Improving the estimation of activation levels in flowing liquids under irradiation and decay

A new methodological approach is proposed to calculate activation levels of flowing liquids under irradiation by coupling CFD analyses with activation physics. It overcomes the limitations of the conventional one, which assumes a simple linear dependency of residence times with flowrate that is only valid in a limited number of fluid-dynamic regimes. The new approach is more generally applicable and, as a consequence, successful in improving the post-analysis of a recent ITER water activation experiment. C/E values which with the conventional approach were very different from unity and showed a clear pattern with flowrate are now close to unity and showing a flat profile. Results are encouraging and suggest that the new approach leads to improved predictive capability for more complex applications in ITER and beyond. It is demonstrated that, under the particular conditions of the above experiment, the conventional approach always leads to under-prediction of activation levels downstream compared to the new one for the case of decay only, whereas it always leads to over-prediction for the case of irradiation and decay. The magnitude of the under- or over-predictions depends intricately on the actual flow patterns, flowrate and nuclide half-life. (paper)