, in which:

P16. 1 shows very diagrammatically a ball mill in which the separation of can fragments from the fuel takes place at the discharge end; said ball mill forms part of an apparatus in accordance with the invention which is designed for the treatment of uranium dioxide fuel elements;

HO. 2, which is similar to FIG. 1, shows diagrammatically an apparatus in which controlled fluorination of the fuel is carried out within the mill itself by countercurrent circulation of the fragments and fluorinating agents.

The apparatus which is illustrated in FIG. 1 and placed within a leaktight shield structure (not illustrated) is designed for the treatment of fuel elements each consisting of a stack of pellets within a stainless steel can, said pellets being fabricated from uranium dioxide U0 or from the mixed oxide U0 PuO Said fuel elements such as the element 10 are cut into fragments 12 by means of a bank of cutting shears 14 which have been illustrated diagrammatically. The fragments 12 fall into a hopper 16 for feeding a grinding mill 18 of the steel ball type. Said grinding mill is composed of a drum 20 which is rotatably mounted by means of journals 22 and set at a small angle of slope in order to induce the forward motion of the fragments of treated fuel elements. Said drum 20 is placed within a stationary shell 24 which serves as a support. Uniformly spaced transverse partition walls 26 are formed within the drum 20 and provided at their periphery with openings 28 through which the fragments are intended to pass.

Said openings are ofsufficiently small'siie to ensurethat' the balls 30 are retained by the partition walls. In order to facilitate the upward motion of the balls along the cylindrical wall and to increase their falling distance, the internal face of the wall is provided with a number of longitudinal ramps 32 (six, for example) which are welded along generator lines.

The drum 20 is intended to feed a delivery spout 34 which discharges the can fragments and the powdered fuel onto a screen 36 which is endowed with vibratory motion by means which are not shown in the drawings. The empty can fragments slide over the screen 36 and are removed whereas the powdered fuel passes through the screen and is retained in a collector 38.

The operation of the device is readily apparent: when the drum 20 is set into rotation by a motor 40, the fragments which progress along the drum are subjected to a hammering action by the balls 30. The fuel progressively crumbles and passes out through the ends of the can fragments which move along the drum and are gradually freed of their contents. As they pass out of the grinding mill, said fragments fall onto the vibrating screen 36 which serves to retain within a collector 38 the powdered fuel which has thus been separated from the empty can fragments.

There now follows a description of one example of application of the process according to the invention, this example being given without implied limitation. The fuel elements to be treated are made up of a stack of sintered U0, pellets 5.7 mm. in diameter placed within a stainless steel can having a thickness of 0.9 mm. and an external diameter of 6.7 mm.; these fuel elements are cut into lengths of approximately 15 mm. and introduced into the ball mill, the drum of which rotates at a speed of 60 r.p.m.

The processing time, that is to say the time taken to remove all the fuel from can fragments, is 15 minutes when the balls employed are 31 mm. in diameter and each weigh 120 g. The processing time is only l0 minutes if 10 balls 40 mm. in diameter and each weighing 270 g. are employed in order to remove the fuel contained in can fragments. However, in the case last mentioned, a few particles of U0 have not been removed and remain within some of the can fragments. It is thus apparent that an increase in the weight of balls makes it possible to reduce the processing time. However, this weight cannot be increased without limitation as this would give rise to a degree of swaging of the can fragments at the extremities which would tend to close them and prevent the release of oxide powder. It is therefore necessary to remain within acceptable limits and, in the case considered above, to ensure that the weight of the balls does not exceed a few hundred grams.

On completion of this operation, separated oxide powder is available at the discharge end of the grinding mill for the purpose of carrying out subsequent treatments which are intended on the one hand to produce the volatile hexafluorides UF, and PuF and, on the other hand, compounds containing fission products.

As has been stated in the foregoing, the process and apparatus according to the invention are perfectly suited to direct fluorination resulting in the formation of UF and PuF which are highly volatile and therefore readily separable from the fluorides of fission products. This fluorination treatment can be carried out in several steps by means ofa number of alternative solutions, some of which will have the advantage of requiring only a single apparatus for the purpose of carrying out either simultaneously or successively the decanning of fuel elements and a part of the subsequent fluorination treatment which results in the formation of UF and PuF l. A first solution consists in carrying out the complete fluorination treatment in two steps, namely (a) by oxidation at about 400 C. to U and PuO- within the grinding mill followed by fluorination to UF and PuF outside the mill, (b) by controlled prefluorination by hydrofluoric acid to UF and PuF, within the grinding mill followed by fluorination.

