The simulation of the BR2 test A/400/1 was undertaken to support the safety analysis of the conversion of the BR2 research reactor to low-enriched uranium (LEU) fuel and to extend the validation basis of the RELAP code for analysis of the conversion of research reactors from highly enriched fuel to LEU. This test was characterized by a steady-state peak heat flux of 400 W/cm2 , total loss of flow without loss of system pressure, reactor scram, flow reversal, and reactor cooling by natural convection. This paper presents the RELAP analysis of test A/400/1 and the comparison of code predictions with experimental measurements of peak cladding temperatures during the transient at different axial locations in an instrumented fuel assembly. The simulations show that accurate representation of the pump coastdown characteristics and of the power distribution, especially after reactor scram, between the fuel assemblies and the moderator/reflector regions are critical for correct prediction of the peak cladding temperatures during the transient. Detailed MCNP and ORIGEN simulations were performed to compute the power distribution between the fuel assemblies and the moderator/reflector regions. With these distributions, the predicted peak cladding temperatures were in a good agreement with experimental measurements.