Limer LMC and Thorne MC, Assessment Calculations for Radon for ...

6 Summary of Assessment AssumptionsThe assumptions made in this assessment are summarised in Table 6-1. An audit of theassessment against the LLWR FEP list, as given in Lean and Willans (2010), is given inAppendix A.Table 6-1: Rn-222 Gas Assessment AssumptionsAspect of theSystemSource term:release of Rn-222from the waste(Section 2)Inventory ofRn-222 in soilzone(Section 3)Indoorconcentration ofRn-222(Section 4)Inhalation DoseFactor(Section 1)AssumptionActivity of Rn-222released from thewaste is lower thanthe rate ofproduction from theRa-226 inventorydisposed.No attenuation inthe capAttenuation in thecapEmpiricalrelationshipbetween soil gas andindoor air Rn-222concentrationsEffective(UNSCEAR, 2000)DescriptionThe trench monitoring data (e.g. Hornsby andBechelli, 2007) support the hypothesis that there is asignificant degree of decay of the Rn-222 before it isreleased from the waste.The analysis of bulk gas transport in the capindicates that if direct channels (e.g. vents, cracks)between the waste and above-ground atmospherewere to occur, gas transport would be dominated byadvection (Limer et al., 2011). In such an instance, itis cautiously assumed that any Rn-222 released fromthe wastes is transported directly to the surface.The analysis of bulk gas transport demonstrates thatin the absence of any direct channels to the surfacethat gas migration would occur via moleculardiffusion (Limer et al., 2011). Travel times throughthe cap material indicate substantial attenuationfactors as a result of prolonged travel times throughthe cap.A scaling factor is applied to soil Rn-222concentrations to determine indoor concentrations,based upon work carried out by the BritishGeological Survey and the Health Protection Agencyto analyse UK data and to develop empiricalrelationships between uranium and radium in soil,radon in soil and radon in dwellings (Appleton et al.,2011).Using the ventilation rate method the calculatedindoor concentrations would be a factor of sixgreater.This is an intermediate dose factor, between factorsderived using epidemiological and dosimetricmethods (ICRP, 1993; UNSCEAR, 2000)54