Canadian legacy in Nuclear: CANDU

As a Canadian, I can’t start up a nuclear blog without talking about the CANDU reactor design. A reactor design that has stood the test of time in its decades of operation racking up not only an impressive safety record but also have been shown to be able to run for over 890 days continuously. Check out Unit 7 of the Pickering Nuclear Plant in Ontario Canada if you don’t believe me.

Now look, I don’t want to just regurgitate the Wikipedia article about the CANDU. They cover it in absurd detail and expose every aspect of it, positive and negative for the world to see. So instead I want to just touch on a few things that make the CANDU design special.

The primary item that helps make the CANDU special is the use of heavy water instead of regular water. Sounds like a really small difference, doesn’t it? Just add a neutron to the hydrogens and make it into water? But here’s where it gets interesting. Heavy water, known more scientifically as Deuterium Oxide, doesn’t absorb neutrons as easily as regular water. This means that since they are using it as both a moderator And a heat exchange fluid, they don’t lose as many neutrons by being absorbed in the liquid. This is important because it means that CANDU reactors can do something very few other reactors can do (ba-dum-tsh) which is use unenriched uranium, previously used fuel from other more specialized reactors, decommissioned nuclear weapon material, or mixed oxide fuels. That’s right, CANDU reactors can get rid of nuclear weapon materials and the most dangerous of “nuclear waste” but can it really be described as “waste” if it can still be used?

The second thing that makes CANDUs special is that unlike standard pressurized water reactors, they do not use the same vessels for moderation and for steam production. The steam production takes place in highly pressurized tubes that contain the fuel bundles and some heavy water, but these tubes are submerged in a large tank of heavy water called the calandria. This double layer design allows the moderating heavy water to be at low pressure while the steam producing water is kept at high pressure. This is the best of both worlds because it minimizes the size of the pressure vessels required which helps to improve safety and minimize costs.

Having the calandria at low pressure also allows easy access to the reaction area for multiple safety mechanisms such as: adjuster rods which are normally used even in standard operation to control the rate of fuel usage and change the desired output of the reactor, emergency shut-off rods that are basically just more adjuster rods that only drop into the calandria when the station loses power in emergency situations due to the electromagnets that hold them in place letting go because they don’t have power. And finally, there is even a small tank of a water-soluble, neutron absorbing liquid that can be dumped into the calandria on the off chance that the first two lines of defence aren’t able to completely smother the reaction.

I could keep going about the safety and the impressive design considerations like how if you have to drain and flood the calandria with regular water, like in the worst case meltdown Fukushima style scenario, the reactor still won’t start heating up again because the light water eats up too many neutrons compared to the heavy water it is designed to use. Or like how since the pressure vessels are horizontal rather than vertical that the reactor can be refuelled and serviced while still in full power operation. But as I said I don’t want to just regurgitate facts.

The sad part is is that we’ve had this technology since 1971, and yet all anyone thinks about nuclear reactors is how they all explode. We’ve been able to make effectively meltdown proof reactors since BEFORE Chernobyl exploded but we haven’t been allowed to build them because people keep gnashing their teeth and wringing their hands screaming “Think of the children!” without bothering to think that, maybe, scientists actually know what they are doing and take steps to think ahead and make improvements to things?  

The CANDU design is important to history as proof that we can make a reactor that is safe and efficient to operate, even in the effective infancy of the industry. 40+ years later it still stands as one of the most reliable designs. Just imagine what we would be doing with it right now if we had been allowed to progress and innovate the nuclear industry? People keep harping that we can build a renewable powered world if only we put more effort into researching better technology for it… so what would have happened if we had done that for nuclear?

3 thoughts on “Canadian legacy in Nuclear: CANDU

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  1. Excellent article. The father of Indian nuclear program, Homi J. Bhabha selected CANDU as the backbone of his 3 stage nuclear program. Rajasthan-1 was first Indian CANDU reactor, which was replica of Douglas Point.
    India was able to adopt CANDU technology because of its simplicity. Pressure tubes are easy to manufacture, and does not require mega-forges. Also, large scale continuous uranium enrichment is not needed to run a CANDU reactor.

    Today it is the pressure tube heavy water reactors which hold world record for continuous operation of 962 days (That is almost 3 year continuous run by a India’s CANDU derived design!)

    As a Indian, I feel bad that India’s atomic show-off has negatively affected CANDU reactors. I’m not an expert; but, I think CANDU is more proliferation resistant than a LWR, and CANDU was not used to make Indian bombs.

    CANDU Non-Proliferation and Safeguards: “A Good Story Seldom Told”

    Click to access whitlock-slides-01-10.pdf

    Liked by 1 person

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