20. How Nuclear Energy Works
The, following content, is provided under, a Creative Commons license. Your, support will help MIT, OpenCourseWare continue. To offer high, quality educational, resources, for, free, to, make a donation or to view additional materials, from hundreds, of MIT courses visit, MIT. Opencourseware at. Ocw.mit.edu. So, today I wanted to give you some context, for why we're learning about all the Neutron stuff and go over all the reactor types that until. This year the first time you learned about the non light water reactors, at MIT was once you left MIT I, remember. That as an, undergrad as well the only exposure, we had to non light water reactors, is in our design course because we decided to design one so, I wanted to show you guys all the different types of reactors are out there how, they work and start. Generating and, marinating. In all the different variables and nomenclature that we'll use to. Develop the neutron transport, and Neutron diffusion, equations, the. Nice part is now until, quiz 2 you, can pretty much forget about the concept, of charge so 802, can go back on the shelf because, every interaction we do here is neutral, charge neutral. There'll be radioactive decays, that are not the case but, everything Neutron is neutral, doesn't. Mean it's going to be simple it's just gonna be different but. In the meantime today is not. Going to be particularly, intense, but I do want to show you where we're going and this. Goes with the. Pedagogical, switch, that we made in this department starting this year and you guys are the first trial, of this we're. Switching to context, first in theory second I personally. Find it much more interesting to study the theory of something. For which I know the application exists. Who, here would agree. Just. About actually. Everybody ok yeah that's what I thought too so in the, end we. Had arguments amongst, the Faculty of all well you have to learn the theory to understand, the application. And. That works really well when you say it behind the closed office door by yourself but, the fact is I'm in it for, yeah. I'm, in it for a maximum, subject. Matter retention, so in whatever order that works the best and sounds, like for you guys this works the best that's, what we're doing with the whole undergrad, curriculum, not just this class. So. Let's launch into all, the different methods of making, nuclear power both fission, and fusion and, to, switch gears since we're dealing with neutrons.
I Don't know what happened with the oh there we go the. Idea here is that neutrons. Hit things like uranium and plutonium the, fizzle isotopes, that you guys saw in the exam and caused, the release of other neutrons. As we come up with these variables, I'm, gonna start laying them out here it, might take more than a board to. Fill them all and. I'll warn you ahead of time this is the only time in this course that we're gonna have V and knew the, Greek letter nu on the board at the same time and I'm gonna make it really, obvious. Which. One is nu and which, one is V. So. This parameter. That. Describes. How. Many neutrons, come out from each fission reaction we refer to as nu or the average number you'll. See in the data tables, as nu bar and so, as we come up with these sorts of things I will start. Going over them and, the idea here is that each. Uranium-235. Or, plutonium, or whatever nucleus, B gets two to three neutrons the, exact, number for which is still under a hot debate and I don't think it actually matters we'll. Make a couple of fission products that take away most of the heat of the nuclear reaction, and. I just want to stop there even though you know there's, going to be a chain reaction and that's what makes nuclear power happen, and. We can go over the timeline of what actually happens, in fission and what, kind of a nuclear reaction it really is, so. In this case this is a reaction where a neutron. Is heading towards, this, time are actually gonna give it a label a. Uranium-235. Nucleus. And it. Very temporarily, like I showed you yesterday forms. A compound, nucleus. Some. Sort of large excited, nucleus, that lasts. For. About ten to the minus fourteen, seconds. So. It doesn't instantly fizz apart there's actually a neutron, absorption, event some, sort of nuclear instability. At which, point. Your. Two fission products break off. Notice. You don't have let's, call them fission product one and fission product to notice. You don't quite have any neutrons, yet Neutron. Production is not instantaneous. For. The following reason. If. You remember back to nuclear stability when we plotted, let's. Say n I think, that was maybe Z, and this. Was N and I think this was a homework problem and, you, had to come up with some sort of curve of best fit for the, most stable, come. Combination. Of n and Z. For a nucleus it was not a straight, line it was something on the order of like N. Equals. Like what, is it 1.00. 5, 5 z plus. Some. Constant, something, with a rather small slope, well. If you, have a heavy, nucleus like uranium-235. And, you. Split it apart evenly. Let's, just pretend it splits evenly for now you're. Kind of splitting, that. Nucleus, along a rather unstable, line and as. You saw in the semi empirical mass formula a, little, bit of instability goes a really long way towards. Making the nucleus extremely, unstable so. Let's say you'd make a couple of fission products that, just cleave. That just cleave that nucleus, with the same proportion, of protons and neutrons how. Would they decay, or how can they decay there's, a couple different ways what do you guys think.
