Improvement of modelling capabilities for assessing urban contamination: the EMRAS Urban Remediation Working Group.

The Urban Remediation Working Group of the International Atomic Energy Agency's Environmental Modelling for Radiation Safety (EMRAS) programme was established to improve modelling and assessment capabilities for radioactively contaminated urban situations, including the effects of countermeasures. An example of the Working Group's activities is an exercise based on Chernobyl fallout data in Ukraine, which has provided an opportunity to compare predictions among several models and with available measurements, to discuss reasons for discrepancies, and to identify areas where additional information would be helpful.

Model testing using data on 131I released from Hanford.

The Hanford test scenario described an accidental release of 131I to the environment from the Hanford Purex Chemical Separations Plant in September 1963. Based on monitoring data collected after the release, this scenario was used by the Dose Reconstruction Working Group of BIOMASS to test models typically used in dose reconstructions. The primary exposure pathway in terms of contribution to human doses was ingestion of contaminated milk and vegetables. Predicted mean doses to the thyroid of reference individuals from ingestion of 131I ranged from 0.0001 to 0.8 mSv. For one location, predicted doses to the thyroids of two children with high milk consumption ranged from 0.006 to 2 mSv. The predicted deposition at any given location varied among participants by a factor of 5-80. The exercise provided an opportunity for comparison of assessment methods and conceptual approaches, testing model predictions against measurements, and identifying the most important contributors to uncertainty in the assessment result. Key factors affecting predictions included the approach to handling incomplete data, interpretation of input information, selection of parameter values, adjustment of models for site-specific conditions, and treatment of uncertainties.

Model testing using data on 137Cs from Chernobyl fallout in the Iput River catchment area of Russia.

Data collected for 10 years following the Chernobyl accident in 1986 have provided a unique opportunity to test the reliability of computer models for contamination of terrestrial and aquatic environments. The Iput River scenario was used by the Dose Reconstruction Working Group of the BIOMASS (Biosphere Modelling and Assessment Methods) programme. The test area was one of the most highly contaminated areas in Russia following the accident, with an average contamination density of 137Cs of 800,000 Bq m-2 and localized contamination up to 1,500,000 Bq m-2, and a variety of countermeasures that were implemented in the test area had to be considered in the modelling exercise. Difficulties encountered during the exercise included averaging of data to account for uneven contamination of the test area, simulating the downward migration and changes in bioavailability of 137Cs in soil, and modelling the effectiveness of countermeasures. The accuracy of model predictions is dependent at least in part on the experience and judgment of the participant in interpretation of input information, selection of parameter values, and treatment of uncertainties.

Modelling radionuclide distribution and transport in the environment.

Mathematical models of radionuclide distribution and transport in the environment have been developed to assess the impact on people of routine and accidental releases of radioactivity from a variety of nuclear activities, including: weapons development, production, and testing; power production; and waste disposal. The models are used to estimate human exposures and doses in situations where measurements have not been made or would be impossible or impractical to make. Model results are used to assess whether nuclear facilities are operated in compliance with regulatory requirements, to determine the need for remediation of contaminated sites, to estimate the effects on human health of past releases, and to predict the potential effects of accidental releases or new facilities. This paper describes the various applications and types of models currently used to represent the distribution and transport of radionuclides in the terrestrial and aquatic environments, as well as integrated global models for selected radionuclides and special issues in the fields of solid radioactive waste disposal and dose reconstruction. Particular emphasis is placed on the issue of improving confidence in the model results, including the importance of uncertainty analysis and of model verification and validation.

Opportunities for the testing of environmental transport models using data obtained following the Chernobyl accident.

The aftermath of the Chernobyl accident has provided a unique opportunity to collect data sets specifically for the purpose of model testing, and with these data to create scenarios against which environmental transport models may be tested in a format constituting a blind test. This article serves as an introduction to three test scenarios designed for testing models at the process level: (1) surface water contamination with radionuclides initially deposited onto soils; (2) contamination of different aquatic media and biota due to fallout of radionuclides into a body of water; and (3) atmospheric resuspension of radionuclides from contaminated land surfaces. These scenarios are the first such tests to use data sets collected in the former Soviet Union. Interested modelers are invited to participate in the test exercises by making calculations for any of these test scenarios. Information on participation is included.

Determining the collection efficiency of gummed paper for the deposition of radioactive contaminants in simulated rain.

