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June 03, 2006

CANDU Plants

Something I didn’t know (well, OK, there’s lots of things I don’t know. This is one more specific thing I didn’t know).

Its pressurised heavy-water reactors, known as CANDU, are similar to ordinary pressurised-water reactors (or light-water reactors, as they are sometimes known) but they contain water in which the hydrogen atoms have been replaced by their heavier cousins, deuterium. Heavy water is expensive. However, the fuel used by CANDU is cheap. Light-water reactors rely on enriched uranium. The thing enriched is a rare but highly fissile isotope of the element. Enrichment is an expensive process. CANDU, by contrast, uses natural uranium.

Not having to enrich the fuel? That rather changes those equations about the energy costs (and carbon life cycle) of nuclear, doesn’t it?

June 3, 2006 in Nuclear | Permalink


Older CANDU designs have always operated like this. For a country with abundant uranium, it makes sense to just shove a load of in there. The Advanced CANDU Reactors use slightly enriched uranium (~2%) to improve margins.

Of course, as long as they are heavy water moderated, and they have their CANFLEX refuelling, they offer fantastic fuel cycle flexibility. Plutonium, MOX, thorium, spent LWR fuel, the garden shed. You name it'll burn it.

I'm hoping we see some ACR-1000s built over here.

Posted by: Josh | Jun 3, 2006 5:07:34 PM

Errm, Tim,
What's your source? I'm not arguing, just curious about where/who "its" is/are.

Tim adds: not quite sure what you’re asking but my reference was to those people who say that nuclear offers 40 % of the emissions of gas....based on the costs (energy costs) of mining and enrichment.

Posted by: Winston Smith | Jun 3, 2006 9:25:32 PM

Don't know about the original citation, but it's at the AECL website.

I think one of the earlier SLS papers used diffusion enrichment as part of the ploy to inflate inputs (although those numbers too were exaggerated), but a later version switched more properly to centrifuge enrichment, which is two orders of magnitude more energy efficient. Of course, if enrichment facilities were nuclear powered like they are in France, we wouldn't have any carbon coming from it. The same goes for a lot of other inputs.

Posted by: Josh | Jun 4, 2006 11:53:59 AM

I haven't a clue about this, but wouldn't more, less rich fuel mean more waste to deal with at the end?


Posted by: The Remittance Man | Jun 4, 2006 9:13:11 PM

Check out this site


I've not studied it in detail, but I'll see if I can work things out.

Waste isn't an issue of the amount, but the type of radio-nucleides. It could be more or less depending on the type od reaction.

Posted by: Rob Potter | Jun 5, 2006 3:34:27 AM

Thanks Rob, I was just thinking in terms of volume.

Sadly, I suspect that that's also how the clueless media and enviro-industry will portray this idea as well. And they won't follow your links to read further either.


Posted by: The Remittance Man | Jun 5, 2006 10:07:47 AM

Of course being a loyal guest of South Africa I'd also have to punt pebble bed reactors as the real way to go nuclear.



Posted by: The Remittance Man | Jun 5, 2006 10:19:31 AM

RM, the CANDU has numerous advantages, but also is more expensive for the more complicated core and the requirement to use heavy water. It is a reason why light water reactors have endured against CANDU and will continue to do so. And remember, it is physically impossible to get criticality from natural uranium and light water moderation.

Higher enrichment is, all things being equal, the more efficient way to go, at least as far as thermal uranium reactors are concerned. It means you get more fission out of a sample at the expense of producing more depleted uranium, which is just fine and dandy.

That is why the move is towards higher enrichment. The Advanced CANDU's use slightly enriched uranium and GenIII+ uses ~5% enriched uranium. Greater efficiencies in enrichment, notably the move to the centrifuge method have made the efficiency gains in use of enriched fuel worth the extra expense of higher enrichment.

As to the PBMR (BTW, they use 8% enrichment), I can see a plethora of uses for the Very High Temperature Reactors. Particularly useful is for localised industrial use, providing heat and work in lieu of fossil fuels. Helium is an excellent coolant and it does not activate so it remains clean of radioactivity.

However, the strict open cycle nature of the PBMR means that other Gen IV fast reactors will also have their markets I can see fast breeders being used to supply PBMRs. Of course, with thorium, the story might be different.

Posted by: Josh | Jun 5, 2006 1:05:56 PM

"CANDU has numerous advantages, but also is more expensive for the more complicated core and the requirement to use heavy water".

I recently found an obscure reference stating that you can use carbon dioxide as a moderator, presumably liquid to start with and supercritical at operating temperatures.

Posted by: P.M.Lawrence | Oct 30, 2008 8:51:25 AM