Nuclear Technology / Volume 193 / Number 2 / February 2016 / Pages 247-258
Technical Paper / dx.doi.org/10.13182/NT14-118
When an accident occurs, operators in nuclear power plants (NPPs) must follow emergency operating procedures (EOPs) or severe accident management guidelines (SAMGs). However, EOPs and SAMGs are symptom-based procedures and guidelines to cope with severe transients and accidents. Operators depend on real-time operating parameters of NPPs to perform each action in EOPs or SAMGs. When a beyond-design-basis accident like the Fukushima Daiichi accident of 2011 occurs, EOPs or SAMGs cannot be performed effectively without adequate information. One lesson learned from the Fukushima accident is that such a situation requires advance preparation regarding the key indicators, the water supply, reactor pressure vessel (RPV) depressurization, and containment venting strategies so actions can be performed with limited manpower and time. After the Fukushima accident, Taiwan Power Company established ultimate response guidelines (URGs) and has implemented them in three operating NPPs. An URG is an event-based guideline developed to manage accidents caused by a compound disaster beyond the design basis. The purpose of this study is to find out the differences of RPV depressurization strategies between EOPs and URGs and to discuss the effect of different RPV depressurization strategies on fuel integrity. The plant responses and accident physical phenomena are simulated using MAAP5. The results show that the RPV water level should be maintained as high as possible and the RPV pressure should be controlled sufficiently low at the beginning of RPV emergency depressurization to avoid core uncovery and assure fuel integrity. The URG provides the better RPV depressurization strategy to respond to a beyond-design-basis accident and mitigate an anticipated severe accident consequence as early as possible.