Modeling a potential atmospheric release from a waste disposal facility at the savannah river site as an area source.

The Saltstone Facility was designed at the Savannah River Site (SRS) to treat and dispose of certain low-level liquid radioactive wastes. The final product of Saltstone is several large concrete vaults. As part of the performance assessment for Saltstone, reduction of dose to receptors downwind of the vaults have been estimated for treating the vaults as an area atmospheric source as opposed to a point source. The CAP88 model has the ability to handle area sources, but the methods are not appropriate for receptors close to the source such as those modeled at 100 m. Use of the area source as opposed to the point source can reduce the dose by as much as a factor of 5 depending on vault size. A method for quickly assessing the dose from an area source for near-in exposures is demonstrated here.

Method for estimating ingestion doses to the public near the Savannah River site following an accidental atmospheric release.

At the Savannah River Site, emergency response computer models are used to estimate dose following releases of radioactive materials to the environment. Downwind air and ground concentrations and their associated doses from inhalation and ground shine pathways are estimated. The emergency response model (PUFF-PLUME) uses real-time data to track either instantaneous (puff) or continuous (plume) releases. A site-specific ingestion dose model was developed for use with PUFF-PLUME that includes the following ingestion dose pathways pertinent to the surrounding Savannah River Site area: milk, beef, water, and fish. The model is simplistic and can be used with existing code output.

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.

CAP88-PC tritium ingestion model inconsistencies.

CAP88-PC is an Environmental Protection Agency (EPA) code approved for demonstrating compliance with National Emission Standard for Hazardous Pollutants (NESHAP), 40 CFR Part 61. A detailed look at the dose methodology for tritium revealed that there are several inconsistencies in the ingestion model when compared with th ingestion model for other radionuclides within the code. The inconsistencies include use of an out-dated tritium ingestion dose conversion factor and hard coding of out-dated food consumption parameters for tritium. Correction of these values could result in approximately a 50% reduction in dose from tritium for certain cases.

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.

Determination of optimum time interval of meteorological data used with atmospheric dose modeling at SRS. Savannah River Site.

Measured tritium oxide concentrations in air were compared with calculated values using routine release Gaussian plume models for different time intervals of meteorological data. These comparisons determined an optimum time interval of meteorological data used with atmospheric dose models at the Savannah River Site (SRS). Meteorological data of varying time intervals (1 y to 10 y) were used for the comparison. Insignificant differences are seen in using a 1-y database as opposed to a 5-y database. Use of a 10-y database results in slightly higher or more conservative estimates. For meteorological databases of length 1 y to 5 y the mean ratio of predicted to measured tritium oxide concentrations is approximately 1.25 whereas for the 10-y database the ratio is closer to 1.35. Currently at the SRS a meteorological database of five year's duration is used for all dose models. This study suggests no substantially improved accuracy using shorter or longer time intervals.

Radiostrontium in the Savannah river site environment.

The Savannah River Site has produced plutonium, tritium, and other special nuclear materials for national defense, other government programs, and some civilian purposes. Strontium has been released to the environment during the operation of five reactors, two radiochemical processing facilities, and other supporting facilities. During the period 1954-1996, 110 GBq of 90Sr were released to the atmosphere and 5,400 GBq were released to site streams. The maximum individual effective dose equivalent at the site boundary was estimated to be 72 microSv from atmospheric releases and 25 microSv from liquid releases. The 80-km population dose was 0.3 person-Sv.

Three-parameter model for estimating atmospheric tritium dose at the Savannah River Site.

The models used in the NRC approach to assess chronic atmospheric releases of radioactivity generate deterministic dose estimates by using standard assumptions about exposure conditions and environmental transport mechanisms. This approach has been used at the Savannah River Site since 1983. Total dose to off-site maximally exposed individuals at the SRS from atmospheric releases has been on the order of 1 microSv y(-1), three orders of magnitude lower than the applicable dose limit. When estimating atmospheric dose many parameters remain unchanged each time calculations are performed. These parameters, therefore, are essentially unimportant with regard to routine modeling. It is proposed, therefore, that transport and dosimetry models can be reduced to simple functions of a few parameters that essentially determine dose at all locations across the site. The three-parameter transport and dosimetry model developed in this work is useful for quick and easy estimates of chronic atmospheric tritium dose that are within a factor of 2 of estimates by more sophisticated models. The three parameters critical to estimating annual average concentration at the Savannah River Site are wind-direction frequency, downwind distance, and physical stack height. The model is bounded by physical stack heights between 10 and 61 m and downwind distances between 800 m (0.5 mi.) and 32 km (20 mi.) and should not be used outside its intended domain. It requires knowledge of wind-direction frequency, downwind distance, and physical stack height to estimate an Atmospheric Dose Factor (ADF; in units of microSv GBq(-1)) for the conversion of long-term release activity to maximum individual effective dose equivalent. This concept is being carried forward to the development of a reduced model for particulate emissions from SRS stacks.

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.