Development and validation in water of FLUNED, an open-source tool for fluid activation calculations

Abstract

In nuclear fusion installations, high-energy plasma neutron activates the cooling and breeding fluids, generating a mobile and distributed source of radiation. In water-cooled fusion reactors, such as ITER and DEMO, the most concerning radioisotopes present in the coolant are the 16N and 17N. Their prompt emission impacts the design, safety, and, ultimately, the economics of the plant as it can be the dominant source of radiation in regions far from the plasma. The flowing condition makes the calculation of radioisotope concentrations within an irradiated cooling circuit particularly challenging as it requires the coupling of the neutronics and fluid dynamics characteristics of the problem. Recent works showed that, except for simple and limited cases, the coupling requires the implementation of Computational Fluid Dynamics (CFD) methods. Nevertheless, the use of ad-hoc methodologies or the application of proprietary codes has hampered the participation and formation of common benchmarks among the neutronics community. In this work, we present the development of an open-source tool called FLUNED that studies the evolution of the concentration of radioactive species inside a fluid and that considers the fluid dynamics of the carrier. The tool is based on the open-source CFD code OpenFOAM and makes use of its modular nature to complement the fluid continuity equation with the radioisotope decay and production terms. In the second part of this work, we show the code validation using experimental data from the water activation experiment performed at FNG in Frascati in December 2019. The validation was performed by producing a computational chain that simulated the main components of the water circuit and estimated the counts in the two experimental detectors caused by the decay of the 16N and 17N isotopes. Finally, the positive comparison between the calculated and the experimental values allowed us to establish the validation of FLUNED for studies of fluid activation in water-cooled fusion experiments and reactors.