It. Can emit neutrons, if. It's. Really, unstable, at. Which point it would just go down a neutron, number or how else could it decay. Alpha. Decay let's. See yeah a lot of those will the, heavier ones tend to do alpha decays what would it do at alpha decay for, alpha you. Guys have we go in that direction right. Yeah. You know what I'm not gonna rule that out yet so let's let's go with that how. Else could they decay yeah. Through. Beta decay let's say in that direction. Pretty. Much all these happen just. Not necessarily in this order when. You have a really, really, asymmetric, nucleus a lot, of these fission products. Will. Emit neutrons. Almost. Instantaneously. In the realm of like 10 to the minus 17, seconds, some. Incredibly. Short time line you'll, start to decay downwards, a little bit, but. You're not quite at the stability line which is why a lot, of the fission products. Then. Go on and they. Deposit. Their kinetic energy by bouncing, around the different atoms and material creating. Heat but, a lot of them will also send off. Betas. Or gammas. And. It. May take you know ten to the minus thirteen, seconds, for them to. Whatever, the half-life, of that particular isotope is and after, around like ten to the, let's. Say 10 to the minus 10 to 10 to the minus six seconds. Depending. On the isotope in the medium, those. Two fission products will. Stop. And. Let's. Just say that they stop there. So. The whole process of fission it's actually quite a compound, process, first. The neutron, is absorbed, forming. A compound nucleus then, it splits apart then, those individual, fission products undergo what ever decays suit them best and that's, the source of the neutrons, in fission sometimes. One of those fission products might be particularly, unstable and it might send off two neutrons in, other cases that I don't know of it one off the top of my head it might be none but. This is the whole timeline of events in fission and the, justification. For why this happens, straight. From the first month to 2201. And. I wanted to pull up some of the nuclear data so you can see what these values tend to look like. And also where to find them I've. Got to do that screen. Cloning, thing again. Hey. There we go so, I've already pre, pulled up the. Janus. Library I've already clicked on uranium-235. Thanks, to you guys I have all the data now on my shirt but, so you can see a little better I also have it on the screen so let's. Look at this value right here a new bar total. Neutron. Production, and. I'll make it bigger so it's easier to see did. I click on the right one yeah. So. Take a look at that then, total number of neutrons, produced. During. U-235. It's for, most energies, it's hovering around the. 2.4. Or so there's, been arguments about whether it's two point four three or two point four four and. That's. A linear scale, that's not very helpful let's, go to a logarithmic scale that's. More like what I'm used to seeing, most. Of the fission happens. For u-235, in the thermal, region in the region where the neutrons, are at values, let's say the, cutoff usually about one electron, volt or lower in average, energy and noubar. Is fantastically. Constant. At that level then. As you go up and up in energy you, start to make more and more neutrons, why do you guys think that would be the case. What, are you doing to that compound, nucleus as you, increase, the incoming, Neutron energy. It's. Gonna have more energy itself you, might excite other nuclear, states that can then lead to other sorts of decays or other neutron emission so. To me, that's the reason why, once, you hit about 1 MeV you can start to see a lot more neutrons. Being given off the. Reason we usually, treat this as a constant, notice, I haven't given it an energy dependence, is because. Most of the fission that happens is, at thermal energies for. That I want, to show you the fission, cross-section. There's. A lot of cross-sections, and, it's.
Probably Gonna be in a different graph because it's in different units and this, gives you a rough measure per atom what's the probability, of fission happening as, a function, of incoming Neutron energy, at, those high energies, you. Have relatively, low cross sections or low probabilities. Of fission happening then, there's this crazy resonance, region that looks like a sideways, mustache but. Then as you get down to the lower energy levels it gets much more in fact exponentially. More likely that fission will happen so, almost all the fissioning, in a light water reactor, or any sort of other thermal reactor happens, at thermal energies and, that's why we take nu bar as a constant, you. Don't have to especially. If you're analyzing what's called a fast reactor, or, a reactor whose Neutron population, remains fast on purpose. And. So with that I want to launch into, some of the different types of reactors, that you. Might see, you. Guys already did those calculations, in problem, set 1 so I don't have to repeat them for you. Let's. Get right into the acronym, so if you haven't figured this out already nuclear. Is a pretty acronym, dense field. Does. Anyone can anyone say they know all. The acronyms on this slide. You're. Gonna know about 90%, of them in about 90 minutes so. It's, okay or you'll have seen them at least. And. You look completely unfamiliar. Most. Of them well. Let's knock them off so. KN last Thursday already showed you the basic layout of a boiling water reactor one. Of the types of light water reactors, and the reason that this is a thermal. Reactor is because it's full of water water, as we saw in our old Q equation, argument. Is very good at stopping neutrons, because. If you guys remember this the. Maximum, change. In energy that a neutron can get is. Related. To alpha times its incoming energy where. This alpha is just a minus, 1 over a plus. 1. Squared. And I. Think this should actually be a 1 minus. Right. There a is. That mass number of whatever the. Neutrons are hitting