PERFORMANCE OF THE VARSKIN 5 (v5.3) ELECTRON DOSIMETRY MODEL.

A new electron skin dosimetry model was developed for the VARSKIN 5 tissue dosimetry code. This model employs energy deposition kernels that provides for improved accuracy of energy deposition at the end of electron tracks. The Monte Carlo code EGSnrc was utilized to develop these energy deposition kernels such that scaling of electron energy loss is dependent on effective atomic number and density of the source material, electron range and conservation of energy. This work contrasts VARSKIN's electron dosimetry model to several existing deterministic and Monte Carlo dosimetry tools to determine the efficacy of these improvements. Comparison results are given for a wide range of scenarios that extend beyond the typical use of VARSKIN, including mono-energetic electrons and a homogenous water medium. For planar and point sources in contact with the skin, VARSKIN produces results equated to other dosimetry methods within 10%. However, it appears that VARSKIN is unable to account accurately for electron energy loss with the introduction of a cover material or an air gap. The comparisons herein confirm that VARSKIN provides accurate electron dose calculations for skin-contamination scenarios.

Building protection- and building shielding-factors for environmental exposure to radionuclides and monoenergetic photon emissions.

We describe a simplified method for calculating both building protection- and shielding-factors for generic one- and two-story housing-unit models that are source-term dependent. Typically, radionuclide-independent factors are applied generically to external dose coefficients to account for the radiation shielding effects of indoor residences. In reality, the shielding effectiveness of each housing-unit would change over time as the radionuclide mixture and gamma-ray energy spectrum change due to physical effects such as deposition, radioactive decay, weathering effects, and decontamination efforts. Thus, it is necessary to develop factors designed for multiple photon energy spectrums to generate a more realistic estimate of the shielding effectiveness of a particular building. It is impractical to develop factors specific to a spectrum of photons emitted by each radionuclide of interest. Therefore, Monte Carlo simulations have been performed for sixteen monoenergetic photon energies from 0.10 to 3.0 MeV to characterize the 3D radiation fluence through each housing-unit produced by two idealized, yet realistic, environmental exposure scenarios. Results of these simulations were then used to develop fitted logarithmic functions (extrapolated to 0.0 MeV) to correlate an estimated factor to any monoenergetic photon energy up to 3.0 MeV. To verify these functions, another series of Monte Carlo simulations were performed for a select set of radionuclides to develop radionuclide-specific building protection- and shielding-factors. Good agreement is achieved between factors estimated using the presented functions and those calculated directly using Monte Carlo methods. Factors predicted by these functions are found to be in general agreement with other study results reported on similar structures which applied various computational methods and source-terms. This study only focuses on generic one- and two-story homes to provide a practical application that can contribute to improve the preparedness for and the response to a nuclear or radiological emergency.

Contaminant deposition building shielding factors for US residential structures.

This paper presents validated building shielding factors designed for contemporary US housing-stock under an idealized, yet realistic, exposure scenario from contaminant deposition on the roof and surrounding surfaces. The building shielding factors are intended for use in emergency planning and level three probabilistic risk assessments for a variety of postulated radiological events in which a realistic assessment is necessary to better understand the potential risks for accident mitigation and emergency response planning. Factors are calculated from detailed computational housing-units models using the general-purpose Monte Carlo N-Particle computational code, MCNP5, and are benchmarked from a series of narrow- and broad-beam measurements analyzing the shielding effectiveness of ten common general-purpose construction materials and ten shielding models representing the primary weather barriers (walls and roofs) of likely US housing-stock. Each model was designed to scale based on common residential construction practices and include, to the extent practical, all structurally significant components important for shielding against ionizing radiation. Calculations were performed for floor-specific locations from contaminant deposition on the roof and surrounding ground as well as for computing a weighted-average representative building shielding factor for single- and multi-story detached homes, both with and without basement as well for single-wide manufactured housing-unit.

Cloud immersion building shielding factors for US residential structures.

