Sandra Brereton

Operations on the National Ignition Facility

Abstract The National Ignition Facility (NIF) is a high-energy-density-physics, experimental user facility that focuses up to 1.8 MJ of UV light in 192 laser beams onto a mm-sized target at the center of a target chamber. This paper describes how we conduct experimental shots on the NIF. We review processes and tools used to facilitate experiment planning and operations. Safety and radiological aspects of NIF’s operations are discussed. We also describe efforts to continuously improve operational efficiency and further increase shot rate.

Analysis of decay dose rates and dose management in the National Ignition Facility.

AbstractA detailed model of the Target Bay (TB) at the National Ignition Facility (NIF) has been developed to estimate the post-shot radiation environment inside the facility. The model includes the large number of structures and diagnostic instruments present inside the TB. These structures and instruments are activated by neutrons generated during a shot, and the resultant gamma dose rates are estimated at various decay times following the shot. A set of computational tools was developed to help in estimating potential radiation exposure to TB workers. The results presented in this paper describe the expected radiation environment inside the TB following a low-yield DT shot of 1016 neutrons. General environment dose rates drop below 30 &mgr;Sv h−1 within 3 h following a shot, with higher dose rates observed in the vicinity (∼30 cm) of few components. The dose rates drop by more than a factor of two at 1 d following the shot. Dose rate maps of the different TB levels were generated to aid in estimating worker stay-out times following a shot before entry is permitted into the TB. Primary components, including the Target Chamber and diagnostic and beam line components, are constructed of aluminum. Near-term TB accessibility is driven by the decay of the aluminum activation product, 24Na. Worker dose is managed using electronic dosimeters (EDs) self-issued at kiosks using commercial dose management software. The software programs the ED dose and dose rate alarms based on the Radiological Work Permit (RWP) and tracks dose by individual, task, and work group.

PLANNING TOOLS FOR ESTIMATING RADIATION EXPOSURE AT THE NATIONAL IGNITION FACILITY

Abstract A set of computational tools was developed to help estimate and minimize potential radiation exposure to workers from material activation in the National Ignition Facility (NIF). AAMI (Automated ALARA-MCNP Interface) provides an efficient, automated mechanism to perform the series of calculations required to create dose rate maps for the entire facility with minimal manual user input. NEET (NIF Exposure Estimation Tool) is a web application that combines the information computed by AAMI with a given shot schedule to compute and display the dose rate maps as a function of time. AAMI and NEET are currently used as work planning tools to determine stay-out times for workers following a given shot or set of shots, and to help in estimating integrated doses associated with performing various maintenance activities inside the target bay. Dose rate maps of the target bay were generated following a low-yield 1016 D-Tshot and will be presented in this paper.

Shielding Analysis for X-Ray Sources Generated in Target Chamber of the National Ignition Facility

Abstract Prompt doses from X-rays generated as result of laser beam interaction with target material are calculated at different locations inside the National Ignition Facility. The maximum dose outside a target chamber diagnostic port is ~10 mSv for a shot utilizing the 192 laser beams and 1.8 MJ of laser energy. The dose during a single bundle shot (eight laser beams) drops to ~0.4 mSv. Doses calculated outside the target bay (TB) doors and inside the switchyards (SYs) [except for the 5.33-m (17-ft 6-in.) floor level] range from a few microsieverts to ~110 μSv for 192 beams and scale down proportionally with a smaller number of beams. At the 5.33-m (17-ft 6-in.) floor level, two diagnostic ports are directly facing two of the TB doors, and the maximum doses outside the doors are 0.5 and 0.16 mSv, respectively. Shielding each of the two TB doors with 6.35-mm-thick Pb (1/4-in.) reduces the dose by a factor of 50. One or two bundle shots (8 to 16 laser beams) present a small hazard to personnel in the SYs.

Activation Analysis of the Final Optics Assemblies at the National Ignition Facility

Abstract Commissioning shots have commenced at the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory. Within a year, the 192 laser beam facility will be operational and the experimental phase will begin. At each shot, the emitted neutrons will interact with the facility’s surroundings, activating them, especially inside the target bay where the neutron flux is the highest. We are calculating the dose from those activated structures and objects in order to plan and minimize worker exposure during maintenance and normal NIF operation. This study presents the results of the activation analysis of the optics of the Final Optics Assemblies (FOA), which are a key contributor to worker exposure. There are 48 FOAs weighting three tons each, and routine change-out and maintenance of optics and optics modules is expected. We found that the effective dose from any optics is negligible 6 days after the last shot, and that the effective dose from frames is low but should be minimized not to reach the dose limit.

Post-Shot Radiation Environment Following Low-Yield Shots Inside the National Ignition Facility

Abstract A detailed model of the Target Bay (TB) at the National Ignition Facility (NIF) has been developed to estimate the post-shot radiation environment inside the facility. The model includes large number of structures and diagnostic instruments present inside the TB. These structures and instruments are activated by the few nanosecond pulse of neutrons generated during a shot and the resultant gamma dose rates are estimated at various decay times following the shot. The results presented in this paper are based on a low-yield D-T shot of 1016 neutrons. General environment dose rates drop to below 3 mrem/h within three hours following a shot with higher dose rates observed at contact with some of the components. Dose rate maps of the different TB levels were generated to aid in estimating worker stay-out times following a shot before entry is permitted into the TB.

