Patent Number: 
Section: description

In the various figures, the same reference notations denote identical or similar components. The process of the invention, for the use of non-free-flowing UO2 powder, comprises basically a process for the manufacture of (U,Pu)O2 mixed oxide fuel pellets, that is to say overall (FIG. 2). dosing and first blending (step 1) of PuO2 powders and/or UO2 powders and/or fuel manufacturing scrap; micronization (step 2) of this first blend, particularly by milling, and forced sieving (step 3) of its product, for example through a 250 xcexcm screen mesh; additional dosing and second blending (step 4) of the first blend thus treated, UO2 and, where appropriate, fuel manufacturing scrap; addition, and blending with the resulting second blend of one or more lubricants and/or poreformers (step 5), the latter step possibly being completely or partly combined with step 4; compression (step 6) of the second blend into pellets using pelletizing presses; and sintering (step 7) of the pellets thus formed, preferably in an atmosphere of moistened argon (or nitrogen) and hydrogen. This mixed oxide fuel pellet manufacturing process may also usually include, for the pellets thus obtained, steps of: dry grinding (step 8); visual inspection (step 9); stacking up to length (step 10); loading the pellets into a cladding and welding the latter so as to form a fuel rod (step 11, FIG. 1); pressurizing the rods; nondestructive testing/examination of the rods (step 12); and assembling of the rods (step 13). Said process of the invention furthermore includes (FIG. 2) a prior mechanical granulation treatment of all or part of the nonflowing UO2 (step 29). This treatment may comprise, for example: either (FIG. 3) steps of compressing the non-free-flowing UO2 into tablets (step 30) and of crushing these tablets (step 31) and, where appropriate, of sieving the crushed material (step 32) in order to form free-flowing granules having properties suitable for being incorporated as the basic constituent in the second blending operation (step 4) or, in a variant, in both blending operations (steps 1 and 4), while maintaining the original chemical composition and original particle size of the original UO2; or an agglomeration/precompaction/granulation step by forcing the non-free-flowing UO2 powder through a screen or sieve (step 29), the amount of additive (s), the mesh size of the screen or sieve and the pressure exerted on the powder being adjusted in order to form granules having the suitable properties described above. A few nonlimiting parameters of the pellet manufacturing process are given below by way of example: batch/campaign operation rather than continuous operation; plutonium content of the first blend: 20 to 40% (step 1); milling (step 4) in 60 kg batches for a minimum effective time of 5 hours; use of non-free-flowing UO2 powders coming from a wet conversion (for example, ex-ADU or ammonium diuranate powder) or from a dry conversion (said conversions being known to those skilled in the art); addition of 0.2 to 0.5% of zinc stearate and 0 to 1% of an AZB pore former (known to those skilled in the art); pelletizing compression (step 6) at a pressure between 400 and 700 MPa; sintering (step 7) for at least 4 hours at a temperature between 1600 and 1760xc2x0 C. in an argon atmosphere containing 5% hydrogen, with an H2/H2O ratio of 10 to 30; and dry centerless grinding (step 8). By way of nonlimiting example, the compression step (step 30) may be carried out at a pressure of between 50 and 200 MPa, this being tailored according to the characteristics of the non-free-flowing powder. These pressures are therefore higher than the granulation pressures (4 to 10 MPa) generally used in UO2 nuclear fuel manufacturing plants. Some binder and/or lubricant, both well known to those skilled in the art, may be incorporated into the non-free-flowing UO2 powder before compression: by way of nonlimiting example, the compression may thus be carried out at a pressure of between 40 and 100 MPa. Also by way of nonlimiting example, the aforementioned tablets may be crushed in one or more jaw crushes or roll mills of 200-250 xcexcm aperture. This crushing may be followed by sieving it the crusher lets through, or runs the risk of letting through, granules having a size greater than 250 xcexcm. The fines possibly resulting from the crushing may usefully be incorporated as raw material into the first blending operation (step 1). By way of yet another nonlimiting example, the operation of forcing the powder through a sieve (step 29) may be carried out in a machine of the kind used in MIMAS-type processes (step 3) to fill the first blend (after the micronization of step 2) before the second blending (step 4). Such machines, which combine agglomeration/precompaction upstream of the sieve and control of the maximum granule size by passing the powder through this same sieve, may produce granules of the desired characteristics directly. Experience has shown the Applicant that a non-free-flowing powder treated according to the processs forming the subject matter of the invention can be used in existing MOX manufacturing plants, by passing the parameters of this second blending operation (step 4), the pelletizing (step 6) and the sintering (step 7), within the adjustment limits routinely used to optimize the manufacturing process according to the characteristics of the various free-flowing UO2 powders used for MOX fuel manufacture. The process of the invention therefore makes it possible to extend the range of UO2 powders which can be used to manufacture MOX fuel, without loosing the benefit of the similarity between the MOX fuel produced according to the invention and the UO2 fuel manufactured on an industrial scale by the processes known hitherto, starting from the same non-free-flowing UO2 powder. It should be understood that the present invention is in no way limited to the methods of implementation described above and that many modifications may be made thereto without departing from the scope of the claims given hereafter. The non-free-flowing UO2 conditioning process may especially be applied to UO2 coming from a conversion other than the conversion of uranium hexafluoride into UO2.