Emergency core cooling system

Purpose: To protect heavy water in primary coolant circuits against degradation in heavy water moderated pressure tube reactors upon issue of erroneous failure signals. Method: Heavy water in a calandria tank is injected into primary coolant circuits from the time of generating a signal for loss of coolant accidents in the primary coolant circuits to a time where the amount of the moderators is decreased to a certain level. Then, the water-take valve is automatically switched to inject light water in the condensate storage tank. Accordingly, if the accident signal is judged to be error, the operation of the ECCS system can be interrupted by the judgement of an operator. If the interruption can be made before the switching of heavy water to light water, dilution of heavy water in the primary coolants with light water can be prevented. (Ikeda, J.)