Conceptual Design and Characteristic Analysis of Thorium-Based Long-Life Reactor Core

The design of a long-life reactor must not only achieve high burnup and closed fuel cycle utilization but also account for system heat transfer efficiency and inherent safety characteristics, thereby enhancing the long-term operational reliability of the reactor. To address these requirements, this paper proposes a long-life core design scheme based on spent-fuel plutonium-thorium nitride fuel and lead-bismuth eutectic coolant. This scheme exploits the excellent breeding and burnup performance of thorium-based fuel under a fast neutron spectrum and employs the neutronics calculation code DRAGON and the subchannel code COBRA-PB to carry out corresponding core physics analysis and safety characteristic assessment. The results show that the core can achieve a 25-year long refueling cycle, with mild reactivity swing over the entire lifetime and low control difficulty; the core radial power gradually shifts inward with increasing burnup, achieving power flattening and thereby mitigating safety risks caused by localized deep burnup; both the control assembly and shutdown assembly possess sufficient reactivity margins, with a single set of control mechanisms capable of achieving effective shutdown upon independent insertion; all key reactivity coefficients of the core are negative, demonstrating negative-feedback regulation capability and inherent safety characteristics. Calculations indicate that the cladding temperatures of both the average channel and the hottest channel remain within limits, leaving ample safety margins, ensuring long-term cladding mechanical integrity, and effectively preventing high-temperature nitride fuel from melting through the cladding, resulting in outstanding overall core safety performance. This core design scheme can provide a theoretical basis and design reference for the engineering application of long-life reactors. It should be noted that this study is an exploratory investigation at the conceptual design stage of the core. The analysis is conducted on the basis that the fuel supply remains stable, that steady-state calculation models are adopted, and that the performance data of key materials are taken from published literature. Accordingly, the conclusions drawn still require further support from experimental verification and detailed design. Nevertheless, the proposed core design can provide a theoretical basis and design reference for the engineering application of long-life reactors.