Migration of radionuclides in a gas cooled solid state spallation target

The current design of the ESS (European Spallation Source) program proposes a rotating solid tungsten target cooled by helium gas and a pulsed beam of protons. For safety reasons any design has to address whether or not the induced radionuclidic isotopes in the target migrate. In this paper we have investigated the diffusion of (primarily) tritium in solid tungsten to see if a pulse driven short-term variation in temperature (temperature peaks separated by one turn of the wheel(2.36 s)) could possibly give rise to wave-like migration of the radionuclides, possibly accelerating the overall release. In order to calculate the diffusion in the solid tungsten target two approaches have been used. One neglecting the time structure of the beam and thermal cycling of the target, and one numerical, discrete time step simulation to capture the effects of the thermal cycling on the diffusion behavior. We found that the time structure of the of the temperature has a negligible impact on the diffusion,and that the radioactive release at the surface can be calculated safely by solving the differential equation(Fick’s law) using an appropriate temperature to calculate the diffusion constant.© 2014 Elsevier B.V. All rights reserved.