Studies of unprotected transients and alternative decay heat removal system for the Gas cooled Fast Reactor (GFR)

The Gas cooled Fast Reactor (GFR) is considered by the French "Commissariat a VEnergie Atomique" as a promising fast neutron spectrum system, with the potential to fulfill the Generation IV criteria. Due to the high core power density and the limited core thermal inertia, the core Decay Heat Removal (DHR) is a key design and safety issue. In case of Loss Of Flow Accidents (LOFA), the DHR can be achieved by natural convection in dedicated loops. In case of Loss Of Coolant Accidents (LOCA), the use of a "close-containment" enclosing the primaiy system allows to maintain some gas pressure and the DHR can be achieved by "limitedpower" forced convection systems. In this paper, one will present complementary DHR studies. In a first part, exploratory analyses of unprotected transients (transients without reactor SCRAM) are reported. Both ULOFA and ULOCA (U for Unprotected) have been calculated using the TRACE computer code. It is shown that the "normal" DHR systems can be used to limit and stabilize the core temperatures. It is expected that for these temperature levels no significant core degradations would occur, the core geometry being still coolable. A specific research program is ongoing to determine the real limits of the innovative GFR fuel. In a second part, an alternative DHR design is presented using the main primaiy loops. This system is based on air-cooled heat exchangers connected to the main secondary circuits. The preliminary system design and arrangement are provided. A dedicated CATHARE modeling have been developed and used to study some typical accidental transients. The results obtained confirm the interest of this alternative system.