FR2476368A1 - METHOD FOR RECOVERING NUCLEAR FUEL MATERIAL FROM AN IRRADIA AND SHEET NUCLEAR COMBUSTIBLE ELEMENT - Google Patents

Abstract

THE INVENTION CONCERNS THE RECOVERY, FROM AN IRRADIATED NUCLEAR FUEL ELEMENT, OF NUCLEAR FUEL CONTAINED WITHIN A PROTECTIVE METAL SHEATH IN AUSTENITIC STAINLESS STEEL OR A CUBIC NICKEL ALLOY WITH A CENTERED FACE. THE OUTER LAYER OF THE SHEATH IS STRENGTHENED BY CONTACT WITH A NITRURING AGENT (PREFERREDLY AMMONIA). A BENDING STRESS FOLLOWED BY REVERSE BENDING OR SHEARING ACTION IS THEN APPLIED TO THE SHEATH FOR BREAKING IT USING, FOR EXAMPLE, A CYLINDER DRESSING MACHINE. THE COMBUSTIBLE MATERIAL (U ETOR PU) CAN THEN BE EXTRACTED BY DISSOLUTION AND TREATED FOR REUSE.

Description

The present invention relates to a method for recovering fuel

  from irradiated nuclear fuel elements, where the fuel

  is contained in a protective metal sheath

  In particular, the invention relates to the recovery of nuclear fuel from nuclear fuel elements when the protective metal sheath is made of austenitic stainless steel.

  or a face-centered cubic nickel alloy.

  Before treating the used nuclear fuel to separate the fission products, it is usual

  to separate the nuclear fuel from its protective sheath

  trice, and the current practice is to shear or section mechanically in short lengths the fuel elements and then extract, by preferential dissolution, the combustible material. However, the maintenance of a mechanical sectioning equipment exposed to a radioactive environment is difficult, and the sectioning operation generates fine dust, which contaminates the immediate area surrounding the cutting machine and must be mastered. After dissolving the nuclear fuel, the solution obtained is further processed to separate the fission products from the reusable nuclear fuel (uranium and plutonium) that is recovered and that can be used to manufacture other fuel elements for a nuclear reactor. Separation methods are well known in the art and include solvent extraction methods in which

  using tributyl phosphate as the

traction.

  According to the present invention, a method of

  recovery of nuclear fuel from a

  The spent nuclear fuel method, wherein the nuclear fuel is contained in a protective metal sheath of austenitic stainless steel or a face-centered cubic nickel alloy, comprises the steps of contacting the sheath with a nitriding agent which introduces nitrogen atoms in the metal of the sheath to cause the embrittlement of the outer surface layer of the sheath, then to break this sheath by subjecting it to a bending stress followed by an inverse bending or shearing action. By "embrittlement" it is not meant to simply denote a decrease in ductility but a loss of ductility such that a break occurs without appreciable plastic deformation. A small amount

  plastic deformation is tolerable.

  The flexural stress followed by reverse bending and shearing section can be imparted by passing the sheathed nuclear fuel, after the nitriding, in a roller or cylinder straightener, which machine comprises a multi-roll cage.

  cylinders having a number of upper cylinders

  and lower rollers each of which is offset by half the diameter of a roll in the direction of the dressing passage so that the first upper roll is disposed midway between the first and second lower rolls). Such machines are

  more commonly used for finishing metals.

  Nitriding or the introduction of nitrogen atoms into steel through the metal surface

  is a commonly used process for the treatment of

  ferritic steels in order to increase the surface hardness thereof, and this method can be applied similarly to austenitic steels. The process relies on the dissolution of nitrogen in the steel network and the formation of metal nitride precipitates formed with both iron and alloying elements, such as chromium, which are in solution

  solid in the crystal lattice of steel. These phenomena

  but they increase the hardness of the steel and diminish its ductility; they usually go to completion as the nitrogen enters the steel, and thus, by adjusting the nitriding time, nitriding only the outer layer of a steel sheath. Thus, the nitriding of the outer surface layer only of a steel sheath has the effect of producing a composite material formed of two layers having quite different mechanical properties. The outer layer,

  nitrided and weakened, easily cracked by

  ring pressure for example, but the inner layer, still non-nitrided, retains the ductility of the material of the initial sheath. By subjecting the composite material to bending stress as described above, however, cracks can be propagated all around the sheath and up to its entire depth. Thus, by treating nuclear fuel elements according to the invention, they can be broken down into many pieces which will be little or not imprisoned by deformed sheath material, which allows a

  easy extraction by lixiviation of the combustible material.

  The nitriding rate depends on a number of factors. It can be conveniently adjusted by limiting the nitriding temperature and the exposure time to

  the nitriding agent and by choosing this agent.