This paper presents validated building shielding factors designed for contemporary US housing-stock under an idealized, yet realistic, exposure scenario within a semi-infinite cloud of radioactive material. The building shielding factors are intended for use in emergency planning and level three probabilistic risk assessments for a variety of postulated radiological events in which a realistic assessment is necessary to better understand the potential risks for accident mitigation and emergency response planning. Factors are calculated from detailed computational housing-units models using the general-purpose Monte Carlo N-Particle computational code, MCNP5, and are benchmarked from a series of narrow- and broad-beam measurements analyzing the shielding effectiveness of ten common general-purpose construction materials and ten shielding models representing the primary weather barriers (walls and roofs) of likely US housing-stock. Each model was designed to scale based on common residential construction practices and include, to the extent practical, all structurally significant components important for shielding against ionizing radiation. Calculations were performed for floor-specific locations as well as for computing a weighted-average representative building shielding factor for single- and multi-story detached homes, both with and without basement, as well for single-wide manufactured housing-units.

Experimental shielding evaluation of the radiation protection provided by the structurally significant components of residential structures.

The human health and environmental effects following a postulated accidental release of radioactive material to the environment have been a public and regulatory concern since the early development of nuclear technology. These postulated releases have been researched extensively to better understand the potential risks for accident mitigation and emergency planning purposes. The objective of this investigation is to provide an updated technical basis for contemporary building shielding factors for the US housing stock. Building shielding factors quantify the protection from ionising radiation provided by a certain building type. Much of the current data used to determine the quality of shielding around nuclear facilities and urban environments is based on simplistic point-kernel calculations for 1950s era suburbia and is no longer applicable to the densely populated urban environments realised today. To analyse a building's radiation shielding properties, the ideal approach would be to subject a variety of building types to various radioactive sources and measure the radiation levels in and around the building. While this is not entirely practicable, this research analyses the shielding effectiveness of ten structurally significant US housing-stock models (walls and roofs) important for shielding against ionising radiation. The experimental data are used to benchmark computational models to calculate the shielding effectiveness of various building configurations under investigation from two types of realistic environmental source terms. Various combinations of these ten shielding models can be used to develop full-scale computational housing-unit models for building shielding factor calculations representing 69.6 million housing units (61.3%) in the United States. Results produced in this investigation provide a comparison between theory and experiment behind building shielding factor methodology.

The new VARSKIN 4 photon skin dosimetry model.

A new photon skin dosimetry model, described here, was developed as the basis for the enhanced VARSKIN 4 thin tissue dosimetry code. The model employs a point-kernel method that accounts for charged particle build-up, photon attenuation and off-axis scatter. Early comparisons of the new model against Monte Carlo particle transport simulations show that VARSKIN 4 is highly accurate for very small sources on the skin surface, although accuracy at shallow depths is compromised for radiation sources that are on clothing or otherwise elevated from the skin surface. Comparison results are provided for a one-dimensional point source, a two-dimensional disc source and three-dimensional sphere, cylinder and slab sources. For very small source dimensions and sources in contact with the skin, comparisons reveal that the model is highly predictive. With larger source dimensions, air gaps or the addition of clothing between the source and skin; however, VARSKIN 4 yields over-predictions of dose by as much as a factor of 2 to 3. These cursory Monte Carlo comparisons confirm that significant accuracy improvements beyond the previous version were achieved for all geometries. Improvements were obtained while retaining the VARSKIN characteristic user convenience and rapid performance.

A review of contemporary methods for the presentation of scientific uncertainty.

Graphic methods for displaying uncertainty are often the most concise and informative way to communicate abstract concepts. Presentation methods currently in use for the display and interpretation of scientific uncertainty are reviewed. Numerous subjective and objective uncertainty display methods are presented, including qualitative assessments, node and arrow diagrams, standard statistical methods, box-and-whisker plots,robustness and opportunity functions, contribution indexes, probability density functions, cumulative distribution functions, and graphical likelihood functions.

Uncertainty of the thyroid dose conversion factor for inhalation intakes of 131I and its parametric uncertainty.

