An experimental environmental uptake model was developed for the purpose
of estimating the final harvest concentration of contaminant following an acute
accidental release of radiocesium onto an eastern Oregon winter wheat crop. The
system was developed using the PATHWAY environmental uptake model as
presented by Whicker and Kirchner (1987) for its basis. The experimental system was
constructed and operated with the STELLAᵀᴹ modeling software.
Two source terms were considered for the experiment. The first was a mixture
of 62.8% ¹³⁷Cs and 37.2% ¹³⁴Cs, and the second was ¹³⁷Cs by itself. Both were
assumed to be in the form of cesium chloride. These isotopes of cesium were chosen
because of their presence within the nuclear industry and as fission products.
A primary alteration to the PATHWAY model was the incorporation of the
growing degree day (GDD) concept. This allowed the model to adjust the length of the
growing season based on the average temperature to which the crop was exposed. It
was found that higher average growing temperatures resulted in increased radiocesium
concentrations (Bq kg⁻¹) in the harvested crop.
The day on which the contaminant was dispersed during the growing season
also changed the final harvest concentration. The day during the growing season
which results in the highest final concentration is referred to as the optimum day of
dispersal (ODD). During the 261 day growing season for winter wheat, with an
average temperature of 9.58°C, the ODD was on day 96 for ¹³⁴/¹³⁷Cs and day 85 for
¹³⁷Cs. The area contamination which resulted in a harvest concentration equal to the
derived intervention level for radiocesium contaminated foods (1200 Bq . kg⁻¹) was
1.50 x 10⁴ Bq m⁻² and 1.43 x 10⁴ Bq m⁻², respectively.
The exposure rates at 30 cm for 1.50 x 10⁴ Bq m⁻² (¹³⁴/¹³⁷Cs) and 1.43 x 10⁴
Bq m⁻² ('¹³⁷Cs) were calculated to be 5.05 x 10⁻⁵ mR hr⁻¹ and 6.70 x 10⁻⁵ hr⁻¹
respectively, excluding natural background. These levels suggest that conventional
area surveys could not detect the level of radiocesium contamination which would
require the embargo of an eastern Oregon wheat crop.
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