(a) In the case of the oxidation, the oxygen reacts with the uranium dioxide U0 at a temperature of approximately 400 C. to form the higher oxide U 0 in the state of fine particles. The advantage of this solution lies in the fact that U 0 is much more readily converted to hexafluoride by the action of fluorine than the dioxide U0 even when this latter is in the finely ground state. Moreover, U 0 is even more brittle than U0 and the oxidation process facilitates decanning if it is carried out at the same time.

(b) In the case of prefluorination by hydrofluoric acid followed by complete fluorination, the following reactions will take place at about 400 C.:

This process will have the advantages of reducing the consumption of fluorine and permitting a further decontamination inasmuch as some fission products which form volatile fluorides with hydrofluoric acid are removed within the ball mill during this step and are therefore not present in the uranium or plutonium tetrafluorides which are formed and are in the solid state.

In both cases, the apparatus shown in FIG. 1 is employed for the first step either continuously or in a batch process. In continuous operation, the gaseous reagent is fed countercurrent to the fragments while decanning is carried out. In the batch process, the reagent is introduced at the end of the decanning operation which is carried out on a charge of fragments.

It will be possible in both cases to employ the ball mill drum as a heating jacket. For the purpose of continuous operation, the apparatus described in FIG. 1 must comprise a pipe for the admission of reagent at the downstream end (on the righthand side of FIG. 1) and a pipe for discharging the volatile products of the reaction and excess reagent at the upstream end. The angle of slope of the apparatus will be chosen with a view to ensuring that the rate at which the fragments travel down is such that the fuel remains within the ball mill for a sufficient period of time to allow the fluorination reaction to proceed to completion. It is also apparent that the means for loading and unloading the fragments must be provided with lock chambers in order to prevent any leakage of gaseous products which circulate within the ball mill.

For the purpose of batch operation, there will be added to the apparatus described in reference to FIG. 1 a pipe for the admission of reagent as well as a pipe for the discharge of volatile products which is fitted with filters in order to prevent the powders from being carried away. The products obtained, that is to say the small oxide particles of U 0 and PuO in one case and the tetrafluorides in the other case, are in the solid state and can be discharged into a suitable reaction vessel for the complete fluorination reaction.

2. A second solution consists in carrying out the fluorination treatment in three steps:

firstly, an oxidation process which will form the oxides U 0 and PuO and which is carried out within the ball mill;

secondly, a prefluorination by hydrofluoric acid in order to obtain the solid tetrafluorides UF, and PuF which can also be carried out within the ball mill;

and finally, fluorination by fluorine which forms the volatile hexafluorides UP and PuF The two first steps of this treatment within the ball mill can be carried out either in batch operation by successive introduction of the reagents or in continuous operation by making use ofa mixed reaction vessel which will have a number of successive reaction zones.

FIG. 2 shows diagrammatically an apparatus for carrying out continuously within the ball mill enclosure the steps of oxidation and prefluorination of the complete three-stage process which has just been described. For the sake of enhanced simplicity, the corresponding elements of FIGS. I and 2 are provided with the same reference numeral to which is assigned the prime index in FIG. 2.

The apparatus comprises a cylindrical shell 24 in which is placed the ball mill drum 20'. Only a first portion 42 of said drum is provided with partition walls 26' for retaining the balls 30'. The second portion of the drum which is not provided with partition walls is surrounded by heating means (not shown) for the purpose of regulating the temperature therein. A pipe 48 for the supply of hydrofluoric acid opens into said second portion at the downstream end of the drum. A pipe 50 for the supply of oxygen opens into the drum at an intermediate point. Provision must evidently be made for a rotary seal through which said two pipes penetrate into the drum. The zone 44 which is located between the last partition wall 26 and the outlet of the pipe 50 may be regarded as an oxidation-prefluorination zone and the terminal zone 46 may be regarded as a prefluorination zone. A lock chamber 52 for supplying fragments and a lock chamber 54 for discharging empty can fragments and solid reaction products are evidently provided.

