Radiation dose assessment for plutonium contamination at Area 13 of the Nevada Test Site using Nevada Applied Ecology Group data and the RESRAD code

A radiation dose assessment is performed for plutonium contamination at Area 13 of the Nevada Test Site (NTS). Area 13 is one of five safety shot areas at NTS. Safety shots were experiments that tested the ability of nuclear weapons not to fission in the case of accident, storage, handling, and transport situations. As a result of the safety shots, plutonium, the majority of which is Pu-239, has contaminated surface soil of the five safety shot areas. This dose assessment determines approximate radiation doses that an industrial-worker or residential- farmer could receive at Area 13, and analyzes compliance of the potential doses with Department of Energy (DOE) standards.

The Nevada Applied Ecology Group (NAEG) determined that the ratio of plutonium ingestion to plutonium inhalation at NTS is approximately 100 for a residential- farmer. At this ratio, inhalation is responsible for approximately 100% of the dose to the lungs and 95% of the dose to the whole body and bone; lungs and bone were determined to be the critical organs. External dose was not considered by the NAEG. Because of results of the NAEG, this dose assessment only considers inhalation.

The RESRAD code is used to calculate radiation doses. RESRAD calculates dose by modeling environmental transport of radioactive contaminants from the source to a human receptor and using dose conversion factors (dose per unit intake of radionuclide). The parameters used in the RESRAD code are subjected to sensitivity analysis to analyze variations in radiation dose that could occur from variations in the parameters, "Sensitive" parameters are the mass loading factor (mass of soil suspended per unit volume of air), inhalation rate, distribution coefficient (radionuclide adsorbed onto soil versus that in infiltrating water), contaminated zone thickness, evapotranspiration coefficient (percentage of precipitation that is lost by evaporation and transpiration), and soil mixing layer (depth to which surface soil undergoes mixing).

Values for committed effective dose equivalent (whole body dose from internal radiation) were calculated for a generic scenario (i.e., parameters were for normal weather conditions and/or were not specific to a given land-use scenario). The radiation doses reach a peak at 73.3 years, at which point the dose is 0.334 mrem/yr per pCi/g of initial plutonium contamination. Area 13 is divided into six contaminated strata, which range in plutonium activity from 14,000 pCi/g in stratum six to 36 pCi/g in stratum one. For year 73.3, the calculated doses would, therefore, be 4680 mrem/yr at stratum six and 12 mrem/yr at stratum one.

Results from the generic scenario were used to analyze compliance of the residential-farmer and industrial-worker scenarios with DOE standards, DOE Order 5400.5 states that the effective dose equivalent (whole body dose from internal and external radiation) to the public should not exceed 100 mrem/yr; this limit would apply to the residential-farmer. At year 73.3, the doses at four of the six Area 13strata exceed this limit. The DOE Radiological Control Manual states that the effective dose equivalent to a worker should not exceed 5 rem/yr. At year 73.3 (or any other year), none of the doses calculated for Area 13 strata exceed this limit.