and. That. One comes, directly from the neutron mass number. If. You remember this was the, simplest. Reduction, of the Q equation, the generalized, Q. Equation, or kinematics. That we looked at when, I said let's do the general form and okay let's take the simplest form Neutron, elastic scattering here's. Where it comes back if, a. Neutron. Hits water which is made mostly a hydrogen, and a, is one then it can transfer a maximum, of all of its energy to the. Let's. Say to that hydrogen atom therefore. Given the neutron no energy and thermal izing, it or slowing it down very. Quickly. To. Show you what one of these things actually, looks like that's. The underside of a BWR I don't know if K did K and show you this before okay you've already seen what this generally looks like what. About the turbine does anyone actually seen, a turbine. This size close up gigawatt. Electric turbine. Trying. To see which one of those pixels is a person. So. I'm person-sized. I don't see anything persons oh there's a ladder that looks to be about 6 feet tall, so. Give. You guys a sense of scale of the, sort of turbines, that we say oh yeah we, draw a turbine, on our diagram well it's not actually not simple, these. Things take up entire hallways, they're kind of airport hangar size buildings, never, seen one of the you but I've seen one in Japan there, was a lot cleaner than this but, otherwise it looked pretty much the same and the, way this actually works for those who haven't taken any thermo. Classes, yet is, this, turbine is full of different. Sets, of blades that are curved at an angle so that when steam shoots in it transfer, some of its energy to get the turbine rotating, and there's, going to be a generator. Found, of an eye like an alternator, to generate the electricity there which, looks to be roughly, a hundred feet away just.
To Give you a sense of scale for this stuff, as. Kayne showed you a pressurized, water reactor, it's another kind of light water reactor, with what's called an indirect, cycle so, this water stays pressurized, it, also stays liquid which is good for Neutron. Moderation, or slowing down because. In addition to the probability. Of any interaction. Some. Probability, Sigma if you, want to get the total reaction probability you have to multiply by its number density. To. Get a macroscopic, cross-section. This. Is why I introduced, this stuff Y at the beginning of class so, you'd have time to marinate in it and then bring it back and remember what it was all about and so. Every single reaction. That goes on in. A nuclear reactor, has. Got its own cross section will. Probably need half the board. For. This one. You. Can say you have a, total. Microscopic. Cross-section, these. Are all going to be as a function of neutron energy what's. The probability of anything happening at all and. These are actually tabulated, up. On the janice website, so. Let's unclick. That get. Rid of neutron, production and go all the way to the top n. Comma. Total, so, all this stuff is written in nuclear reaction, parlance, where if you have let's say n comma. Total, that. Means a neutron comes in and that's. The reaction that you're. Looking at. So. This data file here, once I open it up will. Give you the probability that, anything, at all will, happen. You. Can see as the. Neutron energy gets higher the probability of anything, happening at all gets less and less less and it, follows the shape of most of the other cross-sections. And I'm going to leave this up right there you've. Also got a few different kinds of reactions, like. You can have a scatter. Let's. Call that scatter. Which. We've already said can either be elastic, or inelastic. It. May not matter to us from the point of view of. Neutron. Physics whether the collision, is elastic or, inelastic. All. That matters is the neutron goes in and a slower Neutron comes out because. What we're really concerned, with here is tracking. The full population of. Neutrons. At any point in the reactor so we'll, give this a position, vector R which. Has just got x y. And z in. It or. Whatever. Other coordinate, system you might happen to you is I prefer Cartesian, because it makes sense at. Every, energy. Going. In any direction, so we now have a solid angle vector that's. Got both theta and Phi. In it at, any given time and. The. Whole goal of what we're going to be doing today and all of next week is to find out how do you solve for and simplify, this. Population. Of neutrons. Let's. See make sure to fill that in as velocity. Yes. And so, a lot of let's see let. Me get back to the cross sections and stuff if. We want to know how, many neutrons, are in a certain, little volume. Element, in some. D volume, in. Some certain little, increment, of energy, de, traveling. In some very small, solid. Angle D Omega. Supposedly. If you have this function then you know the direction. And location and speed of every single Neutron everywhere in the reactor and this. Is eventually what the goal of things like Ben and cords group does the computational, reactor physics group is, solve for this or a simplified, version of it over. And over and over again for different sorts of geometries, and in. Order to do so you need to know the, rates of reactions. Of every. Kind of possible reaction, that could take a neutron, out. Of its current position like if it happens to be moving which most of them are out of its current energy group, which. Pretty much any reaction. Will, cause the neutron to lose energy, what's. The only reaction, we've talked about where the neutron loses absolutely. No energy. It's. A type of scattering. Yep, exactly, forward. Scattering, so, for forward scattering, for that case where, theta. Scattering. Equals zero again, that's. You. Missed the. Neutron didn't actually change, direction at all and therefore it didn't transfer any energy, but. For everything else for every other possible, reaction, there's, gonna be an energy change associated, with, it and probably.
