Limer LMC, Assessment Calculations for C-14 Labelled Gas and ...

calculated annual ingestion dose for the reference PEG (C) of 20 µSv (i.e. a doseequivalent to the risk guidance level of 1E-6 y -1 ; EA et al., 2009).The time periods following cap completion that it takes the average fluxes to dropbelow that level are given in Table 5-3. These times are similar to those for the RDAassessment (relevant values from the RDA reference case are provided forcomparison).Table 5-3: Time period following completion of final cap before average flux dropsbelow that associated with an effective ingestion dose of 2.0E-2 mSv for thereference PEG (C)CaseTime period following completion of final cap beforeaverage flux drops below that associated with a risk of1E-6 y -1 for PEG ATrenches RDA Vaults EDA VaultsReference Case (Case2_InvA) ~ 100 y ~ 350 y ~ 300 yCase2_InvA_C14rel ~ 100 y ~ 300 y ~ 300 yCase2_var1_InvA ~ 100 y ~ 300 y ~ 150 yCase2_InvA_S3M ~ 100 y ~ 350 y ~ 350 yCase_InvB ~ 100 y ~350 y ~ 400 yReference Case for RDA assessment ~ 100 y ~340 y N/AThe timescales upon which calculated annual effective ingestion doses for the referencePEG drop below the target are broadly similar in each case.For Case2_var1_InvA, where the flow through the wastes are assumed to be higherdue to increased infiltration rates, the time for the C-14 fluxes to drop below the targetis substantially shorter for the EDA vaults (see also Figure 5-7b). When the Stage 3decommissioning metals are assumed to be subject to a slower release rate of C-14 thanother metals, the tail of the C-14 flux from the wastes takes longer to decline, leading toit taking about 400 years for the flux to drop below that associated with the target.24