Inhalation exposures of 131I may occur in the physical form of a gas as well as a particulate. The physical characteristics pertaining to these different types of releases influence the intake and subsequent dose to an exposed individual. The thyroid dose received is influenced by the route through which 131I enters the body and its subsequent clearance, absorption and movement throughout the body. The radioactive iodine taken up in the gas-exchange tissues is cleared to other tissues or absorbed into the bloodstream of the individual and transferred to other organs. Iodine in the circulatory system is then taken up by the thyroid gland with resulting dose to that tissue. The magnitude of and uncertainty in the thyroid dose is important to the assessment of individuals exposed to airborne releases of radioiodine. Age- and gender-specific modelling parameters have resulted in significant differences between gas uptake, particulate deposition and inhalation dose conversion factors for each age and gender group. Inhalation dose conversion factors and their inherent uncertainty are markedly affected by the type of iodine intake. These differences are expected due to the modelling of particulate deposition versus uptake of gas in the respiratory tract. Inhalation dose estimates via iodine gases are very similar and separate classifications may not be necessarily based on this assessment.

A modified ICRP 66 iodine gas uptake model and its parametric uncertainty.

Intakes via inhalation may occur from radionuclides released in the form of a gas. The chemical characteristics pertaining to the release influence the intake and subsequent dose to an exposed individual. Gases are taken up or absorbed in the entire respiratory tract and the associated uptake mechanisms are quite different from deposition of particulates. Gaseous iodine can exist in various chemical forms, e.g., elemental iodine, inorganic, and organic iodine compounds. These different chemical species play an integral role in the gaseous uptake o f iodine in t he respiratory tract. Gas uptake in the various regions of the respiratory tract results in the intake of iodinated material into the body. The radioactive iodine taken up in the gas-exchange tissues is absorbed into the bloodstream of an individual and subsequently transferred to other organs. Iodine in the circulatory system can then be taken up by the thyroid gland, with resulting dose to the thyroid. The magnitude and uncertainty in regional gas uptake is important in the assessment of individuals exposed to airborne releases of radioiodine. The current ICRP 66 model is rudimentary and estimates regional gas uptake based on solubility and reactivity of the different radionuclides entering the respiratory tract. The modified model proposed here employs methodology and a mathematical structure to determine estimates of fractional gas uptake rather than defaulting to literature values, as in the current ICRP model. Model parameters have been assigned input distributions and estimates of uncertainty have been determined. A sensitivity analysis of these parameters has been performed to demonstrate the importance of each of these parameters. The sensitivity analysis ranks the model-input parameters by their importance to estimates of regional gas uptake. The model developed herein may be used for improved estimation of gas uptake in the respiratory tract and subsequent dose estimates from the different chemical forms of radioiodine.

Uncertainty in transport factors used to calculate historical dose from 131I releases at the Savannah River Site.

During the 1950's, atmospheric release of 131I was one of the largest contributors to offsite dose at the Savannah River Site. Computer models used to estimate offsite dose involve the use of many parameters with wide ranges of uncertainty. The overall uncertainty in dose can be estimated by propagating the uncertainty of each parameter through the model. A major component of the calculational model can be solved for a given release scenario and condensed into a transport factor, which, when multiplied by the air concentration (or deposition) and the appropriate dose conversion factor, can be used to estimate a specific pathway dose. Uncertainties are estimated for the period of 1955-1961 for all parameters contributing to the 131I transport factor for each pathway. The overall transport factor including all pathways has ranges characterized by maximum-to-minimum ratios (95% to 5%) of about 40. The parameter shown to have the greatest impact on the transport factor calculation was the fraction of elemental iodine released.

Age-specific uncertainty of the 131I ingestion dose conversion factor.

