Laying the groundwork for a UK repository

Image: One of the boreholes for sealing at the Harwell site (Photo credit: RWM)

RADIOACTIVE WASTE MANAGEMENT (RWM), A subsidiary of the UK government’s Nuclear Decommissioning Authority (NDA), is responsible for developing a UK Geological Disposal Facility (GDF) to permanently dispose of higher-activity waste. As well as the public focus on a willing community and suitable site, a vast amount of technical underpinning is required to ensure a GDF will be safe and secure for thousands of years.

The company has recently embarked on public discussions in west Cumbria as part of its mission to identify a suitable site for a GDF. In line with a government policy set out in 2018 the process is based on community consent, and it will be preceded by a period of fact-finding and local engagement that is expected to last several years.

Although the two groups where RWM is currently embarking on discussions are in West Cumbria — the region around Sellafield where around 75% of UK radioactive waste is already stored — RWM has been raising general awareness across the rest of England for two years and anticipates that other groups will step forward to open talks in due course.

Waste policy varies among the UK’s devolved administrations. Scottish policy is for long-term management in near-surface facilities, while Wales endorses deep geological disposal but has a separate approach to engagement.

The process of finding a willing community and suitable site, and securing relevant consents, will be lengthy and comprehensive: it could be more than 10 years before a ground-breaking ceremony takes place.

A GDF comprises a series of highly engineered tunnels and vaults up to 1000 metres below the surface or below the seabed. Waste containers will be surrounded by man-made engineered barriers and natural solid, stable rock, safely isolating and containing the waste.

A GDF can feasibly be constructed in one of three broad rock types commonly found across the UK: higher-strength rock (eg, granite), lower-strength sedimentary rock (eg, clay) or evaporite (eg, rock salt).

Technical preparations

RWM’s 65-strong Technical Directorate is the heart of the organisation, fundamental to ensuring that a GDF will be constructed safely and successfully.

Ahead of a site being identified, RWM scientists and engineers have already spent years developing generic designs that will provide the necessary safety barriers. They have carried out exhaustive research and collaborated with organisations overseas. Regulators provide scrutiny at all stages.

Once a prospective location is known, the focus will switch to developing site-specific designs so that the engineered barriers work within a real geological environment (on land or under the seabed) to provide the necessary long-term safety.

Last year saw a collaboration established with the universities of Manchester and Sheffield that will harness research in disciplines such as geological disposal, environmental and materials sciences, radiochemistry, advanced manufacturing and communicating science.

This Research Support Office (RSO) will receive £20 million of funding from RWM over 10 years. Its programme will include wholly funded research and research part-funded with the BEIS-sponsored UK Research and Innovation (UKRI), as well as other academic and industry partners.

The RSO’s remit covers all aspects of deep geological disposal. It has to build a comprehensive bank of solid evidence and ensure any knowledge gaps are plugged. A further aim is to nurture a generation of experts to keep the UK at the forefront of global GDF science. The first cohort of RSO PhD students will embark on their research projects 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.

Image: 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)

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.”

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.