Numerical Simulation Study on Boiling Heat Transfer Characteristics of Annular Fuel

Enhancing reactor safety and economic efficiency represents a key trend in nuclear fuel element technology development. Various nuclear fuel suppliers continuously optimize fuel assembly designs to improve fuel performance. The performance potential of conventional rod-shaped fuel is now approaching its limits. Annular fuel assembly has emerged as one of the key directions for developing high-performance fuel. Annular fuel assembly has two cooling channels, increasing the heat transfer area of the fuel element and reducing the heat transfer distance within the fuel pellet. This structural feature effectively reduces the pellet temperature at the center, enhancing reactor safety while significantly increasing reactor power output without altering the reactor volume. Funded by the U.S. Department of Energy, MIT spearheaded annular fuel assembly research, evaluating its feasibility for constructing next-generation advanced nuclear power plants and retrofitting existing facilities. The study encompasses reactor physics analysis, thermal-hydraulic safety assessment, irradiation performance evaluation, and manufacturing process research. Led by the Korea Atomic Energy Research Institute, a feasibility study was conducted on applying annular fuel to the OPR-1000 reactor. This research encompasses reactor core design, fuel pellet manufacturing, cladding preparation, irradiation tests, thermal-hydraulic analysis, and fuel assembly fabrication. The results indicate a potential 20% increase in reactor power. Beyond the U.S. and South Korea, Iran, Egypt, Canada, and Singapore also conducted a series of related studies. The findings indicate that annular fuel can enhance reactor power while extending refueling cycles. China has systematically advanced annular fuel research since 2010. To date, significant progress has been achieved with breakthroughs in multiple key technologies including design, manufacturing, and testing. Regarding thermal-hydraulic safety, as an advanced fuel type, annular fuel exhibits more complex boiling heat transfer characteristics. In this paper, a numerical calculation model of a single annular fuel rod was established based on computational fluid dynamics (CFD) method. By analyzing the heat flux distribution characteristics of the inner and outer channels, the sequence of boiling criticality occurrence, and the enhanced heat transfer effect of the spacer grids, the boiling heat transfer characteristics of annular fuel under typical operating conditions were studied. The results show that under the boiling criticality condition, the heat flux distribution of the inner and outer channels of the annular fuel are 47% and 53%, respectively. Flow distribution ratio (φ) significantly affects the triggering sequence of boiling criticality in the inner and outer channels, and when φ = 1.6, both channels reach the boiling critical state simultaneously. The spacer grids have a significant enhanced heat transfer effect and can effectively alleviate the sub-channel boiling crisis. The results of this study can provide support for the development of annular fuel assemblies.