EXPLOSIVE REACTIONS DURING REPROCESSING OF REACTOR FUELS CONTAINING URANIUM AND ZIRCONIUM OR NIOBIUM

Small particles of zirconinm formed during mechancal operations have caused serious industrial explosions. Particles of this type will not be encountered during aqueous chemical reprocessing. However, small panticles of metallic phases rich in zirconium or niobium, produced by selective leaching of a more reactive uranium-rich phase, can enter into violently explosive reactions. The conditions under which explosions with zirconium-bearing alloys may be expected during chemical reprocessing have been defined in the literatare. The uranium-zirconiaum alloys containing less than 30 wt.% zirconium are hazardous when they contact nitric acid. Laboratory work has shown that potentially explosive zirconium alloys may be safely dissolved in nitric acid if enough fluoride ion is added to maintain a mole ratio of fluoride to dissolved zirconium of four. Niobium-bearing fuel alloys can also explode after contacting nitric acid, but the conditions which produce explosions have not yet been studied thoroughly. Dissolution studies with the EBWR alloy (93.5% U, 5% Zr, 1.5% Nb) indicate that hazardous niobium alloys can also be processed safely in nitric acid by addition of enough fluoride to prevent formation of surface deposits. The ternary phase diagram for zirconium -uranium -oxygen shows that the epsilon phase is unstable at the interface between a zirconium or a Zircaloy-2 cladding and a uranium oxide core. These fuels can be processed by techniques which require contact with nitric acid without fear of epsilon-phase explosions. (auth