A compelling case
Andy Moffat raises some important issues regarding deep borehole disposal (DBD) (Storage not disposal. Geoscientist 33(1), 18-19, 2023) in response to our original discussion (Borehole disposal. Geoscientist 32(3), 10-11, 2022). However, some of his assertions and resulting concerns do not account for developments in drilling technology, or that the depths for DBD would be ‘modest’ (<5,000 m) compared to the very deep and ultra–deep, albeit relatively small diameter, boreholes (>9,000 m), such as the Kola and KTB boreholes (both of which we have personal experience of). It is the combination of the depth and the larger diameters that may present a challenge, but only for the largest sizes of waste packages.
The concept of DBD has two distinct stages: The drilling and construction of the fully cased borehole is separate from the disposal stage of waste emplacement and post-closure safety. Disposal will not take place unless and until the fully lined borehole is stable and thoroughly checked (with checks repeated regularly throughout the emplacement programme).
During the drilling stage, anisotropic rock stress at depth can create challenges. However, with modern technology and knowledge from very deep, and ultra–deep, research boreholes drilled into crystalline rocks, it is believed that these can be overcome. Eleven such boreholes were drilled across the former USSR. The deepest of these, at Kola, was started in 1970 and drilled over some 20 years and 12 attempts, reached 12,261 m. The drilling technology available at that time was not able to counter stress issues at depth. However, in the 1990s, the KTB borehole in Bavaria, which was drilled to 9,001 m, benefitted from the development of a Vertical Drilling System, such that the borehole was drilled to about 7,000 m with an inclination of less than 1o from the vertical. Such downhole tool technology is now used widely in the drilling industry as Rotary Steerable Systems for accurate directional drilling. Such technology would be used to construct vertical boreholes for DBD.
For the depths required for DBD, breakout, hole enlargement and non-circularity due to the horizontal stress anisotropy are not considered to be significant issues (as discussed in our previously referenced papers). The possibility of encountering significant highly permeable zones and fluid loss is always a risk in any drilling, while a lack of near-surface brines coupled with their presence at depth supports the argument for a groundwater salinity gradient that would constitute a natural barrier to the escape of radionuclides to the biosphere. Regardless, each of these potential challenges – geomechanical influences, permeable zones, fluid loss, the absence of dense brines at depth or the presence of any other detrimental geochemical issues – would be determined using small-diameter, preliminary investigation boreholes to either confirm suitability or eliminate the prospective site. If a site proves unsuitable at any stage, the cost of abandoning a borehole (investigative or ‘disposal’) would be relatively insignificant compared with the investment required to dispose of high-level waste in a Geological Disposal Facility (GDF).
For the sealing and post-emplacement stages, we can be confident that the integrity of the seals will survive for the necessary timescale (with the sealing process discussed in our previously referenced papers). Likewise, any risk of leakage from the waste cannisters and overpacks in the first few hundred years (when the decay heat is high) is minimal and, should a leak occur, radionuclides would not reach the biosphere. The key point is that the primary containment required of the packaging need only survive until the natural geological barriers have recovered from the perturbations caused by drilling and waste emplacement: this is likely to be on a scale of decades to centuries.
Concerns relating to cementing are well known, but it is considered that adequate cementing of the upper casings can be achieved during borehole construction. For the DBD concept we discussed, the final casing to the bottom of the borehole would be perforated and not cemented during construction to avoid the hydrostatic pressure issues that would otherwise arise. Additionally, the proposed containment and seals for our DBD concept do not rely on conventional oilfield cementing, an acknowledged weak point in well construction in the oil and gas industry. Instead, for DBD, a tremie system would be used to cement the large-diameter casing and provide short-term stability. A recently trialled down-hole tool to deliver bentonite dry to depth is also available for such seals, but this approach has not yet been used in the oil and gas industry. Seals above the disposal zone are proposed as part of a redundancy design and may include ‘rock welding’ in granitic rocks (Gibb & Travis, 2015).
Exposure to seismicity is possible, but given that shear waves do not disrupt the saline groundwater density gradient and the powerful barrier it provides to upward flow, any such event is unlikely to disqualify the safety case.
The case for a small-diameter investigation borehole and a full-scale non-nuclear demonstration borehole to establish public and political confidence in the viability of DBD as an alternative to a GDF for high-level radioactive waste is compelling. With the quantity of waste from conventional nuclear power stations and potentially small modular reactors continuing to increase, as well as the potential to dispose of legacy waste in just 10 to 15 years at under 10% of the cost of a GDF, DBD must be an attractive and responsible option.
Director, Marriott Drilling Group. He has researched the concept of deep borehole disposal for 35 years including studies for SKB, Sweden, the UK Nuclear Decommissioning Agency and the US Department of Energy in association with Sandia National Laboratories, Albuquerque, USA.
Emeritus Professor of Petrology & Geochemistry in the Department of Materials Science & Engineering at The University of Sheffield. He has researched the geological disposal of radioactive wastes for over 30 years and pioneered the renaissance of deep borehole disposal. He was a member of the UK Committee on Radioactive Waste Management (CoRWM) from 2007 to 2012.
- Gibb, F.G.F. & Travis, K.P. (2015) Sealing deep borehole disposal of radioactive waste by “rock welding”. International High Level Radioactive Waste Management 2015, Charleston, SC, April 12-16.