Quantitative Risk Analysis of Hydrogen Events at WTP: Part 1 of 2 — The Operational Frequency Analysis Model

Abstract

A Quantitative Risk Analysis (QRA) model has been developed to determine the frequency and severity (potential combustion loads) of postulated hydrogen event types in piping systems and proposed as a design-informing tool for the U.S. Department of Energy’s (DOE) Hanford Tank Waste Treatment and Immobilization Plant (WTP) at the Hanford Site near Richland, Washington. Specifically, the QRA provides a systematic, comprehensive methodology for assessing hydrogen events, including deflagrations, detonations, and deflagration-to-detonation transition (DDT) types in piping systems containing legacy nuclear waste streams being processed for vitrification. The events considered include normal operations as well as postulated upset conditions as a result of internal and external accidents. The QRA approach incorporates three sequential phases, including Operational Frequency Analysis (OFA), Gas Pocket Formation (GPF) and Event Progression Logic (EPL) models in the form of an integrated logic framework. The WTP piping design will be evaluated on a specific piping route basis using a probabilistic sampling approach, with the QRA providing the quantitative dynamic loads for evaluation according to the frequency and type of hydrogen event. The OFA is based on an industry standard fault tree computer model, CAFTA, and analyzes the frequency of combustible gas pocket formation in a piping system from three primary sources: (1) normal operations; (2) piping system-specific upset conditions affecting transfer operations; and (3) plant-wide initiating events such as fire and seismic accidents. A second output from the OFA is duration time for each event, quantifying the length of time that a gas pocket is likely to develop before the initiating event is terminated, with the information provided directly to QRA event tree models for assessing gas pocket growth. A team of safety, operations and engineering, developed the underlying logic of the fault tree model with the overall modeling approach following applicable nuclear/chemical industry guidance and standards for performing QRA applications. Primary inputs to the OFA module are initiating event, equipment reliability, and human/operator error data and their characteristic distributions, and are drawn from Hanford Site safety documentation, government and commercial sector sources, and related nuclear/chemical industry experience. This paper discusses the overall OFA module, its inputs, the outputs to the GPF and EPL modules, the relative importance of different initiating event conditions, key insights obtained to date, upcoming supporting uncertainty/sensitivity analyses, and summarizes technical peer review assessments.