Some Corresponding. Change in angle because, a neutron can't just be moving, and hit something and continue. Moving more slowly there's. Got to be some, change. In momentum to, balance along with that change of energy and. It might slightly move in some different direction, and all, this is happening as a function of time as. You can see this gets pretty hairy pretty quick and, that's why we put the full equation for this on our department t-shirts but. No one ever solves the full thing what we're gonna be going over is how, do you simplify it into something you can solve with like pen and paper or, possibly a gigantic, computer but. It's, not impossible. So. Inside, this Sigma total we talked about different scattering. And then. You could have. Absorption. In. All. Its different forms. What. Sort of reactions, with a neutron, would cause it to be absorbed. Yes. Vision thank. You, so. There's going to be some Sigma fission cross-section as, a, function, of energy, and. If. It doesn't fizz. But. It is absorbed, we'll. Call that capture. But, capture. Can mean a whole bunch of different things to right there, could be also a whole bunch of other nuclear, reactions, like. There could be a reaction, where one Neutron comes in. Two. No neutrons, go out, like. We looked at with beryllium in, the Chadwick. Paper from the first day, we're. Like what actually does exist, for. This, stuff so. Janice. Doesn't like multi-touch, so have, to bear with me on the small print on the screen, but. There should be up here it is, cross-section. Number 16 there. Is a probability, that one, Neutron goes in that Z right there is, whatever. Your incoming, particle, happens to be and in this case we know it's a neutron because, we picked incident Neutron data and, two n means two neutrons come out let's. Plot that cross-section. You. Can see that the value is zero until. You hit about four or five oh it's, actually five. Point two nine seven seven eight one MeV. So. That's the Q value at which this particular, reaction happens to turn on. Might. Be responsible, for a little, bit of the blip in the, total cross-section. So technically. If we were to turn on every single cross-section. In this database it should, add up to that. Red line right there, so. You can start to get an idea for how much of all the reactions, of uranium-235. Are due to fission that's, the one we want to exploit. So. Let's find fission. Right. Down there oh wow. There's a 3n reaction, I want to see that. That. Doesn't happen until 12, MeV. Yeah. So, neutrons. Don't typically tend to hit 12 MeV, in, a fission reactor so, this is a flute and the perfect flimsy, pretext to bring in another variable, it's. Called the Chi spectrum, or. What's. Called the fission birth spectrum. Yeah. We've. Already talked about the neutrons being born and how many there were but. We're didn't, say at what energy they're born in fusion. Reactors, this, is pretty simple. You've. Already looked at this case. What. Is it fourteen point seven MeV, that's. A lot simpler, that's, the, fusion.
For. Fission it's not so simple. For. The case of fission if you. Draw energy versus. This Chi spectrum. It. Takes an. Interesting. Looking curve from about one MeV. To. About ten MeV. With. The most likely energy, being around to anything so. You aren't really going to get neutrons. At the energy required for a three n reaction, in. A regular fission reactor just not, gonna happen but. It's good that you know that that exists, so let's, go and answer my original, question how much of the total cross-section. Is due to fission. Most. Of it especially. At low energies. So. Let me get rid of those two n and 3n ones because they're kind of ruining. Our data, it's. Making it harder to see. That's. Better. So. You can see at energies below, around. Let's say a KETV. Or so almost all of the reactions, happening, with neutrons, in uranium-235. Are fission this, is part of what makes it such a particularly, good isotope, to, use in reactors, the other one is you can find it in the ground. Unlike. Most the other fissile, isotopes. Unlike. I think any of the other fissile, isotopes. Thorium. You got to breed and turn it into uranium 233, have. To think about that one. But. Then you can start to look at what are the other components, of this. Cross-section like, ZN. Prime in elastic scattering which. Doesn't, turn on, until. About. 0.002. MeV. But. Later on is one of the major contributors, and actually is responsible for and I've brought this for a reason is, responsible. For that little bump in the total cross-section, so, eventually all these things do matter but let's. Think about which ones we actually care about at. All. Because. What we eventually want to do is. Develop. Some sort, of Neutron balance, equation. If. We, can measure the change in the number of neutrons as a function of position, energy. Angle. And, time. As. A. Function of time and that would, probably be a partial, derivative because there's like seven, variables, here. Before. I write any equations. It's just going to be a measure of the gains minus, the losses. And. While. Every particular, reaction, has its own cross section there's, only going to be a few that we care about like they'll, only be one. Or two types, of reactions that. Can result in a gain of the neutron population, into a certain. Volume with a certain energy with a certain angle and for.
