Abstract
High-density fuels have been proposed as a possible replacement for uranium-dioxide as a fuel for light water reactors (LWRs) due to their increased loading of fissionable material. The objectives of this proposal are (1) to increase reactor cycle length and reactor power, and (2) to offset any neutronic penalty associated with advanced cladding systems. Of the high-density fuels under consideration, there is particular interest in triuranium disilicide (U3Si2) due to its increased metal density and favorable thermal properties as compared to UO2. However, there are concerns regarding the chemical compatibility of U3Si2 with water and steam as used for LWR coolant.
This paper summarizes research on fuel-coolant chemical compatibility for UO2 LWR fuel during a cladding breach and highlights that generally, because of its chemical inertness, UO2-coolant reactions are of little consequence to reactor operation. However, the volumetric expansion associated with the reaction of UO2 and oxygen is a concern for possible conditions encountered during air ingress of dry storage. These same concerns arise for U3Si2, which exhibits greater volumetric expansion than UO2 when exposed to water or steam. These reactions ultimately result in increased fuel volume that the cladding must accommodate, as well as additional heat generated as the fuel reacts.
The BISON fuel performance code was used to perform a comparative analysis on the behavior of UO2 and U3Si2 under normal operation. Silicide fuel simulations were then extended to demonstrate how varying thermodynamic and chemical kinetics influence fuel expansion and subsequent cladding performance during a cladding breach. These simulations were further extended to a 3D subsection of a fuel rod to demonstrate the characteristics of the resulting cladding crack.
