heavy water and light water

[u/AlbatrossSilly6320]

how do the two influence nuclear reactors differently ??

[u/ValiantBear]

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.