In 2019, the International Atomic Energy Agency (IAEA) launched the 5-year Cooperative Research Project (CRP) I31033 to advance the understanding and characterization of sources of uncertainty and to investigate their effects on the key figures of merit (FOMs: response parameters of interest) of the severe accident code predictions for water-cooled reactors. Twenty-two institutions from 18 member states participated in the CRP, and as a result, TECDOCs are being developed for the relevant benchmark exercises. The respective TECDOCs address a specific exercise and outline relevant research results pointing to best practices for the uncertainty and sensitivity analyses of the currently available severe accident codes.

For the uncertainty analysis exercise, the CRP participants defined their own analysis scope and framework, including target plant, severe accident code, accident scenario, and relevant FOMs. According to each respective framework, participants independently carried out their own exercise, including plant modeling and nodalization, simulation of reference cases, and relevant uncertainty and sensitivity analyses.

Finally, general conclusions were made based on an analysis of the results in view of the best-practice application of the uncertainty and sensitivity methods. Among them, this paper summarizes the main results of the uncertainty and sensitivity exercise performed for both pressurized water reactors and integral light water small modular reactors in the frame of the IAEA CRP with relevant insights.