It's not because they necessarily "interact more readily", it's just that the kinematics is more favorable when they do interact.

If you want to slow something down, you want to take as much kinetic energy away from it as possible with each collision, and simple kinematics shows that the optimal way to do that is for the neutron to collide with a nucleus of roughly the same mass, so ideally a proton.

That's why hydrogen, and hydrogen-containing compounds are very good neutron moderators. The lighter the nucleus the better.

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Would that mean that the liquid hydrogen is even better?

Liquid hydrogen is awkward to work with and water has a higher density of hydrogen atoms.

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How is that water has higher density of hydrogen if it contains also oxygen in it?

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You generally don't want to fill a nuclear reactor with a highly explosive liquid.

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Surprisingly, water is 14x denser than liquid hydrogen. The bonding angle of the H-O-H makes the O slightly positive and the H slightly negative which then tends to attract another H from the other H-O-H and holds it closer (surface tension). Even at cryogenic level water will have more H than liquid H.

I wonder whether water is considered denser than liquid hydrogen because it's heavier.

Liquid hydrogen has a density of 71 kg/m^3, water is at roughly a thousand, so it's 14 times denser. They both have 2 hydrogen atoms per molecule and water molecules are 9 times more heavy than H2, so in water there's just more molecules per unit of volume.

Between water molecules there's a much stronger attractive force than between hydrogen molecules, so they are pulled much closer together. So much that liquid water is famously even denser than ice.

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liquid hydrogen or liquid methane is great moderator when you need very cold neutrons. remember you can cool down neutrons only down to energy of thermal motion - liquid hydrogen is pretty cold at 20K

also you need to keep your material chemically simple, because when interacting with neutron proton is kicked out of molecule. this leaves water, because all products of that can react back forming water again, same goes for hydrogen. this does not work for oil, for example, that is if you wanted to put oil in nuclear reactor for some reason

Water has a hydrogen atom number density of 111 mol/L, while that of liquid hydrogen is 70.3 mol/L. No, it's not just due to water having a higher molecular weight.

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So, BeH2 and liquid NH3 will be slightly better. Of course, applying water is much easier.

Imagine a billiards ball hitting a wall (basically a very heavy object); it will bounce back with roughly the same energy it had going into the collision (alternatively think of a ping pong ball hitting a billiards ball). Now imagine a billiards ball hitting another billiards ball of equal mass; it will efficiently transfer energy to the target and will come to a standstill. The kinematics of moderation (reducing energy of the neutron) favor targets that have as close a mass as possible to the neutron.

As long as you don't allow any oxygen in, hydrogen is pretty tame. That's one of the safety issues with compromised water cooled reactors; the heat causes the water to split into it's component atoms and you end up with pure hydrogen, pure oxygen and a whole lot of heat.

That's why it's important to keep water on your core.

And if you want ultracold neutrons, superfluid isotopically-pure Helium-4 is by far the best, with deuterium ice in second place.

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The way the navy explains it in nuclear power school is pretty simple. Imagine you want to slow a speeding golf ball down. Shoot it into a room full of bowling balls, in every collision it only loses around 10% of its momentum. Shoot it into a room full of golf balls, in every collision it loses about 50% of its momentum.

The BeH2 molecule has a very linear structure with the H atoms bonded "inline" so they cannot be as densely packed as H2O. Further, it's actually a solid and form a kind of crystalline (lots of open space). NH3 (amonia) bonding is something vague for me but I think it has something to do with the electron orbital in O that give is a higher charge than N thus allowing it to be more attractive than N in that configuration. I don't think it comes even close.

But they do. Respectively, 118 and 120 mol H per L.

> isotopically-pure Helium-4

Does that need any special purification effort? Helium-3 is already a tiny fraction of the helium we extract.

You can distill them apart, it's part of how dilution refrigerators work

I know it's possible, but I'm asking if it's necessary for this application.

Sorry for taking so long to reply - I first wanted to get the numbers right and then kinda forgot about this comment.

> Does that need any special purification effort?

Yes. Specifically something called "superleak" or "superfluid helium filter" is used. It's basically a filter with material so tightly compressed that only a superfluid can go between the particles. As helium-3 reaches superfluidity at a much lower temperature level than helium-4, the isotopes can be thusly separated.

>Helium-3 is already a tiny fraction of the helium we extract.

Yep, but the less of it, the better. The natural content is about 10^(-6), whereas by using a superleak you can get something around 10^(-11), which is much better.

Your question is based off a false premise. Neutron interactions have nothing to do with isotopic mass. The isotope with the highest neutron interactions cross section is Xe-135. Lead just happens to not be that good. Other elements than interact strongly with neutrons are beryllium, carbon, hafnium, cadmium. On the other hand deuterium and helium have fairly low interaction cross section. Water is good because it's very cheap and has lots of protons which are good at shielding neutrons. Same reason concrete is used. Cheap and abundant. Also doesn't because significantly radioactive when interacting which is another plus for shielding.