Spent nuclear fuel

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See also the World Nuclear Association's page: "How is used nuclear fuel managed?" and the US NRC's page "Backgrounder on Storage of Spent Nuclear Fuel"

What a nuclear fuel assembly is made up of (Image: United States Department of Energy)

A conventional nuclear reactor uses fuel in solid form, comprising pellets of uranium oxide (or sometimes MOX: mixed uranium and plutonium oxides) clad in zirconium in long tubes, and made into bundles. When they are fresh and unused they have very low radioactivity and can be handled easily, but once in a working reactor their fissile isotopes, such as uranium 235, get transmuted into many other isotopes, many of which have very short half lives and are thus very radioactive. Some of these "fission products" are good at absorbing the neutrons which are needed to keep the chain reaction going.

Over the years in a reactor the proportion of such neutron-absorbing fission products builds up, and the proportion of 235-U in the fuel, which started out enriched to 3 – 5%, decreases, until it gets to a point where the fuel is no longer productive. At this point it is removed. In light water reactors such as PWRs and BWRs the reactor is shut down for refuelling outages; this typically happens every 18 months or so, although only a proportion of the reactor's total fuel is replaced at each stop. In other designs such as CANDU reactors refuelling can be done while the reactor is still running.

Short term storage: cooling pool

When a reactor is shut down, the fuel still produces heat at around seven percent of the thermal power the reactor had while it was running. In an hour this heat production drops to 1.5 percent, and in a week it drops to 0.2 percent. For a reactor which produces 3 GW (3,000 MW) of thermal power (which would generate about 1 GW of electrical power) its heat output would have dropped to about 60 megawatts one week after shutdown. (That's still about as much as some Small Modular Reactors are designed to give at full power.)

When a fuel assembly is removed from a rector it is extremely hot, both thermally and radioactively, due to the heat produced by highly radioactive fission products (i.e. ones with short half lives). This fuel is immediately transferred to a pool (also known as a pond) which cools the fuel and shields its radioactivity from its surroundings (including people). It remains in the pool for a few years — normally between 2 and 10 years.

Central Interim Storage Facility (CLAB), Sweden
Spent fuel pool danger - xkcd what-if.png



xkcd creator Randall Munroe discusses the safety (or otherwise) of swimming in a spent fuel cooling pool on his "What If" website.

Trials at the Dungeness A Magnox power station in the UK which is being decommissioned have shown that divers can safely work in spent nuclear fuel pools to cut up items before retrieving them[1]

Interim storage: dry cask

Dry cask storage outdoors in the USA
Dry cask storage indoors at Zwilag, Switzerland

After cooling the spent fuel is moved to interim storage. This can be another pool, or "dry-cask" storage, where it can safely be stored for decades. At this point, air cooling is enough to keep the spent fuel from overheating, and it poses no danger to people near it.

See also Wikipedia article "Dry cask storage"

Reprocessing

In some countries spent nuclear fuel is reprocessed to recover unburned uranium, and plutonium, which are used to make new Mixed Oxide (MOX) fuel. This leaves a relatively small amount of radioactive material as waste to be disposed of.

This video describes the process:

Long term disposal

Various options have been proposed for final disposal of spent nuclear fuel and other high level nuclear wastes. The mainstream option is burial in deep geological repositories, but deep boreholes have been proposed as an alternative. A different approach is to "burn up" highly radioactive isotopes by transmutation in fast neutron reactors, which produce much smaller quantities of waste which has a much shorter lifetime: of the order of decades rather than centuries.

Deep geological repositories

This type of repository consists of an underground mine into which waste to be disposed of (nuclear or otherwise) is packed. The mine may be excavated for the purpose of constructing the repository or may be re-purposed from a disused existing mine.

The waste is held in suitable containers which are surrounded by material such as Bentonite clay, which strongly binds to any radioactive material which might escape the container. The mine is divided into many passages or caverns and when one is filled it is sealed shut.

The scientific consensus is that deep geological repositories are a safe and effective approach to permanently disposing of spent nuclear fuel and high-level radioactive waste[2].

K+S forklift loading drums.jpg
K+S truck loading bags.jpg

Deep geological repositories are an accepted method of long-term disposal of waste containing arsenic, cyanide, mercury, and other toxic chemicals[3].

