Elsevier

Annals of Nuclear Energy

Volume 159, 1 September 2021, 108296
Annals of Nuclear Energy

A quantification methodology of Seismic Probabilistic Safety Assessment for nuclear power plant

https://doi.org/10.1016/j.anucene.2021.108296Get rights and content

Highlights

  • We proposed a quantification methodology of Seismic PSA.

  • The proposed quantification methodology can generate minimal cutsets (MCSs) equal to the CDF which is calculated by Monte Carlo Simulation.

  • As a case study, we performed the Seismic induced LOOP accident with the proposed quantification methodology.

  • It is expected that these methods would contribute to developing the Seismic PSA.

Abstract

A Seismic Probabilistic Safety Assessment (SPSA), unlike other internal or external event analyses, has a very high probability of basic events when a strong earthquake occurs. Because of this high probability, the Core Damage Frequency (CDF) might be overestimated by the Rare Events Approximation (REA) and Minimal Cutset Upper Bound (MCUB) method for general quantitative analysis. To compensate for this overestimation, the exact CDF value can be derived with post-processing software such as an Advanced Cutset Upper Bound Estimator (ACUBE) or Fault Tree Evaluation using Monte Carlo simulation (FTeMC). However, the Minimal Cut Sets (MCSs), which has the same result as the calculated CDF, cannot be derived with both methods. In this paper, we propose a Monte Carlo Simulation Allocation Method (MCSAM) to derive the exact CDF and MCSs. We expect that the MCSAM will easily identify the plant's vulnerabilities to review the exact MCSs consistent with CDF results without additional post-processing or Monte Carlo Simulation.

Introduction

A Probabilistic Safety Assessment (PSA), together with deterministic safety analysis, is used to assess the risks associated with operating nuclear power plants. The PSA consists largely of internal and external events including fires, seismic events, and floods in the case of external events. Since the Fukushima nuclear accident caused by the Great East Japan Earthquake in 2011, the importance of Seismic Probability Safety Assessment (SPSA) has been emphasized. Seismic events have different characteristics from other external events, which can cause simultaneous failure in many Structures, Systems and Components (SSCs). Because the probability of seismic-induced failure has a value of close to 1 (total failure), the Core Damage Frequency (CDF) can be overestimated with the conventional methodology. Several solutions to this problem have been proposed, but all of them have methodological limitations. In this paper, we examine the seismic quantification methodologies developed so far and propose a new methodology that can overcome the deficiencies of the existing methodologies. In Chapter 2, we examine the seismic event quantification methodologies currently used and compare their strengths and weaknesses. In Chapters 3 and 4, we explain our proposed methodology, discuss the results of its application, and explain the pros and cons of this new methodology. Finally, in Chapter 5, we summarize our results.

Section snippets

Comparison of recent SPSA methodologies

In this chapter, we examine three seismic quantification methodologies used in SPSA. These methods include the Binary Decision Diagram (BDD), the Rare Events Approximation (REA) and Minimal Cutset Upper Bound (MCUB) method, and the Monte Carlo Simulation method.

Monte Carlo Simulation Allocation method (MCSAM) for SPSA

Our proposed method combines the Monte Carlo Simulation and the REA and MCUB methods described above. The Monte Carlo Simulation method is to compute the exact CDF and MCSs generated by the REA and MCUB method are allocated by suggested Allocation Rules. The MCS (Minimal Cut Set) database through the REA and MCUB method represents a general solution for exact MCSs, and the Monte Carlo Simulation method provides an accurate CDF and the combinations of events that cause core damage. The flow

Case study for a seismic-induced loss of offsite power (LOOP) accident

To verify the proposed quantification method, we developed a simplified model for a seismic-induced Loss of Offsite Power (LOOP) and applied it to this case study. The offsite power system is most vulnerable to earthquakes and the LOOP may happen when an earthquake with seismic acceleration exceeding OBE occurs in most nuclear power plant. In addition, LOOP shows the steps required to achieve a typical safety shutdown procedure during normal power plant operation.

Conclusions

In this work, we proposed a new quantification methodology for SPSAs. MCSAM was proposed to overcome the shortcomings of over-estimating the CDF and the inaccuracy of MCS derivation with existing methods. The characteristics of MCSAM are as follows:

  • Contributes to the accuracy of a Level 2 PSA by deriving the exact probability of each sequence.

  • Identifies the most vulnerable SSCs by deriving the exact CDF and MCSs.

  • Intuitive analysis is possible as the number of MCSs is significantly reduced

CRediT authorship contribution statement

Junghyun Ryu: Methodology, Validation, Formal analysis, Investigation, Visualization, Writing - original draft. Moosung Jae: Conceptualization, Methodology, Validation, Writing - review & editing, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Nuclear Safety Research Program through the Korea Foundation of Nuclear Safety (KOFONS), granted financial resource from the Multi-Unit Risk Research Group, Republic of Korea (No. 1705001).

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