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

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From a design perspective, it is intended that the LLWR cap will have a ventbetween the gas-collection layer and the surface during the period of site control.This will enhance release of gases through the cap and prevent any build up ofgas. It is anticipated that gas generation will have reduced to low levels before thesite is closed, since the most readily degradable material will have fully degraded.The vent will be engineered out before site release, i.e. before the period ofinstitutional control ends, and after that gas movement will be diffusive only.For C-14, although the cap will delay the release of C-14 labelled methane, the durationof the delay is limited, so that it is not possible to justify the assumption that the capwould significantly reduce the impact of C-14 in the biosphere. For this reason, theC-14 labelled gas assessment assumes that any C-14 labelled gas released from thewaste is transported directly to the biosphere, i.e. without any radioactive decayoccurring between any release of gas from the wastes and that gas entering thebiosphere.These conclusions have important implications for Rn-222 gas, because the gas travelthrough the cap may or may not be large relative to the half-life of Rn-222, and thedominant transport mechanism could change during the post-closure period, e.g. dueto exposure of the cap vent at the ground surface through erosion, or drying out andcracking of the cap moisture retention barrier during summers. A slump or intrusioninto the cap, that directly connects the gravel gas collection layer to the atmosphere,could behave like a vent. If the entire cap were assumed to be substantially damagedsuch that a building on top of the wastes had only 1 m of coarse material between thewastes and the base of the building, then Limer and Thorne (2011) showed that thecharacteristic travel time through the coarse material would be of the order of 3.9 days;the degree of attenuation associated with this is a factor of 5.0E-1. For an intact cap of3 m thickness which comprised of some fine materials as well, the characteristic traveltime would be longer, thereby reducing the potential impacts of any Rn-222 gasreleased.2.3 Impact Assessments2.3.1 C-14For C-14 labelled gaseous carbon dioxide and methane, the principal exposurepathway identified in both UK and overseas studies involves uptake by plants inphotosynthesis following the gas entering the soil zone and subsequent ingestion byhumans. This is because, for a given flux of gaseous C-14 entering a specified area, thehuman dose from inhalation is many orders of magnitude lower than the dose arisingfrom the ingestion of contaminated crops (Limer et al., 2010). These plants may be6
QRS-1443ZH-1, Version 3.0consumed directly, e.g. vegetables and fruit, or indirectly in the form of animalproduce from livestock that have grazed on the contaminated area.Although the exposure calculations consider exposure as a result of inhalation, for thereasons described above the focus is upon potential exposure groups (PEGs) whoconsume food grown on the cap. The exposure pathways are shown in Figure 2-1. ThePEGs chosen are cautious but reasonably plausible estimates of human habits on thecap area; the descriptions of these PEGs are given in Thorne (2009) and are the samePEGs as were used in the RDA C-14 labelled gas assessment (Limer et al., 2011).Ingestion of animal produceInhalationIngestion of cropsC-14 gas enters building( 14 CO 2 , 14 CH 4 )C-14 uptake by crops( 14 CO 2 )C-14 uptake by pastureand ingestion by animals( 14 CO 2 )14CH 4 oxidised to 14 CO 2Not to scaleC-14 gas enters soil zone ( 14 CO 2 , 14 CH 4 )Figure 2-1: Schematic of Exposure Model for C-14 Labelled Gas Assessment (Limeret al., 2011)For convenience the ingestion PEGs are summarised below.PEG A: The smallholder with 4 to 12 hectares of land who keeps several head ofcattle and uses them to provide meat and milk products.PEG B: The smallholder with 1 to 3 hectares of land who keeps a single cow formilk production only.7
Page 3: Les thèmes du portail Leçon 3Néa
Page 7: QRS-1443ZH-1, Version 3.0SummaryThe
Page 11 and 12: QRS-1443ZH-1, Version 3.01 Introduc
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Page 19 and 20: QRS-1443ZH-1, Version 3.0relation t
Page 21 and 22: QRS-1443ZH-1, Version 3.0In the RDA
Page 24 and 25: Table 3-2: Rn-222 Inventory Informa
Page 26 and 27: Please see Table 5-1 of Limer et al
Page 28 and 29: contribute substantially to the tot
Page 30 and 31: Calculated Effective Ingestion Dose
Page 32 and 33: (a)Annual Effective IngestionExposu
Page 34 and 35: calculated annual ingestion dose fo
Page 36 and 37: Figure 5-7: Average Decay Corrected
Page 38 and 39: Figure 5-9: Calculated Effective An
Page 40 and 41: Table 5-5: Calculated Annual Effect
Page 42 and 43: Table 5-7: Calculated Annual Effect
Page 44 and 45: Table 5-9: Calculated Effective Inh
Page 46 and 47: Table 5-11: Calculated Effective In
Page 48 and 49: throughout. The reference case is h
Page 50 and 51: EDAVaults_NoCap@10000y and Vaults15
Page 52 and 53: area of the cap over vault 8 were t
Page 54: the Nuclear Decommissioning Authori

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