Stochastic Risk Index for Radionuclides

To classify this mixed radioactive and hazardous chemical waste based on risk, a composite risk index giving the sum of the risk indexes for chemicals that cause deterministic effects, chemicals that cause stochastic effects, and radionuclides must be evaluated. The first two elements of the composite risk index were evaluated in Sections 7.1.7.4 and 7.1.7.5, respectively. In this Section, the stochastic risk index for 137Cs in the waste is computed. 

The external dose to an inadvertent intruder who is assumed to be exposed to uncovered waste for a period of 1,000 h at the time of facility closure can be estimated as follows. For a 137Cs source assumed to be uniformly distributed in surface soil with its decay product 137mBa in activity equilibrium, and taking into account the decay branching fraction of 0.946 (Kocher, 1981), the external dose rate per unit concentration is 2.9 X 10 11 Sv s 1 per Bq g 1 (Eckerman and Ryman, 1993). Multiplying this external dose coefficient by the assumed concentration of 137Cs (4.8 Bqg ) and exposure time (1,000 h) gives a total dose for the assumed scenario of 5 X 10 4 Sv. 

The stochastic risk corresponding to the estimated dose is obtained using the nominal probability coefficient for fatal cancers of 0.05 Sv-1 (ICRP, 1991 NCRP, 1993a). Since this coefficient is intended to represent a best estimate, rather than a conservative upper bound, the value is not increased by a factor of 10, as in the adjustment of the slope factors for chemicals that induce stochastic effects (Section 7.1.7.5). Therefore, the calculated stochastic risk due to 137Cs in the waste is (5 X 10 4 Sv)(5 X 10 2 Sv-1) = 2.5 X 10 5. 

Given the assumption that an acceptable stochastic risk from disposal in a hazardous waste facility is about 10 3 (see Table 7.1), the stochastic risk index due to the presence of radionuclides in the electric arc furnace waste is (2.5 X 10 5)/10 3 = 0.025. Since this result is much less than unity, the waste clearly would be classified as low-hazard due only to the presence of 137Cs, and there is no need to perform a less conservative analysis. 

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