D.M. Hamby

a comparison of sensitivity analysis techniques

Modeling the movement and consequence of radioactive pollutants is critical for environmental protection and control of nuclear facilities. Sensitivity analysis is an integral part of model development and involves analytical examination of input parameters to aid in model validation and provide guidance for future research. Sensitivities of 21 input parameters have been analyzed for a specific-activity tritium dose model using fourteen methods of parameter sensitivity analysis. This report demonstrates, for each sensitivity method, the required calculational effort, the sensitivity ranking of parameters, and the relative method performance. The sensitivity measures include the following: partial derivatives, variation of inputs by 1 standard deviation (SD) and by 20%, a sensitivity index, an importance index, a relative deviation of the output distribution, a relative deviation ratio, partial rank correlation coefficients, standardized regression coefficients, rank regression coefficients, the Smirnov test, the Cramer-von Mises test, the Mann-Whitney test, and the squared-ranks test.

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 ofthe former Soviet Union, maintains a population of more thanone-half-million persons and is heavily dependent on LakeIssyk-Kyol, both to draw tourists to the area and for itsutilization by some as a food and recreation source. Historical surveys, conducted primarily for geologicalexploration, have indicated that localized areas ofshoreline on Lake Issyk-Kyol have relative radiation levelsin excess of ambient background by as much as a factor often. Uranium mining operations in the mountains borderingthe lake to the south may have resulted in the contaminationof a number of areas on the lakes southern shore. Concentrations of naturally occurring uranium, thorium, andpotassium are present in these soils in elevated quantities. This paper presents the results of an investigation of soilconcentrations along the shoreline of Lake Issyk-Kyolrelative to previously discovered areas of high exposurerate.

A probabilistic estimation of atmospheric tritium dose

A radiation dose distribution has been calculated for an individual exposed to tritium oxide (tritiated water vapor) in the atmosphere. A specific activity model was used that is similar to that detailed in the Nuclear Regulatory Commissions Regulatory Guide 1.109. The values of 19 parameters are sampled consistent with assigned probability distributions using a Latin Hypercube approach. Dose was estimated assuming a concentration of tritium in atmospheric moisture of 0.13 +/- 0.16 Bq mL-1. Conventional dosimetric methods for tritium that were consistent with the Regulatory Guide result in an estimate of 0.63 microSv for annual exposures to airborne tritiated water. For the same atmospheric tritium concentration, the estimated dose distribution is lognormal and ranges from 0.08-3.2 microSv (at 95% confidence) with a median value of 0.59 microSv and a 95th percentile of 2.6 microSv.

Predicted versus measured tritium oxide concentrations at the Savannah River Site

Measured tritium oxide concentrations in air at various offsite locations are compared with concentrations predicted by three computer codes that are utilized at the Savannah River Site to estimate doses to maximally exposed offsite individuals. Annual average concentrations calculated by the computer models were compared with measured average concentrations taken from monitoring data collected over the last 10 y. The computer programs used for the comparison are AXAIRQ, MAXIGASP, and CAP88. The 10-y averaged ratios of predicted-to-measured tritium oxide air concentrations using AXAIRQ, MAXIGASP, and CAP88 are 1.89+/-0.56, 1.70+/-0.48, and 1.40+/-0.39, respectively. The difference in ratios is primarily due to different wind speed averages used within each of the models. These results show exceptional agreement, considering Gaussian plume models typically over predict annual average air concentrations by a factor of two to four.

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

Abstract— 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.

Decision making for risk management: a comparison of graphical methods for presenting quantitative uncertainty.

Previous research has shown that people err when making decisions aided by probability information. Surprisingly, there has been little exploration into the accuracy of decisions made based on many commonly used probabilistic display methods. Two experiments examined the ability of a comprehensive set of such methods to effectively communicate critical information to a decision maker and influence confidence in decision making. The second experiment investigated the performance of these methods under time pressure, a situational factor known to exacerbate judgmental errors. Ten commonly used graphical display methods were randomly assigned to participants. Across eight scenarios in which a probabilistic outcome was described, participants were asked questions regarding graph interpretation (e.g., mean) and made behavioral choices (i.e., act; do not act) based on the provided information indicated that decision-maker accuracy differed by graphical method; error bars and boxplots led to greatest mean estimation and behavioral choice accuracy whereas complementary cumulative probability distribution functions were associated with the highest probability estimation accuracy. Under time pressure, participant performance decreased when making behavioral choices.