EVALUATION OF PROMPT DOSE ENVIRONMENT IN THE NATIONAL IGNITION FACILITY DURING D-D AND THD SHOTS

Abstract Evaluation of the prompt dose environment expected in the National Ignition Facility (NIF) during Deuterium-Deuterium (D-D) and Tritium-Hydrogen-Deuterium (THD) shots have been completed. D-D shots resulting in the production of an annual fusion yield of up to 2.4 kJ (200 shots with 1013 neutrons per shot) are considered. During the THD shot campaign, shots generating a total of 2x1014 neutrons per shot are also planned. 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. The Target Chamber shielding, along with Target Bay and Switchyard walls, roofs, and shield doors (when needed) will reduce dose levels in occupied areas to acceptable values during these shot campaigns. The calculated dose values inside occupied areas are small, estimated at 25 and 85 μrem per shot during the D-D and THD shots, respectively. Dose values outside the facility are insignificant. The nearest building to the NIF facility where co-located workers may reside is at a distance of about 100 m from the Target Chamber Center (TCC). The dose in such a building is estimated at a fraction of a rem during a D-D or a THD shot. Dose at the nearest site boundary location (350 m from TCC), is caused by skyshine and to a lesser extent by direct radiation. The maximum off-site dose during any of the shots considered is less than 10 nano rem.

Analysis of Activated Air Following High Yield Shots in the NIF

Abstract During the ignition experimental campaign, the National Ignition Facility (NIF) is expected to perform shots with varying fusion yield (up to 20 MJ or 7.1 x 1018 neutrons per shot) and a maximum annual yield of 1200 MJ. A detailed MCNP model of the Target Bay (TB) and the two switchyards (SY) has been developed to estimate the post-shot radiation environment inside the facility. During D-T shots, a pulse of 14.1 MeV neutrons streaming outside the Target Chamber (TC) will activate the air present inside the TB and the argon gas inside the laser tubes. Smaller levels of activity are also generated in the SY air and in the argon portion of the SY laser beam path. The activated TB air will be mixed with fresh air from the Operations Support Building (OSB) before release through the stack. Flow of activated air from the Target Bay is controlled by the heating, ventilating, and air conditioning (HVAC) system. 16N (T1/2 = 7.13 s) dominates the radiation levels during the first minute following the shot. It is expected that 16N will decay away during the confinement time before releasing the TB air through the stack. The other major contributors are 13N (T1/2 = 9.97 min) and 41Ar (T1/2 = 1.83 h). In general a low dose rate of < 1 μSv/h is expected near the stack during the first few hours following a 20 MJ shot. The amount of activated Target Bay air released through the stack is very small and does not pose significant hazard to personnel or the environment. In the mean time, due to a very small leakage rate out of the laser tubes, the activated argon gas decays within the tubes and any resulting release to the environment is insignificant.

Radiological design aspects of the National Ignition Facility.

AbstractThe National Ignition Facility (NIF) has been designed to accommodate some challenging radiological conditions. The high prompt neutron source (up to 1.6 × 1019 neutrons per shot) results in the need for significant fixed shielding. Concrete shielding approximately 2 m thick is used for the primary (target bay) shield. Penetrations in this shield, including those required for 192 laser beams, utilities, diagnostics, and 19 shielded personnel access doors, make the design challenging. An additional 28 shield doors are part of the secondary shield. In addition, the prompt neutron pulse results in activated air within the target bay, requiring special ventilation considerations. Finally, targets can use a number of hazardous and radioactive materials including tritium, beryllium, and depleted uranium (the latter of which results in the generation of small quantities of fission products). Frequent access is required to the associated potentially contaminated volumes for experimental setup, facilitating the need for local exhaust ventilation to manage these hazards. This paper reviews some of these challenges, design considerations, and the engineering solutions to these design requirements.

Impact of Target-Material Activation on Personnel Exposure and Radioactive Contamination in the National Ignition Facility

Detailed activation analyses are performed for the different materials under consideration for use in the target capsules and hohlraums during the ignition campaign of the National Ignition Facility (NIF). Results of the target-material activation are additionally used to estimate the levels of contamination within the NIF Target Chamber (TC) and the workplace controls that are necessary for safe operation. The analysis examined the impact of using Be-Cu and Ge-doped hydrocarbon capsules on the external dose received by workers during maintenance activities. Five days following a 20-MJ shot, dose rates inside the TC due to the two proposed capsule materials are small (~ 0.01 ¿Sv/h). Gold and depleted uranium (DU) are considered as potential hohlraum materials. Following a shot, gold will be 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 cryopumps. The dose rates inside the TC due to activated gold or DU, at five days following a 20-MJ shot, are about 10 ¿Sv/h. Dose rates in the vicinity of the cryopumps (containing noble ¿fission¿ gases) drop off to about 10 ¿Sv/h during the first 12 h following the shot. Contamination from activation of NIF targets will result in levels in the NIF TC above U.S. Department of Energy surface contamination limits. Objects removed from the TC will need to be managed as radioactive material. However, the results suggest that airborne contamination from resuspension of surface contamination will not be significant and is at levels that can be readily managed by negative ventilation when accessing the TC attachments.