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

Abstract In this paper, formulas for evaluating expansion reactivity based on reactivity coefficients in sodium-cooled fast reactors are derived, and the approximation errors used in the derivation are also evaluated. This research provides formulas for both Z-axis expansion reactivity and radial expansion reactivity. These reactivities are separated into the effect of change in geometric shape and that of change in the cross-section. Since the major approximation in the present derivation is the separation of effects due to geometric shape changes and cross-sectional area changes, the approximation error due to this separation was evaluated using results of neutron transport calculations for fast reactor. This evaluation was performed for two geometries with different ratios of the reactor core’s height to diameter. The results show that the approximation error is less than 20 pcm. Therefore, it can be expected that expansion reactivity can be accurately evaluated using reactivity coefficients.