Safety systems SS

ISSUE TITLE ECCS sump screen adequacy (SS 1) 

The containment is equipped with sumps to collect the water of the primary circuit and of the ECCS water storage tank after a LOCA in order to recirculate the water in the second phase of the accident. The openings of the containment bottom plate are covered with a screen which is intended to prevent debris penetration to the suction of the ECCS and containment spray system. 

The insufficient design of thermal insulation of equipment and pipelines inside the containment can, under LOCA conditions, lead to a common mode failure by clogging the sump screens and/or the ECCS heat exchangers with a high risk of losing ECCS recirculation. In this situation, the function can be questionable (or disabled in extreme situations) for scenarios within the DB envelope. 

The ECCS function can also be affected by inadequately screened debris. 

At the Kozloduy NPP, tests have been performed using the spray system. For these tests, particles and debris were carried by the system and reached the filtering system. The results of these tests have already led to a change in the design of the horizontal device at the entrance of the sump filtration system for avoiding their blockage. 

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ISSUE TITLE Reliability of the motor-operated valves in safety systems (SS 16)... 

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