The Optimized Method and System of Promptly Determining Protective Actions During Multi-reactor Accident of Nuclear Power Plant Based on OIL

In the situation of nuclear accidents of nuclear power plants(NPP), the protective actions should be taken by the public are ascertained by Generic Intervention Level(GIL) and the controls should be placed in food are ascertained by Generic Action Level(GAL). GIL and GAL were not designed to be used during an emergency; they cannot be promptly measured in the field and do not address facility conditions. Operational Intervention Level(OIL) which can easily be measured during an emergency, and on which the need for protective action can be rapidly ascertained, was introduced and described in detail by International Atomic Energy Agency(IAEA). According to the approach of calculation and revision proposed by IAEA, the adaptive OILs are studied and given in each nuclear power plant in China. Further, software system for calculation and revision of OILs and determining protective actions have been also established and applied in these NPPs. After Fukushima nuclear accident, emergency response preparedness for multi-reactor accidents is taken seriously. As the generic assessment procedures for determining protective actions during a reactor accident based on OILs mentioned above are not fully applicable to the situation of multi-reactor accident, it is necessary to optimize the approach of calculation of OILs and determining protective actions for multi-reactor accident. The paper first introduces and summarizes the basic concept and approach of OIL during a reactor accident. In this approach, environmental data are assessed primarily through the use of OILs which are quantities directly measured by the field instruments. The default OILs which have been calculated in advance on the basis of the characteristics of severe reactor accidents are used to assess environmental data and take protective actions until sufficient environmental samples are taken and analyzed to provide a basis for their revision. This approach allows data to be quickly evaluated, and decisions on protective actions to be promptly made. The paper studies the essence of OILs during single reactor accident and presents the method for OILs calculation during multi-reactor accident. The core idea of the method is to evaluate the environmental radioactive consequence of each single reactor accident which can be overlaid together finally during a multi-reactor accident. Using the final environmental radioactive consequence, the suitable OILs can be calculated in the original calculation formula. As the default assumptions are not related to characteristics of reactor and site, the default OIL5 of IAEA is suitable for multi-reactor accident. The default of other OILs (OIL1$1 \sim$OIL4, OIL6$6 \sim$OIL9) can be calculated in this optimized method. The paper also indicates that the approach of revision of OILs during a single reactor accident is also suitable for multi-reactor accident without optimizing. In this method, the procedures and strategies of determining protective actions are also optimized. Different from sector areas based on distance and direction during a reactor accident, regions based on administrative boundaries and natural boundaries are considered as unified units for taking a determined protective action. On the basic of the optimized method, the corresponding software system is also designed and implemented. Suitable for both single-reactor and multi-reactor accident, the software provides complete process for use of OILs which include the function of data management, environmental data input, calculation, revision and decisions on protective actions. The optimized method and system in this paper have been applied to several NPPs in China, e.g. Qinshan, Ningde, Sanmen, Taishan and Yangjiang. In the application, the method has been verified and the system has also been constantly improved.