The technique, developed in the UK by researchers at the University of Sheffield, involves drilling a borehole around 0.6 metres wide and five kilometres deep, and lowering the waste into it.
Known as deep borehole disposal (DBD), the technique is much cheaper than the mined repository proposed by the UK government for burying the country’s nuclear waste, according to its pioneer Fergus Gibb, emeritus professor of petrology and geochemistry at the University of Sheffield.
Each borehole would cost a few tens of millions of dollars to drill, compared to hundreds of millions to tens of billions for a mined repository. Around six boreholes would be enough to store all of the UK’s existing high level waste, said Gibb, with each taking less than five years to drill, fill and seal.
Deep borehole disposal should also be safer than a mined repository, which at 500 metres deep would still be within the zone of circulating ground water, meaning any leakage caused by an earthquake, for example, could potentially return to the surface, he said.
“By going down several kilometres, it means the disposal zone at the bottom of the borehole is in a geology which is below, and isolated from, the normal ground water,” he said. “So even if the waste eventually leaks out of the containers into the surrounding rock and water, it will never come back to the surface, instead it will leak into waters in the deep rocks that have been isolated from the surface waters for millions of years.”
This should make it easier to gain public acceptance for a burial site, he said.
Although not a new idea, DBD has only been made possible by recent advances in drilling by the oil and gas and geothermal energy industries, he said.
To ensure the waste is protected from leakage, corrosion, or theft after it has been buried, the researchers have been developing two distinct sealing techniques, which they recently presented at a conference held by the American Nuclear Society in Charleston in the US.
Firstly, the waste would be slowly lowered down the hole in metal containers, and then a specially-developed sealing and support matrix would be poured in with it.
For “hot” radioactive waste - that which gives off heat of over 190 degrees Celsius as it decays - metal alloy shots that melt into a solder to seal the waste in the casing would be used, said Gibb.
But for waste that does not give off as much heat, Gibb’s colleague, Dr Nick Collier, is developing a special type of cement that can be poured down the hole, which will not set until it reaches the bottom, up to five hours later.
Once the waste has been sealed in position, the hole is then filled above the disposal zone with crushed granite, said Gibb. An electric heater is used to melt the granite, which is then cooled slowly to allow it to re-crystalise, in a process known as rock welding. “So we create a weld that is both continuous with, and identical to, the host rock around the borehole,” he said. “This completely seals the hole, just as if you’ve never drilled through it.”
The US trial, which is being run by Sandia National Laboratories for the Department of Energy (DoE), will take place at a yet to be determined site in late 2016.
If it proves successful, the DoE plans to build a much smaller, 22 centimetre-wide hole to dispose of small capsules of highly radioactive cesium and strontium being held at the Hanford nuclear facility in Washington State.