Slovenia has moved a step closer to deciding what to do with its spent nuclear fuel, after a new cost study found that burying the waste from the country’s research reactor and its one power plant in the same deep underground repository would be significantly cheaper than building two separate facilities. The assessment, carried out by the US firm Deep Isolation for Slovenia’s Agency for Radioactive Waste Management (ARAO), looks at a disposal method that has been talked about since the 1950s but has yet to be used anywhere in the world: the deep borehole.
What the Study Found
Slovenia operates a 1960s-era TRIGA II research reactor at the Jožef Stefan Institute in Ljubljana, which produces medical isotopes and trains nuclear scientists. Its waste inventory is small, just 84 fuel elements, enough to fit in a single disposal canister. Building a dedicated repository for that alone would cost around €65.6 million (about $73.5 million), excluding contingency and VAT.
The study found that adding the TRIGA II fuel to a planned deep borehole repository for the much larger Krško Nuclear Power Plant inventory would add only a marginal cost. The reason is simple: a repository, borehole or otherwise, has heavy fixed costs that have to be paid whether you are burying 84 fuel elements or thousands.
Sandi Viršek, chief executive of ARAO, said the figures gave the agency “a clear comparison of disposal pathways” and showed that squeezing the research reactor fuel into the Krško plan carried no real safety or scheduling penalty. Deep Isolation added that the study gives Slovenia a solid basis for its long-term planning, though it stressed that safety assessment, implementation and regulatory engagement all still lie ahead.
What a Deep Borehole Actually Is
Deep boreholes are narrow, vertical shafts drilled deep into the Earth’s crust, sometimes with a horizontal section at the bottom. The idea is to place sealed waste canisters into rock that has sat undisturbed for tens or hundreds of millions of years, then plug the hole above them with layers of rock, bentonite clay and other materials.
Deep Isolation’s version uses off-the-shelf directional drilling technology borrowed from the oil and gas industry. Corrosion-resistant canisters, roughly a foot wide and 14 feet long, are lowered down a steel-lined shaft that gradually curves until it is almost horizontal. The horizontal “disposal section” can run for up to 3.2 kilometres, several thousand feet below the surface. For the TRIGA II fuel alone, the standalone options modelled were a short vertical “microdepository” drilled 1.5 kilometres down, or a longer, narrower horizontal borehole designed around 84 bespoke canisters.
A Shared Reactor and a Shared Waste Problem
The Krško power plant is itself unusual. A Westinghouse pressurised water reactor on the Sava river, it is jointly owned by Slovenia and neighbouring Croatia, a legacy of their shared Yugoslav past when the plant came online in 1981. Krško supplies up to 40% of Slovenia’s electricity and has had its operating life extended to 2043, the same year TRIGA II is due to shut down. That alignment is one reason both countries have been quietly working with Deep Isolation on borehole options for several years.
Slovenia is not alone. Deep borehole concepts have been studied in Denmark, Sweden, Switzerland and the United States, and a 2024 Deep Isolation study commissioned by Norwegian Nuclear Decommissioning examined boreholes as an option for high-level waste from five countries. Deep Isolation itself launched a full-scale “commercialisation pilot” of the technology in January 2026.
For small nuclear nations with modest waste inventories, the appeal is obvious. A traditional mined repository, of the kind Finland has pioneered at Onkalo, is a decades-long megaproject. A borehole is faster and uses equipment the drilling industry already has lying around. Whether regulators will one day sign off on it is another question entirely.
World Nuclear News (2026). Slovenia’s deep borehole disposal options studied. 23 October 2026. https://wnn.world-nuclear.org/articles/slovenias-deep-borehole-disposal-options-studied
Agency for Radioactive Waste Management (ARAO), Slovenia: https://arao.si/en/
Deep Isolation, Cost Assessment of Deep Borehole Disposal Options for TRIGA II Spent Fuel in Slovenia (case study, 2026): https://www.deepisolation.com/wp-content/uploads/2026/05/20260526-Slovenia-Case-Study-03-DIGITAL.pdf