Experimental investigation of isothermal temperature coefficient in ALWR-type UOx and full MOX cores

From 1995 to now on, the French ''Atomic Energy Commission'' (CEA) and the Japanese governmental agency ''Japan Nuclear Energy Safety Organization (JNES, formerly NUPEC) launched an intensive and extensive collaboration focused on 100%-MOX recycling in advanced LWRs through 3 main first-of-a-kind programs: MISTRAL (from 1996 to 2000), BASALA (from 2000 to 2002) and FUBILA (from 2005 to 2006), developed to the determination of core parameters of ALWR Generation III and III+ Pressurized and Boiling Water reactor types. MISTRAL and BASALA programs, implemented in the EOLE critical facility of the CEA-Cadarache, comprised various over moderated core configurations where the isothermal temperature coefficient (ITC) between 5degC and 85degC was measured by using both subcritical method and critical mass or boron concentration adjustment. In particular, a positive ITC has been measured in MISTRAL-1 UO2 core between 5degC and 10degC, and in the BASALA-C/REF MOX core between 5degC and 30degC. Even if this behaviour has no influence on the hot nominal conditions, its interpretation should bring additional information on the different parameters (nuclear data, models, etc....) and give an important feedback on the code validation. (author)