Abstract
Ultimate Response Guidelines (URGs) was proposed by Taiwan Power Company to mitigate the so called “Fukushima type accident” of nuclear power plants. As specified in URGs, if it is necessary, operators will depressurize the reactor coolant system to bring in the low pressure water. Decay heat is then removed via containment venting. These collective actions are called DIVing (depressurizing, injecting, and venting) in URGs. The actions are designed to avoid large scale evacuation of public around the damaged plant during the accident. In the present study, the effectiveness of URGs to prevent the large release of radionuclide to the environment is assessed using RELAP5 and MAAP5 codes. The plant analyzed is Lungmen Nuclear Power Station. The purpose of the study is to identify the acceptable initial state for operators to carry out depressurization actions. The success criterion is the peaking cladding temperature is kept below 1500 ℉ during the transient. It can be noticed that the peak cladding reached depends on the timing of emergency depressurization, the flow rate of low pressure injection, water level and pressure upon the initiation of emergency depressurization. The results provide guidance for operators when the depresurization actions of URGs are executed. Based on the results of RELAP5 simulation, a simplified computer program is developed to help opeator identifying the proper conditions of depressurization.