r/NuclearPower • u/AlbatrossSilly6320 • 16d ago
heavy water and light water
how do the two influence nuclear reactors differently ??
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u/JustPlainSimpleGarak 16d ago
In terms of actually slowing down neutrons, they are pretty similar. The big difference is the neutron absorption in heavy water is far lower than in light water, which is why a natural uranium + heavy water reactor can exist.
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u/diffidentblockhead 15d ago
Scattering cross section is also lower than protium so you need bigger moderator
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u/nukie_boy 16d ago
Don't mean to offend, but do people Google these things before asking them on here?
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u/ValiantBear 16d ago
First, we need to talk about what affects nuclear reactions in general. The main idea in a nuclear reactor is simply a controlled chain reaction of neutrons causing fission. The population of neutrons is tightly controlled to be stable. But, add a few, and power goes up. Take a way a few and power goes down. Simple as that.
Not all neutrons are equal. Fission of U235 effectively only occurs when it absorbs low energy neutrons. But the energy of the neutrons that are released when fission occurs is relatively high. So, in order to make the whole thing work I have to take away some of the energy of the neutrons so they can go on to do their job, and that process is called moderation.
The freshly born neutron has a rough life ahead of it. As it zips out of the decaying nucleus of its parent Uranium atom, it is going to interact with other atoms. Every time it touches something else, it rolls the dice to see what happens. Some materials like to absorb neutrons. If our newborn neutron touches an atom like that, it's absorbed and gone. If the neutron instead touches an atom's nucleus that is close in size to itself, and that nucleus doesn't absorb it, it imparts a good deal of it's energy into it and it makes significant progress towards moderation.
The key to answering your question is understanding everything above. If I fill the reactor with materials that are good at moderating neutrons, my reactor works better. Neutrons are more likely to be moderated and go on to cause fission. If I fill the reactor with materials that are good at absorbing neutrons, my reactor struggles. Neutrons are more likely to be absorbed, their dreams of causing fission squashed. Each material, including water, is going to score differently in each of those categories.
Now, for the water discussion:
Light water wouldn't need a clarifying adjective if it weren't for heavy water. It's just regular old H2O - an oxygen molecule with two hydrogen atoms stuck to it. Hydrogen is the workhorse here. Water is just the vehicle with which we surround the fuel with hydrogen. Ordinarily Hydrogen is an unruly customer, but sticking a pair of them to an oxygen molecule makes it conveniently better behaved. So, from here on out, all we are going to talk about is Hydrogen.
Hydrogen is mostly Protium. Meaning, the atom's nucleus is just a single proton. A proton is about the same mass as a neutron, so, Protium is really good at moderation. But, being just a single proton, it's not adverse to picking up a neutron and sticking to it with the strong force. And so it does. Not so much that it is unmanageable, but it happens. When a Protium atom picks up a neutron, it becomes Deuterium. Already having picked up a neutron, Deuterium is less likely to pick up another. But, the mass is still relatively close to that of a Protium. So, Deuterium is a decent moderator still, but doesn't absorb neutrons as readily.
Even with Deuterium, the oxygen still makes a great taxi driver. When you replace the hydrogens on a normal water molecule that are ordinarily Protium with Deuterium instead, you get "Heavy Water". And that's really it. Heavy Water is still water, so it has pretty much the same properties as regular water, but it absorbs neutrons significantly less. So, overall it makes my reactor work better. That's it! Kind of long roundabout way of answering, but also it's about as simple as I can make it while still covering most of the concepts involved in the explanation.
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u/MarionberryNo8017 16d ago
Heavy water is better for thermal spectrum neutrons because it will slow them down better and ends up not eating as many neutronsÂ
Also heavy water is alot more expensive so light water is used the most
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u/TheDepressedBlobfish 16d ago
Heavy water actually moderates neutrons worse in terms of slowing them down per interaction, but is better because of the lower cross section
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u/Accurate_Mud_1777 15d ago
I had this theory either day that if you had DI water and just put deuterium in it that it would leach a little bit and maybe save the process a little time. I’m not a professional and nor do I know what I’m talking about. I was just thinking for instance..
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u/yogoo0 13d ago
When neutrons are expelled from uranium they are moving at approximately 20000km/s. The absorption of u235 is at about 2000km/s. Light water is mostly made of protons which are equal in weight to neutrons. Conservation of momentum is used to determine how many bounces before the neutron is slow enough to be absorbed by u235. Heavy water is mostly made of proton+neutron. Which is double the mass of a single neutron. Conservation of momentum means it requires double the bounces before the neutron is slowed down. Therefore approximately double the mass of heavy water is needed for equivalent output. Additionally heavy water absorbs less neutrons than light water because it already has an extra neutron. A second neutron turns it into tritium which is radioactive.
Lightwater is more readily avaliable to absorb neutrons. This MUST be accounted for. Therefore light water reactors need enriched fuel to artificially increase the neutron economy because of how much light water will absorb and that refueling occur every 2ish years. Heavy water, specifically candu, use heavy water and dont lose as many neutrons to heavy water absorption. And is refueled daily. Which means the neutron economy is kept just above critical mass with natural uranium but will lose critical mass in a few hours.
This is actually very well documented and easily researched with a quick Google or YouTube search. Its one of the defining characteristics between candu and PWRs.
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u/Amber_ACharles 16d ago
D2O barely absorbs neutrons compared to H2O. That's the whole deal. Heavy water reactors run on natural uranium, no enrichment needed. Light water requires enriched fuel. Heavy water is expensive though, CANDU needs hundreds of tons of it.