Publications Details

Methods for APT source term estimation: Approach and illustrative results

Williams, D.C.

An approach is described for performing scoping calculations of radionuclide release fractions from target materials being considered for use in the Accelerator Production of Tritium (APT) project, and some illustrative results are presented. The releases are evaluated for postulated accident scenarios involving severe overheating of either of two neutron source target materials, tungsten and lead. The potential for vaporization of radionuclides produced by spallation and neutron capture reactions is evaluated using a model that includes production of volatile species by reaction with steam, hydrogen, and/or oxygen. Emphasis is on release from the neutron source materials themselves, with a more limited treatment being given for radionuclides produced in other parts of the target/blanket assemblies. In the tungsten neutron source target, the low rate of diffusion within the tungsten is shown to limit releases of even volatile species to small values in a chemically inert or reducing environment. However, oxidative ablation of tungsten could permit considerably larger releases of volatile species in steam-rich or oxygen-rich environments. Tungsten radionuclides would dominate the source term for accident conditions considered the most plausible. For the lead neutron source target, the releases are predicted to be dominated by mercury radionuclides. Quantitative source term evaluation for this target is complicated because, in any accident sufficiently severe to be of much concern, lead melting will likely result in loss of target geometry. Hence, results presented for release from the lead must be carefully qualified. Extensive parametric results are presented for release from both neutron source materials. These results may be used to perform scoping estimates of radionuclide releases for additional APT accident scenarios as the controlling parameters for these scenarios become better defined.