A BBC programme about the “Telemark Heroes” describes how a bunch of very resourceful guys helped foil the Nazi heavy water project in Norway. This, the programme explained, was a key part of their larger project to develop nuclear weapons. Which is where my question comes. What use is heavy water in the development of nuclear weapons?
I’m not sure how it would lead to nuclear weapons, but heavy water (deuterium) is used in nuclear (fission) reactors to control the reaction. Deuterium slows down the neutrons released by the fissile material better than regular water.
Heavy water isn’t needed to produce nuclear weapons as such, but it is needed to run the type of reactor where you get the plutonium for your nuclear weapons from.
I thought heavy water was part of the cooling system, not part of controlling the reaction which I thought was done with some solid material that absorbed neutrons. Can you explain this more?
The CANDU reactor system is not a boiling water reactor, and uses heavy water as the core moderator. The fuel is [mostly] un-enriched uranium.
You may be thinking of graphite. I don’t know a whole lot about it eris, but I do know is that there are a few kinds of moderators. Water, heavy water and graphite being a few (all?) of the things used to absorb neutrons. Depending on the type of reactor one or more of these materials is used to control or balance the fission reaction. The good thing about using water is that it doesn’t catch on fire like graphite (Cherynobyl). 
Hey, check it out, I just found a SDStaff report on Deuterium! Apparently beryllium is another moderator.
Nice find, Demo. And also the modesty-inducing, “So that’s what CANDU is all about” form Cerowyn. Sometimes, you just have to face up to your own laziness in researching topics you’re interested in and swallow any pride you had previously. And to think, my mom is dating a guy that works with nuclear power. I hope this doesn’t get back to him…
In any case, still very interesting. Nuclear power is fascinating.
The staff report explained something I never knew before:
Sorry to hijack this OP, but it raises another question, which does tangentially relate. Countries seek to obtain uranium-enrichment equipment and facilities under the guise of nuclear power. With something like this, though, isn’t it totally obvious they’re lying? If no one needs enriched uranium for nuclear reactions, then… 
(It’s rare that I find an opportunity to give anything resembling an educated answer on anything at all, so pardon my verbosity :))
Coolant and moderator serve two distinct purposes: Coolant transfers the energy of the reaction to the steam generators (glorified boilers) and keeps the reactor from overheating in the process; the moderator slows down the neutrons in order to increase the reaction rate.
I will describe pressurized water reactor moderation as an example to better describe its role and the mechanism involved.
In your typical PWR, a fuel atom splits and forms lots of random bits, and throws off a handful of neutrons. The random garbage thrown off carries a significant amount of kinetic energy, which eventually heats up the coolant and drives the plant. The neutrons speed on their way to other fuel atoms for more fissions. As it would be, if the neutrons are going too swiftly, they cannot interact as well with the fuel. What this means is that the slower the neutrons travel, the better chance they have of striking another fuel atom and causing another fission.
They call these slower neutrons “thermal” neutrons, because they are traveling at a speed that is consistent with the surrounding temperature.
It is the job of the moderator to slow down neutrons, thereby increasing the chance of interactions with fuel atoms and thereby increasing the reaction rate. If the moderator weren’t there (and something else were used to cool the reactor), the reaction would cease.
Water does this job quite nicely: How do you slow down a speeding neutron? If it strikes something big, it will simply ricochet off, like a ping-pong ball striking a bowling ball. If it strikes something tiny, it will plow through, like a bowling ball through a pile of marbles. It turns out that the best way to slow down a neutron is for it to strike slower objects of identical size. The proton in the nucleus of a hydrogen atom fills the bill well. When a neutron strikes a few protons, it is similar to a cue ball striking a few billiard balls; it gives up much of its velocity in each collision. Water has plenty of hydrogen atoms, so it makes an excellent moderator. Since water has such a high heat capacity, it also excells as a coolant. Pressurized water reactor design is thus quite simple and stable. Oh, and about that heavy water – that’s just water with an extra proton in the hydrogen atoms. All the better for neutrons to slow down.
A nice side effect of using water as a moderator is that temperature changes cause the distance between nucleii to change (due to expansion), thereby allowing coolant temperature to automatically throttle power – as power is generated in excess, the coolant heats, expands, and acts as a poorer moderator, throttling down power; as more power is used than is being generated, the coolant cools, gets more dense, and therefore moderates more efficiently, boosting the reaction rate to meet the demand. Very stable.
Holy simulposts, and thank you.
Just a WAG (that I might use to explain it, were I wanting to have a facility for producing enriched U ): Graphite rods seem like they’d be a LOT cheaper than a boatload of deuterium as a moderator. In the SDStaff report it quote heavy water as being $300/kg for D2O (roughly $300/liter) and I imagine graphite rods would be significantly less than that.
As a bit of a tangent, the German heavy water was eliminated while being transported from Norway to Germany. The Allies got permission from the king of Norway to incur some civilian casualties, and they snuck aboard the ferry I think the day before it sailed and put a roll of plastique at the very bottom of the ship. The ferry was the only point that didn’t have heavy SS security along the trip.
The ferry sank in the middle of a fjord, in very cold water, in the mid 1940’s (obviously). It was gone, along with the barrels of heavy water.
The assumption that the German heavy water production could have resulted in a bomb, or was even directed toward creating a bomb, is not necessarily correct.
Like I always say, there’s nothing like good 'ol American CANDU!
PSSST! The CANDU is a Canadian design, eh?
CANDU = Canadian deuterium uranium.
I think you mean an extra neutron in the hydrogen atoms. After all, if my hydrogen had two protons, it’d be my helium.
How could that possibly have slipped in there? I’m certain somebody must have switched my keyboard to Dvorak, causing “neutron” to come out as “proton” Gack!
Of course, unlike the movie, a lot of people died in the ferry. 
slight hijack: what happens to someone who drinks heavy water?
Here are some MSDS’s for heavy water. Heavy water looks exactly like normal water, obviously, and there have been times when I’ve had the opportunity to drink some, if I didn’t take the ‘don’t drink anything you find in a lab’ rule so seriously.
Deuterium is one major exception to the rule that isotopes are chemically identical. Since deuterium is twice as heavy as hydrogen, many compounds containing deuterium react differently than the same compound containing hydrogen. If you were to drink heavy water, some of the deuterium would be incorporated into biomolecules, and some of these would react differently. Proton transfers go more slowly, for example. However, since most hydrogen in the body is just in the form of cytoplasmic or intracellular water, you’d have to drink quite a lot of D[sub]2[/sub]O to get yourself labeled.
Another effect: if you replaced all your hydrogen with deuterium, you’d be invisible to MRI. =)