There are currently no long term storage facilities for civilian high level nuclear waste (although there is a repository for military nuclear waste in the USA[4] and two repositories for low- and medium-level waste in Germany[5]).

A deep geological repository for spent nuclear fuel is currently under construction at Onkalo in Finland, and others are at various stages of planning in Sweden, the UK[6] and elswehere.

Onkalo, Finland

What happens if Finnish nuclear repository leaks cf banana.png

The Finnish Radiation Safety Authority (STUK) assessed several safety evaluations during the preparations for the Onkalo repository. The the worst-case scenario[7] from the externally reviewed Posiva 2009 Biosphere Assessment Report[8] requires:

  • someone to spends all of his or her days – from birth to death – in the single worst contaminated one square meter plot around the repository, while:
  • eating nothing but the most contaminated food available, with a diet that maximizes radionuclide intake; and
  • drinking only the most contaminated water and nothing else.

The resulting maximum exposure of 0.00018 milli-sieverts per year (much less if any one of the above requirements aren’t met) also requires that the copper canisters which house the spent fuel effectively vanish after 1,000 years, while the bentonite clay barrier (which, alone, is a very effective catcher of radioactive particles) must also disappear somewhere, and the groundwater must move towards the surface. Note that even if the canisters begin to leak immediately, the maximum exposure occurs only after some 10,000 years as it will take time for the radioactive materials to migrate to the surface. After AD 12,000, doses will fall steadily.

Even allowing for reasonable scepticism about assessments made by the company responsible for building the repository, it seems that safety margins are nevertheless considerable. The Finnish Radiation Safety Authority (STUK) accepted the assessment and gave Posiva a permit to proceed with construction in 2015.

Borehole disposal

Deep borehole disposal proposes disposing of spent nuclear fuel in extremely deep boreholes (as much as 5Km underground), relying primarily on the thickness of the natural geological barrier to safely isolate the waste from the biosphere for long enough that it would not pose a threat to living beings.

The Wikipedia article has more information on this approach.

Transmutation

Although "spent" nuclear fuel can no longer be used productively in the type of reactor it has been removed from, it still contains fissionable Uranium-235 and Plutonium isotopes, and various other radioactive isotopes which emit energy as they decay. These can be used in fast neutron reactors to produce more energy. The fast reactor still produces its own spent fuel waste but it is smaller in quantity and shorter-lived in activity.

This approach has not been used to date because fast reactors are significantly more expensive to build than regular (so called "thermal" or "thermal neutron") reactors, and the cost of Uranium fuel has been so low that it has not justified building fast breeder reactors to "close the fuel cycle" by breeding fuel from waste, on economic grounds.

How does the danger from spent nuclear fuel change over time?

Activity of spent fuel over time.jpeg

The activity of the spent fuel decreases rapidly when it is removed from the reactor. In the graphic above it is compared with natural uranium. After about a thousand years the spent fuel is dangerous to humans mainly because it is a somewhat toxic heavy metal, not because of its radioactivity. The main precaution then needed is not to eat it.

  1. Divers take the plunge in spent fuel pond Nuclear Decommissioning Authority; UK Government; 4 Nov 2016
  2. See e.g. "Management and Disposal of High-Level Radioactive Waste: Global Progress and Solutions" by OECD Nuclear Energy Agency and "Managing Nuclear Projects" by H.P. Berg, P. Brennecke, 2013, cited in Science Direct: Deep Geological Disposal
  3. See e.g. Underground disposal page of K+S Aktiengesellschaft website
  4. Waste Isolation Pilot Plant Wikipedia
  5. Morsleben radioactive waste repository and Asse II mine
  6. See e.g. The UK's nuclear waste could be buried underground in Cumbria by Adam Vaughan in New Scientist (4th Nov 2020)
    This (paywalled) article discusses the borough of Copeland in Cumbria considering hosting the UK's Geological Disposal Facility
  7. Graphic and text (adapted) from What does research say about the safety of nuclear power? by J. M. Korhonen
  8. Biosphere Assessment Report 2009 Hjerpe et al.; Posiva; 2010 - p.137 in particular