anyway, cheese details? I guess I can explain neutron activation analysis - here's the version I give on reactor tours. which is probably somewhere near low enough of a level for me to get :P So everybody here knows what an atom's made of, right? Nucleus in the center made of protons and neutrons, electrons flying around the outside. (insert power point slide with smiley-faced elementary particles) e:) n_n In most atoms found in nature, the nucleus is what we call 'stable' -- that is, the protons and neutrons are in the right balance. p_p If you have too many neutrons or too few neutrons, the atom is radioactive -- it can't stay like that forever. Eventually it will decay, emitting some kind of radiation and turning into some other type of atom. For example, it could emit an alpha particle, which is two protons and two neutrons stuck together. Or it could emit a beta particle, which looks like either an electron or a positron. q: q: q: yay antimatter. Most likely it will also emit energy in the form of a gamma ray. the classic particle used for making things grow, shrink, and gain superpowers. Now, the cool thing about this is that the energy of that gamma ray is characteristic of the original nucleus. ~~~~~:D>-< So if you started with Sodium-24, you'll get gamma photons of one energy. If you started with Cesium-137, you'll get a different energy. What that means is, you can take a radioactive sample and put it under a gamma spectrometer, and get a graph of the spectrum of gamma rays being emitted. You look at that spectrum and you'll see peaks corresponding to these characteristic energies -- look that up on a table, and you can see exactly what you're dealing with. so, it's kinda like a heating up a sample and using a spectrometer, but on a nuclear scale? Uuq~~~ D: Essentially! But the spectrum you're looking at is entirely gamma rays. So you'll only see it with radioactive samples. Stable atoms never decay, so they aren't emitting gamma radiation at all. Which is inconvenient, because most of the time, if I want to perform elemental analysis on some kind of sample, it's not radioactive. For example, I could take some drinking water and ask whether there's any arsenic in it. This method works for extremely tiny trace amounts of various elements, by the way. You can do a quantitative analysis, in some cases with parts-per-billion accuracy. cool. But, okay, it only works for radioactive stuff. That's where the reactor comes in. that's good accuracy. Water treatment only goes down to parts per million accuracy... A nuclear reactor is essentially just a place where you have a crapton of neutrons flying around. (If anyone's interested in how the reactor actually works, I can give that spiel too when I finish this one.) sounds good. n_n n_n n_n n_n n_n n_n n_n n_n So if I take a sample of something that's not radioactive and stick it down next to the reactor, it'll get smacked by all those neutrons. So let's say I start with a sodium atom. (>n_n)>H2O<(n_n<) Sodium's stable abundant form is Sodium-23 -- all sodium atoms have eleven protons, and sodium-23 has 12 neutrons. 11+12=23, hence the name. hmmkay. Now, if my sodium atom happens to get hit by a neutron, there's a decent chance that it will absorb it. It doesn't have to happen that way -- the neutron might just bounce off. Each nuclide has a different probability of absorbing a neutron, which is why some things become more radioactive than other things when you irradiate them. (>n_Na Sodium activates pretty well, so it's fairly likely that our Na-23 atom will absorb that neutron. So that means it's now a Na-24 atom. which now *is* radioactive? Thing is, Sodium-24 is radioactive. Exactly. that's a neat trick. Now, Sodium-24 has a half-life of about fifteen hours. What that means is, each atom eventually has to decay, but there's no particular time that has to happen. It's completely random, and completely unpredictable. But on average, Sodium-24 atoms decay after fifteen hours. (:e Na~~~~:D>-< So that means if I have a chunk of radioactive sodium, after fifteen hours half of the atoms in it will have decayed. So I'll have half as much Sodium-24 as I started with. After thirty hours, I'll have half of that -- so a quarter of what I started with. But anyway. So we've taken our stable sample and stuck it in the reactor, making it radioactive. We shut down the reactor and pull out the sample, and stick it under the gamma spectrometer. And we observe peaks at particular gamma energies, so we know exactly what radioactive elements we have. :O Mg We subtract one neutron to account for the fact that we probably made it radioactive, and what we're left with is a really good elemental analysis of what we started with. But the best part is that after a while -- in this case, about a week -- almost all of the radioactive atoms have decayed, and the sample is no longer radioactive. The atoms that used to be radioactive have decayed to something else, but of course that's the vast minority -- for all intents and purposes, you've got your original sample back. good stuff... So the story with the cheese went basically that way. Somebody wanted to perform neutron activation analysis on a cheese sample. Did some quick calculations to estimate how radioactive it would be when it was pulled