Baldwin TD and Hicks TW, Assessment Calculations for Human ...

0977-6Version 1majority of the intrusion events considered. However, dose to the siteoccupier is dominated by exposure to radon (from 226 Ra and 234 U decay), anddose to the site smallholder is dominated by ingestion of 14 C and 36 Cl in plantmaterial and goat’s milk.• External irradiation exposure to 108m Ag, recently identified as requiringinclusion in post-closure radiological assessments for the LLWR (Baldwin andHicks, 2011), contributes up to a few per cent of total dose in the short termafter closure.• Should the site not be eroded as expected, but exist in the very long term(10,000 years after disposal), then inhalation of radon, and the dose associatedwith exposure to its daughters, becomes significant for occupancy events:radon concentrations increase in the long term as radium is ingrown from thedecay of 234 U disposals via 230 Th. However, ingestion of 14 C and 36 Cl in plantmaterial and goat’s milk by the smallholder is the most significant of all theexposure pathways and events.• The calculated dose for human intrusion into the vaults for all events for theCase A and Case B inventories is similar.• Different radionuclides dominate dose calculations for the extended facilitycompared to the reference waste management case (Hicks and Baldwin, 2011).The dominant radionuclides in the RDA are typically 232 Th and daughters228 Th and 228 Ra, rather than 239 Pu, 240 Pu and 94 Nb, which dominate in the EDAcalculations. In addition, ingestion of 14 C and 36 Cl in plant material dominatesthe calculated dose to the smallholder for the extended facility, whereas radonexposure is the dominant pathway for the RDA. However, the maximumcalculated annual doses for intrusion into the EDA for the Case A inventoryare smaller than the maximum annual doses calculated for intrusion in to theRDA for each intrusion event considered.Requirement R7 of the NS-GRA (Environment Agency et al., 2009, §6.3.40) alsorequires the operator to show that dose thresholds for severe deterministic injury toindividual body tissues are unlikely to be exceeded as a result of human intrusion intothe facility. Consistent with HPA guidance, as discussed in the NS-GRA(Environment Agency et al., 2009, §6.3.16), deterministic effects would need to beassessed if the estimated effective dose received over a year or less is greater than100 mSv. The human intrusion assessment calculations have shown that estimateddoses from intrusion into the facility do not exceed this threshold for consideringdeterministic effects.The GoldSim implementation of the human intrusion model has been subjected tocomprehensive and systematic verification checks throughout its development andapplication.Galson Sciences Ltd 49 21 March 2011