Pressure tube type heavy water reactor

A water injection source disposed at the outside of a reactor container and a water injection pump independent of existent safety facilities are connected by way of water injection pipeline with a valve. Water in a purified water storage tank and a filtered water tank of existent reactor facilities is provided as water injection sources. A fire extinguishing pump can be used as a water injection pump in the same manner. If such a severe accident that all of safety systems are not operated should occur, nuclear fuels in pressure tube assemblies are melted and the fuels are flown out to the calandria tank filled with heavy water. In this case, the valve is opened by detecting the lowering of the water level of a steam drum caused by the reduction of the reactor coolants and water is injected to the calandria tank. This can cool the fuels flown out to the calandria, and the accident is ceased with no discharge of the molten fuels. (I.N.)