Shaheen A. Dewji

Effective Dose Rate Coefficients for Immersions in Radioactive Air and Water

The Oak Ridge National Laboratory Center for Radiation Protection Knowledge (CRPK) has undertaken a number of calculations in support of a revision to the United States Environmental Protection Agency (US EPA) Federal Guidance Report on external exposure to radionuclides in air, water and soil (FGR 12). Age-specific mathematical phantom calculations were performed for the conditions of submersion in radioactive air and immersion in water. Dose rate coefficients were calculated for discrete photon and electron energies and folded with emissions from 1252 radionuclides using ICRP Publication 107 decay data to determine equivalent and effective dose rate coefficients. The coefficients calculated in this work compare favorably to those reported in FGR12 as well as by other authors that employed voxel phantoms for similar exposure scenarios.

COMPARISON OF MONOENERGETIC PHOTON ORGAN DOSE RATE COEFFICIENTS FOR STYLIZED AND VOXEL PHANTOMS SUBMERGED IN AIR

As part of a broader effort to calculate effective dose rate coefficients for external exposure to photons and electrons emitted by radionuclides distributed in air, soil or water, age-specific stylized phantoms have been employed to determine dose coefficients relating dose rate to organs and tissues in the body. In this article, dose rate coefficients computed using the International Commission on Radiological Protection reference adult male voxel phantom are compared with values computed using the Oak Ridge National Laboratory adult male stylized phantom in an air submersion exposure geometry. Monte Carlo calculations for both phantoms were performed for monoenergetic source photons in the range of 30 keV to 5 MeV. These calculations largely result in differences under 10 % for photon energies above 50 keV, and it can be expected that both models show comparable results for the environmental sources of radionuclides.

Estimated dose rates to members of the public from external exposure to patients with 131I thyroid treatment

PURPOSE Estimated dose rates that may result from exposure to patients who had been administered iodine-131 ((131)I) as part of medical therapy were calculated. These effective dose rate estimates were compared with simplified assumptions under United States Nuclear Regulatory Commission Regulatory Guide 8.39, which does not consider body tissue attenuation nor time-dependent redistribution and excretion of the administered (131)I. METHODS Dose rates were estimated for members of the public potentially exposed to external irradiation from patients recently treated with (131)I. Tissue attenuation and iodine biokinetics were considered in the patient in a larger comprehensive effort to improve external dose rate estimates. The external dose rate estimates are based on Monte Carlo simulations using the Phantom with Movable Arms and Legs (PIMAL), previously developed by Oak Ridge National Laboratory and the United States Nuclear Regulatory Commission. PIMAL was employed to model the relative positions of the (131)I patient and members of the public in three exposure scenarios: (1) traveling on a bus in a total of six seated or standing permutations, (2) two nursing home cases where a caregiver is seated at 30 cm from the patients bedside and a nursing home resident seated 250 cm away from the patient in an adjacent bed, and (3) two hotel cases where the patient and a guest are in adjacent rooms with beds on opposite sides of the common wall, with the patient and guest both in bed and either seated back-to-back or lying head to head. The biokinetic model predictions of the retention and distribution of (131)I in the patient assumed a single voiding of urinary bladder contents that occurred during the trip at 2, 4, or 8 h after (131)I administration for the public transportation cases, continuous first-order voiding for the nursing home cases, and regular periodic voiding at 4, 8, or 12 h after administration for the hotel room cases. Organ specific activities of (131)I in the thyroid, bladder, and combined remaining tissues were calculated as a function of time after administration. Exposures to members of the public were considered for (131)I patients with normal thyroid uptake (peak thyroid uptake of ∼27% of administered (131)I), differentiated thyroid cancer (DTC, 5% uptake), and hyperthyroidism (80% uptake). RESULTS The scenario with the patient seated behind the member of the public yielded the highest dose rate estimate of seated public transportation exposure cases. The dose rate to the adjacent room guest was highest for the exposure scenario in which the hotel guest and patient are seated by a factor of ∼4 for the normal and differentiated thyroid cancer uptake cases and by a factor of ∼3 for the hyperthyroid case. CONCLUSIONS It was determined that for all modeled cases, the DTC case yielded the lowest external dose rates, whereas the hyperthyroid case yielded the highest dose rates. In estimating external dose to members of the public from patients with (131)I therapy, consideration must be given to (patient- and case-specific) administered (131)I activities and duration of exposure for a more complete estimate. The method implemented here included a detailed calculation model, which provides a means to determine dose rate estimates for a range of scenarios. The method was demonstrated for variations of three scenarios, showing how dose rates are expected to vary with uptake, voiding pattern, and patient location.

