Simulation of subcooled flow instability for high flux research reactors with ATHLET

The ATHLET code has been improved and validated for the safety analysis of high flux research reactors against experimental data of the Thermalhydraul ic Test Loop of Oak Ridge National Laboratory (ORNL). Therefore, an extensi ve series of "stiff mode" experiments were recalculated. The comparison bet ween experiments and ATHLET posttest calculations shows that the extended c ode simulates accurately the thermalhydraulic conditions and the flow insta bilities in a wide range of inlet velocity, heat and mass flux.