S. R. Biegalski

Analysis of data from sensitive U.S. monitoring stations for the Fukushima Dai-ichi nuclear reactor accident

The March 11, 2011 9.0 magnitude undersea megathrust earthquake off the coast of Japan and subsequent tsunami waves triggered a major nuclear event at the Fukushima Dai-ichi nuclear power station. At the time of the event, units 1, 2, and 3 were operating and units 4, 5, and 6 were in a shutdown condition for maintenance. Loss of cooling capacity to the plants along with structural damage caused by the earthquake and tsunami resulted in a breach of the nuclear fuel integrity and release of radioactive fission products to the environment. Fission products started to arrive in the United States via atmospheric transport on March 15, 2011 and peaked by March 23, 2011. Atmospheric activity concentrations of (131)I reached levels of 3.0×10(-2) Bqm(-3) in Melbourne, FL. The noble gas (133)Xe reached atmospheric activity concentrations in Ashland, KS of 17 Bqm(-3). While these levels are not health concerns, they were well above the detection capability of the radionuclide monitoring systems within the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty.

Investigation of Stochastic Radiation Transport Methods in Binary Random Heterogeneous Mixtures

Abstract This paper reviews existing Monte Carlo techniques for performing neutron transport simulations in binary random heterogeneous fissile fuels and presents a new approach offering superior efficiency at little cost in fidelity for problems involving densely packed, optically thick absorbers. The accuracy of the chord-length sampling technique is demonstrated to be a function of the total optical thicknesses and optical scattering thickness of the constituent materials as well as the packing density of the fissile kernels. The results of this parameter assessment provide a foundation for an original hybrid algorithm that combines homogeneous and explicit geometry models within a single Monte Carlo simulation. The geometry model utilized is selected according to the energy-dependent optical thickness. By partitioning the geometry representation within a single Monte Carlo simulation into homogenous and heterogeneous energy-dependent models, acceptable ensemble average results are obtained in a fraction of the run time of the detailed explicit geometry benchmark method.

Examination of radioargon production by cosmic neutron interactions

Radioargon isotopes, particularly (37)Ar, are currently being considered for use as an On-Site Inspection (OSI) relevant radionuclide within the context of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). In order to understand any soil air measurements taken during an OSI, the radioargon background due to cosmic ray induced activation along with other sources must be understood. An MCNP6 model was developed using the cosmic ray source feature within the code to examine the neutron flux at ground level as a function of various conditions: date during the solar magnetic activity cycle, latitude of sampling location, geology of the sampling location, and sampling depth. Once the cosmic neutron flux was obtained, calculations were performed to determine the rate of radioargon production for the main interactions. Radioargon production was shown to be highly dependent on the soil composition, and a range of (37)Ar production values at 1 m depth was found with a maximum production rate of 4.012 atoms/sec/m(3) in carbonate geologies and a minimum production rate of 0.070 atoms/sec/m(3) in low calcium granite. The sampling location latitude was also shown to have a measurable effect on the radioargon production rate, where the production of (37)Ar in an average continental crust is shown to vary by a factor of two between the equator and the poles. The sampling dates position within the solar magnetic activity cycle was also shown to cause a smaller change, less than a factor of 1.2, in activation between solar maxima and solar minima.

UTEX modeling of radioxenon isotopic fractionation resulting from subsurface transport

The underground transport of environmental xenon (UTEX) model is a finite-difference code that was developed at the University of Texas at Austin to simulate the transport of radioxenon from an underground nuclear detonation to the surface. UTEX handles a time dependent source term and includes the effects of radioactive decay to determine isotopic signatures of the various radioxenon species as a function of release time. The model shows that significant perturbations in the isotopic signatures are possible under some geologic conditions. Transport of radioxenon gas in UTEX is driven in large part by atmospheric pumping. A study was undertaken to characterize the dependence of resulting isotopic signatures on the various geologic and physical parameters that define the system model. Additionally, the model was used to roughly simulate isotopic measurements at various depths and position; the potential dependence of isotopic radioxenon fractionation on sampling depth and lateral position between fractures was examined.

Noble gas migration experiment to support the detection of underground nuclear explosions

A Noble Gas Migration Experiment injected 127Xe, 37Ar, and sulfur hexafluoride into a former underground nuclear explosion shot cavity. These tracer gases were allowed to migrate from the cavity to near-surface and surface sampling locations and were detected in soil gas samples collected using various on-site inspection sampling approaches. Based on this experiment we came to the following conclusions: (1) SF6 was enriched in all of the samples relative to both 37Ar and 127Xe. (2) There were no significant differences in the 127Xe to 37Ar ratio in the samples relative to the ratio injected into the cavity. (3) The migratory behavior of the chemical and radiotracers did not fit typical diffusion modeling scenarios.