Losses There's. Only one we really care about total. Because, any interaction. With a neutron is going. To cause that. Neutron to leave this little group of, perfect. Position, energy, and angle. So. That's where we're going we'll probably start, down that route on Tuesday, because. I promised you guys context. Today. You've. All been to the MIT research, reactor, a couple of you are you running it yet. Awesome. Okay, yeah. Say. That Bo so yeah so Sarah and Jared's doing that anyone else training or trained, no. I'd. Say the folks. Usually pretty scared when they find out mit has a reactor, and they're even more scared when they find out you guys run it what. They don't realize is there's been basically no problem, since 1954. The, only one I know of as someone fell asleep at the controls once, and forgot to push the don't call FoxNews, button and it called Fox News or something so. There was a big story about asleep at the helm, Nora. Knew. Alarms. And passive, safety systems, and backup operators, and everything else that actually made, sure that nothing happened but. Nowadays correct, me if I'm wrong you actually have to get up every half hour reach, around a panel and hit a button right. So. You want to hit it before it beeps at you. It's reminding, you. Okay. Okay. Yeah I'd heard the buttons every half-hour. Gotcha. Cool. Yeah. So for all use watching, on camera, whatever just know that these guys got it under control. So. Onto some gas cooled reactors, and to explain some of these acronyms, there, are some that use natural uranium though. All the ones pretty much all the ones in this country you need to enrich the uranium to. Get enough u-235. To, turn the reaction, on but. That's not actually you don't have to do that in every case and you'll, also see these acronyms, Leu, meu, or h EU standing, for low medium. Or high enrichment. The, accepted, standard for what's low enriched uranium is 20%, or below an. Interesting, fact though you can't, have something at 19, what at 19 point 99 percent. Enriched. Uranium and expect it to be low enriched uranium because. Every measurement technique has some error and what really determines if, it's Leu is when, an inspector, comes and takes a sample it better, be below 20%, including. Their error so. You'll usually see 19, point 75%. Given. As the Leu limit because there's always some processing. Error in homogeneities, measurement. Error head, your bets pretty, much like, in. England or the UK the advanced gas reactors, have been churning along for decades, they. Actually use co2. As, the coolant which is relatively, inert and they, use graphite, as the moderator, so, in this case the coolant in the moderator are separate, unlike the light water reactors, we have so. This way the, graphite, right here just sits in solid, form and slows down those neutrons, not quite as good as water but pretty good there. Is an issue though that. Co2. Just. Like anything has a natural, decomposition. Reaction. Where. Co2. Naturally. Is in equilibrium. With CO and O 2 and. O. 2 plus graphite. Yields. Co2 gas. Graphite. Was, solid. And talking. With a couple folks from the National Nuclear Laboratory. They said that 40 years later when they took the caps off these reactors, a lot. Of that graphite, was just gone, with. A good, explanation it. Vaporized, very, very very slowly over 40 earth years or so due, to this natural. Recombination. With whatever. Little bit of o2 is an equilibrium with co2 and possibly, some other leaks I'm sure I wouldn't have been told that if there was a leak so, I'd, say the feasibility is high because they've been running for almost half a century the. Power density is very. Low why, do you guys think that's the case. Yeah. Mm-hmm. Absolutely. So well, let's say you need the same cooling capacity, but you're right co2.
Even, If pressurized, is not a good at heat transfer medium as water water's, dense it's, also got one of the highest heat capacities, of anything we've ever seen the. Other reason is. Right here if, you want enough reaction. Density, it not, only matters what, the per atom density is but what the number. Density is and if, you're using gaseous, co2 coolant. Even if it's pressurized there, are fewer reactions, happening, per unit volume because, there are a few co2 molecules, per unit volume than, water would have so. That's why we pressurize, our light water reactors, to, keep water in its liquid state where it's a great heat absorber, takes, a lot of energy to boil it and it's really dense so it's a very effective dense, moderator. These. Have been around forever I think when did Windscale happen. When, scale was also the source of an interesting, fire. That's. You guys might want to know about it's one of those only, nuclear disasters, that hit seven, on the arbitrary, unit scale I don't, quite know how they determine, what's a seven but. There was a fire at the wind scale plan due to the build-up of what's, called Wigner, energy it. Turns out that when neutrons. Go slamming, around in the graphite, they, leave behind radiation. Damage and when. My family always, explained. What do you do for a living and I just can only think well they don't know radiation damage they've, watched Harry Potter I like, to say radiation, like dark magic, leaves traces well. It leaves traces in the graphite in the form of atomic. Defense which, took energy to create so. By causing, damage, to the graphite you, store energy in it which is known as Wigner energy and you, can store so much that. It just catches fire and explodes, sometimes that's. What happened here at Windscale. Eleven. Tons of uranium ended, up burning because. All of a sudden the temperature in the graph I just started going up for no reason no reason that they understood at the time it. Turns out that they had built up enough radiation damage, energy that, it, started releasing more heat and releasing. More heat caused, more of that energy to be released and it was self-perpetuating. Until it just caught fire and burned. 