Research On Nuclear Power Plant Seismic Risk Assessment Method

Nuclear power is proved to be clean, efficient, economical and steady,which contributes a lot to the settling of energy and environment problem.Nuclear power develops rapidly now in China despite the potential risk of nuclear fuel’s strong radioactivity,decay heat,high temperature and high pressure water and some other characters. So the security issue is particularly important.Although full consideration about the capability against natural disasters has been taken into the design of nuclear power plant and measures have been taken to prevent and mitigate beyond design basis accidents,beyond design basis earthquake would imperil the redundancy associated with nuclear facilities and the design concept of defence in depth because of the common cause failure,which threatens the nuclear power plant seismic design basis. As a result,seismic risk assessment of nuclear power plants is necessary.In this paper, it summarizes current research on nuclear power plant seismic risk assessment domestic and overseas through related reports, conferences and other literature.lt illustrates the main systems and corresponding functions of nuclear island and conventional island of Pressure Water Reactor nuclear power plant.On the basis of plant familiarization^proposes the nuclear power plant earthquake catastrophe system and risk assessment process as well as relationship between them,enlightened by dam disaster system and associated concepts of natural disaster risk assessment.The article focuses on event sequence simulation and analysis in nuclear power plant seismic risk assessment process. Firstly, it determines the safety function, which the power plant needs to maintain when an earthquake occurs. Afterwards, the systems to perform the relevant functions are identified. Then it identifies possible initiating events.After that, it uses event tree to simulate event suquences and proposes extra factors due to earthquake when establishing event trees.In addition, simplifying and modifying methods are put forward.System analysis issues and difficulities due to system dependency involved in the nuclear power plant seismic event trees are settled by fault tree analysis.The paper lists additional factors under the influence of earthquake as an external event in the fault tree model. In consideration of intensive calculation and large memory of complex fault trees of nuclear power plant, it proposes a preprocessing of simplification and modularization.After the static and dynamic modulars are identified,they are respectively analysed using binary decision diagram and Markov chain.In the binary decision diagram analysis,a direct generation method is used to avoid the space explosion due to variable ordering.Then an example of nuclear power plant pressure tank system is used to illustrate the fault tree construction and analysis process.In the end,the article proposes corresponding methods of combining fault trees and event trees for different relationship types among systems,which completes nuclear power plant event sequence simulation and analysis procedure under earthquake.