This article on fast breeder reactors is expertly timed to coincide with everyone forgetting what I said about regular nuclear reactors a couple of months back.

A brief recap: you start with a big chunk of uranium mined out of the ground. This chunk of uranium will be 0.3% U-235, which is fissile, and 99.7% U-238, which is fertile. The difference between fissile and fertile nuclear material is that fissile uranium can be induced to split fairly easily by bombarding it with what are called slow “thermal” neutrons. In order to slow neutrons down so that they trigger nuclear fission nuclear reactors cores are surrounded by a moderating material such as graphite or water which rob neutrons of their energy as they repeatedly bounce off of water/graphite molecules. Once they have been bled dry of energy the slow neutrons then interact with U-235 nucleii, producing energy, fission byproducts and more neutrons which themselves are slowed by the moderator and continue the chain reaction. At this point the reaction is self sustaining your main challenge is in controlling the rate of neutron absorption/fission so that your nuclear reactor doesn’t melt into a pool of highly dangerous radioactive sludge, usually with control rods composed of a neutron poison such as boron that stops the neutrons dead in their tracks and halts the nuclear reaction.

Now, if we were to try to use fast neutrons to start a chain reaction in our fissile material we’d run into a big problem. Fast neutrons have what’s called a neutron absorption cross-section (basically the chance that a neutron fired at a U-238 atom will be absorbed by it) that is far lower than that for thermal neutrons and U-235, which means you’d need a very high density of fissile material – in other words, a big critical mass – in order to induce a chain reaction. While a light water reactor using slow neutrons needs less than 5% of its fuel to be enriched U-235, a fast neutron reactor needs a much larger 20% plus quantity of enriched fuel. Fuel enrichment is one of the most expensive parts of the whole nuclear reactor energy generation process, so using highly enriched fuel in a reactor like this is both costly and inefficient. So why would we bother?

The answer is: so that we can get at the energy locked up in the 99.7% of natural uranium that is merely fertile, and not fissile. Fast breeder reactors are called “breeder” reactors because they convert the otherwise-useless fertile U-238 that comprises 80% of the fuel material into fissile P-239 by hitting them with fast high-energy neutrons. Set it up right and a fast breeder reactor can potentially create more useable fissile fuel than it consumes; the concept of fast breeder reactors in general would increase our useable stocks of uranium by a factor of about 100. However, there are some drawbacks to the fast breeder process that have prevented the practical development of the concept beyond a few test reactors.

The main stumbling block currently facing fast breeder reactors is an economic rather than a technological one: with the high upfront cost involved in building a breeder reactor (25% more than an equivalent light water reactor) and the additional ongoing costs involved in enriching nuclear fuel to the point where it can sustain a chain reaction using fast neutrons, creating new nuclear fuel out of fertile material using a fast breeder reactor is currently more expensive than just digging a lump of uranium out of the ground and enriching it the old fashioned way. This is just a temporary barrier, though, as sooner or later (assuming no discovery of previously unknown uranium deposits, between 50-100 years from now) the supply of naturally-occurring fissile U-235 is going to start drying up and the cost of producing fuel that way is going to exceed the cost of making it out of U-238 in a breeder reactor. So at some point we are going to be doing this; it’s just a matter of time.

Then you have the technological and political challenges inherent in building a fast breeder reactor. The technological hurdle is that you can’t use water in a fast breeder reactor. Like, at all. It’s a very good neutron moderator which makes it great for light water reactors, but it’s absolutely what we don’t want when we’re trying to saturate our reactor core in fast neutrons. This means that you have to rely on an exotic coolant like liquid sodium/molten salt, described in my post on LFTR reactors. Liquid sodium actually has some advantages over using water as a coolant, but it also makes reactor maintenance a bit trickier as it’s rather corrosive, not to mention potentially catastrophic if the reactor ever sprung a leak as liquid sodium will ignite on contact with air.