11 tonnes of uranium out in the countryside this was 1957. So. Again, a seven on the scale, with no units, of nuclear disasters. Argue. It's probably not as bad as Chernobyl so they might want a little bit of sort. Of resolution, in that scale, there's. Another type of gas cool reactor called, the pebble bed modular, reactor a much more up-and-coming, one or, each fuel. Element, you don't have fuel rods you've actually got little pebbles full. Of tiny kernels, of fuel so, you've got a built in graphite, moderator, tennis, ball sized thing with lots of little grains, of sand of uo2 cooled. By, a bed of flowing. Helium, or something like that and, then, that helium or the other gas, transfers. Heat to water which. Goes into make steam and goes into the turbine like I showed you before. So. This is what's the fuel actually looks like inside each one of these tennis balls spheres. Of mostly graphite, there's, these little kernels, of uranium, dioxide about, a half a millimeter across. Covered. In layers of silicon, carbide a really, strong and dense material, that keeps the fission products in because. The biggest danger, from nuclear, fuel is the, highly radioactive fission, products. That. Due to their instability, are giving off all sorts of awful, for, anywhere, from milliseconds. To mega. Years after. Reactor operation, and so, if you keep those out of the coolant then the coolant stays relatively, non radioactive and, it's safe to do things like maintain, the plant. Then. There's the very high temperature, reactor the ultimate, in acronym, creativity, it operates, at a very high temperature which. Has been steadily. Decreasing over. Time as, reality. Has caught up to expectations. When, I first got into this field they, were saying we're gonna run this at 1,100. Celsius, then, I started studying material, science and I was like yeah nothing wants to be at 1,100, Celsius, by, that time they downgraded, it to a thousand, now, they've asked some toda at around 800 850. Due. To some actual problems in, operating. Things in helium, it's. Not the helium itself but the impurities, in the helium that could really mess you up and the. Sorts of alloys that they need to get this working these nickel.
Super Alloys like alloy 230, they, can slightly carburized. Ich arbor eyes depending, on the amount of carbon and the helium coolant either. Way you go you lose the strength that you need. So. I'll say feasibility, is low to medium because, well I haven't really seen one of these yet. Then. Onto water-cooled, reactors, has anyone heard hear heard of the reactors they have in Canada. The. CANDU reactors that's my favorite acronym, hope. That was intentional, is what, yeah. It's. Not like the well they're not sorry about anything but whatever, at. Any rate one, of the nice features about this is, you can actually use natural, uranium. Because. The moderators, heavy water you. Have to look into what the sort of cross-sections, are even, though. Deuterium. Won't. Slow. Down neutrons as, much as hydrogen will or, my alpha thing oh it was right here all along even. Though a is, two instead of one for deuterium its, absorption. Cross-section or, specifically, yeah because, it doesn't fish in its, absorption, cross-section is, way, lower than that of water so, you actually it. Actually functions as a better moderator because fewer of those collisions. Are absorption, and because. You have a better Neutron population, and less absorption, you don't need to enrich your uranium you. Also don't need to pressurize, your moderator, so, you can flow some. Other coolant, through these pressure, tubes and just, have a big tank of close. To something room temperature unpressurized, e2o. As your, moderator, problem. With that is d2. Is expensive. Anyone. Priced out, deuterium. Oxide before. Probably. Have it the reactor because I know you have drums, of it. A couple. Thousand a kilo it's an expensive, bottle of water it'll, also mess you up if you drink it because. A lot of that even if it's you know crystal-clear, filtered, d2o, a lot, of what if sell your machinery depends. On the diffusion, coefficients, of various. Things in water those solutes, in water. And if you change the mass of the water than, the diffusion coefficients, of the water itself as well as the things in it will change and. If you depend on let's say exact sodium, and potassium concentrations. For your nerves to function a little, change in that can go a long way towards giving you a bad day and. There's. Actually we have a little piece of one of these pressure tubes upstairs, if anyone wants to take a look there's, all these sealed fuel bundles, inside. What they call a calandria, tube just, a pressurized, tube that's horizontal. The, problem with some of these is if, these spacers, get knocked out of place which they do all the time those, tubes can start to creep downward, and get. A little harder to cool or touch. The sides and change thermal, and now, getting into material signs it's it's. A mess, then. There's the old RBMK. The. Reactor that caused chernobyl, you. Can also use natural uranium, or, low enriched uranium here, the, problem, though that, led to turn out one of the many, problems led to Chernobyl was you've. Got all this moderator, right here so if you lose your coolant let's. Say you had a light water reactor, and your coolant goes away your. Moderator, also goes away which. Means your. Neutrons, don't slow, down anymore. Oh that, one, reaction, is messing up there, we go which, means your neutrons don't slow down anymore, which, means the probability of fission happening could be like, 10,000, times lower so. Losing coolant, and a light water reactor, might. Temperature, might go up but it's not going to give you a nuclear, bad day in the. RBMK reactor, it, will and it did and in, addition, the. Control, rods which was supposed to shut down the, reaction made. Of things like boron for carbide, or hafnium, or something with a really high, capture. Cross-section. We're. Tipped with graphite, to help them ease in so. You've got moderator, tipped rods which.
