Development and testing of a high-temperature treatment facility mock-up for pyroprocessing of mixed uranium-plutonium nitride SNF at reprocessing module of experimental and demonstration energy complex

The paper presents results of testing a mock-up high-temperature facility for the treatment of mixed uranium-plutonium nitride (MUPN) spent nuclear fuel (SNF) from the BREST-OD-300 reactor. The mock-up facility is designed to test selected high-temperature treatment (HTT) process parameters using model nuclear fuel (MNF) and allows design development testing of pyroprocessing equipment for fast neutron reactor SNF at the Reprocessing Module of the Experimental and Demonstration Energy Complex. The performance of the mock-up facility was preliminarily evaluated using MNF dummies; temperature distribution, tightness, and dummy product unloading efficiency under specified HTT conditions were obtained. The mock-up tests on the uranium nitride were carried out to prove the feasibility of the MNF nitriding/denitriding and its oxidation to U 3 O 8 , followed by the reduction to UO 2 . After the tests, the pulverized fuel was characterized for the particle size distribution, the bulk density, the angle of repose, and the phase composition by X-ray diffraction analysis. A material balance of uranium-containing product distributed over the units of the facility was drawn. The nitriding/denitriding processes proved ineffective to pulverize the MNF (25% of the loaded amount remained in claddings); nevertheless, X-ray diffraction showed a finely-pulverized UN-U 2 N 3 mixture in the powdered product. The nitride MNF oxidized in the Ar–10% O 2 gas mixture and the U 3 O 8 reduced to UO 2 in the Ar–4% H 2 atmosphere completely. According to the X-ray diffraction analysis, the produced UO 2 did not contain a U 3 O 8 phase and was a coarse powder, i.e. the fraction of particle size less than 100 µm was only 12.5%. The quantity of uranium in the product removed from the mock-up facility was 58% of the total amount used for the tests. The product loss was caused by the product deposits in the retort (13%) and a significant dust carryover from the retort during the tests (31%). The design of the mock-up HTT facility requires improvements to reduce the internal deposits and the dust carryover from the facility together with off-gases.