Improvement of cooling following a non-isolable primary circuit rupture

After a non-isolable primary circuit rupture, the residual reactor heat must be removed over a long period of time while either the fuel is unloaded or the leak repaired. The reliability studies which Electricite de France has been performing since 1980 have shown that, during this period, the probability of a total loss of the safety injection system and the residual heat removal system is high. Since it is essential that this function be assured over a long period, operating procedures have been developed (the so-called H4 and U3 procedures) which have recourse to mobile means of dealing with total failure of the low pressure safety injection pumps and total failure of the containment sprinkler system pumps. The Nuclear Safety and Protection Institute of the Commissariat a l'Energie Atomique has, as part of a probabilistic safety analysis of a 900 MW(e) PWR, carried out probabilistic assessments for a loss of coolant accident. The results obtained show that, in the short term, in the event of a small primary circuit rupture, the risk of a failure of the low pressure safety injection pumps makes a major contribution to the overall core meltdown probability. Thus, one must ask oneself whether the short term redundancy level of the low pressure safety injection system should be increased by the use of additional resources. The quickest way of providing such resources would be to install permanent H4 and U3 equipment. (author). 1 fig., 3 tabs