Within Work Package 4 of the SASPAM-SA project, funded under the Horizon-EURATOM programme, some established severe accident codes are assessed as to their capability of modelling In-Vessel Corium Melt Retention in two candidate Small Modular Reactor designs. Project participants were asked to set up a Best-Estimate-Plus-Uncertainty analysis by simulating accident transients in a stand-alone Lower Plenum model, with a set of agreed initial and boundary conditions. This paper reports results of analyses that have been carried out around the task set in SASPAM-SA, with the Severe Accident code ASTEC V3.1.2 and its Uncertainty-Analysis front end SUNSET, for the integrated PWR design #1 of SASPAM; an external reactor vessel cooling loop driven by natural convection is part of the model to explore the functioning of the “ERVC pipe” feature of ASTEC. Five Uncertainty and Sensitivity Analyses of 500 simulations each have been carried out, three for an initial set of 15 uniformly distributed uncertain model variables and two where this set of uncertain input parameters has been varied to target the investigation. Their results have been evaluated regarding their impact on the residual thickness of the partly ablated Reactor Pressure Vessel wall, on the forming of a heavy-metal layer at the bottom of the Corium pool, and on other target model outputs. The global sensitivity of these target values has been analysed by correlation-based (Pearson coefficients) and by variance-based (Sobol’ indices) methods. The results discussed indicate that the code produces plausible results for the melt retention scenario. It is shown that natural convection in the ASTEC model of the external reactor vessel cooling loop is established and that all residual heat is removed through the vessel wall and to the ultimate heat sink. The mean value of the remaining, stationary, minimum vessel thickness at the end of a 20,000 s transient is typically more than 40% of the original thickness (judging by the 5% percentile), and 23% in the most extreme single simulation. Key sensitivities are towards the uncertainties that determine the level of residual heat and the heat resistance of the cooling path. In addition to these key findings, the results provide perspectives into the In-Vessel Melt Retention processes and the related ASTEC models.

BRUMM Stephan; 

2026-05-18

Zenodo online

JRC145715

https://publications.jrc.ec.europa.eu/repository/handle/JRC145715,