Review of recent events and regulatory guidances concerning emergency core cooling system strainer blockage at boiling water reactors in the U.S.A

On July 28, 1992, an event occurred at Barsebaeck Unit 2, a Swedish BWR, which involved clogging of emergency core cooling system(ECCS) strainers resulting from the insulation material dislodged by steam from a safety relief valve that spuriously opened. The issue of the ECCS strainer clogging was recognized as important to safety in the late 1970's and potential for loss of emergency recirculation cooling due to clogging of containment sump screens at PWRs and suppression pool suction strainers at BWRs was identified as Unresolved Safety Issue(USI) A-43 in the U.S.A. A number of experimental and analytical studies had been performed before Regulatory Guide(RG) 1.82, 'Water Sources for Long-Term Recirculation Cooling Following a Loss-of-Coolant Accident', was revised in 1985(Revision 1). Since similar events occurred at U.S. BWRs after the Barsebaech-2 event, however, the U.S. Nuclear Regulatory Commission(NRC) recognized the safety significance of BWR ECCS strainer clogging and issued regulatory guidance documents to alert or request addressees to take appropriate actions. In 1996, the NRC revised RG-1.82 again to alter the evaluation guidance for suction strainer blockage at BWRs(Revision 2). The review of recent events indicates that the strainer blockage would be caused by debris generated by a LOCA such as insulation debris, that generated during normal plant operation such as corrosion products, that stemming from fibrous filters and labeling materials installed in the containment, that stemming from maintenance-type materials such as tools and plastic bags, etc. In addition to the strainer design changes, therefore, preventive or mitigative actions to be taken. This report provides brief descriptions for the Barsebaeck-2 event and the subsequent ones at U.S. BWRs, describes the NRC's responses, and summarizes the revised RG-1.82. Also, the contents of the NRC's guidance documents issued and the RG-1.82 Revision 2 are given. (J.P.N.)