Table of Contents
CANDU Basics
Cernavodă Nuclear Power Plant
Romania
Gentilly Nuclear Generating Station
Canada
Embalse Nuclear Power Station
Argentina
Wolseong Nuclear Power Plant
South Korea
Darlington Nuclear Generating Station
Canada
Qinshan Nuclear Power Plant
China
Bruce Nuclear Generating Station
Canada
Pickering Nuclear Generating Station
Canada
Welcome to a tour of the world of CANDU! What’s a CANDU you ask? It’s neither a strange Canadian animal nor an exclamation of determination. CANDUs are a type of nuclear reactor developed by Canada during the 1950s and 1960s. The acronym stands for CANada Deuterium Uranium. Obviously, Canada is rather fond of its achievement so it had to make sure to remind the world that this reactor belongs to them…
CANDUs don’t just exist in Canada though. They are used across the world. Our interactive map will tell you exactly when and where these reactors have been popping up.
CANDUs don’t just exist in Canada though. They are used across the world. Our interactive map will tell you exactly when and where these reactors have been popping up.
The reactor itself is not the world’s most powerful but is designed to work alongside multiple other reactors. The thing that really set it apart from its peers back in the day is that it doesn’t have to be shut down to be refuelled. This allowed it to achieve a much higher ‘capacity factor’, which essentially means it spends a higher percentage of time producing electricity than other designs could. Unfortunately for our Canadian friends, this gap has closed over the decades thanks to improvements in outage management on other station types. CANDU reactors have lost one of their main unique selling points as time has gone on.
However, CANDUs have had a fantastic track record of safety across their 50+ years of operation around the globe and remain a unique and clever design to this day.
However, CANDUs have had a fantastic track record of safety across their 50+ years of operation around the globe and remain a unique and clever design to this day.
Note – Marker size denotes the station power output. Marker colour denotes the number of reactor units (green = 1, orange = 2, purple = 4)
CANDU Fuel
Next up on our CANDU tour; It’s unique use of fuel. Most reactors are designed to use a very specific type of fuel and nothing else. Not the CANDU! Originally it was designed for natural uranium fuel because it is cheap and relatively easy to obtain (At least for a power station. Ordering hundreds of tons of uranium to your home might not go quite as smoothly).
Using unenriched fuel makes a power station more independent from foreign parties which might otherwise be required for fuel enrichment. And that’s not even considering that enrichment is a very expensive process. Highly enriched materials can potentially be used to make nuclear weapons. This is not something most of the world wants, so a reactor that doesn’t touch the stuff seems like a pretty neat idea.
Using unenriched fuel makes a power station more independent from foreign parties which might otherwise be required for fuel enrichment. And that’s not even considering that enrichment is a very expensive process. Highly enriched materials can potentially be used to make nuclear weapons. This is not something most of the world wants, so a reactor that doesn’t touch the stuff seems like a pretty neat idea.
But the mighty CANDU doesn’t even stop there! It not only doesn’t create more enriched fissile material, it can actually consume it. CANDU reactors can use a mix of different fissile materials, such as mixtures of uranium and plutonium. This means it could consume plutonium from reprocessed fuels or even from dismantled nuclear weapons. This eliminates the proliferation hazard plutonium would otherwise pose, and world peace is swiftly restored (well maybe not quite).
CANDU reactors can also use reprocessed uranium which has been extracted from used fuel. This can significantly increase the amount of energy extracted from uranium.
The reactor core, which you will be seeing later on our tour, is made up of many pressurised tubes holding fuel bundles. To refuel, a single pressurised tube can be depressurised and opened. In modern versions of the reactor this refuelling is done by big machines which attach to the tube, open it up, and push in new fuel, which pushes out the old fuel on the other end. Doesn’t sound so complicated, does it? This refuelling system also makes it possible to quickly remove damaged fuel, bringing down the levels of radiation within the primary loop and keeping our workers safe.
Heavy Water?
The tour has been going on for a while. Perhaps it’s time for a refreshment break. However, I wouldn’t use this water to rehydrate. Heavy water is different to the water we are familiar with. All of its hydrogen atoms are the isotope deuterium, which is why it’s also called Deuterium Oxide. What this means for you and me is that it can be lethal in higher concentrations as it inhibits cell division.
