r/nuclear • u/SteelHeid • 7d ago
Closing the fuel cycle – how fast CAN DU-terium get?
To properly breed Pu239 and burn even-numbered nuclides and other actinides we need a fast neutron spectrum. Liquid metals are somewhat tricky to work with in FBRs. At some point, the Germans were experimenting with a light water fast-ish reactor, that would use less water to reduce moderation. Meanwhile, heavy water only has 70% moderation efficiency compared to light water. So…
What would happen if you took the standard CANDU design and just removed the calandria moderator, and maybe rearranged the tubes/fuel elements too? How fast could the spectrum get, would it be enough to effectively burn actinide waste and get a breeding ratio above 1? And what would the controlability of this thing be?
I found this document that describes an experiment with fast neutrons inside a small cavity within a CANDU core. I’m thinking of the whole thing running on fast neutrons, using MOX at whatever the concentration is needed to work with a fast spectrum. Or a pressure vessel design using heavy water and less moderator density. I presume there are reasons no one is pursuing a “PHW-FBR” and going with sodium/lead instead.
How effective are the current CANDUs at burning LWR reactor grade Pu and actinides?
On a related note, there seem to be limits to how big a CANDU can be scaled up due to how heavy water and the calandria design work, with MONARK still using 480 channels and barely getting to 1GW. Would the design above allow for more capacity?
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u/Goofy_est_Goober 7d ago
If you remove the moderator, you'd have to massively increase fuel enrichment, which kind of defeats the purpose of the CANDU. As a pure coolant, water (light or heavier) is mostly inferior to liquid metal. A few major reasons are:
-Liquid metal has a much higher boiling point, and can run at atmospheric pressure
-Thermal conductivity is much higher, increasing heat transfer effectiveness
-Lower moderating power
Your fuel to moderator ratio would have to be very high to harden the neutron spectrum, you'd need a very high flow rate and have small thermal margins, or run at very low power.
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u/SteelHeid 7d ago
If you remove the moderator, you'd have to massively increase fuel enrichment, which kind of defeats the purpose of the CANDU
To be extra clear, my goal is not to burn natural uranium, but to burn reprocessed reactor grade plutonium from other water reactors - as well as increase PU239 breeding ratio itself. I will put as much of it in the fuel as needed, just like other FBRs.
-Liquid metal has a much higher boiling point, and can run at atmospheric pressure
-Thermal conductivity is much higher, increasing heat transfer effectiveness
-Lower moderating powerYes, but liquid metals also have drawbacks like corrosion, opacity, flammability with sodium. The Russians themselves said a sodium reactor isn't economically feasible compared with a LWR - yet. That's why apparently the Germans were experimenting with hardening the spectrum with water. I'm not against sodium reactors, I just want to explore other possibilities.
Your fuel to moderator ratio would have to be very high to harden the neutron spectrum, you'd need a very high flow rate and have small thermal margins, or run at very low power.
Yes. The question was how much. Or how hard would it get without the moderator (in the calandria) in a current CANDU.
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u/Goofy_est_Goober 7d ago
Idk man, model it in OpenMC. Unless someone here already did that, you're not going to get a real answer.
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u/neanderthalman 7d ago
Can candu burn reprocessed plutonium?
Son.
It can burn unprocessed PWR spent fuel.
CANDU burns dirt.
Throw whatever you want in there. Natural fuel. Spent fuel. Enriched fuel. Weapons grade. Plutonium. Thorium.
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u/Godiva_33 6d ago
If it decays it plays in a candu.
Joke in the office is only thing it can't burn is lead.
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u/SteelHeid 7d ago
Yes, I myself said DUPIC is my favorite CANDU feature, but DUPIC is still mostly about burning the remaining U235 in the fuel. Plutonium doesn't burn well with thermal neutrons, you get a lot of Pu240 which then spawns all the other actinide garbage.
That's why I specifically asked "How effective are the current CANDUs at burning LWR reactor grade Pu and actinides?" - compared to a metal cooled Fast Reactor.
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u/karlnite 7d ago
It’s extremely hard to design a new reactor type, probably harder to take existing CANDUs and augment them that much, than to just start from scratch.
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u/Izeinwinter 3d ago edited 2d ago
Sodium's problems can be, and has been dealt with.
Option 1: while sodium catches fire if you look at it wrong, those fires are not very energetic. This gives you the straightforward (Russian) approach of multiple steam generators in separate fire bunkers and when one of them inevitably catches fire, you cut the loop to it off, shovel some sand on it and fix the leak.. without turning off the reactor.
Option 2: "Don't have leaks". Superpheonix eventually got there. The BN's, which are still fire bunkered also reportedly don't actually have any fires anymore.
Option 3: Don't put the sodium near water. This is the Astrid concept. Core loop sodium, secondary loop, nitrogen under higher pressure. Any leaks between those two loops is nitrogen into the sodium.. where it just bubbles out before it gets back to the core, and the hot nitrogen then is fed into the steam generator which is also lower pressure than the nitrogen loop, to prevent the N loop from getting wet.
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u/Godiva_33 6d ago
There is a limit to how big they can get without a significant redesign. But there is no physical reason you can't go bigger.
Monark and CANDUs in general have a practical side (they are known as the physicists/engineers reactor). They are being designed working off the knowledge of hundreds of years of OPEX and by the time they start getting built 10 successful refurbs. More if you count pickering which hasn't started yet.
To go bigger means a new core, new columns, new FM head. Whole lot of new untested designs.
And why? The utilities most likely to buy it (Bruce Power and OPG) want 4 GW with a certain availability. The next nice whole numbers of reactors to get that is 2 GW monsters which you only build 2 of to reach the nameplate.
But it also means that whenever you have one done in an outage you lose 50% of that nameplate. If you are at 1 GW you only lose 25%.
Candu in the 1 GW range give a balance between economy of scale and economy of manufacturing.
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u/Eywadevotee 6d ago
Eaaiest method would be beryllium lined channels that effectively multiply the neutron flux, then put whatever you want to irradiate with a fast flux inside.
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u/InTheMotherland 7d ago
You remove the moderator, it's no longer economically feasible. Essentially, you're describing a different reactor. Modifying a design this significantly is more work than just designing a different reactor.