Regional transport of radioxenon released from the Chalk River Laboratories medical isotope facility

An examination of proposed sampling sites near Chalk River Laboratories in Ontario, Canada is performed by considering the regional transport of radioxenon using atmospheric dispersion modeling. The local geography is considered, as are the local meteorological conditions during the summer months. In particular the impacts of predicted conditions on the imprinting of atmospheric radioxenon into the subsurface are considered and weighed against site proximity, geography, and geology.

A 24-element Silicon PIN diode detector for high resolution radioxenon measurements using simultaneous X-ray and electron spectroscopy

The measurement of atmospheric radioxenon is an important tool for monitoring nuclear weapons testing. The development of new and improved xenon detection methods supports the monitoring program of the Comprehensive Test Ban Treaty Organization (CTBTO). In the current work we have developed a 24-element Si PIN diode detector to measure both the characteristic X-rays and the high energy mono-energetic conversion electrons emitted by the xenon radioisotopes. The low noise properties and ultra-thin entrance window of the PIN diodes are well suited for resolving the relatively low energy X-ray lines while simultaneously measuring the high energy conversion electrons with high collection efficiency and near-Gaussian peak shapes. The use of coincidence gating between the X-rays and conversion electrons can further improve the detection sensitivity, which we show to rival the current HPGe and scintillator based xenon detection systems that rely mostly on Gamma-ray and Beta/Gamma coincidence detection, respectively. The Si PIN detector arrangement offers others advantages compared to current xenon detection methods, such as compact construction, intrinsically low background, and the lack of any memory effect from previous measurements. We discuss the construction of the detector and present measurements performed with 131mXe, 133Xe, 133m Xe and 135Xe. Finally, we make an estimate of the minimum detectable concentration (MDC) for each isotope and compare with the CTBTO requirements.

Measurement of Fukushima aerosol debris in Sequim and Richland, WA and Ketchikan, AK

Aerosol collections were initiated at several locations by Pacific Northwest National Laboratory (PNNL) shortly after the Great East Japan earthquake of May 2011. Aerosol samples were transferred to laboratory high-resolution gamma spectrometers for analysis. Similar to treaty monitoring stations operating across the Northern hemisphere, iodine and other isotopes which could be volatilized at high temperature were detected. Though these locations are not far apart, they have significant variations with respect to water, mountain-range placement, and local topography. Variation in computed source terms will be shown to bound the variability of this approach to source estimation.

Analyzing nuclear fuel cycles from isotopic ratios of waste products applicable to measurement by accelerator mass spectrometry

Abstract An extensive study was conducted to determine isotopic ratios of nuclides in spent fuel that may be utilized to reveal historical characteristics of a nuclear reactor cycle. This forensic information is important to determine the origin of unknown nuclear waste. The distribution of isotopes in waste products provides information about a nuclear fuel cycle, even when the isotopes of uranium and plutonium are removed through chemical processing. Several different reactor cycles of the pressurized water reactor, boiling water reactor, Canada deuterium uranium reactor, and liquid-metal fast breeder reactor were simulated for this work with the ORIGEN-ARP and ORIGEN2.2 codes. The spent-fuel nuclide concentrations of these reactors were analyzed to find the most informative isotopic ratios indicative of irradiation cycle length and reactor design. Special focus was given to long-lived and stable fission products that would be present many years after their creation. For such nuclides, mass spectrometry analysis methods often have better detection limits than classic gamma-ray spectroscopy. The isotopic ratios 151Sm/146Sm, 149Sm/146Sm, and 244Cm/246Cm were found to be good indicators of fuel cycle length and are well suited for analysis by accelerator mass spectroscopy.

Detection in subsurface air of radioxenon released from medical isotope production

In order to better understand potential backgrounds of Comprehensive-Nuclear Test-Ban Treaty on-site inspection relevant gases, a sampling campaign was performed near Canadian Nuclear Laboratories in the Ottawa River Valley, a major source of environmental radioxenon. First of their kind measurements of atmospheric radioxenon imprinted into the shallow subsurface from an atmospheric pressure driven force were made using current on-site inspection techniques. Both atmospheric and subsurface gas samples were measured and analyzed to determine radioxenon concentrations. These measurements indicate that under specific sampling conditions, on the order of ten percent of the atmospheric radioxenon concentration may be measured via subsurface sampling.