Technical Review of the 2011 ESC for the LLWR near Drigg. Issue Resolution Form ESC-TQ-ASO-009 TECHNICAL QUERY

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1 Content Title Date raised 12/07/2013 Acknowledgment required by 17/07/2013 Response required by 30/08/2013 Related issue numbers Originated by Current owner Review group Approved by Reply to LLWR s response to ESC-TQ-ASO- 009 Well calculations ESC-TQ-ASO-009 ASO History Status Date Description Raised 19/07/2013 Acknowledged 19/07/2013 LLWR Response 13/09/2013 Response Assessed 30/01/2014 Transferred 12/02/2014 Closed 13/02/2014 Statement of Technical Query This Technical Query (TQ) specifically relates to our response to information submitted by LLWR to ESC-TQ-ASO-009 concerning the estimation of radionuclide concentrations for well pathway calculations and their impact on potential exposures calculated. This TQ should be read in conjunction with ESC-TQ-ASO-009. It is presented as a discrete TQ for clarity. For quality assurance purposes, this should be considered as one TQ. assessment of LLWR Ltd response The following TQ relates to the calculation of impacts via the well pathway in the 2011 Low Level Waste Repository (LLWR) Environmental Safety Case (ESC). We seek further clarification of a number of key assumptions used in these calculations in order to provide reassurance that the calculations are not overly optimistic. Potential exposures estimated via the well pathway are significant in the context of the 2011 LLWR ESC. We note that the revised calculations presented by LLWR Ltd in the response to this TQ now estimate peak risks associated with the well pathway as 1.9E-07 yr -1. Estimation of radionuclide concentrations for use in well pathway calculations LLWR Ltd s approach to estimating radionuclide concentrations in abstracted groundwater averaged the groundwater concentrations over the area of the assumed plume (i.e. the area between the coast and the site). The assessment calculations carried out in GoldSim, as part of the 2011 ESC, used these averaged concentrations in estimating radiological impact. We are uncertain as to the magnitude of variation of radionuclide concentrations that may exist between the calculated average and the range of values present in the plume (as Page 1 of 5 Last updated 07/07/2015

2 represented by the various GoldSim pipes 1 ). We are therefore also uncertain whether any differences could be significant in understanding the pattern of potential exposures. We also note that in its response to this TQ LLW Repository Ltd provided an analysis which states that, assuming homogeneous properties, the maximum width of a well s capture zone is around 74 m. This suggests that mixing across the entirety of modelled plume width is unlikely to happen, as is implied by LLW Repository Ltd s approach of considering average plume concentrations. Furthermore, the calculations undertaken within the GoldSim assessment model assume that the geological strata are homogenous and so the hydrogeological behaviour is also assumed to be homogeneous. However, the geoscientific understanding presented by LLW Repository Ltd elsewhere in the 2011 ESC recognises the substantial heterogeneity exhibited by the geological strata around the site. We are aware that ConnectFlow simulations have been undertaken which explore the impacts of this spatial variability and heterogeneity. In these simulations the particle tracking revealed areas of both preferential and reduced groundwater flow in response to variations in the hydraulic conductivity realisations generated. We cannot find any information reported on the implications of the variation of modelled groundwater concentrations on the approach used in the well pathway calculations or a commentary on their comparison to conditions expected at the site. Therefore, we are unable to judge the acceptability of LLW Repository Ltd s approach to only use the calculated average radionuclide concentration in its assessment until we have further information. Range of potential exposures associated with use of a well In order to support our assessment we also require a range of potential exposures to be presented to us with the likelihood that they will be received (c.f. GRA para ). Given the inherent uncertainties in characterising future human behaviour we do not consider it inappropriate to include a range of potential exposures associated with abstracted groundwater in our assessment, given the importance of this exposure pathway. We would therefore expect that LLW Repository Ltd provide us with information on the potential exposures associated with abstracted groundwater arising from various scenarios. We note that in its response to the initial TQ, LLW Repository Ltd has provided information on the implications of considering Potential Exposure Groups with additional exposure pathways (for example, water consumption by livestock). However we suggest that further scenarios be considered which should include, but not be limited to, consideration of the impacts of the variations of groundwater concentrations (as discussed above) as well as assuming a specific location for an abstraction well (for example, close to the site boundary). Such scenarios could be presented as supplementary calculations, accompanied by any appropriate discussion of potential likelihoods and any implications for the ESC. We consider that the presentation of such information could help the EA in its assessment decision-making and in any engagement with local communities. In presenting these results it would be informative if LLWR Ltd were able to provide a breakdown of potential exposures which reports both dose and risk. 1 The GoldSim model represents groundwater flow processes using as series of pipes. Page 2 of 5 Last updated 07/07/2015

