Uniform Thermal Expansion Reactivity Evaluation Based on Reactivity Coefficient for Sodium-Cooled Fast Reactor

Abstract This paper presents a method for calculating reactivity changes caused by uniform thermal expansion in sodium-cooled fast reactors using reactivity coefficients. The reactivity changes due to both Z-axis and radial expansion are separated into two effects: the shape change effect and the macroscopic cross section change effect. These effects are expressed using the reactivity coefficients of materials in the reactor core. The reactivity change due to Z-axis expansion considers the expansion of the fuel, cladding, and wrapper tube, while the radial expansion considers the fuel pin, structural materials, and reactor core support structures. To confirm the accuracy of this approach, calculations were performed for simplified reactor geometry with different height-to-diameter ratios. The results show that reactivity changes behave almost linearly for expansions of up to 2%. Additionally, the difference between the expansion reactivity obtained by using reactivity coefficients and those by direct calculations considering core expansion is within a few percent.