Storage not disposal
Deep boreholes may not be a long-term solution for nuclear waste disposal, writes Andy Moffat
I read with interest Beswick and Gibb’s proposal for disposing of higher grade long-lived nuclear waste in deep c.5.0 km boreholes (Geoscientist 32(3), 10-11, 2022). I have no experience with nuclear waste, but I have over 40 years’ experience of drilling (onshore and offshore to 7.3 km) learning much about the subsurface behaviour of rocks, and the potential risks to borehole stability. Rocks don’t always behave as expected during and post-drilling, despite often extensive evaluation and modelling of nearby borehole data.
Metamorphic high-grade basement complex rocks may initially appear ideal for the long-term, stable storage of nuclear waste in boreholes. Yet, case studies such as the very deep KTB (9,101 m) borehole drilled in Bavaria, Germany, and the ultra-deep Kola SG3 (12,262 m) borehole drilled in the Kola Peninsula, north-western Russia, show that this is not necessarily so. These scientific boreholes were plagued with stability problems mainly due to horizontal stresses, with numerous breakout caving zones. The consequent borehole rugosity made vertical drilling difficult with frequent stuck pipe incidents, the severity of which sometimes led to expensive drill string severing, plugging-back and side-tracking the holes.
These boreholes also encountered zones of unexpected permeability, with the loss of drilling circulation fluids and brine flows at depth in seemingly hard, impervious rocks. The Kola SG3 borehole recorded copious influxes of hot, mineralised brine flows between 4.5 and 9.0 km, as well as a hydrogen gas influx to surface. Underground blowouts (cross-flow), where fluids flowing out from a higher-pressure influx zone migrate into the uncased open hole or behind the casing and into lower-pressure loss-circulation zones, are usually tricky to deal with and have the potential for serious post-drilling long-term consequences. In the petroleum industry, once the hole has been drilled and cased-off, cement is pumped into the annulus between the casing and borehole to seal off the permeable zones. However, achieving a perfect ‘cement job’ to ensure zonal isolation is often difficult. Shear zones can also occur, with brittle deformation. Earthquakes and seismic tremors from post-glacial isostatic recovery can cause ‘slippage movements’ along deep sub-seismic faults or re-opening fractures allowing groundwater movement and hydraulic connectivity.
Drilling a slim-diameter, pilot hole to establish the suitability of subsurface geology is pre-requisite for a deep borehole, yet the deep geology of the pilot hole may not be entirely representative – I have frequently observed faults, loss-circulation zones and variations in rock properties in the drill hole that were not evident in the adjacent pilot or abandoned hole section. Maintaining verticality in deep and hard basement rocks can be difficult meaning the pilot hole may be crooked (as observed at the KTB and Kola SG3 holes) resulting in significant lateral offset between the two holes. The project may have to budget for the worst-case scenario of abandoning a costly deep borehole due to unsuitable geology that was not evident from the pilot hole, meaning deep boreholes are not always that cost effective.
Once the borehole has been drilled and stabilised, and the nuclear waste disposed of, the borehole is sealed. But do we really know what the long-term lifespan of such a borehole and its sealing elements will be? Oil and gas wells have only been in existence for around a century. Expecting the seals and casing of nuclear waste boreholes to maintain integrity for tens or hundreds of thousands of years seems like a reckless act of faith. Damaged casing or leaking waste cannisters coupled with future fluid migration could be disastrous. The migration of radioactive fluids, which could lead to cross-flow or reach the surface, would be very difficult or impossible to fix. Future generations will not thank us.
Trying to retrieve radioactive waste from a deep borehole is likely to be hazardous, if not impossible if the casing has deformed or ruptured. I think our radioactive waste should be stored in a relatively shallow, monitored underground warehouse repository from which retrieval is possible – storage not disposal. French law currently requires companies to build a retrievable scheme; meaning that for the first few hundred years at least, they can remove the waste should future generations find a better disposal method.
Andy Moffat is a retired geologist living in Strathpeffer, Highland Scotland.
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