The production of weapons-grade nuclear materials and their by-products has resulted in a number of releases from United States Department of Energy facilities. 131I, a fission by-product, is one of the most common radionuclides generated and released to the environment. It is known that there are differences in various physiological parameters over all age groups when considering biokinetic modeling of iodine. The establishment of age-specific dose conversion factor uncertainty is necessary for accurate internal dose assessment. The 131I dose conversion factor determined herein is log-normally distributed with varying age-specific distribution characteristics. The two most important parameters for determination of the dose conversion factor, in all age groups, are thyroid mass and iodine uptake fraction. These parameters are assumed to be highly correlated with a relationship that is quite important to dose conversion factor uncertainty. Dose estimates to individuals exposed to radioiodine can be determined more accurately with an increased understanding of the correlation between thyroid mass and uptake fraction. Improved dose estimates following oral intakes of 131I can be made from the consideration of age-specific dose conversion factors and their input parameters.

Age-specific uncertainty in particulate deposition for 1 microm AMAD particles using the ICRP 66 lung model.

The ICRP 66 lung model may be used to determine age-specific dose estimates for members of the public via the inhalation pathway. A significant source of uncertainty in internal dosimetric modeling is due to particulate deposition in regions of the respiratory tract. Uncertainties in estimates of particulate deposition are present because of the inherent variability in the deposition model and its various input parameters. An improved understanding of the uncertainty in particulate deposition will further guide research efforts and improve our ability to quantify internal dose estimates. The ICRP 66 lung deposition model is most sensitive to breathing rate when 1 microm AMAD particles are inhaled by members of the public. Uncertainties in deposition fractions are shown to span an order of magnitude with their distributions varying by age and sex for a particular lung region. Age-specific uncertainties of deposition fraction demonstrate increased estimates of extrathoracic deposition in younger age groups due to the decreased size of the respiratory tract airways. This age-specific trend becomes more pronounced for very young children and infants. The largest fractional deposition occurs in the alveolar and extrathoracic regions, regardless of age and sex.

Uranium, thorium, and potassium in soils along the shore of Lake Issyk-Kyol in the Kyrghyz Republic.

The Kyrghyz Republic, located in the southeastern region of the former Soviet Union, maintains a population of more than one-half-million persons and is heavily dependent on Lake Issyk-Kyol, both to draw tourists to the area and for its utilization by some as a food and recreation source. Historical surveys, conducted primarily for geological exploration, have indicated that localized areas of shoreline on Lake Issyk-Kyol have relative radiation levels in excess of ambient background by as much as a factor often. Uranium mining operations in the mountains bordering the lake to the south may have resulted in the contamination of a number of areas on the lake's southern shore. Concentrations of naturally occurring uranium, thorium, and potassium are present in these soils in elevated quantities. This paper presents the results of an investigation of soil concentrations along the shoreline of Lake Issyk-Kyol relative to previously discovered areas of high exposure rate.

The Gaussian atmospheric transport model and its sensitivity to the joint frequency distribution and parametric variability.

Reconstructed meteorological data are often used in some form of long-term wind trajectory models for estimating the historical impacts of atmospheric emissions. Meteorological data for the straight-line Gaussian plume model are put into a joint frequency distribution, a three-dimensional array describing atmospheric wind direction, speed, and stability. Methods using the Gaussian model and joint frequency distribution inputs provide reasonable estimates of downwind concentration and have been shown to be accurate to within a factor of four. We have used multiple joint frequency distributions and probabilistic techniques to assess the Gaussian plume model and determine concentration-estimate uncertainty and model sensitivity. We examine the straight-line Gaussian model while calculating both sector-averaged and annual-averaged relative concentrations at various downwind distances. The sector-average concentration model was found to be most sensitive to wind speed, followed by horizontal dispersion (sigmaZ), the importance of which increases as stability increases. The Gaussian model is not sensitive to stack height uncertainty. Precision of the frequency data appears to be most important to meteorological inputs when calculations are made for near-field receptors, increasing as stack height increases.

Uncertainty in particulate deposition for 1 microm AMAD particles in an adult lung model.

