Radiological assessment of target debris in the National Ignition Facility

Abstract

Detailed activation analysis and dose rate calculations are performed for the different materials under consideration for use in the target capsules and hohlraums used during the ignition campaign on the National Ignition Facility. This analysis examined the impact of using Be-Cu and Ge-doped CH capsules on the external dose received by workers during maintenance activities. Five days following a 20 MJ shot, dose rates inside the Target Chamber (TC) due to the two proposed capsule materials are small (∼ 1 mrem/h). Gold and depleted-uranium (DU) are considered as potential hohlraum materials. Following a shot, Au will most probably get deposited on the TC first wall. On the other hand, while noble-gas precursors from the DU are expected to stay in the TC, most of the noble gases are pumped out of the chamber and end up on the cryo-pumps. The dose rates inside the TC due to activated Au or DU, at 5 days following a 20 MJ shot, are about 1 mrem/h. Dose rates in the vicinity of the cryo-pumps (containing noble “fission” gases) drop-off to about 1 mrem/h during the first 12 hours following the shot. A wait period of about 12 hours is recommended before transporting the fission gases outside the Target Bay for radchem analysis.