The operation of the apparatus will already have become clear from the foregoing description: the fuel-element fragments are successively decanned within the portion 42: the uranium dioxide is oxidized to form U 0 and partially fluorinated within the zone 44, maintained by the heating means at a temperature of the order of 400 C. and swept by the mixture of hydrofluoric acid and oxygen which are supplied respectively through the pipes 48 and 50; finally, prefluorination to form UF and Pul] is completed in the zone 46, maintained by the heating means in the vicinity of 500 C. and contacted in countercurrent flow with hydrofluoric acid I-IF which is introduced in the pure state. The empty can fragments, the fission products which have not undergone the fluorination reaction and the mixture of tetrafluorides are collected by the lock chamber 54. The volatile fission products together with the excess hydrofluoric acid and oxygen pass out through the branch tube 22 and through the outlet pipe 56. The fluorides which are collected can be purified and converted to hexafluorides in the conventional manner.

The solution described above provides a number of different advantages. In the first place, the pretreatment with hydrofluoric acid for which it is known to provide satisfactory rotary seals leads to the subsequent economy of fluorine which is a costly reagent. Since the final treatment is carried out in the presence of hydrofluoric acid in the pure state and not of a mixture l-IF-O which would result in a product whose oxide content would still be high, a substantial proportion of the uranium and plutonium is prefluorinated to form tetrafluoride. Partial decontamination is carried out as a result of the release of the volatile fission products through the pipe 56.

The invention permits of a large number of variants, in particular in regard to the nature of the fluorinating agents employed for the purpose of converting the oxide fuel to fluoride. The temperatures to be maintained within the apparatus obviously differ according to the nature of these agents (hydrofluoric acid, fluorine, bromine pentafluoride and so forth).

It is readily apparent that the variants which have been contemplated in the foregoing as well as any other alternative form which remains within the definition of equivalent means are intended to come within the broad purview of this patent.

What is claimed is:

l. A process for the treatment of irradiated ceramic fuel material contained in an elongated can comprising the successive steps of cutting said fuel elements into short fragments, then introducing said fragments into a rotating drum of a ball mill, maintaining said fragments in said ball mill until the ceramic fuel material is crumbled and flows out of the can fragments in powdered form and then removing said material in powdered form from the ball mill separate from the can fragments.

2. A process as claimed in claim 1, wherein the particles of powdered fuel material are removed from the can fragments progressively as they are separated therefrom by falling through holes in the wall of the drum of smaller dimensions than the dimensions of the can fragments.

3. A process as claimed in claim 1 for the treatment of elements in which the fuel material is constituted by uranium dioxide or by uranium dioxide and plutonium dioxide, including the step of introducing an oxidation reagent into said ball mill and maintaining said reagent therein at a temperature of the order of 400 C. and simultaneously and progressively crumbling and converting the material to U 0 and PuO- 4. A process as claimed in claim 1, wherein a fluorination reagent is introduced into the ball mill so as to prefluorinate the fuel material to form UF, and PuF 5. A process as claimed in claim 4, wherein said fluorination reagent consists of HF or BrF 6. A process for the treatment of irradiated ceramic fuel material contained in an elongated can comprising the successive steps of cutting said fuel elements into short fragments, then introducing said fragments into a rotating drum of a ball mill and maintaining said fragments in said ball mill until the ceramic fuel material is crumbled and separates from the can fragments in powdered form, the drum of said ball mill being inclined for progressive forward motion of the can fragments and fuel material and for discharging the can fragments and fuel material from the drum and then screening said material in powdered form from the can fragments.

Claims

2. A process as claimed in claim 1, wherein the particles of powdered fuel material are removed from the can fragments progressively as they are separated therefrom by falling through holes in the wall of the drum of smaller dimensions than the dimensions of the can fragments.
3. A process as claimed in claim 1 for the treatment of elements in which the fuel material is constituted by uranium dioxide or by uranium dioxide and plutonium dioxide, including the step of introducing an oxidation reagent into said ball mill and maintaining said reagent therein at a temperature of the order of 400* C. and simultaneously and progressively crumbling and converting the material to U3O8 and PuO2.
4. A process as claimed in claim 1, wherein a fluorination reagent is introduced into the ball mill so as to prefluorinate the fuel material to foRm UF4 and PuF4.
5. A process as claimed in claim 4, wherein said fluorination reagent consists of HF or BrF5.
6. A process for the treatment of irradiated ceramic fuel material contained in an elongated can comprising the successive steps of cutting said fuel elements into short fragments, then introducing said fragments into a rotating drum of a ball mill and maintaining said fragments in said ball mill until the ceramic fuel material is crumbled and separates from the can fragments in powdered form, the drum of said ball mill being inclined for progressive forward motion of the can fragments and fuel material and for discharging the can fragments and fuel material from the drum and then screening said material in powdered form from the can fragments.