One of the key elements in probabilistic risk assessment is identification and characterization of uncertainties. This paper aims to suggest a procedure to identify influencing factors for uncertainty in source term evaluation, which are important to risk of public dose. We propose the following five steps for the identification: (1) derivation of variable parameters by expansion of dynamic equation for the system and scenario to be investigated, (2) extraction of uncertainties in variable factors, (3) selection of influencing factors for uncertainty among variable parameters by analysis of existing knowledge and sensitivity study, (4) identification of influencing factors for uncertainty using expert judgement, and (5) integration of selected factors in the aforementioned steps and determination of input parameters in the uncertainty evaluation of the source term evaluation tools relevant to selected factors. The approach is applied to select factors for a risk dominant accident scenario in direct cycle High Temperature Gas-cooled Reactor (HTGR) plant. As a first step, this approach is tested to evaluation of fuel temperature in a depressurized loss-of-forced circulation (DLOFC) accident and failure of mitigation systems such as control rod systems in a representative HTGR plant from the view point of reactor dynamics and thermal hydraulic characteristics. The procedures (1) and (2) are trial to investigate in this paper. As a result, the variable parameters and the relevant uncertainties are successfully extracted in accordance with the suggested procedure. The result will be used for determination of input parameter and uncertainty distribution of these parameters in the uncertainty analysis of source term evaluation.

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