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

0977-6Version 13 Treatment of Spatial VariabilityThe impact of future human intrusion at any particular location in the disposal facilitywill depend on the local properties of the waste and barrier system and theirevolution, as well as the type of intrusion event. Radionuclide activities, wastevolumes, and wasteform and barrier properties will vary significantly across thevaults, and such variability requires consideration in the human intrusion assessmentcalculations. The approach to addressing spatial variability in the GoldSim model isoutlined in this section.3.1 Spatial Variability in Waste Activities and VolumesThe effects of spatial variability in radionuclide activities and waste volumes acrossthe vaults have been investigated in the human intrusion assessment by specifying amodelling solution grid across the facility. The approach involves assigningradionuclide activities and waste volumes to each grid element. The discretisationmethod and the treatment of radionuclide inventories and waste volumes areconsistent with the approach being developed for the separate assessment of theradiological impacts of coastal erosion (Towler et al., 2011).Past and future disposals of waste packages to the trenches and vaults in the RDA arediscussed in by Hicks and Baldwin (2011, §6.1.2). The discretisation of the wastesaimed to capture the variability in inventories and variability in package stack height.The approach led to the specification of spatially varying parameters in 44 regions ina model grid across the disposal area for the RDA.The engineering data provided by LLWR for the extended repository places the sixadditional vaults to the south-east of the RDA. Considering the uncertain nature ofthe variant inventories for the extended area, it was considered appropriate todiscretise the EDA at the vault scale, giving rise to six new regions (vaults 15 to 20)and resulting in a total of 50 regions in the model grid across the disposal area. Thisis shown in Figure 3.1. The rows indicated on the grid represent the following:Row A: Trenches 6 and 7, and Vaults 16 and 19Row B: Trenches 4 and 5Row C: Trench 1, and Vaults 15 and 18Row D: Trenches 2 and 3Row E: Eastern half of Vaults 8 to 14, and Vaults 17 and 20Row F: Western half of Vaults 8 to 14The numbers shown on the trench elements in Figure 3.1 indicate the bays included inthose elements. The vault locations in the grid are also indicated. Vaults 15 to 20 ofthe EDA have been allocated to Rows A, C and E in Columns 10 and 11 of the grid.Radionuclide activities (available per trench bay and per vault) and waste volumes areallocated to each grid element based on this partitioning (Figure 3.1).Galson Sciences Ltd 7 21 March 2011