Assessing internal contamination after the detonation of a radiological dispersion device using a 2×2-inch sodium iodide detector

The detonation of a radiological dispersion device may result in a situation where individuals inhale radioactive materials and require rapid assessment of internal contamination. The feasibility of using a 2×2-inch sodium-iodide detector to determine the committed effective dose to an individual following acute inhalation of gamma-emitting radionuclides was investigated. Experimental configurations of point sources with a polymethyl methacrylate slab phantom were used to validate Monte Carlo simulations. The validated detector model was used to simulate the responses for four detector positions on six different anthropomorphic phantoms. The nuclides examined included (241)Am, (60)Co, (137)Cs, (131)I and (192)Ir. Biokinetic modelling was employed to determine the distributed activity in the body as a function of post-inhalation time. The simulation and biokinetic data were used to determine time-dependent count-rate values at optimal detector locations on the body for each radionuclide corresponding to a target committed effective dose (E50) value of 250 mSv.

Effective dose rate coefficients for exposure to contaminated soil

The Oak Ridge National Laboratory Center for Radiation Protection Knowledge has undertaken calculations related to various environmental exposure scenarios. A previous paper reported the results for submersion in radioactive air and immersion in water using age-specific mathematical phantoms. This paper presents age-specific effective dose rate coefficients derived using stylized mathematical phantoms for exposure to contaminated soils. Dose rate coefficients for photon, electron, and positrons of discrete energies were calculated and folded with emissions of 1252 radionuclides addressed in ICRP Publication 107 to determine equivalent and effective dose rate coefficients. The MCNP6 radiation transport code was used for organ dose rate calculations for photons and the contribution of electrons to skin dose rate was derived using point-kernels. Bremsstrahlung and annihilation photons of positron emission were evaluated as discrete photons. The coefficients calculated in this work compare favorably to those reported in the US Federal Guidance Report 12 as well as by other authors who employed voxel phantoms for similar exposure scenarios.

Assessment of the Point-Source Method for Estimating Dose Rates to Members of the Public from Exposure to Patients with 131I Thyroid Treatment.

AbstractThe U.S. Nuclear Regulatory Commission (USNRC) initiated a contract with Oak Ridge National Laboratory (ORNL) to calculate radiation dose rates to members of the public that may result from exposure to patients recently administered iodine-131 (131I) as part of medical therapy. The main purpose was to compare dose rate estimates based on a point source and target with values derived from more realistic simulations of a human source and target. The latter simulations considered the time-dependent distribution of 131I in the patient and attenuation of emitted photons by the patient’s tissues. The external dose rate estimates were derived using Monte Carlo methods and two representations of the Phantom with Movable Arms and Legs (PIMAL), previously developed by ORNL and the USNRC, to model the patient and a nearby member of the public. Dose rates to tissues and effective dose rates were calculated for distances ranging from 10 cm to 300 cm between the phantoms. Dose rates estimated from these simulations are compared to estimates based on the point-source method, as well as to results of previous studies that estimated exposure from 131I patients. The point-source method overestimates dose rates to members of the public in very close proximity to an 131I patient but is a broadly accurate method of dose rate estimation at separation distances of 300 cm or more at times closer to administration.

