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High-level radioactive waste management

Management and disposal of highly radioactive materials

  • 23 min read
  • 3 chapters

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Through Radioactive waste, Biosphere and Radioactive decay

Among the constituents of spent nuclear fuel, neptunium-237 and plutonium-239 are particularly problematic due to their long half-lives of two million years and 24,000 years, respectively. Handling high-level radioactive waste requires sophisticated treatment processes and long-term strategies such as permanent storage, disposal, or conversion into non-toxic forms to isolate it from the biosphere. Radioactive decay follows the half-life rule, which means that the intensity of radiation decreases over time as the rate of decay is inversely proportional to the duration of decay. In other words, the radiation from a long-lived isotope like iodine-129 will be much less intense than that of short-lived isotope like iodine-131.

Governments worldwide are exploring various disposal strategies, usually focusing on a deep geological repository, though progress in implementing these long-term solutions has been slow. This challenge is exacerbated by the timeframes required for safe decay, ranging from 10,000 to millions of years. Thus, physicist Hannes Alfvén identified the need for stable geological formations and human institutions that can endure for extended periods, noting the absence of any civilization or geological formation that has proven stable for such durations. Other scientists contest the requirement for millennia-stable human institutions, stressing that a waste depository can instead be made fail-safe with regards to groundwater intrusion and therefore eliminating the requirement for active maintenance. Indeed, the removal of the management burden from future generations, while ensuring as best as possible they are not exposed to significant radiological hazard, is a key principal behind intergenerational equity.

The management of radioactive waste not only involves technical and scientific considerations but also raises significant ethical concerns regarding the impacts on future generations. The debate over appropriate management strategies includes arguments for and against the reliance on geochemical simulation models and natural geological barriers to contain radionuclides post-repository closure.

Despite some scientists advocating for the feasibility of relinquishing control over radioactive materials to geohydrologic processes, skepticism remains due to the lack of empirical validation of these models over extensive time periods. Others insist on the necessity of deep geologic repositories in stable formations. Forecasts concerning the health impacts of long-term radioactive waste disposal are critically assessed, with practical studies typically considering only up to 100 years for planning and cost evaluation. Ongoing research continues to inform the long-term behavior of radioactive wastes, influencing management strategies and national policies globally.

Deep geologic disposal

The selection process for permanent repositories for high-level radioactive waste and nuclear spent fuel is underway in several countries. The only purpose-built deep geological repository that is currently licensed for disposal of nuclear material is the Waste Isolation Pilot Plant (WIPP) in the USA, but it does not have a licence for disposal of used fuel or HLW. Plans for disposal of spent fuel are particularly well advanced in Finland, as well as Sweden, France, and the USA, though in the USA there have been political delays. In Canada and the UK, deep disposal has been selected and the site selection processes have commenced.The basic concept involves identifying a large, stable geological formation and using mining technology to excavate a deep tunnel or shaft using tunnel boring machines, between 500–1,000 metres (1,600–3,300 ft) below the surface, where rooms or vaults can be created for the disposal of radioactive waste. The aim is to isolate the waste permanently from the human environment. Concerns about immediate stewardship cessation of such disposal methods have been raised with suggestions that continuous management and monitoring would be preferable.

Given that certain radioactive isotopes have half-lives exceeding one million years, even minimal rates of container leakage and radionuclide migration must be taken into account. It is estimated that several half-lives may pass before some nuclear materials diminish in radioactivity to levels that are not harmful to living organisms. A 1983 review by the National Academy of Sciences endorsed the Swedish nuclear waste program’s estimate that isolation of waste might be necessary for up to one million years.

The land-based subductive waste disposal method proposes disposing of nuclear waste in a subduction zone accessible from land. This method is not restricted by international treaties and is recognized as a feasible and advanced technology for nuclear waste disposal.

At the natural nuclear fission site discovered at the Oklo mine in Gabon, where nuclear fission reactions occurred 1.7 billion years ago, fission products have moved less than three meters. This minimal movement is attributed possibly more to retention within the uraninite crystal structure than to actual insolubility or sorption by moving groundwater. The uraninite crystals at Oklo are better preserved than those in spent fuel rods, likely due to the incomplete nuclear reactions that make the reaction products less vulnerable to groundwater.

The horizontal drillhole disposal method involves drilling over one kilometer vertically and two kilometers horizontally into the earth’s crust to dispose of high-level waste forms such as spent nuclear fuel and specific isotopes like caesium-137 or strontium-90. Following placement and a period of retrievability, the drillholes would be sealed. In 2018 and again in 2019, a US-based private company conducted tests demonstrating the emplacement and retrieval of a test canister in a horizontal drillhole, though no actual high-level waste was used in these tests.

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