Studies of interaction of GEN-IV advanced fuels with metallic coolant (Na, Pb) in operating conditions

The main challenge for the development of innovative fast reactor fuels, envisioned in the international GEN-IV program, comes from the necessity to incorporate the minor actinides (Np, Am, Cm) into the fuels up to significant concentrations. Though various types of fuels, e.g. nitrides, carbides, metals, are potential candidates, oxide fuels are still currently considered as reference for Sodium-cooled Fast Reactors for which worldwide industries have already accumulated substantial experience in terms of fabrication, reactor operation, reprocessing and risk assessment. Though extremely rare during normal operation conditions, the fuel can hypothetically come into contact with the sodium if a breach occurs in a defective cladding. The work already performed in the past showed that sodium can react with (U,Pu)O2 fuels to form sodium urano-plutonate compounds which have a lower density and thermal conductivity than the fuel, leading to a local temperature and stress increase in the cladding and potentially enlarging the original breach up to a pin failure. The compatibility of a fuel with the reactor coolant is a basic criterion, which must be fulfilled for its deployment in a reactor system. This is a safety relevant issue, which requires no loss of fuel into the coolant in the event of a fuel pin breach. Incorporation of minor actinides will introduce a more complex fuel-coolant chemistry for which virtually no data are available. We are currently investigating the implications for safety when minor actinides are introduced in the fuels up to a significant concentration and come into contact with the metallic coolant. The study of the interactions of AmO2-x, and (Pu,Am)O2-x compounds (historically embedded in inert matrices Mo, MgO) with sodium and lead for different compositions and O/An ratio (An = Pu, Am) have been initiated. Tests were performed at 550 deg. C, 648 deg. C in sodium and 800 deg. C in lead. X-ray radiographs and X-ray diffraction experiments were performed prior to the heat treatments. Compositions and stoichiometry (when known) are listed. These fuels are composites of Mo and MgO and are foreseen as targets for deployment in FR systems (Futurix irradiation-EUROTRANS Programme)