out, and went ahead and irradiated it. Trouble is, they botched the math or something (this was before my time, I don't know the details) and it came out far too radioactive, because of all the sodium -- like I said, it activates easily. So what they got back was a chunk of extremely radioactive cheese that would take a while before it could be disposed of. Na~~~~~8)<-< Long story short, we wound up using it as a check-source around the lab -- it's good to have something radioactive around to test your instruments with, so for a few days we were using this chunk of extremely hot cheese. (>o_o)>~~~~~~ D: hah, that is like, the best story about a nuclear reactor I have ever heard. ~~~~~<(o_o<) ,_ We tend to accumulate a lot of those -- it's that kind of workplace, for some reason. The folks who work there are extremely good at what they do, but they're also mostly ridiculous. (>o_o)>~~~~ (>o_o)>~~~~ it's probably a trend. people who are incredibly skillful, and yet hilarious to be around. so, ah, the reactor itself? Oh right! Actually, it's surprisingly simple. I would imagine. So I mentioned how you can take an atom and whack it with a neutron. In most cases, if the neutron is absorbed, the atom becomes radioactive. unh hunh. [|||] (reactor core) But in a few cases, with certain heavy nuclei, if the neutron hits the nucleus with just the right energy, the atom will undergo fission. The nucleus splits apart into two large chunks, each of which goes flying off with a tremendous amount of energy. And you also get two or three stray neutrons which fly off at high speed as well. and that's why fizzy drinks are better than fission drinks. Now, the thing is, those stray neutrons could hit other fissionable atoms. (< >) ,`(>n_n)> Which would then fission, producing more energy and more neutrons. And those neutrons would cause even more fissions. So what you get is a self-sustaining nuclear chain reaction. <(n_n<) <(n_n<) (< >) (>n_n)> Now, in some reactors, the interesting part is all that energy being produced. [|||] On the large scale, it manifests itself as heat. If you use that heat to boil water, you can produce steam, which drives a turbine, which powers a generator, which produces electricity. That's essentially how a nuclear power plant works. I'd hit that. with a beryllium rod. [|:|] (As you're probably aware, the fissionable material used in these cases is uranium-235.) (Which fact I failed to stick in somewhere earlier.) ah, 235. a magical number. of joy. (>U_U)> <(n_n<)`, But the place I work is a research reactor. We don't boil water, and we don't have a turbine, because we're not interested in producing electricity. We operate at much lower power levels -- a typical power reactor might run at three gigawatts; our maximum licensed power is 250 kilowatts. <(n_n<)`, (>? ?)> What heat we do produce, we carefully cool back off with a cooling system that releases the heat to the environment. What we're actually interested in is the neutrons. Some of those won't go on to cause more fissions; some will leak out of the core. So we can put our experiments in some equipment set up in and near the core, and they'll get hit by all those neutrons leaking out. [|:|] `,(>n_n)> D: Trouble is, what I described isn't actually a reactor. What I described, under some circumstances, is a nuclear bomb. The reason being that the way I described it, the chain reaction isn't just self-sustaining, it'll accelerate. It'll accelerate extremely quickly -- each of those generations takes place in about 10^-5 seconds. ~~~~{:*:]~~~ D: D: D: So you've got more and more heat building up faster and faster, and that's how a nuclear explosion work.s fun. In a reactor, obviously, there are mechanisms to keep that from happening -- one is the control rods. ||| :D We have three rods that we can move in and out of the core. They're made of boron, which is a neutron poison; basically, all that means is that it absorbs neutrons really well. (>n_|||n<) So when the control rods are fully inserted in the core, there's a pretty good chance that a neutron coming off a fission won't hit another uranium atom; it'll hit a boron atom instead. here I thought boron would be boring. :P guess not! What that means is that while the control rods are fully inserted, the chain reaction can't sustain itself. As we pull the control rods out, the probability of a fission neutron hitting another uranium atom increases. And so that means the chain reaction gets stronger. so rods in = full stop rods out = warp 6 Not quite full stop - there are still a few neutrons bouncing around, we can't stop them completely. But mostly. (or at very least, full impulse power) (>n_n)> (>n_n)> (>n_n)> (>n_n)> (>U_U;)> <(n_n<) <(n_n<) <(n_n<) <(n_n<) <(n_n<) So as we pull the rods out, we'll eventually reach a particular point where the number of neutrons going on to cause fissions in a particular generation is equal to the number of neutrons that started that generation. so what happens when two neutrons hit each other? anything magical? It's really unlikely. If it happened, they'd just bounce off. hmmm, okay. `,(>n_n)> <(n_n<).` Anyway, at that point, the chain-reaction is precisely self-sustaining. The neutron population is neither increasing nor decreasing. <(n_n<).