[en] Described herein is a special sheathed heater assembly simulating the fuel assembly of an LWR. It is used for the LOCA test, an item of LWR safety investigation. The behavior of the fuel assembly in case the primary coolant within a reactor pressure vessel abruptly drains out can be investigated through this simulated fuel assembly. It has a zircaloy cladding containing gaps to be filled with He gas to simulate an actual fuel assembly in structure. Experiments with this assembly have been made several times by Japan Atomic Energy Institute, and part of the results has been published. Further experiments are to be continued in expectation of valuable results. (auth.)