2015 年 81 巻 827 号 p. 15-00117
A cooling method is proposed of residual power released after the shutdown of nucleate reactors even when all electricity becomes unavailable. The cooling source is a water reservoir that the initial level is 20 m high from the sea. Only turbine driven pumps are used for circulation of coolant in the reactor. The high-pressure injection system is used to remove residual power directly from the reactor vessel, then cooled with the heat exchanger in the residual heat removal system, to which a turbine-driven pump in the reactor core isolation cooling system supplies water from the suppression pool. The flow system of cooling water consists of feeding pipe, tubes in the heat exchanger and an orifice that controls flow rate. The simulation reveals that the temperature of the suppression pool has its maximum at about 50 hours from the shutdown, though the flow rate is steadily decreasing. It is results from the decrease of residual power. The temperature increases again in the last phase of cooling because the flow rate of cooling water diminishes. The delay of the start of cooling contributes to make the cooling water exit temperature cooler at the last phase of cooling. Two restrictions were applied to minimize the area of the reservoir. One is that the temperature of the suppression pool does not exceed designed temperature of the containment vessel. The other is that the cooling water exit temperature is less than 60°C. The minimized area is 3094 m2, a possible area to build. This cooling system is considered very important for reactors located on seashore, where tsunami attack may destroy the residual heat removal sea water system.
