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

0977-6Version 16 ResultsCalculations have been undertaken to evaluate potential doses from inadvertenthuman intrusion into the additional vaults for each of the two inventory casesdiscussed in Section 5.1. The starting point for evaluating the impacts of intrusion isthe time of withdrawal of regulatory control from the site, which is assumed to beabout 100 years after all engineering and decommissioning of active systems iscomplete and the site has been closed. For the Reference Disposal Area considered inthe previous report this led to an assumption of the end of the period of managementand regulatory control at 2180. Assuming it takes 3 years to complete cappingfollowing completion of disposals to Vault 20 in the Extended Disposal Area, the siteclosure date for inventory cases A and B will be 2130, indicating an end toinstitutional control at 2230. Thus, doses from the geotechnical investigation events(B1), construction and occupancy events (B3) and smallholder event (B7) for theExtended Disposal Area have been evaluated from 2230.The earliest erosion contact of the site is assumed to be 300 years after present (AP),which is considered unlikely but credible. The latest time at which the site will becontacted by erosion, which includes the possibility that some measures are taken toinstall and maintain coastal defences for a period, is about 3,000 years AP. It isexpected that the site will be completely destroyed and dispersed by coastal erosionby about 5,000 years AP. Thus, the intrusion events are considered from the year2230 to the year 7000.The hazard presented by the disposal system in the longer term has also beenconsidered assuming that the facility is not eroded. Such calculations are importantbecause the inventory of some radionuclides will increase in the long term, notably226 Ra and its daughters growing in from 234 U via long-lived 230 Th. The longest timefor which it is considered credible to continue calculations for the occupancy andsmallholder events B3 and B7 is 10,000 years AP (i.e. until the year 12,000). Thestate of the disposal system cannot be predicted with any confidence on such longtimescales. Therefore, the characteristics of the system, such as cap and profilethickness, are assumed to be the same as at closure. Note that the human intrusionassessment ignores any leaching of radionuclides in infiltrating water and subsequentmigration away from the site, which is very cautious on such long timescales.The following subsections present results for each intrusion event for inventory casesA and B. Results for all the intrusion events have been produced using Version 1.2 ofthe GoldSim EDA human intrusion model. The code runs are numbered 1.2.0 (CaseA) and 1.2.1 (Case B).Dose calculations are shown in terms of results at different GoldSim model grid cells,which map to trenches and vaults as indicated in Figure 3.2 and summarised in Figure6.1. The model grid was developed for the Reference Disposal Area human intrusionassessment calculations (columns 1 to 9) and has been used for the Extended DisposalArea (columns 10 and 11) to ensure consistency between the assessments.Galson Sciences Ltd 20 21 March 2011