Analysis of Radionuclide Deposition Ratios from the Fukushima-Daiichi Incident.

This paper will examine deposition patterns of four radionuclides following the Fukushima-Daiichi accident. For nearly 13 d following the event, fission products were released into the environment through planned venting procedures and hydrogen explosions. To assist the government of Japan (GOJ) in the assessment of the releases, the National Nuclear Security Administration's Consequence Management Response Team (CMRT) deployed and took nearly 3 mo of measurements using airborne radiation sensors, fixed monitors, high purity germanium (HPGe) detectors, and health physics survey equipment. From the HPGe detector in-situ results gathered by the CMRT and GOJ teams, the depositions of Cs, Cs, Cs, and I were examined as a function of latitude and longitude. Deposition ratios were calculated to express how each radionuclide was deposited relative to Cs. In addition, the first 30 d of results were compared to the isotopic ratios listed in the Federal Radiological Monitoring and Assessment Center (FRMAC) Dose Assessment Manual Volume 2 Nuclear Power Plant default scenario. This was completed to analyze how the default FRMAC values compared with actual measurements. For Cs:Cs and Cs:Cs (1 wk after shutdown), the ratios were 0.969 ± 0.025 and 0.13 ± 0.007, respectively. These were significantly different from the FRMAC default values of 1.6 and 0.4, but they were of the same order of magnitude. Spatially, larger ratios with high uncertainties were recorded near Tokyo, over 200 km from the accident site. The I to Cs ratios, as expected, decayed exponentially over time but were significantly higher than FRMAC values. Six ratios were greater than 20 within 10 d after shutdown compared to the FRMAC default value of 9.9. In addition, the highest ratios were located less than 75 km to the southwest of the plant. Comparing all the isotopic ratios to the FRMAC manual illustrated differences between the default values and the actual field results. This is at least partly due to the fact that the FRMAC default values are based on an average between a pressurized water reactor and boiling water reactor release. These results of the comparison illustrate that the Assessment Manual default values should only be used when no other data are available.

Background characterization of an ultra-low background liquid scintillation counter.

The Ultra-Low Background Liquid Scintillation Counter developed by Pacific Northwest National Laboratory will expand the application of liquid scintillation counting by enabling lower detection limits and smaller sample volumes. By reducing the overall count rate of the background environment approximately 2 orders of magnitude below that of commercially available systems, backgrounds on the order of tens of counts per day over an energy range of ~3-3600keV can be realized. Initial test results of the ULB LSC show promising results for ultra-low background detection with liquid scintillation counting.

Development of a customized radiation monitor for livestock screening.

The monitoring and decontamination of livestock has been an emerging topic in emergency response planning in recent years. Under the National Response Framework, the U.S. Department of Agriculture is tasked with providing support to the states during a radiological incident for the "assessment, control, and decontamination of contaminated animals, including companion animals, livestock, poultry, and wildlife." While there are currently no protocols in place on a national level for coordinated animal response, working groups have been developing a command structure and task force procedures, and some states have issued their own guidelines. A customized Bovine Screening Portal was manufactured and tested at Texas A&M University to investigate the operational capabilities in detecting, identifying, and localizing external contamination on livestock. An array of six sodium iodide detectors attached to power-over-Ethernet Multi-Channel Analyzers was used to collect time-stamped count rates, and spectral data were collected as a heifer was led past the detector panel. A 1.85 × 10(5) Bq 137Cs source was placed in four locations on a heifer, which was led through a cattle chute adjacent to the detector panel. The trials were repeated walking the heifer through a walkway with detectors hung on cattle pens lining a walkway. The Bovine Screening Portal observed increased count rates (>10σ) from the 1.85 × 10(5) Bq 137Cs source in live time. The identification capabilities with the intuitive software interface of the BSP are consistent with the requirements of a detection system for radiological emergency management of livestock.

An experimental determination of FIDLER scanning efficiency at specific speeds.

As part of mass remediation efforts across the country some radiation detection systems are now being used in conjunction with data logging and positioning system technology. These systems can be used in the scanning mode, simultaneously recording both count rate and position. Following data analysis, hot spots can be identified and remediation efforts for that particular area can commence. This technique has been used for nearly a decade and has had success in accelerating preliminary remediation work while also reducing potential clean up costs. However, little work has been completed on how the sensitivity of these detection systems are affected when used with this technology because while the intrinsic efficiency of the detector is constant, scanning efficiency can vary depending on data sampling time and scanning speed. To better understand scanning efficiency for a detector attached to such a system, a device was developed which moved soil at a constant speed underneath a Field Instrument for Detecting Low Energy Radiation (FIDLER). Count rate was measured every 2 s as a 241Am source passed under the detector at speeds ranging from approximately 10 cm s(-1) to 100 cm s(-1). A surface source and a buried source were both examined. Experimental detection efficiency was calculated and compared to Monte Carlo generated results. For the surface source, the efficiency dropped to a value of approximately 1% at 100 cm s(-1). At the same speed, the buried source had a detection efficiency of 0.1%, primarily due to attenuation of the low energy photon in the soil. It was also noted that the response time of the meter affected the scanning efficiency. With a response time set at 1 s, higher average efficiencies were recorded but with a large standard deviation from the mean. Higher response time setting had the effect of reducing the variability of the reading but also reducing efficiency.

An innovative technique in scanning land areas with a multi-FIDLER system.

Remediation can be a long and tedious effort. One possible step in this process is the scanning of land to locate elevated areas of radiological contamination. By adapting existing global positioning technology with radiation detection systems, this process can be significantly accelerated. The Field Instrument for Detecting Low Energy Radiation (FIDLER) was used in conjunction with a Global Positioning System (GPS) and Trimble data logger. With this system two different land areas were scanned using two different scanning methods. In the first method, three FIDLERs were attached to a baby jogger and were used to scan a 20-acre site devoid of vegetation. The second technique involved individuals carrying the instruments over a 15-acre site that contained vegetation. Here the FIDLERs were waved in front of the workers in 50-cm arcs. In all cases, radiological and position data were collected by the data loggers. Using these results, accurate maps were generated for each site clearly illustrating areas and spots of elevated activity. By employing this technique over 250,000 data points pertaining to position and count rate were used to map nearly 40 acres of land in under 3 wk.

A comparison between default EGS4 and EGS4 with bound Compton cross sections when scattering occurs in bone and fat.

Computer simulation packages are important tools in understanding how radiation interacts with matter. EGS4 is a photon/electron Monte Carlo transport program that is employed in the health/medical physics field. Due to its high energy roots, the default version of EGS4 treats all electrons as unbound and therefore uses the Klein-Nishina cross section formula to determine Compton scattering angle distributions and the probability of Compton scattering through the branching ratio. Researchers have created improvements to EGS4 that account for the bound Compton cross section as well as other scattering properties. Numerical experiments were performed on both the default code and modified EGS4 to examine output differences in low Z materials such as fat and bone. Four incident photon energies were considered. At higher energies (500 keV and 1 MeV) the default and modified EGS4 codes produced results within 2sigma of one another. At 50 and 100 keV differences in scattering angle distribution and branching ratio values were found. In addition, the number of photoelectric absorptions and Compton scatters were also different at these energies.