` `,(>n_n)> When that's the case, we say the reactor is critical. [|:|] `,(>n_n)> D: If we pull out the rods just a hair, the chain reaction is more than self-sustaining; you'll have more neutrons out of each generation than you started with. So the neutron population is slowly increasing, and we say the reactor is supercritical. Pull the rods out more and the neutron population starts increasing faster, as does your power level. over 9000! So what we ordinarily do is go supercritical for a few minutes until the power level is where we want it for the day, then put the rods back in until we're critical again. [:*:]~~~~~:D>-< At that point the chain reaction will hold steady at that power level, and we can do whatever experiment we want to do. WHAT HAS SCIENCE DONE? cool enough. ^^ The other cool thing, which is a research-reactor-only feature that power reactors don't have. :D like windows XP pro lets to use the control panel. you* Our reactor is inherently safe -- it can't have a runaway chain reaction. Not just because of the safety features built into it, but because it was designed such that the laws of physics prevent it from happening. how so? So I mentioned before that a neutron hitting a uranium atom will cause fission only if it's moving with just the right energy. unh hunh? [|||] (: If it's moving too fast or too slow, it won't cause fission. it doesn't work if it's moving too *fast*? Specifically, the neutron has to be in thermal equilibrium with its surroundings -- it has to have the same energy as everything around it. Exactly! hunh, cool. There's a particular energy peak where you get fission -- too far left or right, it's a no go. Now, the interesting part is, a neutron coming OUT of a fission event has far too much energy to cause fission. So in order to actually get a chain reaction, you need to slow them down to the point where they're at thermal equilibrium and they can cause another fission. The way you do that is with a moderator. In our case, we use zirconium hydride. The zirconium isn't actually the interesting part, it just forms a matrix for the hydrogen atoms. The hydrogen itself is the actual moderator. Picture a neutron and a hydrogen atom for a sec. n_n H Hydrogen nucleus is just a proton -- so it's the same size as a neutron. (>n_n)> <(p_p<) If a neutron with high energy comes flying at it, they're going to collide. (>n_p<) Picture two billiard balls, you shoot one into the other. The second one speeds up, but the first one slows down. So a high-energy neutron, by bouncing off a whole bunch of hydrogen atoms, will eventually slow down to the point where it's at thermal equilibrium. Make sense? sure! p_p p_p p_p n_n; At that point, it will be able to cause fission and you'll get more fast neutrons, which will have to undergo moderation before THEY can cause fission. Now, this is true at any reactor anywhere. p_p p_p p_p (>n_n)> <(U_U;<) The reason our reactor is cool is that zirconium hydride has what's called a prompt negative temperature coefficient. That means that as the moderator gets hotter, it quickly stops being an effective moderator. Which makes sense -- if those hydrogen atoms have higher energy, they won't be able to slow down the neutrons effectively. So picture an uncontrolled chain reaction for a second. Our reactor has mysteriously lost all control and safety mechanisms. ~~~~{:*:]~~~ D: D: D: The control rods have spontaneously vanished, and the chain reaction is unchecked, so it's self limiting. as the reactor starts going faster, it slows it down, like a governor on a steam engine? Exactly! As the neutron population increases, it causes more fissions, which produce more heat. That heat makes the moderator less effective, which stops fissions from being produced. The reactor actually shuts itself down. hunh, wow. cool. or hot, as the case may be. [|||;] Every reactor in the US has a negative temperature coefficient. Most research reactors have a prompt negative temperature coefficient, which effectively means that it shuts itself down faster. By comparison, the reactor at Chernobyl had a positive temperature coefficient. That meant that as the core got hotter, it got more efficient. Which was only one of a whole bunch of design and protocol failures at Chernobyl. <(U_U;;;<) <(n_n<) <(n_n<) <(n_n<) <(n_n<) (>n_n)> (>n_n)> (>n_n)> (>n_n)> (>U_U;;;)> note to self: don't make a machine that gets better at blowing up the closer it gets to blowing up amirite? yeah Yeaaaahhh... If you decide to do that, though, you should probably train your operators in how to run it safely. That might be a good idea. the whole core melted down, turned to lava, flooded everywhere @RK: In my line of work that's a pretty good sign that you've fucked something up. and now it has black mold growing inside which uses radiation as a source of energy wait, what? hehe That's what I've heard. I don't know anything about it but it scares the shit out of me. http://en.wikipedia.org/wiki/Radiotrophic_fungus http://digg.com/general_sciences/Black_Fungus_Found_in_Chernobyl_Eats_Harmful_Radiation oh god. evolution is impressive and holy lagg. firefox has slowed my computer to a halt. meep and my computer's only loaded to like 60%, so i assume it's not doing anything. there we go. anyway, would that be related to your algae? Jesus I hope not.