Simulation of water activation in the KATANA activation loop of the TRIGA reactor

Water is widely used as a coolant in both fission and fusion nuclear reactors. However, when exposed to neutrons, it becomes activated and can turn into a significant radiation source especially in fusion reactors operating with deuterium–tritium plasma. This effect influences reactor design and requires accurate modeling to ensure safe and reliable operation. During the development of the ITER fusion reactor, methods and codes were created to simulate the process of water activation and its flow through the cooling system. However, lack of relevant experiments needed for experimental validation of these codes was highlighted. In 2024, the KATANA water activation facility began operating at the TRIGA Mark II reactor at the Jožef Stefan Institute with the aim to improve understanding of water activation and to validate computational tools used for simulating it. This paper presents an analysis of water activation in the KATANA loop using the open-source FLUNED code, which integrates fluid dynamics with neutron and gamma-ray transport simulations. The analysis focuses on activation calculations based on the latest loop design. Additionally, we examine how uncertainties in some of the main input parameters, i.e. flow rate and the total volume of the loop components, influence the overall uncertainty in the determined activity of water. • Fusion-relevant water activation studies in support of ITER operation. • CFD + FLUNED + MCNP: model water activation in KATANA’s closed loop. • Max saturation (short loop, 250 kW): 16 N: 7.1e7 Bq (0.5 L/s), 17 N: 1.2e4 Bq (0.8 L/s), 19 O: 1.1e7 Bq (0.1 L/s). • Corresponding dose rates: γ ( 16 N): 5.7 mSv/h; γ ( 19 O): 0.31 mSv/h; n( 17 N): 26 μ Sv/h. • Uncertainty from volume (0.05 L) and flow (0.001 L/s) < 1% for ⩾ 0.5 L/s.