Hesham Khater

Measuring x-ray burn history with the Streaked Polar Instrumentation for Diagnosing Energetic Radiation (SPIDER) at the National Ignition Facility (NIF)

We present a new diagnostic for the National Ignition Facility (NIF) [1,2]. The Streaked Polar Instrumentation for Diagnosing Energetic Radiation (SPIDER) is an x-ray streak camera for use on almost-igniting targets, up to ~1017 neutrons per shot. It measures the x-ray burn history for ignition campaigns with the following requirements: X-Ray Energy 8-30keV, Temporal Resolution 10ps, Absolute Timing Resolution 30ps, Neutron Yield: 1014 to 1017. The features of the design are a heavily shielded instrument enclosure outside the target chamber, remote location of the neutron and EMP sensitive components, a precise laser pulse comb fiducial timing system and fast streaking electronics. SPIDER has been characterized for sweep linearity, dynamic range, temporal and spatial resolution. Preliminary DT implosion data shows the functionality of the instrument and provides an illustration of the method of burn history extraction.

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.

Evaluating radiation induced noise effects on pixelated sensors for the National Ignition Facility

The National Ignition Facility (NIF) utilizes several different pixelated sensor technologies for various measurement systems that include alignment cameras, laser energy sensors, and high-speed framing cameras. These systems remain in the facility where they are exposed to 14MeV neutrons during a NIF shot. The image quality of the sensors degrades as a function of radiation-induced damage. This article reports on a figure-of-merit technique that aids in the tracking of the performance of pixelated sensors when exposed to neutron radiation from NIF. The sensor dark current growth can be displayed over time in a 2D visual representation for tracking radiation induced damage. Predictions of increased noise as a function of neutron fluence for future NIF shots allow simulation of reduced performance for each of the individual camera applications. This predicted longevity allows for proper management of the camera systems.

Managing NIF safety equipment in a high neutron and gamma radiation environment.

AbstractThe National Ignition Facility (NIF) is a 192 laser beam facility that supports the Inertial Confinement Fusion program. During the ignition experimental campaign, the NIF is expected to perform shots with varying fusion yield producing 14 MeV neutrons up to 20 MJ or 7.1 × 1018 neutrons per shot and a maximum annual yield of 1,200 MJ. Several infrastructure support systems will be exposed to varying high yield shots over the facility’s 30-y life span. In response to this potential exposure, analysis and testing of several facility safety systems have been conducted. A detailed MCNP (Monte Carlo N-Particle Transport Code) model has been developed for the NIF facility, and it includes most of the major structures inside the Target Bay. The model has been used in the simulation of expected neutron and gamma fluences throughout the Target Bay. Radiation susceptible components were identified and tested to fluences greater than 1013 (n cm−2) for 14 MeV neutrons and &ggr;-ray equivalent. The testing includes component irradiation using a 60Co gamma source and accelerator-based irradiation using 4- and 14- MeV neutron sources. The subsystem implementation in the facility is based on the fluence estimates after shielding and survivability guidelines derived from the dose maps and component tests results. This paper reports on the evaluation and implementation of mitigations for several infrastructure safety support systems, including video, oxygen monitoring, pressure monitors, water sensing systems, and access control interfaces found at the NIF.

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.