An extensive network of gummed paper collectors was used during the 1950s for routine monitoring of radioactive fallout from weapons testing. The experiments reported in this paper were designed to examine the collection efficiency of gummed paper for wet deposition of several types of soluble and insoluble radioactive contaminants under conditions similar to those found during natural rainstorms. The collection efficiencies for each substance were determined over a range of rainfall amounts and at two separate rainfall intensities. The collection efficiency of the gummed paper is highest for large insoluble particles and lowest for soluble ionic substances. The values for 7Be and 131I ranged from about 0.30 (30%) at 2.5 mm of rain to 0.04-0.06 (4%-6%) at 20 mm of rain. These values were negatively correlated with the amount of rain and were unaffected by rain intensity. This suggests that the collection efficiency of either ion is simply a matter of rapid saturation and runoff. Neither rain amount nor rain intensity has much effect on the collection efficiency of large insoluble particles. These particles seem to settle readily onto the surface of the gummed paper from which they are not easily removed by additional rain. Analysis of the collection efficiencies of the gummed paper included the activity of both the gummed paper and the standing water on the paper. A large portion of the activity for the ionic substances was found in the standing water. There was less activity by insoluble particles in standing water. This indicates that for estimates of deposition of soluble substances, considerable bias could be introduced into the results if the standing water is discarded prior to analysis.

Modeling the washoff of 90Sr and 137Cs from an experimental plot established in the vicinity of the Chernobyl reactor.

After the Chernobyl event, a large area of land was contaminated following the deposition of radionuclides. This area became a continuing source of radionuclides to natural waters and aquatic ecosystems. In 1986, an experimental plot was constructed in a contaminated area near the Chernobyl Nuclear Power Plant to study the washoff of radionuclides by surface runoff. Concentrations of 137Cs and 90Sr were measured in the top 10 cm of the soil prior to the experiments. During two separate experiments, intense artificial rainfall was applied to the plot. A washoff scenario was then prepared with site-specific information on initial soil contamination, duration and quantities of rainfall and runoff, physicochemical properties of the topsoil, and some climatological data. Modelers were asked to predict (a) the vertical distributions of the initial concentrations of 137Cs and 90Sr in various chemical forms in the topsoil, (b) concentrations of these radionuclides in various chemical forms in the runoff water during each experiment, and (c) the total amounts of these radionuclides that were washed off during each experiment. Stochastically generated local rainfall data were used in a water budget model to generate annual average runoff and infiltration rates. A vertical, one-dimensional, multiphase, multispecies transport model was then developed to simulate the movement of contaminants in the topsoil during the 160-d period between the Chernobyl event and the experiments as well as the washouts of contaminants by runoff during the experiments and during the 24-h period thereafter. The model provided very good predictions of the vertical distributions of total contaminant concentrations in the top 10 cm of the soil; however, the concentrations in individual chemical forms were not predicted as accurately. Initially, the model overpredicted the washout of contaminants for the two experiments and the 24-h period thereafter. Fraction of runoff that flows as interflow and average sediment loading in the runoff were identified as parameters responsible for the overprediction. Calibration of the interflow fraction and adjustment of the average sediment loading in runoff to a level representative of Eastern Europe considerably improved these predictions. The complete modeling approach and comparisons of model predictions with measurements and with predictions from other modelers are presented.

Modeling the resuspension of radionuclides in Ukrainian regions impacted by Chernobyl fallout.

Following the 1986 Chernobyl event, large amounts of radioactive materials were deposited in nearby areas. Concentrations of various radionuclides were measured in air and surface soil. To study the resuspension of radioactive particulate, three different exposure situations were developed on the basis of the collected data under the auspices of the international BIOMOVS II (BIOspheric MOdel Validation Study) project. Modelers were asked to predict seasonal air concentrations and resuspension factors at several locations at different distances from Chernobyl for six successive years following the accident. Measurements of radionuclide deposition on topsoil were provided for each site along with information on soil, vegetation, land use, surface roughness, meteorology, and climate. In this paper, the three exposure situations are described, along with the initial data set provided to the modelers; two modeling approaches used to make the endpoint predictions are also presented. After the model predictions were submitted, the measured air concentrations and resuspension factors were released to the modelers. Generally, the predictions were well within an order of magnitude of the measured values. Time-dependent trends in predictions and measurements were in good agreement with one of the models, which (a) explicitly accounted for loss processes in soil and (b) used calibration to improve its predictive capabilities. Reasons for variations between predictions and measurements, suggestions for the improvement of models, and conclusions from the model validation study are presented.

