Uncertainty calculation of thermal hydraulic simulation of DEC-A accident scenario at VVER-1000 NPP using CIAU method

All thermal hydraulic best estimate simulations of transient scenarios at Nuclear Power Plants (NPPs) are subject to uncertainties. One method to evaluate those uncertainties is implemented by the "Code with the capability of Internal Assessment of Uncertainty" (CIAU-Code), which calculates the uncertainty for the parameters primary system mass, primary system pressure and hot rod temperature by accuracy extrapolation. The present work now presents an extended use of the CIAU method, by deriving the uncertainty of the parameter iodine -131 release to the environment. For the analysis, a steam generator hot header break with the assumption of a stuck open safety relief valve (BRU-A) at the first opening at the VVER-1000 reactor was selected. Such transient constitutes a design extension condition A (DEC -A), characterized by multiple failures of safety systems but with the reactor core remaining intact. The thermal hydraulic best estimate simulation was conducted with RELAP5-3D. Additionally, the Iodine Spiking (IS) phenomenon for the examined accident scenario was simulated using two different empirical IS models (NRC IS Model and BOKU University IS Model). To allow the application of the CIAU method for the IS calculation a theoretical derivation is provided. The results show that for the selected DEC -A scenario a high uncertainty range has to be expected. The analysis allows a conservative estimation of the risk expected in the event of the examined transient sequence.