SAFETY REVIEW FOLLOWING INSIGHTS INTO A MAJOR EVENT AT BAERSEBAECK 2 A Swedish example-course of action

摘要

When Baersebaeck 2 was restarted after the annual outage on 28 July 1992, a safety relief valve opened inadvertently at less than 2% power and 3.0 MPa reactor pressure. This was caused by a leaking pilot valve which had been assembled incorrectly. The leaking pilot valve caused depressurization of the main valve, which then opened. When the main valve opened, the reactor pressure acted on the rupture disc in this valve. When the reactor pressure reached 3 MPa, the rupture disc broke and a jet of steam was released into the containment. The containment was isolated and the containment vessel spray system (CVSS) and the emergency core cooling system were automatically started. The steam jet caused mineral insulation material to be dislodged from the pipes situated close to the safety relief valve. Insulation material was flying about in the containment. The amount of dislodged insulation material was about 200 kg; approximately 100 kg of this material was flushed down into the condensation pool by steam flow and by water flow from the CVSS. This incident did not jeopardize the safety of the plant, but further investigation and remediation were required because of the potential of the incident. In order to restore the safety of nuclear plants with a similar design, five of the Swedish plants were forced by the Swedish Nuclear Power Inspectorate (SKI) to shut down. The conditions for resuming plant operation were that temporary modifications should be made and that the safety function analysis in probabilistic terms should be shown to be equivalent to the original safety analysis. Four of the plants were able to restart approximately six months after the event. At Oskarshamn 1, other safety deficiencies were found during the outage; these required a longer outage period in order to restore the level of safety commonly assumed to be necessary at that time. In the modification and control project for Oskarshamn 1, called FENIX, the first step, FENIX Restart, aimed at performing the modifications required to obtain permission for continuation of plant operation. In order to meet long term demands for safety, the FENIX project also initiated a modernization programme, plans for which had been made in parallel with the implementation of the FENIX Restart programme. This modernization will be performed in several steps during the yearly outages in the period 1996-1999. The deficiencies found at Oskarshamn 1 forced the utility to perform a much larger inspection, control and modification programme than had initially been anticipated. SKI requested that, before restart of the plant, a complementary safety report should be submitted to it, with an overall evaluation of plant safety. Plant modifications made at Barseback 2 within the FENIX project are described in the paper. When the FENIX project reached its major milestone-restart of Oskarshamn 1, it had demonstrated that the plant was in a better state than had been evaluated in 1992, before the event at Barseback. However,