Uncertainty of the tritium dose conversion factor

Environmental releases of tritium oxide at a number of Department of Energy nuclear weapons facilities contribute to a significant portion of environmental dose. Several conversion factors are utilized in the estimation of human impact from these releases, e.g., dispersion coefficients, consumption rates, uptake factors, transport factors, dose conversion factors, and risk coefficients. A probabilistic determination of the tritium dose conversion factor was generated in this work to assess the uncertainty of the internal dosimetry required to estimate dose equivalent given an intake of tritium oxide. The tritium dose conversion factor was found to vary by a factor of about 15 with a median value of 2.2 x 10(-11) Sv Bq(-1) when considering orthovoltage x rays as the standard for estimating the relative biological effectiveness of tritium. The median dose conversion factor increases by about 50%; however, when gamma radiation is considered as the standard. The current deterministic estimate of the tritium dose conversion factor published by the DOE and the EPA is 1.7 x 10(-11) Sv Bq(-1), 25-50% lower than the median probabilistic values. The tritium oxide dose conversion factor model was found to be most sensitive to biological half-life and quality factor and is highly dependent on the standardizing radiation for RBE assessments.

Land and water use characteristics in the vicinity of the Savannah River Site

Routine operations at the Savannah River Site (SRS) result in the release of small amounts of radionuclides to the atmosphere and to the Savannah River. The resulting radiological doses to the offsite maximum individual and the offsite population within 50 miles of the SRS are estimated on a yearly basis. These estimates are generated using dose models prescribed for the commercial nuclear power industry by the Nuclear Regulatory Commission (NRC). The NRC provides default values for dose model parameters for facilities not having enough data to develop site-specific values. A survey of land and water use characteristics for the Savannah River area has been conducted to determine as many site-specific values as possible for inclusion in the dose models used at the SRS. These site parameters include local characteristics of meat, milk, and vegetable production; river recreational activities; and meat, milk, and vegetable consumption rates. The report that follows describes the origin of the NRC default values, the methodology for deriving regional data, the results of the study, and the derivations of region-specific usage and consumption rates. 33 refs., 3 figs., 8 tabs.

Parameter uncertainty and sensitivity in a liquid-effluent dose model.

Radioactive materials which are released into streams on the Savannah River Site (SRS) eventually flow into the Savannah River. Tritium, 90Sr, 137Cs, and 239Pu account for the majority of the radiation dose received by users of the Savannah River. This paper focuses on the dose uncertainties originating from variability in parameters describing the transport and uptake of these nuclides. Parameter sensitivity has also been determined for each liquid pathway exposure model. The models used here to estimate radiation dose to an exposed individual provide a range of possible dose estimates that span approximately one order of magnitude. A pathway analysis reveals that aquatic food and water consumption account for more than 95% of the total dose to an individual.

The Effect of Cognitive Load on Decision Making with Graphically Displayed Uncertainty Information

An experiment examined the ability of five graphical displays to communicate uncertainty information when end users were under cognitive load (i.e., remembering an eight-digit number). The extent to which people could accurately derive information from the graphs and the adequacy of decisions about optimal behaviors based on the graphs were assessed across eight scenarios in which probabilistic outcomes were described. Results indicated that the load manipulation did not have an overall effect on derivation of information from the graphs (i.e., mean and probability estimation) but did suppress the ability to optimize behavioral choices based on the graph. Cognitive load affected peoples use of some graphical displays (basic probability distribution function) more than others. Overall, the research suggests that interpreting basic characteristics of uncertainty data is unharmed under conditions of limited cognitive resources, whereas more deliberative processing is negatively affected.