New Gap Conductance Model for the ATHLET System Code

Abstract

The ATHLET system code, which is part of the AC² software package, is developed by the Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) to analyze the behavior of nuclear reactors, ranging from normal operating conditions and design basis accidents to design extension conditions including severe accidents with core melt. Recent developments by GRS are aimed at enhancing the predictive capabilities of the existing fuel rod model of ATHLET.

The existent simplified gap conductance model of the system code had been observed to overestimate nuclear rod temperatures, particularly under conditions of high burnup, when an unsuitable initial guess for the gap size is supplied. To address this concern, a more sophisticated gap conductance model has been considered and implemented. This new model dynamically evaluates gap sizes, incorporating aspects such as clad elastic mechanical deformations, as well as burnup-dependent fuel deformations, including swelling, densification, and radial fuel relocation after cracking. This enhanced model has been successfully integrated into the recent code version ATHLET 3.4.

The present research gives an overview of the new models considered and implemented in ATHLET. Additionally, it summarizes the verification and validation activities that underpin the code's improved predictiveness for the fuel temperature distribution within nuclear rods. Finally, we discuss further developments planned for the near future for fuel behavior models within ATHLET.