Contribution from Sweden - Undetected failures in safety systems Swedish contribution to the PWG1 of the OECD Nuclear Energy Agency - July 1996

The present report represents the Swedish contribution to the Nuclear Energy Agency (NEA) project with the same title. This project is done within the activities of the Principal Working Group 1 on operating experience and human factors. The scope of the study has been to review the last five years of operating experience of all Swedish nuclear units with the aim to identify, select and understand why deficiencies in safety systems (components, logic circuitry and structures) remained undetected during long time periods. With long time periods means here latent failure or unavailability time of one year or longer. The scope of the study also encompasses the description of the corrective actions that would preclude or reduce the likelihood of similar events according to the identified causes. The analysis of the Swedish operating experiences has been performed based on the review of on the one hand all Licensee Event Reports (LERs) reported during the last five years and on the other hand of the IRS data base. These experiences exhibit a broad safety spectrum, from safety insignificant events to one event (strainer clogging event of 1992) of world-wide safety implication for most LWRs. The selected events have been chosen with consideration taken to the limitation regarding safety systems and to the classification scheme agreed upon by the project group. Accordingly, the events described in the present report represent a sub-group of a somewhat wider experience base