Session: 02-10: Physics and Transport Theory - II

Paper Number: 132277

132277 - Evaluation of Expansion Reactivity Based on Reactivity Coefficient for Sodium-Cooled Fast Reactor 

Abstract: 

This study derives a formula based on reactivity coefficients for evaluating the expansion reactivity considered during transient events in sodium-cooled fast reactors. The derived reactivity evaluation formula is compared with the formula shown in the previous study, and the differences are discussed. Additionally, the study discusses the approximation errors in the evaluation of expansion reactivity based on reactivity coefficients.

In the design of fast reactors, the transient analysis is generally performed to ensure core safety during a design-based accident such as an accident of unprotected transient overpower. The transient analysis may require the consideration of reactivity introduced due to changes in fuel temperature, coolant temperature, core support plate expansion, and fuel rod expansion. The reactivity change due to fuel or coolant temperature variations can be considered by treating the change in the effective cross-sections used for neutron transport calculations. On the other hand, from a computational cost perspective, the shape change due to the expansion of the core support plate and that of the fuel rod is challenging to consider in the transient analysis directly. Therefore, these expansion reactivities are usually treated as external reactivity in the transient analysis using the reactivity coefficient. However, the assumptions used in deriving the reactivity evaluation formula based on reactivity coefficients, and the approximation errors have been less discussed. Therefore, this research focuses on the formula for expansion reactivity based on reactivity coefficients.

The study addresses both axial and radial expansion reactivities. The radial expansion includes reactivity due to coolant flow area enlargement triggered by the core support plate expansion at the bottom of the core. In addition, the expansion of the fuel pin due to the interference between fuel pellets and cladding is considered. The expansion of fuel pellets and surrounding cladding is treated as one region in the present study. The radial expansion reactivity is described as the sum of reactivity contributions due to shape change and macroscopic cross-section change. The contributions from shape change can be evaluated by changing the core size keeping the macroscopic cross-section. The axial expansion reactivity introduced by the core expansion is also expressed by separating the contributions of shape change and macroscopic cross-section change. Compared to previous studies, the expressions are totally similar but different in some expansion coefficients and volume definitions. The approximation error of the expansion reactivity based on the reactivity coefficient was evaluated using Monte Carlo code MVP3.0. The results show that the error from the separation of shape change and macroscopic cross-section change is negligibly small. These results indicate that the expansion reactivity is accurately evaluated by the coefficients-based assessments.

Presenting Author: Satoshi Takeda Osaka University

Presenting Author Biography: Satoshi Takeda, Ph.D.
Assistant professor at Osaka University

Authors:

Satoshi Takeda Osaka University
Takanori Kitada Osaka University
Eiji Hoashi Osaka University
Takafumi Okita Osaka University