Application of the ASYST Code in Estimating Uncertainty of the QUENCH-03 Experiment Calculation

Abstract

Core quenching is the main safety measure for mitigation of the consequences of a severe accident in a light water reactor (LWR), but it results in cladding oxidation with a large release of heat and hydrogen. Produced hydrogen presents a new potential problem for containment integrity since hydrogen mixed with air creates an easily flammable containment atmosphere. Therefore, it is important to accurately estimate the amount of hydrogen released during core quenching.

Hydrogen production and other physical-chemical phenomena in the reflooded core are modeled using specialized severe accident computer codes, but due to the complexity of those phenomena, it is difficult to create precise and accurate computer models. In order to deliver experimental and analytical data to support the development of quench-related models in severe accident computer codes, QUENCH experimental program has been launched in 1996 at the Karlsruhe Institute of Technology (KIT). The QUENCH-03 experiment was conducted in January 1999 with the aim of investigating the reflood behavior of PWR fuel rods with low oxidation.

Although QUENCH experiments have given new insight into severe LWR accidents, there are many sources of uncertainty in quench-related calculations, e.g. modeling of physical phenomena or numerical models of the plant. Those uncertainties are taken into account by the ASYST code, a severe accident code that uses probabilistic methodology based on propagation of input uncertainties. For the observed QUENCH-03 experiment, the calculation and uncertainty analysis were carried out in the ASYST code, and the results obtained by simulation were compared with the experimental values.