The ICRP 66 lung model may be used to determine dose estimates for members of the public via the inhalation pathway. A significant source of uncertainty in internal dosimetric modelling is due to particulate deposition in regions of the respiratory tract. Uncertainties in estimates of particulate deposition are present because model input parameters have their own inherent variability. These sources of uncertainty need to be examined in an effort to understand better model processes and to estimate better the doses received by individuals exposed through the inhalation pathway. An improved understanding of the uncertainty in particulate deposition will further guide research efforts and improve our ability to quantify internal dose estimates. The ICRP 66 lung deposition model is most sensitive to breathing rate when 1 microm AMAD particles are inhaled by members of the public. Uncertainties in deposition fractions are shown to span an order of magnitude with their distributions varying by gender for a particular lung region. The largest fractional deposition occurs in the deep lung alveolar and extrathoracic regions.

Analysis of an internal kinetic model for free and bound tritium.

Internal dosimetry models that currently drive regulatory compliance decisions assume that tritium retention kinetic behavior can be modeled by a single exponential function. This is contrary to the results of a number of modeling techniques, which indicate that while elemental tritium (HT) and tritiated water (HTO) are the most commonly released forms of tritium, organically-bound tritium (OBT) doses can be quite significant. In this paper, a unified two-compartment model of the retention kinetics of HTO and OBT is examined for the purpose of investigating the importance of metabolic routes not considered in the ICRP one- and two-exponent models; namely the transfer of tritium from the HTO compartment to the OBT compartment and vice versa. In particular, the effect of intake ratio is investigated, and a detailed analysis of dosimetric implications is performed. For typical combined intakes of HTO and OBT, the number of disintegrations from the two tritium forms can be roughly equal. This result, when combined with the suggested greater biological effectiveness of OBT, indicates effective doses will be greater than those obtained from a single exponential model. The results of this study corroborate previous findings using the two-compartment model for the cases of HTO-only and or OBT-only intakes and compare well with data taken from studies on animals and human subjects.

Environmental releases of radioactivity and the incidence of thyroid disease at the Ignalina Nuclear Power Plant.

The Ignalina Nuclear Power Plant consists of two Russian-made RBMK-1500 reactors. The plant uses Lake Druksiai as a natural reservoir for cooling water. Within the framework of the revised radiation dose limitation system, site-specific routine release conversion factors and maximum annual effective doses for the dominant radionuclides and pathways were evaluated for both atmospheric and aquatic releases. Using calculated release conversion factors, the locations of the highest predicted activity concentrations were determined for air and for the dilution zone of heated effluent water during the period 1984-1998. Committed effective doses for critical group members were less than 0.001 mSv for Ignalina Nuclear Power Plant airborne releases and less than 0.05 mSv for aquatic releases. These dose estimates are lower than the 1 mSv dose limit for the adjacent population. In the case of Ignalina Nuclear Power Plant, taking into account the uncertainties, a recommendation for the administrative dose constraint is 0.25 mSv y(-1). This dose level may scarcely affect human health. Interestingly, during screening for thyroid disorders, endocrinologists and pediatric-endocrinologists determined a dominance of abnormal thyroids (up to 60%) among school children in the vicinity of Ignalina NPP. The data on neonatal screening for congenital hypothyroidism and transient hyperthyrotropinemia, however, suggested a possibility that the majority of abnormal thyroid cases were related to stable iodine deficiency. Thus, the influence of Ignalina Nuclear Power Plant on thyroid disorders is highly conjectural and unlikely to be associated with the observed levels of childhood thyroid disease.

Ingestion pathway model developed for use with an acute atmospheric dose model at the Savannah River Site.

AXAIRQ is a straight-line Gaussian plume dose model used for prospective accident assessment at the Savannah River Site and currently includes the following dose pathways: inhalation, ground shine, and plume shine. In the event of an accident, another possible pathway for dose would be through ingestion of locally produced contaminated foodstuffs. A model, AXINGST, has been developed that would incorporate this pathway. Currently available ingestion models were referenced as a basis for AXINGST. The model calculates an ingestion dose following an atmospheric release of radionuclides and includes site-specific variables where applicable. AXINGST estimates tritium vegetation concentrations that are within a factor of 20-40 of measured data during previous accidental releases at SRS.