Independent Peer Review of - Low Level Waste Repository Ltd

Peer Review of LLWR Response to S9R2LLWR_2008-8-2Version 13.4.1 Engineering Performance71. Section 6 of LLWR (2008c) summarises the approach taken in Paksy(2008) to considering and quantifying water flows through the engineeredcomponents of the disposal facility. Water flows through the facility andits engineered features have been estimated using parameter values eitherderived from an expert elicitation or from the 2002 PCSC.72. The peer review panel considers that the engineering performanceassessment work conducted to date under the Lifetime Programme has notfully captured the uncertainties associated with how the facility mayactually perform and evolve over time. The SLC has been aware of ourconcerns for a relatively short time and has not had time yet to addressthem, but recognises this as an area for further work.73. Our concerns include:That the set of parameter values used in the engineeringperformance assessment do not form a coherent set. This has partlyarisen because after the elicitation workshop, the newly elicitedparameter values were put together with values from the 2002 PCSCthat had been derived on a different basis.That the recent elicitation process was not given sufficient time.That some rather optimistic assumptions were made, including:- that the vertical drains were assumed not to clog or fail;- that the permeability of the cap could be identified to within arather narrow range, that did not take account of the potentialfor cracking as a result of differential settlement; and- that waters flowing into the facility (e.g., into trench 6) wouldbe able to flow efficiently to the vertical drains on the otherside of the trenches.That there appears to be considerable uncertainty in the degree oflateral flows into the facility from the surrounding geology, andfrom the facility into adjacent drift groundwaters (e.g., along theNorth West perimeter of Vault 8).74. We are pleased, therefore, to see that the response made in submission toRequirement 2 recognises the need for optimisation of the repositorydesign, based on a better flow model for the engineered system. We notethat this model will need to link properly to the surrounding models ofgroundwater flow.TerraSalus Limited 18 2 September 2008