Decrepit nuclear power plants

Around 20 units reaching more than 30 years had been granted a 10-year license extension. Among them, Tsuruga unit 1, Mihama unit 1, and Fukushima I-1 (destroyed in the 2011 reactor accident) reached their 40-year mark, were licensed for another 10-year extension. Primary concern with regard to age related degradation was reactor pressure vessel subject to irradiation embrittlement that could lead to loss of fracture toughness. Ductile-brittle transition temperature test results using surveillance specimen showed seven units of 5 PWRs and 2 BWRs with the transition temperature greater than 50degC. Worst data was 98degC for that of Genkai unit 1 and 40degC plus scattered from prediction curve correlated transition temperature shift versus neutron fluences, that corresponded to 66 equivalent operation years. The author submitted public comment on prediction curve in 2010 addendum and insisted need of reevaluation of prediction curve applicability and choice of reactor decommissioning in case of unexpected data scatter of transition temperature. (T. Tanaka)