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CHAPTER 1
Shutdown Probabilistic Safety Assessment
Marko Čepin
Faculty of Electrical Engineering,
University of Ljubljana,
Tržaška cesta 25,
1000 Ljubljana,
Slovenia[email protected] Probabilistic safety assessment (PSA) is the standard method for assessing and improving safety of nuclear power plants. Shutdown PSA is a version that has been developed relatively later compared to that associated with the assessment of plant power operation. Many common features and some differences exist between power PSA and shutdown PSA. The main PSA methods are presented jointly with the procedures for their application in order to provide a reference for successful realization of shutdown PSA. In addition, simple examples are given to facilitate the understanding of methods and models. Examples of results are shown to highlight what can be expected based on application of the methods. The results show a notable variability connected with the related variability of time durations of specific plant operating states, which affect the related risks, which in turn impact the accident frequency calculation and calculation of importance factors for specific equipment. Consequently, it is more difficult to identify the entities for safety improvements.
1.1.Introduction
Probabilistic safety assessment (PSA) is the standard method for assessing risk associated with nuclear power plants (NPPs) for the purpose of improving their safety [1–9]. The PSA may be understood as a risk analysis, including the assessment of accident frequencies and the estimation of the dispersion of radioactive substances in the environment according to the weather conditions and environmental parameters. The term “probabilistic risk analysis” (PRA) is used sometimes to indicate the same process in some other communities. The main difference between both the terms is psychological. Although one is dealing with the same methods and referring to safety analysis or risk analysis, the term “safety analysis” has somehow a more positive meaning. The term PSA is used here for the safety assessment of the NPPs, except in cases where references and their names with use of PRA are mentioned.
Assessment of safety of NPPs deals with three basic questions [4–9]: What can go wrong? How likely is it? What are the consequences? These questions follow from the definition of risk, which mathematically means the product of probability of an event and the consequences of this event, i.e.,
where Risk(i) is the risk due to event i, P(i) the probability of occurrence of the event i and C(i) the consequences of the occurrence of the event i.
Risk can be additive for the mutually exclusive events. Risk assessment for the same consequences is directly driven by the probabilities of undesired events (see Eq. (1.1)).
Safety is often used as a relative term meaning that a risk is low. Safety can be defined as the ability of an entity under given conditions to not cause undesired events. Our confidence of a higher degree of safety is related to a lower degree of risk. Considering that the probability of an event is the limit of its relative frequency in a large number of trials, the risk of an event can be expressed as a product of its frequency and its consequences.
The methods discussed in this paper are limited to accident frequency assessment in terms of the status of the reactor core. The processes which may follow after the core damage are out of the scope of this work. Consequently, the analysis of dispersion of the radioactive substances in the environment is not under consideration. Similarly, the analyses connected with spent fuel pit or with dry spent fuel casks are out of the scope of this work, although the same methods and procedures are applicable.
The objectives and the state-of-the-art are presented in Sections 1.1.1 and 1.1.2, respectively. State-of-the-art presentation briefly lists and groups the most important previous studies. Section 1.2 describes the overall method of shutdown PSA and the related procedural steps that are needed for the practical application of the method. Specific methods related to specific steps are presented mathematically and practically. Section 1.3 presents selected examples of results. Section 1.4 gives the concluding remarks.
1.1.1.Objective
The objective of this paper is to present shutdown PSA in terms of method, procedure steps and application examples. The examples are used in order to highlight the theory. The main focus is placed on the consideration of internal events in the NPP. However, the method and the procedure steps are broadly applicable and may be used in general terms for internal hazards, which, in addition to internal events, also include internal fires and internal flood. Similarly, the method and the procedure steps can also be used for external hazards such as seismic events, high winds, external floods and other external events if those steps and models of PSA are performed on the same platform as models of internal events.
1.1.2.State-of-the-Art
Activities started approximately 50 years ago with assessing possibilities and consequences of accidents in NPPs [1]. The risks related with building of 100 NPPs were assessed [2]. The first detailed report emphasized the importance of specific initiating events [3]. Guidelines for PSA were issued later [4–8], and many applications followed the regulatory requirements for performing PSA [10–21]. Standards regarding all the necessary features, which need to be included in the PSA, have been issued after important efforts in this field had been joined under the term risk-informed application of the PSA [22, 23]. In addition, guidelines about specific methods included in PSA are available, such as the fault tree method and its applications [25–28], event tree method and its applications [29, 30], human reliability analysis methods and their applications [31–41], common cause failure (CCF) methods and their applications [42–45] and risk informed applications of the results [15–21, 51]. Researchers are also developing new and improved methods, and the number of fields of their applications is increasing in many industries, engineering fields and other sciences, subject to good data collection and analysis [52–59]. In parallel, activities for shutdown PSA have been initiated [46–50]. A separate standard on shutdown PSA has been issued after being in draft version for years [24].
1.2.Shutdown PSA Methods
During NPP shutdown, the plant systems operate primarily to maintain the core and the spent fuel cooling, to keep the fuel subcritical and to ensure containment integrity. The residual heat removal system (RHRS) is used for decay heat removal during cold shutdown and refueling states. This safety function is critical to shutdown safety. In general, the primary safety functions during the cold shutdown and refueling are the following:
—Decay heat removal via RHRS.
—Reactor coolant system inventory control by keeping the reactor coolant system inventory at a level sufficient to sustain core cooling via residual heat removal.
—Reactivity control by keeping the shutdown subcritical conditions through boration of the reactor coolant system and having control rods fully inserted into reactor core.
—Reactor coolant system pressure and temperature control by keeping the reactor coolant system pressure and temperature within acceptable limits to enable continuous RHRS operation and to prevent reactor coolant system boiling. This control is provided by reactor coolant system makeup...
