What do you do with old nuclear power stations?

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1. The past is catching up with us

Ageing nuclear power stations are found around the world, most holding radioactive waste. When they were first built, no one gave much thought to what would happen at the end of their useful lives. But because the radioactivity in the waste is dangerous and can cause cancer, you can't just demolish a nuclear power station.

Fortunately, in a world trying to work out how to deal with its ageing stations, the UK has a head start. Sellafield, the largest nuclear site in Europe, is decommissioning parts of its sprawling estate of nuclear storage, reactors and labs. So how are they carrying this out in a safe way?

2. The best option: Dismantle the power station

Sellafield has begun to be taken apart - dismantling is the best and safest option.

First the radioactive waste is removed. Some can be recycled, for example old fuel cells can be treated to recover the valuable nuclear isotopes for use in a newer station. The most dangerous waste, a mix of many different chemicals, is immobilised in a type of glass and sealed in steel cylinders for long term storage, potentially off-site in a deep geological disposal facility.

Robots are used to access difficult to reach areas. At Sellafield robots accessed pipelines and waste pools where they removed or sealed waste which would pose a health risk to humans.

Then the rest of the station is decontaminated by deep cleaning and checked thoroughly with radiation detectors. Much of the remaining materials – such as the steel and concrete that makes up the bulk of the station – will be safely disposed of or recycled, as they are not contaminated or the radioactivity has decayed to safe levels.

So what can you do when dismantling is too expensive or dangerous?

3. The back up options

The next safest option is safe storage which happens when parts of the station are too radioactive to remove. This involves waiting to allow the radioactive materials to decay.

After 40 years, used reactor fuel will have dropped to a thousandth of its original radioactivity and heat. Whether to wait before dismantling a nuclear power station is a balance between cost, safety, and time.

25 stations in the UK such as Sizewell A are being left in a partially sealed state, and will remain under observation for decades until much of the waste has decayed.

Entombment is the last resort and only considered when it is too expensive or dangerous to clean the site and has never been used in the UK.

At Chernobyl, in Ukraine, the disastrous meltdown in 1986 meant that normal clean-up was impossible. The crumbling concrete dome which was hastily used to seal the reactor after the disaster has since been replaced with a giant steel shell the size of 5 football pitches.

In the entombment process, a nuclear reactor is sealed in concrete indefinitely so the radiation can decay to safe levels without need for active clean-up. However, the concrete may disintegrate long before the radioactivity declines to a safe level. Entombment is not considered a viable option for current or future stations and is the least used method of decommissioning.

4. Long term solution

So you can decommission a nuclear power station relatively safely however there still remains the problem of what you do with the waste.

Waste in the UK will be stored at central locations like Sellafield so that decommissioning of a station results in complete removal of all nuclear waste.

But are we just storing up problems for future generations? Nuclear waste may be reduced down to vitrified glass or buried deep underground but it is still there. We need to find a way of dealing with the waste permanently. One way of doing this would be to fire neutrons at the waste which would convert it into material that is less radioactive.

Today, dismantling plans must be in place when stations are built but plans may also need to include what happens to the waste as well.

5. Making Fukushima safe

In 2011 a tsunami contributed to the Fukushima nuclear power station disaster in Japan. How would you make it safe? Click to reveal.

Dismantle

The station is taken apart, and components are recycled or scrapped. Waste is removed to be stored centrally. Pictured: Berkeley, UK

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Dismantle

The reactors are too hot to dismantle as they were not properly shut down. Getting access to safely dismantle them is impossible.

Safe Storage

The station is sealed to give time for residual radioactive materials to decay. Eventually it will be followed up with dismantling. Pictured: Trawsfynydd, UK

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Safe store

By making repairs and cooling the reactors, it will take around ten years for the reactor to cool enough to be defuelled. This is the best option.

Entomb

The station, or radioactive parts such as the reactor, are permanently sealed in concrete. Pictured: Piqua, US

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Entomb

By reinforcing the floor and filling the reactors with sand the station could be entombed. This is considered a last resort by the Japanese nuclear agency.