Evaluation of SiC cladded annular fuel as accident tolerant fuel for PWRs

Abstract

Nuclear energy produced by Light Water Reactors (LWRs) was in the spotlight in the aftermath of the Fukushima disaster, which led to the development of Accident Tolerant Fuels (ATF). ATF desires to improve fuel safety and performance under normal and accident operating conditions. This dissertation focuses on contributing research towards ATFs and nuclear reactor safety. The study aimed to develop the North Anna (reference) 3D fuel pin and assembly models. The reference reactor uses a solid fuel pin (UO2 pellets) and Zircaloy cladding, and the study seeks to replace the solid fuel pin with annular fuel pins and the cladding with Silicon Carbide (SiC). Two types of annular fuel pins were developed, the water annulus fuel pin and the helium annulus fuel pin. This study is unique because most studies modelled the water annulus fuel pin, whereas the helium annulus fuel pin was developed for this study. Serpent 2.1.0, a continuous-energy, burnup and neutronics code and the Computational Fluid Dynamics (CFD) simulation code Flownex 8.12.8 were coupled externally. An iterative process was used between the two codes to achieve power distribution, fuel temperature, coolant temperature and density convergence. The adaptive relaxation method is applied to prevent divergence between iterations. The water annulus model has a shorter fuel cycle length compared to the solid reference pin model. Based on these facts and the physical design aspects of the water annulus fuel pin, this model was not considered further for coupling. The designed helium annulus fuel pin used SiC cladding, where the thermal conductivity with two different grain sizes were modelled. The two models were developed and coupled to evaluate the effect of this cladding material. Both claddings produced similar temperatures. The helium annulus model, when compared to the solid reference model, performs better in terms of burnup, cycle length and has reduced maximum fuel temperatures during normal operating conditions. The study succeeded in designing helium annulus fuel without altering the assembly dimensions and flow rates. This fuel pin design in a full core can be further investigated and tested under transient operating conditions.