Induce Additional moderation. Which. Helps slow down the neutrons even more to. Where they fission even better and that's what led to what's called a positive, feedback coefficient. So, the more you tried to insert the control rods and the more you tried to fix things the worse things got in the nuclear, sense and, in something like a quarter. Of a second the reactor power went up by like 35,000. Times and, we'll do a millisecond, by millisecond, rundown, of what happened in Chernobyl, after, we do all this, Neutron physics stuff when you'll be better equipped to understand, it but suffice to say there. Were some positive coefficients. Here that are to be avoided at all costs, in all, nuclear reactor design. And. The actual reactor Hall you can go and stand on one of these things, very. Different design from what you're used to I don't think anyone would let you stand on top of a pressure vessel first, your shoes would melt because, they're usually at like 300. Celsius or so and, second of all you probably, get this a little too much radiation but. This is actually what an RBMK reactors, all, looks like for. One of the units that didn't, blow up there. Were multiple units, at that site, then. There's the supercritical, water reactor, let's, say you want to run at higher temperatures, than, regular water will allow you to you, can pressurize, it so much that. Water, goes beyond, the supercritical, point, in the phase sense and starts, to behave not like a slick, wicked liquid not like a gas, but somewhere in between something. That's really really dense so getting. Towards the density of water not quite which, means it's still a great moderator, but, still can cool the materials, quite well to extract, heat to, make power and so on and so on yeah. Ah good. Question, for, a supercritical, water reactor, it most definitely refers, to the coolant, it's. The phase of the coolant words beyond the liquid gas, sort, of separation. Line and it's just something in between any. Of these reactors, can go supercritical. Where. You're producing more neutrons, than you're consuming, and that, is a nuclear, bad day but. The supercritical, water reactor, does not refer to neutron population, just, a coolant good. Question, it's. Never come up before but it's like. Should've. Thought of that and. So then my favorite, liquid metal reactors, like, LBE, or lead bismuth eutectic, it's, a low, melting point alloy of lead and bismuth lead melts at around 330. Celsius, bismuth. 200-something. Put, them together and it's, like a low temperature solder it melted 123, point 5 Celsius you, can melt it in a frying pan this. Is nice because you don't want your coolant to freeze when. You're trying to cool your reactor because. Imagine that you something, happens you lose power the. Coolant freezes, somewhere outside the core you can't get the core cool again that's. Called a loss of flow accident, that can lead to a really bad day and the, lower your melting point is the better sodium. Potassium it's. Already molten to begin with sodium. Melts at like 90 C and when, you add two different metals together you almost always lower, the, melting point of the combination, in. This case forming what's called the eutectic, or a lowest, possible melting, point alloy. So. The sodium fast reactor, has a number, of advantages like you don't really need any pressure as long, as you have a cover gas keeping the sodium, width from reacting, with anything. Like, the moisture in the air or any errant water in the room you. Can just circulate, it through the core and liquid. Metals are awesome, heat conductors, they. Might not have the best heat capacity, as in, how much energy per, gram they, could store like water but, they're really good conductors, with very high thermal conductivity, they. Also are really good at not slowing, down neutrons. So. These tend to be what's called fast reactors, that rely. On the. Ability of other isotopes. Of uranium like. Uranium 238, to undergo what's called fast, vision and. I want to show you what that looks like let's. Pull up u-238. And, look. At its fission, cross-section and. You. Might find it should look a fair, bit different. So. We'll go down to number 18 to, fission. Cross-section. Very. Very different so. U-238. Is. Pretty terrible, at fission at, low energies, it's. Pretty good at capturing neutrons, this is where we get plutonium, 239, like, you guys saw in the exam but, then you go to really high energies, and all of a sudden it gets pretty.
