• fission products release
• inverse uncertainty quantification
• melcor

Severe Accidents (SA) are events in which multiple safety systems fail, potentially resulting in significant consequences for the affected facility’s integrity, the environment, and public health. These accidents typically involve the loss of core cooling mechanisms, leading to nuclear fuel overheating and potential damage to the reactor core. Severe Accidents can escalate to partial or complete meltdowns, accompanied by the release of radioactive gases and particles into the surrounding environment.
In such scenarios, the release of radioactive isotopes into the environment becomes a significant concern, necessitating accurate predictive modeling to understand and mitigate the consequences. MELCOR, a widely utilized simulation tool, stands at the forefront of nuclear safety analyses. Developed by Sandia National Laboratories (SNL), MELCOR is a fully integrated, engineering-level computer code that models the progression of Severe Accidents in Light Water Reactor (LWR) nuclear power plants.
However, uncertainties inherent in input parameters and model formulations necessitate robust Uncertainty Quantification (UQ) studies to qualify simulations’ outcomes.

The purpose of this thesis work is to verify the feasibility of applying an Inverse Uncertainty Quantification (IUQ) methodology, namely CIRCÉ, to perform BEPU analyses of Severe Accidents. The guidelines coming from the OECD/NEA SAPIUM and ATRIUM projects are adopted as a roadmap for achieving this scope.
The complexity of Severe Accident progression, coupled with increasing uncertainties in boundary conditions throughout the accidental sequence, has led this project to focus on a specific aspect of the accident, namely the Early In-Vessel Phase. This phase immediately follows the Design Basis Accident (DBA) sequence, preceding the involvement of complex SA phenomena and operators' actions.
Additionally, the availability of experimental data, essential for conducting IUQ, makes it feasible to specifically address Fission Products Release (FPR) from nuclear fuel prior to core melting and relocation. In this direction, the project is focused on conducting an Uncertainty Quantification analysis of MELCOR’s FPR calculations. It must be highlighted that MELCORS's capabilities in reproducing FPR phenomena are already well-established and validated, and would not necessitate additional UQ analyses. However, the thesis' main objective is to verify the applicability of IUQ methodologies in Severe Accident scenarios, making this phenomenon an ideal candidate for the study.
To be more specific, the simulation tool chosen for this analysis is MELCOR’s Revised CORSOR-Booth release model. The selected Figure of Merit (FoM) is the Cesium (Cs) release fraction, and the uncertainties calculated pertain to the Cesium diffusion coefficient.
Cesium holds particular significance within the context of MELCOR, as its releases are the sole ones directly simulated and computed. Subsequently, the releases of other radionuclides are assessed by simply scaling Cs releases with specific factors. Thus, Cs represents the fundamental of MELCOR’s release model, necessitating a reliable quantification of its associated uncertainties.

In order to achieve this goal, a thorough review of available FPR experiments is undertaken, a robust database is created, and its Adequacy analyzed. The data collected from existing experiments are then compared to simulation results, with each test having a corresponding simulation. Uncertainty bands are then derived from this comparison using the CIRCÉ algorithm. The results are finally validated against a diverse database, encompassing both Small and Large Scale experiments, including PHEBUS FPT-1.