  A preferred nitriding agent is the thermal dissociation of ammonia catalyzed by iron-based alloys. Thermal dissociation can be obtained by

  heating ammonia or a mixture of ammonia and hydro-

  gene at temperatures between 5000 and 900 ° C, preferably between 6000 and 8000C. Thus, it has been shown experimentally that, by exposure to ammonia at

  6000 to 8000C, tubing made of stainless steel

  austenitic type 316 dyke progressively nitrides from the outer surface and that the boundary between the

  nitrided material and the non-nitrided one, which is usually

  clear, moves away from the surface at a speed

  independent of the temperature between 600 and 8000C. How-

  However, if the nitriding agent is a mixture of hydrogen and ammonia instead of ammonia alone, the nitrogen potential will not be high enough for iron nitride to form like chromium nitride. and it has been observed that the penetration rate continues to increase with the temperature above 600 ° C. For example, if the temperature is allowed to reach 8500C, a greater penetration can be obtained in 24 hours than in the case of the use of pure ammonia. Adjusting the ratio of ammonia to hydrogen in the nitriding gas may be an additional means of adjusting the

  process and the speed (and extent) of penetration

  tion. The method of the invention will now be illustrated

  by the following description, given as an example

  only, the initial stages of nuclear fuel recovery from irradiated nuclear fuel elements, when the fuel elements are needles containing the nuclear fuel material within austenitic steel sheaths and

  when a bundle of such needles is located in

  stainless steel casing to form a nuclear fuel subassembly in which the

  The needles are positioned using distance grids.

  The sheaths are made of type 316 austenitic stainless steel typically having a composition having the presence of the following elements: Cr 16 to 18%; Neither 10 to 14%; Mo 2 to 3%; C 0.04 to 0.06%, Mn 1.5 to 2% and Si up to 0.75%, the remainder being Fe and accidental and inherent impurities from the manufacture of steel. Alternatively, the sheaths may be made of "Nipionic" brand alloys such as "Ninonic PE16", which is a face-centered cubic nickel alloy having the following composition: Ni 42 to%; Cr 15 to 18%; Mo 2.5 to 4%; Ti 1.1 to 1.5%; 1.1 to 1.5%; C 0.05 to 0.1% and B 0.005 0.01%; the

  the remainder being iron and impurities

  lace. The irradiated subassembly is first preserved during a cooling period allowing the decay of the radioactivity or the disintegration of the short lived fission products, then the envelope of the subassembly is separated from the fuel needles. which are placed in a container and exposed to pure anhydrous ammonia at a temperature between 6000C and 8000C. The nitrogen formed by the catalytic thermal dissociation of the ammonia enters the outer surface layer of the stainless steel and causes embrittlement, the nitriding continuing until the weakened layer represents

  about 35% of the thickness of the steel sheath, that is,

  say up to a thickness that is typically 130 half

  cron. The hardness of the nitrided layer (weakened) is between 700 and 800 VPN (penetration index) while the hardness of the non-nitrided material is around 250 VPN. The nitrided needles retain the integrity of their structure and are introduced in isolation or in a planar assembly in a roller or cylinder dressing machine which will be described hereinafter. In this machine, needles are clearly broken into many pieces, which allows to separate the material of the sheath

  from the nuclear fuel contained in this sheath.

  A typical machine to train is represented

  schematically in the accompanying drawings on

  which Figure 1 is a perspective view and Figure 2 is a side view. As shown in the drawings, the machine comprises a cage 1 comprising a number of rollers or upper rolls 2 and rollers or lower rolls 3, each offset by a roll half-diameter in the direction of movement of the roll. the material to be treated, so that the first upper cylinder is disposed halfway between the first and

  second lower cylinders, and so on. The map-

  of the upper and lower rolls is adjusted so that a needle 4 can pass between them, but this spacing is slightly smaller than the diameter of the needle, so that the needle is subjected to a bending stress exerted alternately in opposite directions as the needle passes between the rolls, so that it breaks into pieces. Under certain conditions, the sheath and the needle break before the reverse bending, by a shearing action when they are close to a cylinder being

  inclined with respect to the line or the plane of displacement.

  It goes without saying that, without departing from the

  vention, many changes can be made

  to the process described and shown.

Claims (3)

CLAIMS I   A method for recovering, from an irradiated nuclear fuel element, the nuclear fuel contained within a protective metal sheath of austenitic stainless steel or a face-centered cubic nickel alloy, which method is characterized in that it includes the steps of putting the   sheath in contact with a nitriding agent which introduces   Nitrogen atoms in the metal of the sheath to cause embrittlement of the outer surface layer of this sheath, and to then break the sheath by subjecting it to a bending stress followed by a   reverse bending or shearing action.   2. Method according to claim 1, characterized   in that the sheath is broken by the passage of the corbus-   nuclear power jacketed in a straightening machine having cylinders.   3. Method according to one of claims 1 and 2,   characterized in that the nitriding agent is or includes ammonia.