Loss-of-Heat-Sink Transient Simulation with RELAP5/Mod3.3 Code for the ATHENA Facility

ATHENA (Advanced Thermal-Hydraulic Experiment for Nuclear Applications) is a large multipurpose pool-type lead-cooled facility, which is under construction at the Mioveni site, in Romania. It has been identified by the FALCON (Fostering ALfred CONstruction) Consortium to conduct integral tests, to characterize large to full-scale ALFRED components, and to investigate the main thermal-hydraulic phenomena involved in a pool-type system. ATHENA is representative of ALFRED in terms of the difference in height of the thermal barycenters of the heat source and heat sink, i.e., 3.3 m, in order to reproduce the buoyancy forces in the system. As foreseen also for ALFRED, thermal stratification inside the main vessel is limited, even in natural circulation, by the presence of an internal structure, named barrel, that drives the fluid towards the main vessel, avoiding the stagnation of the fluid adjacent to the external wall. Firstly, a steady-state thermal-hydraulic characterization of the facility is presented, along with the description of the numerical model realized with RELAP5/Mod3.3. Then, the work focuses on the transient analysis considering as a reference scenario a Loss-of-Heat-Sink (LOHS) accidental transient, during which the Core Simulator (CS) and the Main Circulation Pump (MCP) are supposed to continue to work. The response of the system following the isolation of the Main Heat Exchanger (MHX) from the secondary loop is analyzed, and the effect of the delay in the scram of the reactor (reducing the CS power to a value representative of the decay heat) on the pin cladding and system maximum temperature is evaluated. The pre-test simulation presented in this paper have shown that the facility is able to achieve a safe shutdown condition thanks mainly to the high thermal inertia of the lead pool.