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

0977-6Version 1• A 3-m-thick engineered cap has been assumed to be placed over the facility ina single dome design, achieved by using profiling material between the wastesand the cap. A minimum 1 m thickness of profile material will be placedabove the vault wastes.• Spatial variability of radionuclide activity, profile material thickness andseveral other wasteform and barrier properties have been addressed in themodel by defining and specifying parameter values at each location in a gridof elements. Note that a cautious approach has been taken for vault elementsin that the profile material above the vaults has not been taken into account(i.e., only the 3-m-thick cap has been assumed to be present).A conservatism in the assessment model is that leaching of radionuclides from thewasteform in infiltrating rainwater and from materials that are exposed as a result ofintrusion has been neglected. This is pessimistic for more mobile long-livedradionuclides such as 14 C and 36 Cl; these nuclides dominate the site smallholder dosethrough the foodstuffs ingestion pathways.The calculation of dose from inhalation of radon is based on a simple empirical modelthat relates average levels of radon in indoor air to average levels of radium in soilsand rocks. The application of the model is pessimistic in that no credit has been takenfor any mixing of excavated materials with uncontaminated materials. Suchconsiderations would result in reductions in calculated dose from radon inhalation.In general, the expressions of dose are simple linear functions of radionuclide activity.Therefore, it has not been necessary to undertaken specific calculations to evaluate thesensitivity of results to parameter value variations because the effects of variationscan be scaled from the available results.A general assumption of the 2011 ESC is that management and regulatory control ofthe site will be maintained until about 100 years after the last disposal to the facility.For the RDA, considered in the previous report, this led to an assumption of siterelease at 2180; for the EDA, disposals continue until about 2130 and assumed siterelease (the end of the period of management and regulatory control and the pointfrom which intrusion into the facility is considered) is assumed to occur at 2230.Key findings regarding calculated doses from human intrusion into parts of the EDAare as follows:• For the geotechnical investigation intrusion events, even if an engineer isinvolved in multiple intrusions, the calculated dose does not challenge the20 mSv/year regulatory guidance value for transitory exposures, or even the3 mSv regulatory guidance value for prolonged exposure. Similarly, singleintrusions for construction and organised material recovery result in smallcalculated doses. Even if deeper excavations are required as part of largedevelopments (e.g. pile foundations several metres deep), the appropriatelyscaled dose would be much less than 20 mSv/year.• Inhalation ( 239 Pu and 240 Pu) and external irradiation ( 137 Cs in the short termfollowed by 94 Nb at later times) are the dominant exposure pathways for theGalson Sciences Ltd 48 21 March 2011