Thermal-Hydraulic Study of the Converter Facility in the FRM II Research Reactor using Serpent 2 and Ansys CFX within the Framework of the LEU Conversion Program

Abstract

The Forschungs-Neutronenquelle Heinz Meier-Leibnitz (FRM II) actively engages in international efforts toward the decrease of Highly Enriched Uranium (HEU) usage in civil cycles. This goal can be achieved by changing the current fuel system to one with high-density, low-enrichment uranium (LEU) fuel. This work is part of the reassessment of the secondary sources located in the moderator tank and are, therefore, likely to be affected by a change in the fuel material. Therefore, investigating the current secondary sources in terms of neutronics and thermal-hydraulics is of utmost importance as it serves as a validation basis for a future assessment. The converter facility is one of the secondary sources located in the moderator tank of the FRM II reactor. Its purpose is the production of fission neutrons from the thermal neutrons that migrate from the reactor core toward the converter plates. These fission neutrons are used for different applications, such as tumor irradiation for palliative patients, radiation hardness testing for electronics, etc. The converter plates exhibit an asymmetric power density distribution due to their tilted geometry and proximity to the fuel element. In this work, the neutronics and thermal-hydraulic aspects of the converter facility and the influence of the asymmetric heating profile on the cooling of the plates are evaluated using Serpent 2 and Ansys CFX. The thermal-hydraulic assessment is the first realistic calculation that uses the asymmetric heating profile. The coolant temperature in the middle of the cooling channel between the two plates reaches about 50°C. The power density asymmetry causes one side of the facility to be warmer than the other, with a difference of approximately 10°C in maximum temperatures.