Evaluation of Radiological Consequence from Air Activation at NLC BDS Tunnel

Radiological concerns from airborne radionuclides produced in the air region of the collimation section of the NLC Beam Delivery System were evaluated. The worker exposure during access after accelerator shutdown and the general public dose from radioactivity released to environment through ventilation system were studied. The FLUKA Monte Carlo code was used to calculate the fluence of particles (photon, neutron, proton and pion) in the air region. The production of activities for H, Be, C, N, O, and Ar was then estimated by folding the particle fluence with various nuclear cross sections. Under the operational condition of twelve accesses per year, the activity concentrations of radionuclides at the end of 1-month operation were compared with the corresponding Derived Air Concentration limits. Assuming that the air activity was released to a meteorological and ecological environment that is the same as SLAC, the effluent dose to the general public was estimated with the CAP88 computer code using conservative and realistic input parameters. The results show that, with a 500-GeV, 10kW beam loss at collimator section, the radiological consequence from the air activation was found to be minor. The main concern comes from the short-half-life radioisotopes of N and O (produced mainly by photons) and Ar (produced mainly by thermal neutron). The hazards can be mitigated by means of decay (e.g., wait one hour prior to entry to tunnel or the operation of ventilation system) or adding local shielding.