Compound Interest: Nuclear waste and its disposal

Today (22 April) is Earth Day. While currently, we’re somewhat preoccupied with a different crisis, the climate crisis remains a pressing concern. Nuclear power is an oft-mentioned alternative to fossil fuels but comes with the associated problem of nuclear waste. Here, Matthew Harris explains some of the storage solutions and puts the problem in perspective.

With the ongoing climate crisis, there is a strong desire to find alternative power sources to carbon-heavy fossil fuels. One alternative is nuclear fission. Opponents of nuclear power point to high-profile accidents, such as those at Chernobyl and Fukushima, and the issue of the radioactive waste nuclear facilities produce. How can the waste be dealt with safely?

Nuclear fuel is typically made of 3% to 5% enriched uranium. This means that 3% to 5% of its mass is uranium-235. This fuel is made into fuel rods. The uranium-235 nuclei are unstable; when neutrons are fired at them inside the nuclear reactor, they split into smaller nuclei, including strontium-90 and caesium-137.

All radioactive isotopes have a property called a half-life. This is the time it takes for the number of the nuclei in a given sample of the isotope to decrease by half. The longer the half-life, the longer it the radioactive isotope sticks around. Isotopes with a half-life of longer than 30 years are called long-lived, and ones with a half-life of less than 30 years are short-lived.

Strontium-90 and caesium-137 both have an intermediate half-life of around 30 years. They pose issues if released into the environment. Caesium-137 is easily spread in nature due to caesium compounds’ solubility, while strontium-90 is less easily spread but is incorporated into bones and bone marrow if ingested by organisms. They are both highly radioactive and are the principal sources of radiation in the Chernobyl exclusion zone.

These and many other isotopes are found in radioactive waste. Waste is typically split up into three different categories, which correspond to its radioactivity: Low Level Waste (LLW), Intermediate Level Waste (ILW) and High Level Waste (HLW). These waste types are disposed of in different ways, based on the hazard posed by their radioactivity.

Waste is regarded as LLW if it has no more than 4 GBq per tonne (4 Billion decays per second per tonne of the object) of alpha activity or no more than 12 GBq per tonne of beta or gamma activity. Most of the nuclear waste produced (around 90% by volume) is low level waste, but only 1% of the total radioactivity of all radioactive waste.

ILW (Intermediate Level Waste) makes up about 7% of all nuclear waste, and 4% of the total radioactivity. It is too radioactive to be regarded as LLW, but doesn’t produce enough heat to be regarded as HLW. Things that have been in close proximity to radioactive sources, and therefore have a high level of contamination, are often classified as HLW. These include control rods, reactor components and chemical sludge from the treatment of liquid radioactive waste.

HLW (High Level Waste) has a high enough radioactivity that it considerably increases its own temperature. This must be taken into consideration when designing facilities to dispose of it. HLW is produced as a byproduct from reprocessing spent nuclear fuel and is typically liquid. This makes up less than 1% of all waste, but 95% of the total radioactivity.

LLW is the easiest waste to deal with. The waste is compacted into large steel canisters. These are sent to landfill if they have a low enough radioactivity, or disposed of by storing them in large concrete vaults underground. The latter is referred to as near-surface disposal. When these vaults are full, they are sealed, covered with topsoil and left. There will never be an attempt to recover any of the waste due to its radioactive nature, and the design of the sites ensure that the waste can be left with no significant radiation reaching the surface.

These sites sometimes have gas vents and drainage systems to stop pressure in the site from building up and to prevent any leaching from the waste collecting in the vault. A very small amount of LLW cannot be disposed of in these vaults. This may be due to site restrictions on the amounts of different types of radioactive nuclei, because the site is too close to its radiation limit, or because the LLW is too difficult to separate from any associated ILW. In these cases, they have to be disposed of as ILW or HLW.

Both ILW and HLW are ultimately disposed of in the same ways. However, they take slightly different routes to get there.

ILW is compacted into large steel containers, which are then filled with concrete to immobilise the contents. These containers make it safe to transport and store the waste, typically in air-conditioned dry storage, until a suitable disposal facility becomes available. Some long-lived ILW may be left in dry storage for as long as fifty years to let the radioactivity decrease.

HLW has a few extra issues. To start with, a large portion of it is in the liquid state, and any hole in the containment will cause the radioactive liquid to leak. A different way of immobilising it is needed, and one that can be relied upon for the thousands of years the waste will remain radioactive.

Concrete is too prone to weathering over these long time frames. Instead, the waste is mixed in a furnace with crushed glass, giving a HLW-infused molten glass. This is then crystallised in large canisters, keeping the radioactive particles suspended in the glass. This process, called vitrification, makes it highly unlikely that the waste will contaminate anything.

However, we still have the heat production of the waste to deal with. The solution is submerging the canisters in storage ponds, deep concrete water basins, for at least five years. After this, they are moved to dry, air-cooled storage. Due to the long half-life of some of the radioactive isotopes in the waste, they’ll often be left for as long as 50 years before finally being disposed of.

The final disposal method agreed upon by most nations for HLW and ILW is deep geological disposal. This involves placing the waste a few hundred metres to a few kilometres underground, using the rock itself as a barrier for the radiation.

Mined repositories are used most widely. These are tunnels and caverns underground in which containers can be placed. They’re then surrounded by rock and soil to act as an additional buffer to the radiation. These caverns are excavated onshore, or near to it in shallow water, in rock that is suitably stable, such as granite. A low groundwater flow minimises contamination problems. The waste is also recoverable, should this ever become necessary.

Sweden’s KBS-3 mined repository site, which is expected to begin operation in 2023, uses copper containers for spent nuclear fuel. Deposits of native copper have shown that copper tends to be widely unchanged when left in the bedrock, in areas of low groundwater flow. The KBS-3 site also uses bentonite clay, which acts as an effective groundwater barrier and radiation blocker. This means the site will only leak a very small amount of radiation and no actual waste.

Another nuclear waste disposal solution is the use of deep boreholes. These boreholes are drilled up to 5000 metres into the basement rock. The bottom 2000 metres is used for storing the waste and the rest is sealed off with cement, bentonite clay, or other similar materials.

Boreholes can be drilled into both crystalline and sedimentary rocks on land, as well as being drilled offshore, so they expand the range of locations that can be used for waste disposal. However, the waste would be non-retrievable, so this option has been ruled out by many countries.

With the issues of radioactive waste, is nuclear power really the answer to our quest for alternative power sources? Many advocate for renewable power sources and recommend we avoid nuclear power. But renewables require large amounts of land space. A nuclear reactor which produces 1800 megawatts of electricity takes up 4.45 square kilometres of space. A typical solar farm would take up almost 54 square kilometres to produce the same power output.

Renewables are clearly a part of the solution, but it’s hard to see how we could transition away from fossil fuels, in the short time required to avert a climate catastrophe, using renewables alone. Meanwhile, we have established low-risk ways of dealing with nuclear waste, and its actual volume is low. It’s estimated that the volume of nuclear waste currently in the UK, and that which would be produced in the country over the next 100 years, would only fill an area the size of Wembley Stadium.

Despite this, the number of nuclear reactors worldwide has plateaued since the mid-1990s. While new plants are under construction in China and Eastern Europe, countries such as France and Germany plan to phase out nuclear power entirely. Will attitudes change over the coming years as the urgency with which we try to move away from fossil fuels increases?

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