Analysis of the sequence of events causing the failure to close the primary isolating valve in the emergency and residual heat removal system of KWB A on December 17, 1987

During a brief outage in December 1987, repair work was carried out in the secondary system of unit A of the Biblis nuclear power station (KWB A). The afterheat of the reactor core was removed by the emergency and residual heat removal system, as is required in this type of plant outage. When the plant is restarted, the emergency and residual heat removal system is shut off. In this case, the closed position of a primary isolating valve was not indicated; this failure was signaled on the computer, but its significance was not appreciated. In the reports in the media roughly one year later, the event, which had been duly reported to the competent authorities and had been investigated, was treated like a near-disaster. In the final analysis presented in this report, probabilistic methods were applied of the type underlying also the assessments and investigations in Phase B of the German Risk Study. (orig.)