PURPOSE: To correctly simulate the natural circulation of the primary fluid of an actual machine by using an electric heater simulating the actual machine at the reactor core section, and making a reactor vessel nearly equal to the actual machine in height and smaller than the actual machine in diameter.

CONSTITUTION: A reactor vessel 12 is arranged with a pressure chamber 18 at the upper section, a steam generator 20 in the middle, a reactor core 62 made of an electric heater at the lower section, and a poison tank 24 around the reactor core 62 respectively. When a circulating pump 26 is driven, the primary cooling water is circulated in a passage 28 and heat-exchanged with the steam generating water passing through the steam generator 20. The water in the pressure chamber 18 is communicated with the cooling water via a cooling fluid passage 38, and only the pressure is transferred to the cooling water. The upper section in the tank 24 is communicated with the cooling water via a water pressure operating valve 44, the operation valve 44 is closed during the normal operation, and it is automatically opened by the gravitational drop of a valve element at the time of an abnormality. The vessel 12 is made smaller than an actual machine in diameter due to its smaller output and made equal to the actual machine in height, the height of the water surface 34 of the cooling water can be made equal, and the natural circulation of the cooling water can be correctly simulated without correcting the data.