Age-dependent comparison of monoenergetic photon organ and effective dose coefficients for pediatric stylized and voxel phantoms submerged in air

Dose rate coefficients computed using the University of Florida-National Cancer Institute pediatric series of voxel phantoms were compared with values computed using the Oak Ridge National Laboratory pediatric stylized phantoms in an air submersion exposure geometry. Simulations were conducted comparing phantoms classified within five ages: newborn, 1-year-old, 5-year-old, 10-year-old, and 15-year-old for both male and female sexes. This is a continuation of previous work comparing monoenergetic photon organ dose rate coefficients, as defined by ICRP Publication 103, for the male and female adult phantoms. With both the male and female data computed for each pediatric phantom age, effective dose rate coefficients and ratios were computed for voxel and stylized phantoms. Organ dose rate coefficients for the pediatric phantoms and ratios of organ dose rates for the voxel and stylized phantoms are provided for eight monoenergetic photon energies ranging from 30 keV to 5 MeV. Analysis of the contribution of the organs to effective dose is also provided. Comparison of effective dose rates between the voxel and stylized phantoms was within 5% between 500 keV and 5 MeV and within 10% between 70 keV and 5 MeV for phantoms  >1-year-old. Stylized newborn effective dose rates were consistently ~20% higher than the voxel counterpart, over all energies.

Radiation Dose in Active Interrogation

The fundamental concepts and quantities used to describe radiation dose continue to be refined to represent the best scientific understanding to protect humans and the environment. The safe implementation of active interrogation systems requires trade-offs between optimizing the use of radiation-generating technology to the benefit of mankind while minimizing the hazards and risks. Predicting the effects of the source and target radiation as it interacts is critical to optimizing the system’s ability to identify the unknown target material with high confidence in an accurate, economical, and time-efficient manner, while ensuring the safe operation of such systems in either open or closed environments. The radiation protection risks posed by the deployment of such systems are varied and potentially dangerous.

Organ and effective dose rate coefficients for submersion exposure in occupational settings

External dose coefficients for environmental exposure scenarios are often computed using assumption on infinite or semi-infinite radiation sources. For example, in the case of a person standing on contaminated ground, the source is assumed to be distributed at a given depth (or between various depths) and extending outwards to an essentially infinite distance. In the case of exposure to contaminated air, the person is modeled as standing within a cloud of infinite, or semi-infinite, source distribution. However, these scenarios do not mimic common workplace environments where scatter off walls and ceilings may significantly alter the energy spectrum and dose coefficients. In this paper, dose rate coefficients were calculated using the International Commission on Radiological Protection (ICRP) reference voxel phantoms positioned in rooms of three sizes representing an office, laboratory, and warehouse. For each room size calculations using the reference phantoms were performed for photons, electrons, and positrons as the source particles to derive mono-energetic dose rate coefficients. Since the voxel phantoms lack the resolution to perform dose calculations at the sensitive depth for the skin, a mathematical phantom was developed and calculations were performed in each room size with the three source particle types. Coefficients for the noble gas radionuclides of ICRP Publication 107 (e.g., Ne, Ar, Kr, Xe, and Rn) were generated by folding the corresponding photon, electron, and positron emissions over the mono-energetic dose rate coefficients. Results indicate that the smaller room sizes have a significant impact on the dose rate per unit air concentration compared to the semi-infinite cloud case. For example, for Kr-85 the warehouse dose rate coefficient is 7% higher than the office dose rate coefficient while it is 71% higher for Xe-133.

Critical issues in radiation protection knowledge management for preserving radiation protection research and development capabilities

Abstract As a hub of domestic radiation protection capabilities, Oak Ridge National Laboratory’s Center for Radiation Protection Knowledge has a mandate to develop and actuate a formal knowledge management (KM) effort. This KM approach exceeds recruitment and training efforts but focuses on formalized strategies for knowledge transfer from outgoing subject matter experts in radiation protection to incoming generations. It is envisioned that such an effort will provide one avenue for preserving domestic capabilities to support stakeholder needs in the federal government and the nuclear industry while continuing to lead and innovate in research and development on a global scale. However, in the absence of broader coordination within the United States, preservation of radiation protection knowledge continues to be in jeopardy in the absence of a dedicated KM effort.