Uncertainty Analysis for Source Term Evaluation of High Temperature Gas-Cooled Reactor Under Accident Conditions—Identification of Influencing Factors in Loss-of-Forced Circulation Accidents

Abstract

One of the key elements in probabilistic risk assessment is the identification and characterization of uncertainties. This paper suggests a procedure to identify influencing factors for uncertainty in source term evaluation, which are important to risk of public dose. We propose the following six steps for the identification in a systematic manner in terms of completeness and transparency of the results using both a logic diagram based on basic equations and expert opinions: (1) identification of uncertainty factors based on engineering knowledge of accident scenario analysis; (2) derivation of factors at the level of physical phenomena and variable parameters by expansion of dynamic equation for the system and scenario to be investigated, (3) extraction of uncertainties in variable parameters; (4) selection of important factors based on sensitivity study results and engineering knowledge; (5) identification of important factors for uncertainty analysis using expert opinions; and (6) integration of selected factors in the aforementioned steps. The proposed approach is tested with a case study for a risk-dominant accident scenario in direct cycle high-temperature gas-cooled reactor (HTGR) plant. We use this approach for evaluating the fuel temperature in terms of reactor dynamics and thermal hydraulic characteristics during a depressurized loss-of-forced circulation (DLOFC) accident with the failure of mitigation systems such as control rod systems (CRS) in a representative HTGR plan. In total, six important factors and 16 influencing factors were successfully identified by the proposed method in the case study. The selected influencing factors can be used as input parameters in uncertainty propagation analysis.