Fracture Probability Assessment for Embrittled Reactor Pressure Vessels Under Ultimate Response Guideline Operation

Abstract

After the Fukushima nuclear accident occurred in Japan on March 11, 2011, the compound disaster beyond design basis which may severely impact the nuclear safety has been noticed and paid much more attention. In addition to the original emergency operating procedures (EOPs) and severe accident management guidance (SAMG) of nuclear power plant, the licensee in Taiwan developed the ultimate response guideline (URG) when EOPs and SAMG cannot be performed effectively due to loss of power and water supply by the Fukushima-like compound disaster. Once the URG procedures are initiated, the operators will conduct reactor depressurization, low pressure water injection and containment venting to strictly prevent the core damage and the release of radioactive material. In the paper, the fracture probabilities of boiling water reactor (BWR) pressure vessels with incremental levels of radiation embrittlement under URG operation are evaluated by probabilistic fracture mechanics (PFM) analysis. First, the models of PFM FAVOR code concerning the beltline shell welds of reactor pressure vessels (RPVs) associated with a very conservative flaw distribution are built. Then, the hypothetical transients of URG operation obtained from the thermal hydraulic analyses for Taiwan domestic BWRs are applied as the loading condition. The analysis results demonstrate that performing URG operation will not cause significant fracture probability for RPV, even at an extremely embrittled condition. The URG procedures can ensure the prevention of core damage as well as maintenance of structural integrity of RPV in the situation of long-term loss of electric power when suffering from the Fukushima-like accidents.