Comparison of doses and risks obtained from dose reconstructions for historical operations of federal facilities that supported the development, production, or testing of nuclear weapons.

Five dose reconstruction projects focusing on historical public exposures from U.S. government nuclear facilities have been completed in the last 12 y (Fernald, Hanford, Nevada Test Site, Oak Ridge, Rocky Flats). Using information available in published reports, doses and excess health risks of the most serious contaminants in each study are compared for representative maximally and typically exposed individuals. For both the representative maximally exposed individual and the representative typically exposed individual, the highest excess risks of cancer incidence were from 131I released from Hanford, Nevada Test Site, and Oak Ridge and 222Rn released from Fernald (with central estimates for maximally exposed individuals approaching or exceeding 10(-2)); the lowest risks for both maximally and typically exposed individuals were from 239/240Pu and carbon tetrachloride released from Rocky Flats. Excess health risks to the representative maximally exposed individual were at or below 10(-40 for releases from Rocky Flats. For representative typically exposed individuals, the excess risks from releases of mixed radionuclides in the Clinch River (Oak Ridge), PCBs in East Fork Poplar Creek (Oak Ridge), and both plutonium and carbon tetrachloride released from Rocky Flats were mostly below 10(-5).

External exposure to radionuclides accumulated in shoreline sediments with an application to the lower Clinch River.

A simple analytical method was developed to estimate external doses from exposure to contaminated riverine shorelines. The method consists of deriving an adjustment factor that accounts for the geometry of the riverine shoreline; the adjustment factor is applied to the dose-rate coefficients already available for infinite contaminated surfaces. Such a geometry factor circumvents very complex radiation transport calculations which would otherwise be necessary to model exposures to a finite contaminated surface. For instance, for radionuclides emitting gamma rays of energies above 600 keV (e.g., 137Cs), the published dose-rate coefficients must be reduced by 75%, 60%, 50%, and 33% for shoreline widths of 4, 10, 20, and 50 m, respectively. The geometry factor changes only mildly with the energy of the gamma radiation. This property allows for the geometry factor to be used for radionuclides emitting multiple gamma rays of various energies. If a quick analysis is desired, the geometry factors derived for 137Cs can be used for all radionuclides. More refined analysis can be performed by deriving geometry factors for each radionuclide according to its gamma spectrum. Also, the mild variation with energy allows the geometry factors to be applied to the case when radionuclides are accumulated in layers under the soil surface, and not only to the case when radionuclides are deposited onto the soil surface. Empirical relationships between the geometry factor and the dimension of the shoreline were provided so that one can obtain values of the geometry factor for any shoreline width. These relationships can be easily used to account for the uncertainty in the dimension of the shoreline. The method was applied to derive similar adjustment factors for contaminated surfaces of other simple geometries (e.g., circular surfaces). An example of how this method can be applied to its full extent is presented for the case of external exposure to the shores of the lower Clinch River. This river received large amounts of 137Cs, 60Co, 106Ru, 95Zr, 95Nb, 144Ce, and 90Sr released during 1944-1991 from the Oak Ridge Reservation in Oak Ridge, Tennessee.

Modelling remediation options for urban contamination situations.

The impact on a population from an event resulting in dispersal and deposition of radionuclides in an urban area could be significant, in terms of both the number of people affected and the economic costs of recovery. The use of computer models for assessment of urban contamination situations and remedial options enables the evaluation of a variety of situations or alternative recovery strategies in contexts of preparedness or decision-making. At present a number of models and modelling approaches are available for different purposes. This paper summarizes the available modelling approaches, approaches for modelling countermeasure effectiveness, and current sources of information on parameters related to countermeasure effectiveness. Countermeasure information must be applied with careful thought as to its applicability for the specific situation being modelled. Much of the current information base comes from the Chernobyl experience and would not be applicable for all types of situations.

Modelling of a large-scale urban contamination situation and remediation alternatives.

