Developed Operational Strategy - Low Level Waste Repository Ltd

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LLWR Developed Operational Strategy10009/LLWR/LTP Date: March 20106.2.1 Closure EngineeringThe capping of the disposal facilities is vital to ensure the long-term environmental protection required.The principle functions of the cap are to:• minimise the infiltration of water into the waste mass;• prevent/discourage bio-intrusion; and,• control gas release.Prior to the installation of the cap it is proposed to install a bentonite cut-off wall around the wastedisposal area. The principle functions of this wall are to:• minimise the ingress of groundwater into the waste mass; and,• minimise the outflow of leachate into the near surface and surface water systems.In addition to the cap and the cut-off wall it is also the intention to install a vertical drain. This drain isintended to act in the event of cap failure to ensure that that any water entering the waste mass is notdischarged to near surface, (easily accessible), water bodies, but is instead, directed towards deepersystems (less accessible), systems.All of these features and the vault structure are the subject of engineering optimisation to ensure thatthey are effective and satisfying their principle functions in addition to demonstrating value within thesystem. This optimisation process will examine innovative strategies for site remediation, trench cappingand closure will be initiated to enhance final, long-term confinement of disposed waste and passiveenvironmental monitoring systems and will be fully documented within the ESC to be submitted to the EAin May 2011.6.2.2 Summary of the Current DesignThe lining system for the new vaults (9-15), described below, is a component in the management ofLLW, the design for which will include a major cut-off wall to depth around all the vaults and trenchesand a final capping system over trenches and vaults. Leachate generated during the operational andmanagement phases will be actively managed by pumping out of the vaults. Ultimately, in the postclosure phase, leachate from the vaults will preferentially overflow the eastern sidewall and be carriedby a series of vertical drains to the regional groundwater unit, subject to future regulatory approval, referto Figure 9. The current design will be assessed in the ESC.Page 46 of 62
LLWR Developed Operational Strategy10009/LLWR/LTP Date: March 2010FIGURE 9 - MODULAR VAULTS SCHEMATIC OF LEACHATE MANAGEMENT (AFTER CIRCA 2150) ASSESSED IN ESCThe new vault basal lining system comprises the following elements, viewed from bottom up:• 500mm thickness of Bentonite Enhanced Soil (BES) secondary mineral liner, overlain by;• 2mm High Density Polyethylene (HDPE) geomembrane, overlain by;• Needle-punched, non-woven geotextile protector, overlain by;• 500mm thickness of free draining non-calcareous aggregate Leak Detection and Collection Layer(LDCL), overlain by;• Separator geotextile, overlain by;• 500mm thickness of BES primary mineral liner, overlain by;• Fixed electrical leak location system, overlain by;• 2mm HDPE geomembrane, overlain by;• Needle-punched, non-woven geotextile protector, overlain by;• 2000 gauge Polyvinyl Chloride(PVC) membrane (shear break and separator layer to preventleaching of concrete into the underlying geotextile during construction) , overlain by;• 350mm thickness of reinforced concrete running surface.The sidewalls to Vaults 9-15 comprise a ‘single composite’ lining system of 1000mm horizontal thicknessof BES sandwiched between two concrete retaining walls, see Figure 9 below. The ‘outer’ (relative towaste body) retaining wall is of standard construction to the western and southern side of Vaults 9-15; itcomprises the southern wall of Vault 8 to the north; and it is a secant pile wall to the eastern side ofVaults 9-15, adjacent to the trenches where geometrical constraints dictated against standard retainingPage 47 of 62
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Developed Operational Strategy - Low Level Waste Repository Ltd

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