Session: 15-05-01: Models and Methods for Probabilistic Risk Assessment

Paper Number: 166955

Simulation of SBO Accident Progression and Core Melt in a Four-Loop PWR Using MAAP 

The increasing emphasis on nuclear safety and risk-informed decision-making has driven extensive research into severe accident management strategies for pressurized water reactors (PWRs). Station blackout (SBO) scenarios, where a total loss of offsite and onsite power occurs, pose significant challenges to plant safety and require a thorough understanding of accident progression and potential radiological consequences.This paper presents a detailed analysis of severe accident management measures during a total station blackout (SBO) scenario in a Zion-like 4-loop Westinghouse pressurized water reactor (PWR). The study utilizes the Modular Accident Analysis Program (MAAP) code to model the plant and quantify the source terms for representative accident sequences identified in the Level 2 Probabilistic Risk Assessment (PRA) study. The characteristics of the source terms depend on the plant's design details, containment features, and the progression of the accident scenario. A risk-informed approach is adopted, utilizing simplified containment event trees and large Accident Progression Event Trees (APET) from the NUREG-1150 study to assess containment response and potential releases. This method integrates key accident sequences and evaluates the effectiveness of potential improvements in severe accident management strategies for station blackout scenarios. Specifically, the analysis categorizes potential containment failure modes into five distinct source term release categories: containment bypass, containment isolation failure, early containment failure, late containment failure, and intact containment. The development of a containment event tree model for the Zion-like PWR considers various representative cases, qualifying a structured analysis of containment performance under SBO conditions. Selected questions from the APET-based methodology, that involves 72 critical questions capturing key uncertainties in accident progression, containment integrity, and radionuclide release, are specifically related to station blackout (SBO) scenarios. These selected questions are focused on the most relevant aspects of SBO, such as the availability of containment safety features , the timing and effectiveness of cooling systems,control of the primary pressure, and the behavior of radionuclide releases under SBO conditions.The integration of these analytical methods aims to improve the understanding of severe accident progression in SBO scenarios and contribute to the development of risk-informed strategies for accident management. The findings of this research offer valuable insights into potential safety improvements, informing the optimization of containment protection measures, emergency response strategies, and mitigation techniques to reduce the consequences of SBO-induced severe accidents in pressurized water reactors. By integrating both deterministic and probabilistic techniques, this study aims to enhance the understanding of source term behavior and guide future risk-informed safety improvements

Presenting Author: Yahya Alzahrani North Carolina State University

Presenting Author Biography: Yahya is a Ph.D. candidate in Nuclear Engineering, specializing in advanced reactor design, thermo-fluid analysis, and integrated dynamic probabilistic risk assessment for nuclear power plants. He earned his B.Sc. in Nuclear Engineering from Oregon State University (2017) and his M.Sc. in Nuclear Engineering from King Abdulaziz University (2021). His research background includes neutronics analysis, computational methods in nuclear corrosion, and small modular reactors (SMRs).

Authors:

Yahya A. Alzahrani North Carolina State University
Akram S. Batikh NC State University
Mihai A. Diaconeasa NC State University