The Urban Remediation Working Group of the International Atomic Energy Agency's EMRAS (Environmental Modelling for Radiation Safety) program was organized to address issues of remediation assessment modelling for urban areas contaminated with dispersed radionuclides. The present paper describes the first of two modelling exercises, which was based on Chernobyl fallout data in the town of Pripyat, Ukraine. Modelling endpoints for the exercise included radionuclide concentrations and external dose rates at specified locations, contributions to the dose rates from individual surfaces and radionuclides, and annual and cumulative external doses to specified reference individuals. Model predictions were performed for a "no action" situation (with no remedial measures) and for selected countermeasures. The exercise provided a valuable opportunity to compare modelling approaches and parameter values, as well as to compare the predicted effectiveness of various countermeasures with respect to short-term and long-term reduction of predicted doses to people.

Modelling the long-term consequences of a hypothetical dispersal of radioactivity in an urban area including remediation alternatives.

The Urban Remediation Working Group of the International Atomic Energy Agency's EMRAS (Environmental Modelling for Radiation Safety) program was organized to address issues of remediation assessment modelling for urban areas contaminated with dispersed radionuclides. The present paper describes the second of two modelling exercises. This exercise was based on a hypothetical dispersal of radioactivity in an urban area from a radiological dispersal device, with reference surface contamination at selected sites used as the primary input information. Modelling endpoints for the exercise included radionuclide concentrations and external dose rates at specified locations, contributions to the dose rates from individual surfaces, and annual and cumulative external doses to specified reference individuals. Model predictions were performed for a "no action" situation (with no remedial measures) and for selected countermeasures. The exercise provided an opportunity for comparison of three modelling approaches, as well as a comparison of the predicted effectiveness of various countermeasures in terms of their short-term and long-term effects on predicted doses to humans.

Gene assignments and syntenic groups in the sacred baboon (Papio hamadryas).

Eighteen genes were assigned to chromosomes in the sacred baboon, Papio hamadryas, by their concordant segregation with the chromosomes in a set of baboon X Chinese hamster somatic cell hybrids. ACY1 was assigned to P. hamadryas chromosome 2 (PHA 2); SOD1 and MDH2 to PHA 3; ME1 and SOD2 to PHA 4; NP, MPI, PKM2, and HEXA to PHA 7; PP to PHA 9; ADA and ITPA to PHA 10; LDHB and TPI1 to PHA 11; MDH1 to PHA 13; ESD to PHA 17; and GPI and PEPD to PHA 20. Regional assignments were possible for ACY1 (PHA 2pter----q1) and MDH2 and SOD1 (PHA 3p). Five other independently segregating markers or syntenic groups (PGD, PGM1; and PEPC; PGM2 and PEPS; IDH1; LDHA and ACP2; and GSR) were also identified. Gene assignments and syntenic groups described in P. hamadryas are compared to those found in P. papio, the rhesus monkey, and man. A possible primate model for human lymphoid disease is discussed.

New gene assignments and syntenic groups in the baboon (Papio papio).

MDH2, SOD2, PEPS, and ITPA were assigned to Papio papio chromosomes 3, 4, 5, and 10, respectively, by their concordant segregation with previously assigned gene markers in a set of baboon X mouse somatic cell hybrids. The linkage of NP, IDH2, SORD, MPI, and PKM2 was confirmed, and three other independently segregating markers (MDH1, ACY1, and PEPB) were identified. Syntenic groups described in the baboon are compared to those found in man and in the rhesus monkey.

A risk-based screening approach for prioritizing contaminants and exposure pathways at Superfund sites.

Contamination at Superfund sites can involve mixtures of chemicals and radionuclides in a variety of environmental media. Determining priorities for evaluation and remediation of various contaminants is an important part of the initial phases of any site investigation. An effective screening analysis at the beginning of the project can help by identifying both those situations in need of immediate remedial attention and those which require further sampling and evaluation. The screening approach discussed here is made up of two sets of calculations designed to provide upper- and lower-bound estimates of health risk to individuals likely to receive the highest exposures. This approach allows rapid identification of contaminants which pose a negligible risk and can be assigned a low priority for remedial attention or which pose a substantial risk and should be given the highest priority for appropriate remediation efforts. Contaminants designated as neither high- nor low-priority should be investigated in more detail prior to making decisions regarding the need for or method of remediation. The utility of this approach has already been demonstrated in the evaluation of contamination in the Clinch and Tennessee River systems originating from historical operations of atomic weapons and energy research facilities near Oak Ridge, Tennessee.

Peer reviewed: environmental models undergo international test.

The science and art of exposure assessment modeling were tested using real-world data from the Chernobyl accident.