Prediction of neutron source, tritium production and activation for long-pulse operation of the ITER neutral beam test facility

A local mixing model (LMM) (H.-D. Falter et al 1992 Proc. 17th Symp. on Fusion Technology (Rome, 1992) p 481) has been utilized to compute the evolution of the hydrogen isotope content within the implantation zone of the CuCrZr target material of the beam-stopping elements of the ITER neutral beam test facility (NBTF), together with the beam-target fusion reaction rate calculated by taking account of the slowing-down of the 1 MeV incoming projectile ion within the implantation layer. An important modification of the LMM code is to treat the tritium reaction product ions, resulting from D-D reactions, as a constituent of the incident beam. Although the treatment of tritium in the LMM is not ideal, this and other simplifying assumptions either do not significantly affect the predictions, or ensure conservatism in the results when used as input to the safety analysis of the facility. For example, it is shown that T → D 'beam-target' reactions always dominate over those of D → T, which overcomes the problem of uncertainty, in the model, of the distribution of tritium trapped within the implantation layer; in contrast there is little uncertainty that this region could approach deuterium saturation after <100 full-length (1 h duration) pulses. Using the computed sources as input, neutronics and activation calculations for the NBTF components have been carried out using the codes MCNP/FISPACT. As expected, long-pulse operational requirements lead to neutron activation and tritium production levels which have non-negligible but manageable radiological consequences.