Royal Society unearths top secret nuclear research In his long-awaited energy white paper, published this May, Tony Blair opined that energy could be "as important to our future as defence". According to secret documents, sealed during World War II, and unearthed by the Royal Society this year, it was ever so. The papers have lain forgotten on a shelf in the Royal Society's …
While it's true that moderators slow down the neutrons participating in the chain reaction, this is not the same as slowing the reaction itself.
Instead neutrons have to be slowed down before they can usefully be used to intiate further fissions, as such the moderator actually accelerates the chain reaction.
It was cutting edge at the time, but basic text book stuff now. I skimmed some of the papers, but not all. Four of the papers were in French (which I am afraid I don't speak). I didn't have time to read the others in detail.
But from what I did see, they were testing how effective various different materials were as moderators, for instance. They were really doing the leg work of figuring out how to build something that could sustain a reaction for long enough to be useful as a power source without going BANG in a really unhelpful, and frankly messy, way.
I'm hoping the Royal Soc will put them online at some point.
Lucy Sherriff
Slight typo in the article - deuterium isn't heavy water, it's heavy hydrogen makes up part of heavy water (deuterium oxide).
And for any Reg readers planning on killing their boss - okay that's most of you then; you can sterilise or even kill someone if you make sure they only drink heavy water rather than the usual stuff. Magners cider has much the same effect and is only marginally cheaper.
I used to think that moderators just kept the neutron flux down to a level where the atomic pile wouldn't blow up, but it turns out that slower (thermal) neutrons are more likely to be captured by a fissile nucleus and thus provoke further fission than fast neutrons.
Not so, The moderators function is to slow down fast neutrons, so that they may then be absorbed by the nucleus of U235 atoms. The slowing down is by transfer of momentum in elastic collisions with other atoms, hopefully without being absorbed by them. The slowed down neutrons are termed "thermal", since they are in thermal equilibrium with their environment. Elements which easily absorb neutrons (ie Boron) are used in the control rods to soak up neutrons to maintain a balance of neutron creation/absorbance.
So the way it works is this. Natural uranium is a mixture of two isotopes- U-238 and U-235. Only U-235 is really useful, because when a neutron hits it, it goes *pop*, falls apart, and emits a few more neutrons. U-238 just swallows the neutron, decays into neptunium and eventually becomes plutonium- but it doesn't emit much energy in the process.
The problem is, then, that U-235 is really scarce- it only comprises less than one percent of natural uranium metal- so if you're to have a fission chain reaction you either need to make pure (what we'd call weapons-grade, or high-enriched uranium) U-235 with as little U-238 in it as possible, or alternatively, give every neutron the absolute best chance of hitting a U-235 atom and causing it to go pop.
Why is this hard? Well, an atomic nucleus is a bit like a drop of water; if you fire a neutron into it at high speed, it won't stick to it, it'll just go straight through. And guess what, the neutrons emitted by a fission *are* going at a fair old lick- fast enough that they don't cause many fissions. This is a problem.
In order to make fission happen at all, therefore, in your big pile of natural uranium, you need a moderator. What this does is to slow neutrons down- by bouncing them around between atoms that are unlikely to absorb them, like deuterium or carbon, so that when they eventually hit a U-235 nucleus, they're going slow enough that they'll stick.
The name "moderator" confuses people a lot. For some reason they think of "moderating" the speed of the reaction, when in fact they allow the reaction to occur in the first place! In some reactors, like British Magnox and AGR reactors, solid bricks of graphite are used as moderator, and in some, like the Canadian CANDU reactor design, a large stainless steel structure full of deuterium oxide (heavy water) is used. In still others, a less efficient but much cheaper fluid- ordinary water- is used, but these need slightly enriched fuel because the moderator absorbs too many neutrons to work well with natural uranium fuel.
Finally, of course, there are reactors like IFR, PFR and DFR- fast breeder reactors- which use no moderator at all. These reactors have to use extremely high enrichment, because most of their neutrons pass straight through the fuel without causing any fission. Technically, a nuclear bomb is a fast reactor, though it involves maintenance issues...
The reaction is *not* controlled using the moderator, in most reactors; indeed, taking the moderator out will stop most reactors- this is one of the safety systems used in water-moderated reactors, since if the coolant disappears the reaction stops. The reaction is controlled using carefully adjusted concentrations of boron and gadolinium in the coolant, and also by using cadmium (and similar material) rods which are drawn in and out of the reactor, absorbing neutrons. These "control rods" (sometimes "drums", on the outside of the reactor) are not part of the moderator.
Hope this clears things up!
Nice to see things haven't changed since the 1940s... " 'ere, someone bunged me these insanely techy, advanced papers of incredibly useful stuff - and I lost 'em under my expense claims for a few months. Don't tell anyone".
Seriously, it's fantastic to see stuff like this survives!!
Jasmine - awesomely simplified explanation of a damn complex subject. Colour me well impressed!!! Certainly not working for BNFL!!!!
Oh, good to see the obilgatory "Wikipedia - Font of All that is Accurate" reference :)
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