Analysis of safety significant events at the Trino PWR after plant modifications: Inadvertent discharge of emergency core cooling accumulators into the reactor water storage tank

In 1987, an audit was performed on the technical and organizational aspects of the operating experience at the Trino PWR plant. Abnormal events which occurred during 1985-1987, after plant restart following a long shutdown for the installation of new safeguards, were reviewed with the purpose of assessing the performance of the implemented systems and the effectiveness of the periodic surveillance programme. A methodology of analysis similar to the one used in IAEA Assessment of Safety Significant Events Team (ASSET) missions was adopted. Among the events selected for in-depth analysis, one was particularly significant in terms of (1) recurrence; (2) interaction between systems; (3) potential safety significance; (4) possible contra-indications to plant modifications. The event caused the discharge of the emergency accumulators into the reactor water storage tank (RWST), as is described in the following. On 3 August 1985, the plant was at 78% rated power when an automatic scram occurred because of a primary pump trip. It was ascertained that the emergency accumulators can discharge whenever the high pressure injection systems are actuated. The extent of the discharge depends mainly on the duration of operation of the injection pumps. As a short term corrective action, the actuation logic of the bypass valves was modified so that they would normally be shut and would open after pump startup. Furthermore, the pertinent emergency procedures were revised with the aim of improving operator actions and to reduce the diagnostic time. Long term measures now under evaluation will include further plant modifications, most likely the installation of a heat exchanger to cool the injection pumps instead of the minimum flow lines currently installed. 3 figs