Cleaning up nuclear waste after decommissioning

Karen Cholmondeley
6 December 2013

Above: The Gentilly Nuclear Generating Station, with the Gentilly-2 reactor on the left and the Gentilly-1 reactor on the right (Bouchecl)

Did you know? Some countries use a special type of nuclear reactor, called a breeder reactor, which uses the radioactive waste from a regular reactor as fuel.

In the wake of the Fukushima nuclear reactor meltdown in Japan, or simply because older plants have become too expensive to maintain, governments across the globe are examining ways to decommission (shut down) older nuclear power plants and deal with the dangerous radioactive waste those plants leave behind. Canada is no exception, as Quebec closed its Gentilly-2 reactor in 2012.

To understand the challenge of decommissioning, you have to understand how nuclear reactors work. The key ingredient is uranium-235, which is packaged in long columns called fuel rods. When the rods are bombarded with a controlled stream of neutrons, the atoms that make up uranium-235 actually split apart! This splitting produces an enormous amount of heat, which is transferred to the surrounding water. The heated water produces steam with enough force to spin a turbine and generate electricity.

The water used in many nuclear reactors is called heavy water because the hydrogen in the H20 molecule contains an extra neutron. Heavy water helps to moderate (slow down) the nuclear reactions and to cool down fuel rods. Regular water would absorb the neutrons released from the uranium, whereas heavy water already has an extra neutron and won’t absorb any more. Regular water can also be used in a nuclear power plant, but only with enriched uranium, which is more dangerous and can be used in nuclear weapons.

Unfortunately, uranium-235 atoms will not keep splitting apart forever. Worse still, even after they stop being useful for energy production, the fuel rods remain extremely hot and radioactive, meaning they continue to emit high energy particles such as electrons or gamma rays. These particles can cause significant damage to cells in living organisms, including humans. The radioactivity of uranium-235 does decrease over time and it eventually becomes harmless. However, this process takes several hundred years! So what can be done with these hazardous fuel rods until then?

The first step is to slowly cool the fuel rods in a storage pond that looks like a giant swimming pool. The water also helps prevent radiation from being emitted. The used fuel rods have to stay in this pond for at least 10 years! After that, rods can be transferred to dry storage containers, usually made of stainless steel or ceramics. The containers are sealed tightly and the rods are stored inside for another 50 years or more.

Back in 2012, Quebec decided to close its Gentilly-2 nuclear reactor due to high operating costs. To make the site safe from radioactivity, all of the most dangerous waste, including the fuel rods, will be removed and stored in the manner described above until it becomes harmless.

Did you know? Each year, a total of 10,000 cubic metres of high-level nuclear waste is produced worldwide. That’s enough to fill four Olympic-sized swimming pools!

Furthermore, the building that housed the nuclear reactor and all of its contents will be put under an airtight seal. No one will be able to enter or exit for about 50 years. This is called putting the power plant in a static state, a process that has already been performed for Gentilly-1, an earlier reactor built at the same power station. Gentilly-1 has been in a static state since 1986, after being closed in 1979. When the 50 years are up, it will finally be possible to safely dismantle the site.

Overall, nuclear reactors are a relatively safe and powerful method of generating energy. In particular, they emit much less pollution than burning fossil fuels. However, the question of how to manage and dispose of nuclear waste remains a major challenge. And as the case of the Gentilly reactors shows, nuclear waste is a problem that must be dealt with for decades after a plant has been decommissioned.

References

How Does a Nuclear Power Plant Work? (Ontario Power Generation) How Nuclear Power Works (Ontario Power Generation) Processing of Used Nuclear Fuel (World Nuclear Association) Quebec's Gentilly-2 nuclear plant shuts down after 29 years (CBC News) Radioactive Waste Management (World Nuclear Association) Taking Canada’s Gentilly-1 to a “static state” (Balarko Gupta, IAEA Bulletin) Types of Waste (Ontario Power Generation)  OPG Deep Geological Repository (Ontario Power Generation)  Waste Management (Canadian Nuclear Association)

Karen Cholmondeley

I recently completed my MSc. and am now actively seeking my next adventure! I love science because it helps offer explanations to so many different questions affecting all aspects of our lives, from health to nature to technology. In my free time I love to read, travel, and be outside (unless it's cold!).

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