Monte Carlo simulation of the prompt dose environment in the National Ignition Facility during low yield D-T shots

The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory is the world's largest and most powerful laser system for inertial confinement fusion (ICF). The NIF is a 192 laser beam facility that is capable of producing 1.8 MJ, 500 TW of ultraviolet light. During the ignition campaign, the NIF is expected to generate shots with varying fusion yield (up to 20 MJ per shot) and a maximum annual yield of 1,200 MJ. The thicknesses of the different shield doors in the facility were optimized such that the annual dose in the NIF occupied areas is limited to < 50 μSv per 20 MJ shot and < 1 mSv for 1,200 MJ of annual fusion yield. Experiments resulting in neutron yield started in 2010. Monte Carlo simulations have been performed to estimate prompt dose values inside the facility as well as at different locations outside the facility shield walls. Evaluation of the prompt dose environment expected during low yield shots (Category A and B shots) has been completed. Deuterium-Tritium (D-T) shots generating up to 10¹⁴ neutrons per shot are characterized as Category A shots. Due to the low neutron yield, no shield doors other than the primary door connecting the Target Bay and the Diagnostic Building are required. On the other hand, D-T shots generating up to 10¹⁶ neutrons per shot are characterized as Category B shots. At this level, all Target Bay primary shield doors are expected to be in place during the Category B shots. No secondary shield doors are required for either shot category. The Target Chamber shielding, along with Target Bay and Switchyard walls, roofs, and shield doors (when required) will reduce dose values in occupied areas to low levels during these shot campaigns. The maximum dose values in potentially occupied areas are small, estimated at 2 and 7.5 μSv per shot during Category A and B shots, respectively. The dose values inside and outside the NIF are also small and do not pose a hazard to workers, visitors or the public. Results of the Monte Carlo simulation indicate that the current NIF shielding is effective in providing the required radiation protection for all phases of the NIF operation. (author)