Good At undergoing. Fission on its own and, so. The basis behind a lot of fast reactors, is a combination, of making their own fuel and the. Fact that uranium, 238, fast. Visions even. Better than at thermal visions so, something good for you to know even though it's not a fissile, fuel that's. Light water reactor people talking you can, get it to fission if the, neutron population, is higher, now. There's some problems with this it. Takes some time for, neutrons. To slow down from. 1. To 10 MeV to. About 0.025. Evie if your, neutrons don't need to slow down and travel. Anywhere it pretty much all they have to do is be born and absorbed, by a nearby uranium, atom the. Feedback, time, is faster, in these, sorts of reactors they're inherently more difficult, to control and you, can't use normal physics, like thermal. Expansion. Of things that might happen on the order of micro to nanoseconds. If it, takes less time than that for, one Neutron to be born and find another uranium, atom you. Can still use it somewhat but not quite as much so. It's something to note backed. Up by nuclear data. That's. What one of them actually looks like these things have been built that's, a blob of liquid sodium on the Monju. Reactor in, Japan and where. I was all last week in Russia they actually have fleets of fast reactors, they're bien 300, and bien, 600. Reactors are 3 and 600 megawatt sodium. Cool reactors, one. Of them in the Chelyabinsk, region they used for much for desalination, down. In the center of Russia where there's no oceans, nearby and probably. Dirty water they. Actually use that to make clean water. They. Also use this for power production and for radiation damage studies so. When. It comes to radiation. Material, science these fast reactors, are really where it's at. Yeah. You, just notice the bottom I went. To Belgium to their National Nuclear labs where they have a slowing, sodium, test loop it's not a reactor, but it's like a thermal hydraulics and materials test loop and I, asked a simple question where. Is the bathroom, and. They started laughing at me they. Said we're not putting any plumbing in a sodium loop building, you'll. Have to go to the next building over and that's. When I noticed there weren't any sprinkler, systems or toilets but, every 15. Or 20 feet there was a giant barrel of sand that's, the fire extinguisher, for a liquid metal fire is you, just cover it with sand absorb, the heat keep the air out of the moisture out wick. Away the moisture whatever, else and does I don't know but. You can't use normal fire extinguishers, to put out a sodium fire. Ah I. Don't know if that would work. Guess. It's worth a shot. With. Glasses and safety and stuff of course and, the ones that I spent the most time working on like I showed you in the paper yesterday is. The LED or LED bismuth fast reactor, this, one does not have the disadvantage, of exploding, like sodium, it, does have the disadvantage like I showed you yesterday of corroding, everything, pretty.
Much Everything and so, the one thing keeping this thing back was. Corrosion, and I say the outlet temperatures medium, but hotter soon hopefully. Someone picks up our work and like yeah that was a good idea because, we think it can raise the outlet temperature of a. Lead bismuth reactor by like a hundred Celsius, as. Long as some other unforeseen, problem doesn't pop up and we don't quite know yet, these. Things also already, exist in the form, of the Alpha class attack, submarines, from, the Soviet Union these. Are the only subs that can outrun a torpedo, so. You know that old algebra, problem if personated, leaves pittsburgh at 40 miles an hour and person B leaves Boston, at 30 miles an hour word, of the trains collide or I forget. How it actually ends well. In the end if a, torpedo leaves an American sub at whatever speed and the, alpha class submarine, notices, it how close do they have to be before. The torpedo runs out of gas so. What I was told by the designer, of these subs, fellow. And by the name of George Ito schinsky, when. He came here to talk about his experience, with his lead bismuth reactors is there is a button on the, sub, that's, the forget, about safety it's a torpedo button, because. If you are have a if, you're underwater it'll lead bismuth reactor and a torpedo is heading at you you, have a choice between, maybe. Dying, in a nuclear catastrophe and, definitely, dying in a torpedo explosion, well. That button is the I like those odds button and. You. Just give full power to the engines and, whatever. Else happens happens the point is you may be able to outrun the torpedo, and. Quite. Popular, nowadays especially in this department, is molten. Salt cooled reactors, that actually use liquid salt, not dissolved, but molten, salt itself as the. Coolant that, doesn't have as many of the corrosion problems, as lead. The exploding problems, is sodium it. Does have a low a high melting point problem though they tend to melt at around 450. Degrees Celsius, but there's one pretty cool feature you can dissolve uranium, in them so. Remember how in light water reactors, the coolant is also the moderator, in. Molten salt reactors, the coolant is also the fuel, because. You can have principally. Uranium. And lithium fluoride salt, Co, dissolved in each other and the, way you make a reactor, is you just flow a bunch of that salt into. Nearby pipes, and then. You, get less what's called Neutron, leakage, or in, each of these pipes once in awhile uranium, will give off a few neutrons most, of them will just come out the other ends of the pipes and you won't have a reaction when. You put a whole bunch of molten salt together most, of those neutrons, find other molten, salt and the. Reaction, proceeds and. It's, got some nice AIF tea features like if something goes wrong just break, open a pipe all, the salt, spills. Out becoming. Some critical, because leakage, goes up it freezes. Pretty quickly and then, you must deal with it but, it's not a big deal to, deal with it if it's already solid and not critical. So. It's actually five, of its 0 of 5 of I'll stop here. Tuesday. We'll keep developing, the many many different variables we'll need to write down the neutron transport, equation at, which point you'll be qualified, to read the t-shirts that this department prints out and then, we'll simplify it so you can actually solve the equation. You.