Preliminary Core Design Analysis of Core Criticality and Heat Transfer of the All-Solid State Micro-Reactor
Authors: Koike Takeshi , Asano Koji , Yamada Mizuki , Kamohara Satoru , Yodo Tadakatsu , Murakami Nozomu 
Published: November 2024

Abstract
Recently, the use of nuclear power gains great attention as a low carbon energy technology in net-zero emission increases momentum. An innovative development toward advanced nuclear reactors, in which advanced technologies to be implemented to increase reliability and safety compared to the conventional light water reactors, has been intensified throughout the world.
Mitsubishi Heavy Industries, Ltd. (hereinafter MHI) is currently developing a multipurpose modular micro-reactor for diverse applications (e.g., power/heat source usage, energy storage, etc.) in isolated off-gird communities. The micro-reactor’s innovative design is based on all-solid-state core concept. The reactor uses a highly thermal conductive graphite-based material that removes heat from the core without liquid coolant. By implementing all-solid-state concept, the micro-reactor eliminates fuel integrity failure in case of a loss of coolant accident and significantly increases reliability and safety of the reactor. Another innovative design aspect of the micro-reactor compared to the conventional light water reactors is its small size and transportability. It can fit inside a container truck and its high transportability enables it to be delivered to isolated off-gird communities. As for safety systems, the micro-reactor has two independent reactivity control systems; control drums and a passive emergency shutdown system that can bring the micro-reactor the shutdown state. As mentioned, the multipurpose modular micro-reactor fundamentally enhances nuclear reliability and safety and widen its applications for zero-carbon energy.
Preliminary core design analysis of core criticality and heat transfer for a micro-reactor of all-solid-state core was performed with various power levels considering its wide applications. The core criticality and heat transfer analysis were performed using Monte Carlo code [1] and the thermal resistance grid method, respectively. The micro-reactor is assumed to be fueled with enriched uranium (up to 19.75wt% 235U), and the volume ration of graphite-based material and fuel was studied from the neutron economy point of view. Considering the constrain of its size, the micro-reactor is designed to implement the control drums in which neutron absorber and neutron reflector are integrated together. The control drums described above are located along the periphery of the core and have a role to mitigate neutron leakage from the core, which is important factor to enhance neutron economy, while ensuring a sufficient shutdown margins to bring the core to the shutdown state. The core criticality analysis confirmed that the micro-reactor can be operated up to around 12 effective full-power years without refueling depends on the power level. Also, the analysis confirmed that the control drums can bring the core to the shutdown state with all control drums, except one with the largest reactivity worth, rotated with the neutron absorber oriented toward the core. The heat transfer analysis confirmed that the core cooling can be achievable up to around 1 MWt. This paper summarizes the preliminary analysis result of core criticality and heat transfer of the micro-reactor.