However, its great for use in CANDU (and some other) reactors. Normal water tends to absorb neutrons which can then no longer keep the chain reaction going. In fact, no amount of natural uranium and normal water can sustain a fission reaction, because the water simply removes too many neutrons from the equation. Heavy water is less likely to absorb neutrons, making the use of unenriched uranium possible.
Even if you wanted to have a small sip of this stuff, you probably wouldn’t want to pay for it. Just one litre of heavy water can set you back thousands of pounds. CANDUs need so much of it that the costs of simply buying enough heavy water can reach into the billions. On top of that all this water needs to go somewhere so some structures, like the containment building, need to be bigger too.
The CANDU Reactor
Here we are finally. The moment you’ve been waiting for. The heart of the power station; the reactor core. CANDUs are pressurised heavy water reactors. Back when they were first designed, the building of large pressure vessels was extremely challenging. So rather than dealing with all the hassle, Canada decided to just do something else. The reactor makes use of smaller pressurised tubes which in turn sit inside a bigger vessel containing unpressurised moderator (remember the heavy water from earlier?). The heavy water is kept at a low-ish temperature of around 70C, and acts purely as a moderator during normal operations. It is not designed to transfer the fission heat away from the core unlike the moderators in most Pressurised Water Reactors (PWRs). Only in case of an emergency will the vast amount of moderator act as a heat sink, to keep the core temperature down.
In fact, the reactor core is filled to the brim with clever safety features to keep everyone safe. Firstly, the reactor is designed to be ‘sluggish’ which means operators are given more time to respond to problems and resolve them. Then, any deformation in the fuel will cause the chain reaction to slow down. Similarly, any deformation in the pressure tubes will cause contact with an outer shell which will efficiently transfer heat to the moderator, thereby bringing down the temperature. If something does go seriously wrong then the core can be cooled with water from any nearby source without worrying about increasing reactivity, as CANDU core cannot reach criticality with light (i.e. normal) water. In most other reactors this is not the case. Most PWRs will happily go critical if you put normal water anywhere near them.
CANDU use numerous other reactivity control mechanisms which are all kept outside the pressurised tubes, meaning they would be less affected if a tube fails and depressurises.
The reactor uses control rod designs that are very similar to other reactor types. These are suspended by electromagnets and will fall into the core if power is cut.
All of these safety measures add up to create a reactor which has had a fantastic safety record across the decades.
Fuel Bundle
Calandria (Reactor Core)
Adjuster Rods
Pressuriser
Steam Generator
Light-water Pump
Heavy-water Pump
Fueling Machines
Heavy-water Moderator
Pressure Tube
Steam going to Steam Turbine
Cold Water returning from Turbine
Containment Building
CANDU Reactor, Primary Coolant Circuit, and Containment
The Future of CANDUs
We are now approaching the end of our little tour. I’m sure you’re wondering what will become of the CANDUs you’ve grown to love. Unfortunately, it’s not looking too peachy. Advanced CANDU designs were attempted at various times but failed to attract significant interest. Other reactor designs have made significant advancements over the decades and the CANDU design is not as unique as it once was. However, not all hope is lost as there are CANDU based Small Modular Reactors (SMRs) in development. So maybe, just maybe, this isn’t the last you see of the mighty CANDU…
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Below you can find references to the information and images used on this page.
Content References
Image References
- JasonParis from Toronto, Canada – Frenchman’s Bay (Pickering – Bay Ridges) – CC BY 2.0
- Ontario Power Generation – https://www.opg.com/powering-ontario/our-generation/nuclear/darlington-nuclear/ – CC0
- Chuck Szmurlo – Own work – CC BY 2.5
- Mrcukilo – Own work – CC BY-SA 3.0
- Atomic Energy of Canada Limited – Attribution
- IAEA Imagebank – Flickr: 04790183 – CC BY-SA 2.0
- Zlatko Krastev – Own work – GPL
- Bouchecl – Own work – CC BY-SA 3.0
- Description of fuel-cycles for CANDU reactor – Attribution
- CANDU Fuel Bundle – Attribution
- A historical sample of “heavy water”, packed in a sealed capsule. Manufacturer: Norsk Hydro – FAL
- CANDU Reactor Face
- CC BY-SA 2.5
- Christopher K. Grigsby – Own work – CC BY-SA 4.0
