In this work, a case of loss of heat removal, caused by a loss of electrical supply, is analyzed for Atucha II PHWR. This reactor is similar to a PWR reactor in many aspects, but differs because it has, inside the pressure vessel, vertical coolant channels inside a moderator tank, which is at primary system pressure but a lower temperature. During the studied event, moderator temperature rises, heated by primary system (acting as a heat sink), until the temperatures are equalized. Given that primary and moderator systems are interconnected, the expansion of the moderator produces a rise in the level in pressurizer. The main aspects of the plant dynamic in the presented case are shown and analyzed. The normal strategy of the plant to cope with this initiating event is also discussed: the systems involved, operational modes and trigger signals are described. Then, three possible sequences are studied: heat removal by steam generators or moderator heat exchangers, and black out (no heat removal). The safety functions required to cope with this initiating event are also determined, as long as the associated safety and auxiliary systems. Representative variables of selected sequences are shown, in order to go further in understanding of the plant behavior. The different final stages are identified. The minimum capability of each system, necessary to achieve a plant safe condition, is also established. It was found that in this particular event, their requirements are less than the ones specified in the design basis. Also the influence of moderator system thermal inertia on transients is determined as a distinctive characteristics of this reactor.

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