In a large fast breeder reactor (FBR), passive safety devices are able to introduce a negative reactivity effect passively to prevent a core disruptive accident during an anticipated transient without scram. A high temperature sodium test facility was developed to investigate and develop the passive safety devices. This facility can be used for liquid sodium tests at a maximum pressure of 1 MPa and 700° C or for sodium boiling tests up to 1,000°C. Preliminary tests using a gas expansion module unit were carried out with the core conditions of a large FBR. Experimental results were obtained as follows. (1) The gas expanding process is similar to an isothermal one. (2) The gas expansion speed increases gradually due to sodium flow resistance and inertia. (3) Since forced convection by the gas expansion promotes heat transfer, the heat transfer increases rapidly at the start of expansion. (4) The above experimental results are used to propose an empirical correlation.