UK | RADWASTE MANAGEMENT


later this year, looking at the performance of cement grouts, bentonite sealing, hydrogen ventilation and microbial gases. Lucy Bailey, RWM’s head of the RSO, says: “I am thrilled to


be leading this exciting initiative. Through the RSO, we will harness the best research expertise across the UK to build the knowledge and understanding required to underpin the safety case to deliver a GDF that deals permanently with the UK’s higher-activity waste.”


Borehole sealing As the GDF siting programme gains momentum through public engagement, RWM is looking ahead to early on-the- ground activities, which will include seismic surveys and borehole drilling to take rock samples. The first step will be non-invasive seismic surveys. This


technology, which involves transmitting sound waves into the earth using highly sensitive acoustic geophone devices and capturing the returning data to build a picture of underground rock structures, has been well established for many decades in oil and gas exploration. A network of geophones was also deployed by the US Apollo missions to explore the structure of the moon’s crust. Exploratory boreholes could then be drilled to potentially


penetrate deep, stable rocks in the zone where a GDF, likely to extend over 20km2


, is subsequently constructed. Such


boreholes will need to be sealed, with carefully chosen materials whose long-term performance is understood. The sealing system must operate in all three rock types and preclude a future route for radionuclides to migrate. That rules out techniques conventionally deployed in mineral extraction industries such as coal, oil or gas. In preparation, RWM has been carrying out research over


the last eight years to devise a system uniquely tailored for the GDF programme. The £5 million borehole sealing project involves scientists, engineers and geologists from RWM, who are working with contractor Jacobs to research, design and build the downhole placement system (DPS), with support from Marriott Drilling and NeoProducts. The DPS has now undergone a three-month full-scale


field trial demonstration in clay-rich sedimentary rock using a pre-existing borehole at the Harwell site in Oxfordshire, managed by NDA subsidiary Magnox Ltd. The objective was to place a bentonite clay seal 300m below ground, using the DPS to deliver the bentonite in the hole at the required depth (the DPS itself was deployed on a drill string, using a drill rig from Marriott Drilling). Bentonite is a common, naturally occurring clay that


swells when it absorbs water. Used in a GDF and in a borehole drilled to investigate rocks, the bentonite will gradually absorb water and swell to fill the surrounding space, providing a barrier with very low permeability. For RWM, the borehole sealing project marked a first opportunity to move from pure research to realistic on-the- ground operational activities. Technical director Mohammed Sammur says: “It’s been hugely exciting to see our research reach this stage, demonstrating that we are prepared for formal site investigation work in the future. “We carry out extensive R&D into all aspects of a GDF to


understand exactly the requirements for designing, building and operating a facility up to 1000m deep. It will need to be safe for many hundreds of thousands of years. “We’re grateful to Magnox for accommodating our


project and assisting us – it was fortunate to locate suitable boreholes so close to our headquarters.”


www.neimagazine.com | May 2021 | 31 The 7m-long DPS was deployed on drill string, with a


hydraulic connection to the ground surface, enabling water to be pumped through. It is suitable for downhole pressures of up to 3000psi, capable of sealing the full range of water- filled site investigation boreholes of interest to RWM (up to 2000m total depth). The DPS joints, housing a chamber pre-loaded with


bentonite pellets, are formed from threaded steel pipe, with Teflon-coated internal surfaces to reduce friction between the contents and the DPS wall. The DPS chamber length can be adjusted by adding further joints. The DPS chamber was lowered to the required depth, then flooded with water via the drill string, rapidly pushing the bentonite out. Once out, the wetted bentonite payload quickly swelled, producing a seal in the borehole. The DPS and drill string design ensured a water flow rate high enough to flood the DPS and release its contents in less than one minute. The short water contact time minimised the potential for premature bentonite swelling before release into the borehole. Colleagues from the Environment Agency, who will need


to be satisfied with GDF site investigations once they are under way, were kept updated during the demonstration and observed work in progress at Harwell. The practical lessons learned included a greater


understanding of the behaviour of clay rock stability when drilled at depth. RWM’s experience will lead to modifications in the planned geological investigations, and will be shared internationally, particularly with Andra and Nagra, the parallel French and Swiss organisations, who are focusing on sites in similar lower-strength sedimentary rocks. “The project was invaluable in enabling us to thoroughly


road-test our research and has highlighted a number of technical insights that we will take account of as we undertake further field trials as part of this important project,” says Sammur. Future project work will involve demonstrating DPS functionality and bentonite behaviour in a 2km deep granite borehole and in a rock salt borehole. As the search for a site gathers pace, RWM’s technical work will intensify both in the UK and through collaborative initiatives with overseas partner organisations where parallel research programmes are adding to the global understanding. ■


Below: The Downhole Placement System (DPS) being trialled in laboratory conditions, lowering bentonite into a Perspex ‘borehole’, to test bentonite deployment and seal evolution Photo credit: RWM


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