The problem of decay heat removal under accidental conditions in the liquid metal cooled fast reactor

The problem of decay heat removal in case of reactor shutdown remains urgent to date, and it has been confirmed by the Fukushima accident. Besides, the problem becomes more severe for the large size reactors. Based on the results of calculations on the reactor decay heat removal transients made by 3D thermohydraulic codes, comparative analysis was made of heat removal effectiveness of the large size reactors cooled by lead and sodium. Two types of decay heat removal systems (DHRS), with different locations of decay heat exchanger (DHX) are compared. In the first option DHX is situated in the reactor upper plenum, and so heat is removed from the core by the coolant flowing in the circuit "DHX – upper plenum - inter-subassembly space (ISS) of the core – upper plenum". In the second case, DHX is located in the gap downcomer of the reactor, and the check valve is provided in the circuit, which does not prevent hot coolant flow from the reactor upper plenum to the DHX inlet in the decay heat removal mode but stops this coolant flow under normal reactor operation. It has been demonstrated that both DHRS options are more effective as applied to the sodium cooled reactor. In case of the large size reactor cooled by lead DHRS with check valve is also capable of removing decay heat without exceeding permissible temperature values. However, in the option with DHX located in the upper plenum, fuel element temperature exceeds safe operation limit for a short time. (author)