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.

Accounting for 222Rn loss during oven drying for the immediate laboratory gamma-ray spectroscopy of collected soil samples.

Drying soil samples in an oven to remove water alters the 222Rn emanation rate. Measurements of the oven drying 222Rn emanation rate from soil were made with a continuous radon monitor and the degree of 222Rn disequilibrium was quantified by laboratory gamma-ray spectroscopy. This paper presents a disequilibrium correction where the 226Ra activity in oven-dried soil samples is inferred from immediate laboratory gamma-ray spectroscopy of 214Bi before 222Rn and its decay progeny reach secular equilibrium.

A comparison of minimum detectable and proposed maximum allowable soil concentration cleanup levels for selected radionuclides.

Regulations on the release of a radioactively contaminated site for unrestricted use are currently being established by the Environmental Protection Agency. The effective dose equivalent rate limit for the reasonably maximally exposed individual was proposed at 0.15 mSv y(-1). The purpose of this study is to investigate whether or not maximum allowable soil concentrations of common radionuclides corresponding to 0.15 mSv y(-1) are readily detectable. These maximum allowable soil concentrations were estimated using RESRAD. The RESRAD estimates account for an effective dose equivalent rate from external radiation plus the committed effective dose equivalent rate from internal radiation delivering 0.15 mSv y(-1) to the reasonably maximally exposed individual. For Michigan and Arizona soil, the minimum detectable activities were calculated for a few radionuclides and compared to the RESRAD estimated maximum allowable concentrations. Considering only gamma-ray spectroscopy, this study found no evidence that concentrations of gamma-ray emitting radionuclides in soil contributing to 0.15 mSv y(-1) were undetectable.

Comparison of in situ and laboratory gamma spectroscopy of natural radionuclides in desert soil.

In situ and laboratory gamma spectroscopy were used to characterize natural background levels of radiation in the soil at eight sites around the Yucca Mountain Range. The purpose of this practical field analysis was to determine if published empirical in situ calibration factors would yield accurate quantitative specific activities (Bq kg(-1)) in a desert environment. Corrections were made to the in situ calibration factors to account for the on-axis response of a detector with a thin beryllium end window. The in situ gamma spectroscopy results were compared to laboratory gamma spectroscopy of soil samples gathered from each site. Five natural radionuclides were considered: 40K, 214Pb, 214Bi, 208Tl, and 228Ac. The in situ determined specific activities were consistently within +/-15% of the laboratory soil sample results. A quantitative discussion of the factors contributing to the uncertainty in the in situ and laboratory results is included. Analysis on the specific activity data using statistical hypothesis tests determined that three nuclides, 214Pb, 214Bi, and 228Ac showed a weak site dependence while the other two nuclides, 40K and 208Tl, did not exhibit a site dependence. Differing radiation background levels from site to site along with in situ and laboratory uncertainties in excess of 10% are two factors that account for the weak site dependence. Despite the good correlation between data, it was recommended that the in situ detector be calibrated by a detector-specific Monte Carlo code which would accurately model more complex geometries and source distributions.