Session: 04-06: SMRs, Advanced Reactors and Fusion

Paper Number: 134125

134125 - Reactivity Feedback Effects of Fuel and Grid Plate Expansion in Gas-Cooled Reactors 

Abstract: 

Abstract:

Gas-cooled reactor, characterized by its lightweight, compact structure, and high thermal conversion efficiency, is considered as an important candidate for micro nuclear power. To achieve a high power-to-mass ratio, the reactor typically employs high-enriched uranium fuel designs. However, as the relative abundance of ²³⁸U in the fuel decreases, the fuel Doppler effect in fast neutron reactors may transition from negative to positive feedback. It is crucial to discuss the reactivity feedback insert by changes in the core structure to ensure the negative reactivity feedback for the entire core and system operation stability.

The relatively large temperature difference (~300°C) at the inlet and outlet of gas-cooled reactors leads to increased differential thermal expansion between the upper and lower grid plates fuel hole that constrain the fuel rods. This non-uniform expansion between the grid plates results in the "umbrella expansion" of fuel bundle, which leading to a decrease in the effective fuel density and an increase in the neutron leakage. Ultimately, the expansion reducing the excess reactivity of the core. This phenomenon manifests as a non-uniform grid plate expansion negative reactivity feedback.

This study utilizes the reactor Monte Carlo analysis software RMC and the commercial finite element analysis software ANSYS Mechanical to model and analyze the Prometheus reactor, which exhibits the positive fuel Doppler effect. The focus was primarily on discussing the reactivity feedback of fuel axial expansion and core grid plate thermal expansion. The calculations revealed that the non-uniform upper and lower grid plates expansion coefficient was approximately 0.406 pcm/K under full-power steady-state conditions, which is only half of the uniform expansion coefficient commonly calculated by using the average coolant temperature. The results showed that grid plate expansion had a minimal impact on the axial power distribution in the core, with an average deviation of only 0.11% before and after expansion. Furthermore, the reactivity feedback induced by the expansion of the upper and lower grid plates was significantly greater than that caused by the axial fuel expansion.

Using the obtained grid plate coefficients, the self-developed transient reactor system analysis program was employed to analyze the transient variations in positive reactivity insert to the Prometheus reactor when considering different expansion effects. Compared to the instantaneity of negative reactivity feedback from fuel axial expansion, the negative reactivity feedback from the grid plates exhibits hysteresis, but both feedbacks can ensure the safe operation of the core.

By providing insights into the reactivity feedback effects of fuel and grid plate expansion, this research contributes to the understanding of gas-cooled reactor behavior and safety. The findings can contribute to the design and optimization of gas-cooled reactor systems, ensuring their reliable and efficient operation in micro nuclear power applications.

 

Keywords: Gas-cooled reactor, reactivity feedback, grid plate feedback, thermal-hydraulic analysis.

Presenting Author: Yugao Ma Nuclear Power Institute of China

Presenting Author Biography: Dr. Ma graduated from the Department of Engineering Physics at Tsinghua University. He is currently employed as an engineer at the China Nuclear Power Design Research Institute, specializing in research and development in the field of advanced nuclear reactor engineering. His research focuses on thermal-hydraulic analysis, safety assessment, and optimization of reactor systems.

Authors:

Yugao Ma Nuclear Power Institute of China
Suyi Zhang Chengdu University of Technology
Jian Deng Nuclear Power Institute of China
Jinyu Wang Nuclear Power Institute of China
Yue Liu Chengdu University of Technology
Shuhua Ding Nuclear Power Institute of China
Muhao Zhang Chengdu University of Technology