Computational and experimental studies of processes of non-activation release of tritium from beryllium-containing materials of research nuclear reactors

Abstract

Under neutron irradiation lithium-6 is produced in beryllium components of nuclear reactors, which further interacts with neutrons to form tritium. The concentration of lithium-6 grows to some equilibrium value, when the rate of its formation compares with the Li burnup rate. Numerous studies of tritium interaction with irradiated beryllium were conducted, where reliable data were obtained that the main process influencing tritium release rate is diffusion. Diffusion processes become significant at high temperatures (above 700℃), when beryllium recrystallization begins. However, it was found that significant amounts of tritium may be released into the water surrounding beryllium reflector. A possible mechanism for such release may be the release of tritium directly from the near-surface layer of beryllium units. Since the reaction Li⁶+n_th -->He⁴+H³
  produces high-energy helium and tritium ions (with energies >2 MeV), they can leave the near-surface beryllium layer in the beryllium units of the reactor (estimates show that the width of such a layer exceeds 2 µm).

The paper presents a method for calculating tritium ions amount emitted from beryllium reflectors of nuclear reactors in non-activation way. Experiments on method verification consisted in irradiation of various lithium-containing samples wrapped in aluminum foils. Irradiation was conducted during 21 days at temperature 50℃ until achieving thermal neutron fluence 9.1·10¹⁹n/cm² and fast neutrons fluence (E˃0.1 MeV) 1.8·10¹⁹n/cm². Tritium amount accumulated in aluminum foils was estimated and compared with calculation. Results showed applicability of proposed calculation method for estimating tritium ions amount emitted from beryllium reflectors of nuclear reactors in non-activation way.