Uncertainties in advanced safety analyses of nuclear facilities

The evaluation of nuclear facilities’ (especially nuclear power plants) performance during accident conditions has been the main issue in thermal-hydraulic safety research worldwide for decades. Advanced best estimate (BE) thermal-hydraulic computer codes have been developed such as ATHLET, CATHARE, RELAP5, and TRACE. With the development of advanced BE thermal-hydraulic computer codes also new advanced safety analysis methods have been proposed like uncertainty and sensitivity analysis, a combination of deterministic and probabilistic approaches, or a combination of thermal-hydraulic and structural analysis. The purpose of the proposed project is advanced safety analysis with uncertainty quantification and demonstration of the methodology to study the influence of different input uncertain parameters representing human actions upon the predicted events evolution, needed in selection process of the reference event scenario. For the advanced analysis, the limiting overcooling event scenario causing thermal load on the reactor pressure vessel (RPV) in a pressurized water reactor (PWR) is proposed as the reference event scenario. The occurrence of thermal loads on the reactor pressure vessel (RPV) under pressurized conditions is generally called pressurized thermal shock (PTS). For simulations of overcooling event scenarios, the advanced TRACE thermal-hydraulic computer code will be used. TRACE input models representing PWR for simulation of overcooling scenarios will be developed. The limiting overcooling scenario will be identified and for it, the uncertainty analysis will be performed. The novel feature is that uncertainties coming from human factors will be considered, which are traditionally not part of code uncertainty evaluation. For example, the time delay of human action could have a very large influence on the transient progression which can change qualitatively. This aspect is important when thermal-hydraulic analyses represent the input for structural and mechanics analyses or any other safety analyses. The main objectives of the proposed project on uncertainties in advanced safety analyses of nuclear facilities are: • Identification of overcooling scenarios with the potential for being PTS challenges; • Independent assessment of TRACE computer code and PWR transient input models development; • Development of a method for sensitivity study to determine the most influential input uncertain parameters, including parameters related to human actions; • Performing advanced safety analysis with uncertainty and sensitivity analysis for the most limiting overcooling scenario to be able better quantify the safety margin for PTS. The project proposal regarding uncertainties in the advanced safety analysis of nuclear facilities is organized in the following four work packages (WPs): WP1: Identification of the overcooling scenarios Identification of the overcooling scenarios being a challenge to pressurized thermal shock of pressurized water reactor will be done based on the literature review. The important input uncertainties due to human actions will be identified which influence the overcooling scenarios. WP2: TRACE assessment and transient input models development for PWR TRACE thermal-hydraulic computer code will be assessed based on separate effects and integral effects test data. TRACE transient input models for overcooling scenarios in a PWR will be developed. WP3: Transient event simulations and sensitivity studies The identified overcooling scenarios will be simulated and based on sensitivity runs. The influence of input uncertain parameters resulting from human actions on key output parameters to PTS analysis will be quantified and the most limiting transient will be determined. WP4: Uncertainty and sensitivity analysis for the most limiting transient TRACE computer runs will be performed and used for uncertainty and sensitivity analysis of the most limiting transient.