3 Technical Query Actions.A1.A2.A3.A4 Provide a comparison of the range of radionuclide concentrations estimated in the GoldSim pipes against the average values used in the ESC assessment calculations. Provide an analysis of the impacts on radionuclide concentrations in the plume arising from considering site specific heterogeneity in hydrogeological properties. Provide supplementary information on the potential exposures from use of a well by considering impacts of variations in groundwater concentrations as well as assuming specific locations for an abstraction well. Provide a breakdown of potential exposures which reports both dose and risk. LLW Repository Ltd Acknowledgement LLW Repository Ltd ESC Project Manager 19/07/2013 The information requested will be provided. LLW Repository Ltd Response Provided by: LLW Repository Ltd ESC Project Manager 13/09/2013 A response is provided in report: AMEC/200719/001 Issue August Assessment of LLW Repository Ltd Response By: Owner: Date: 12/02/2014 Approved by: Date: 13/02/2014 The following assessment relates specifically to the outstanding information provided in response to only. LLW Repository Ltd did not include in its response a comparison of the range of radionuclide concentrations estimated in GoldSim pipes against the values used in the assessment. However, LLW Repository Ltd did present a further explanation of the approach used to derive radionuclide concentrations in groundwater, including the mathematical equation used in the estimations. This adequately addressed our query. Therefore we now consider that the approach LLW Repository Ltd used to estimate radionuclide concentrations is unlikely to have resulted in an unacceptable amount of risk dilution 2. LLW Repository Ltd provided further information on the impacts on potential exposures of considering locating an abstraction well at particular nodes in the GoldSim pipes and also by using the particle tracking simulations undertaken using ConnectFlow and assuming either 2 Risk dilution, as used in the original Technical Query is probably not the correct terminology. What we are referring to is the potential for the calculation to introduce factors which may have the effect of underestimating radionuclide concentrations and potential exposures. Page 3 of 5 Last updated 07/07/2015

4 homogeneous or heterogeneous hydrogeological properties in the geological media. This provided clarity to the presented model outputs. LLW Repository Ltd has provided a useful analysis of the potential impacts of spatial variability on the distribution of doses within the plume area assuming either homogeneous or heterogeneous hydrogeological properties. We consider that these are valuable in presenting supplementary information on the potential variation in doses. We recommend that LLW Repository Ltd investigates the potential to develop this work further such that these estimates are more readily able to be compared directly with the assessment calculations. This might for example be achieved by better restricting the ConnectFlow calculations to the plume area between the site and the coastline (see Recommendation ASS16 of 2015, Review of LLW Repository Ltd's 2011 Environmental Safety Case: Assessments). Within this TQ we noted that only a few spatial heterogeneity simulations were carried out within ConnectFlow and therefore we were uncertain as to whether they represented the full range of variability likely to exist in the potential exposures from the well pathway. In response LLW Repository Ltd stated that it was of the opinion that the greater amount of variability would be within a realisation rather than between realisations. This was based on its experience and no evidence was provided to support this assertion. We have suggested in our review of the 2011 ESC 3 that the number of spatial heterogeneity calculations was limited and would benefit from further assessment work (see Recommendation ASS15 of 2015, Review of LLW Repository Ltd's 2011 Environmental Safety Case: Assessments). LLW Repository Ltd has included in its response a breakdown of potential exposures which reports both dose and risk. We would expect that for future versions of the ESC, the presentation of doses should be accompanied by a clearer indication of the likelihood associated with the potential exposure to enable its context to be more readily understood (see Recommendation ASS10 of 2015, Review of LLW Repository Ltd's 2011 Environmental Safety Case: Assessments). We recognise that the development and configuration of the well pathway assessment are one of the most significant exposure pathway calculations being within a factor of 5 of the risk guidance level for the Reference Disposal Area. The information presented in response to both well pathway TQs (ESC-TQ-ASO-009 and ) are subject to a number of key assumptions which have significant uncertainties associated with them. These uncertainties are common to any attempt to predict speculative future human activities and are common to any ESC assessment. After consideration of LLW Repository Ltd s response to both TQs (ESC-TQ-ASO-009 and ) we recognise that the assessment includes significant uncertainties. However, we consider that these uncertainties have been adequately addressed in the TQ responses and assessment calculations. We recommend that LLW Repository Ltd carries out further investigations into the nature and significance of uncertainties associated with the potential for future groundwater usage in future assessments (see Recommendation ASS17 of 2015, Review of LLW Repository Ltd's 2011 Environmental Safety 3, Review of LLW Repository Ltd s 2011 Environmental Safety Case: Assessments. Page 4 of 5 Last updated 07/07/2015

5 Case: Assessments). We also recommend that LLW Repository Ltd improves the clarity of its presentation of complex assessment areas such as future groundwater usage in future assessments (see Recommendation ASS18 of 2015, Review of LLW Repository Ltd's 2011 Environmental Safety Case: Assessments). This TQ can be closed. Actions completed, Technical Query resolved and Form closed Owner: Date: 12/02/2014 Approved by: Date: 13/02/2014 Page 5 of 5 Last updated 07/07/2015