Water is one of the most commonly used coolants, it is used in both fission and fusion reactors. One of the peculiarities of using water as a coolant for components where there is a significant flux of neutrons is the activation of this water. As a result, the cooling circuit can become a significant source of radiation, affecting the design of reactors. In order to design and license the ITER fusion reactor, methods were developed to simulate activated water as a source of radiation, and at the same time, the need for experiments where these tools could be tested became apparent.
In 2024, the KATANA activation loop, aimed at improving the understanding of water activation processes, started operating at the TRIGA Mark II reactor at the Jožef Stefan Institute. One of the main applications of this loop will be the validation and optimization of computer programs used to calculate water activation and simulate activated water as a radiation source.
This master's thesis presents the analysis of water activation in the KATANA loop using the FLUNED tool, which combines fluid dynamics simulations with neutron and gamma ray transport in activation calculations. In our case, the particle transport was simulated with the Monte Carlo stochastic particle transport program MCNP, while the water velocity fields were calculated with the computational fluid dynamics program OpenFOAM. The activations of the oxygen isotopes $^{16}$O, $^{17}$O and $^{18}$O and the decay of their activation products were taken into account: $^{16}$N, $^{17}$N and $^{19}$O. For each isotope, the saturation activity was estimated at different flow rates and the dose fields were calculated. The results show that in the vicinity of the main measuring position, the activity of all activation products increases with increasing flow, but the growth above the flow of 0.5 L/s is minimal. For all flow rates, the nuclide $^{16}$N contributes the largest share to the dose field, followed by $^{19}$O, and $^{17}$N contributes the least.
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