Fuel Production for LWRs – MOX Fuel Aspects

Plutonium recycling in Light Water Reactors is today an industrial reality. It is recycled in the form of (U, Pu)O2 fuel pellets (MOX), fabricated to a large extent according to UO2 technology and pellet design. The similarity of physical, chemical, and neutron properties of both fuels also allows MOX fuel to be burnt in nuclear plants originally designed to burn UO2. The industrial processes presently in use or planned are all based on a mechanical blending of UO2 and PuO2 powders. To obtain finely dispersed plutonium and to prevent high local concentration of plutonium, the feed materials are micronised. In the BNFL process, the whole (UO2, PuO2) blend is micronised by attrition milling. According to the MIMAS process, developed by BELGONUCLEAIRE, a primary blend made of UO2 containing about 30% PuO2 is micronised in a ball mill, afterwards this primary blend is mechanically diluted in UO2 to obtain the specified Pu content. After mixing, the (U, Pu)O2 powder is pressed and the pellets are sintered. The sintering cover gas contains moisture and 5 v/o H2. Moisture increases the sintering process and the U-Pu interdiffusion. After sintering and grinding, the pellets are submitted to severe controls to verify conformity with customer specifications (fissile content, Pu distribution, surface condition, chemical purity, density, microstructure)