Probabilistic safety analysis

Probabilistic safety analysis provides a comprehensive, structured approach to identifying accident scenarios and deriving numerical estimates of risks. PSAs for nuclear power plants are normally performed at three levels as follows  

Level 1 PSA, which identifies the sequence of events that can lead to core damage, estimates the core damage frequency and provides insights into the strengths and weaknesses of the safety systems and procedures provided to prevent core damage. 

Level 2 PSA, which identifies ways in which radioactive releases from the plant can occur and estimates their magnitude and frequency. This analysis provides additional insights into the relative importance of accident prevention and mitigation measures such as the use of a reactor containment. 

Level 3 PSA, which estimates public health and other societal risks such as the contamination of land or food. 

Level 1 PSAs have now been carried out for most nuclear power plants worldwide. However, in recent years, the emerging standard is for Level 2 PSAs to be carried out for many types of nuclear power plants. To date, relatively few Level 3 PSAs have been carried out. 

Although in many eountries it is not eompulsory to perform a probabihstie safety analysis (PSA), in practiee it has beeome common practice for new plants and for existing ones. Moreover, international requirements include that safety analysis reports include a summary of the PSA study of the plant. A PSA is a complete and well-structured method for identifying accident scenarios and to obtain numerical risk estimates. 

The question of whether PSAs, or probability risk assessments (PRAs) can be used to demonstrate the compliance with numerical safety criteria has been debated at length. It is now believed that their use is not advisable because of the uncertainties in methods, in data and, therefore, in their results. 

However, all those who experienced the probabilistic method, are convinced, at least, of the following positive aspects of it  

It is common for a probabilistic safety analysis to detect weak points of the plant where the normal 

The present trend for the support of plant safety decisions, including those concerning operation, involves both safety analyses the deterministic and the probabilistic. 

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Baranowsky, P.W., A.M, Kolaczkowski, and M.A. Fedele, A Probabilistic Safety Analysis of DC Power Supply Requirements for Nuclear Power Plants, April 1981. 

S15 Papazoglou, I. A. et al., Probabilistic Safety Analysis Procedures Guide, BNL, January 1984,... 

Paptizoglou, I.A., et al, 1990a, Probabilistic Safety Analysis of an Ammonia Storage Plant," PSAM-I, Beverly Hills California, February 4-7. 

It would be quite appropriate to draw here an analogy with the generally accepted approach to safety assessment of nuclear power plants where, along with the probabilistic safety analysis, each NPP is calculated for the maximum possible (i.e. beyond the design basis) accident in order to obtain as conservative assessment as possible. 

A probabilistic safety analysis has been conducted at the Research Center Jiilich for the process heat variant of the German modular HTGR with the purpose to identify differences compared with the electricity generating HTR-MODUL [55]. The process heat HTR-MODUL consists of a pebble bed core with 360,(KX) spherical fuel elements to produce a thermal power of 170 MW. Helium coolant gas inlet / outlet temperatures are 300 and 950 °C, respectively. The system pressure is 4 MPa. The connection to the secondary circuit is given by a He / He intermediate heat exchanger. 

WOLTERS, J. (Ed.), Probabilistic Safety Analysis and Assessment on Possible Urban Siting of the Modular HTR for Process Heat Application, Internal Report KFA-ISR-IB-3/90, Research Center Julich (1990). 

In the nuclear field grading is tied to the consequences of the failure of a system in most of the national and international categorization/classification models An attempt to develop a standard where the probability of occurrence of a failure is also included, i.e. a risk based categorization, was made in lEC. This resulted, however, not in a standard but in a report [10] which contains four examples for a categorization using probabilistic safety analysis PSA. 

Optimum design solutions in the creation of multi-barrier systems for longterm storage and final disposal of radioactive materials should be chosen based on probabilistic safety analysis for each viable option, supplementing the latter with the cost estimates for the implementation of the option, i.e., using the complex criterion risk-benefit. ... 

Guidelines for the probabilistic safety analysis have been issued recently. In the guidelines, the analysis of initiating events in other operational states than full power is recommended if essential contributions to the total core damage frequency are to be expected. Efforts have therefore concentrated on limited shutdown analysis for a typical PWR and BWR. A low power and shutdown analysis for one PWR has been completed, one for a BWR will be finished soon and a third, major PWR analysis has just been started. 

If the investigation ends here we speak of a probabilistic safety analysis (PSA). Its main objective is to identify possibly existing weak points in the plant design and to show how they are efficiently removed. 

As already mentioned, the scope and depth of the analyses may differ. If only the left-hand side of the bow-tie diagram is treated, we are dealing with a probabilistic safety analysis. Its results are the expected frequencies of undesired events. The objectives then are to identify weak points and imbalances in the engineered safety systems as weU as to indicate ways for eliminating them. This is the most work-intensive part of a risk analysis. 

Mosleh, A., 1993. Procedure for Analysis of Common-cause Failures in Probabilistic Safety Analysis, NUREG/CR-5801. US Nuclear Regulatory Commission. 

The SARDB consists ofhundreds ofcalculationresults for significant core damage scenarios by using integrated severe accident analysis codes, for example, MAAP (FAl, 1994) or MELCOR (USNRC, 1990). It includes most of the high frequency accident sequences which are selected from the plant damage state event tree of a level 2 probabilistic safety analysis... 

YVL-2.8, 2003, Probabilistic safety analysis in safety management of nuclear power plants, STUK, Guide. 

CPQRA is a probabilistic methodology that is based on the NUREG procedures. Procedures for probabihstic risk analysis (PRA) have been defined in the PRA Procedures Guide (NUREG, 1983) and the Probabilistic Safety Analysis Procedures Guide (NUREG, 1985). 

Usually the Probabilistic Safety Analysis is performed for a varying extent of initiating events (internal and external initiators). In order to assess for example the impact of external initiators (explosion e.g.), the frequency of occurrence has to be considered. Usually methods like Fault Tree Analysis (FTA), Event Tree Analysis (ETA) and Monte Carlo Simulation (MCS) are applied and combined forthe purpose of evaluating the frequency and probability of initiating events. 

BfS 2005. Safety Codes and Guides - Translations. Safety Review for Nuclear Power Plants pursuant to 19a of the Atomic Energy Act - Guide Probabilistic Safety Analysis. Bundesarat fur Strahlenschutz, Stand August 2005. 

Sets and Boolean algebra laws play an important role in probabilistic safety analysis, and both are presented here [10]. 

Probabilistic safety analysis (PSA) can be used in the design phase to confirm the appropriate classification of structures, systems and components. 

Use of the best estimate codes as recommended for both deterministic and probabilistic safety analysis should be complemented by sensitivity studies and/or by uncertainty analysis. 

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