And then there’s the political angle, which anyone who knows what the letters “P-239” should have picked up on. P-239 is plutonium 239, otherwise known as the primary constituent of nuclear weapon designs the world over. Widespread deployment of fast breeder reactors would vastly increase the worldwide availability of plutonium and would almost certainly lead to increased proliferation of nuclear weapons, especially since the fast breeder fuel cycle involves regularly reprocessing the fuel cores in order to extract the converted P-239. My attitude towards this is that there are far cheaper and easier ways of creating widespread devastation than detonating a ghetto nuclear weapon, and that terrorists/rogue states would be better off looking at those before trying to develop nuclear weapons and their associated delivery systems, but then I don’t live in a world where such weapons are widely available. One suitcase nuke isn’t much of a threat, but a hundred of them could seriously muss up a country’s hair.

I should finish by saying that unlike LFTR reactors, fast neutron reactors do exist even if they’re not specifically set up to breed nuclear fuel. They’re used because fast neutron reactors have a very efficient neutron economy (that is, very few neutrons are lost to absorption or other processes allowing the reactor to run a on a comparatively small amount of fissile material) which means they can be used either in compact applications like nuclear submarine reactors, or else in commercial reactors that don’t want to bother with actually enriching nuclear fuel a la the CANDU series. Actual fast breeder reactors have also been built, although further construction was discontinued after it became apparent they were not economically efficient. Expect that to change at some point in the near future; many Generation IV nuclear reactor designs – that is, the ones that we will be building over the next half-century – are of the fast breeder type, and eventually we’re going to have to switch over to breeder reactors if we want to continue using uranium as an energy source.

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Today, children, we’re going to learn all about nuclear power plant safety physics. Or rather, about how nuclear power plants will kill you if you ignore safety physics. Are you sitting comfortably? Well too bad, I’m starting anyway.

Nuclear power is safe. Very safe. It’s not perfectly safe, and things can still go very, very wrong in a nuclear reactor — especially when you add panicky humans into the mix — but an awful lot of work has been invested in producing modern nuclear power plant designs that, if operated correctly, are physically impossible to break barring some unforeseen nightmare catastrophe of such force and power that a nuclear meltdown would be the least of your problems.

(As a brief aside here, if there’s one thing that gives me second thoughts about the safety of nuclear power it’s the fact that most of them are run by privatised energy companies with a definite incentive to try screw with these safe designs and operating procedures in order to improve their profit margin. Next time there’s a nuclear fuckup – and it probably will happen again at some point in the next fifty years because you can’t fulfil the energy requirements of rapacious decadent first world societies without taking risks that are eventually going to bite you in the ass – I would be willing to bet quite a substantial amount of money it’ll be either because corners were cut during the construction of the plant itself, compromising its safe design, or else because the company that owned it pushed it beyond its safe operating parameters in terms of regular maintenance and upkeep. YES TEPCO I’M TALKING TO YOU.)

Most of the time the safety features built into nuclear reactors work as designed; hundreds of not-exactly-cheap megawatts are pumped out of the plant and into your house, and nobody really gives it a second thought. It’s only when things go horribly, horribly wrong that anyone notices nuclear power plants in a big way, and that happens because one or more of these safety features have failed or weren’t implemented in a manner that would be darkly comical if it wasn’t resulting in the death or displacement of thousands of people. For example, the poster child for nuclear “accidents” is Chernobyl. From the perspective of a physicist, the design of the Chernobyl plant was like having a huge radioactive stack of nitroglycerin sitting in the middle of the Ukraine. The way it was operated on the night of the disaster was like somebody walking up to this stack of nitroglycerin and whacking it as hard as they could with an iron crowbar. It was an inherently flawed design that was just waiting for somebody to come along and do something stupid to it, and it’s worth examining the Chernobyl incident in detail to see just how the design features of the plant conspired to produce the worlds’ worst nuclear disaster.

The Chernobyl reactor used a comedy RMBK design. The RMBK reactor core uses graphite rods as the neutron moderator instead of light water. Like light water, graphite slows down neutrons to the point where they can split u-235 nucleii. Unlike light water, a graphite moderator outright absorbs a far lower percentage of neutrons, resulting in a much more efficient reaction because there’s a higher number of neutrons bouncing around inside the reactor. It’s so efficient, in fact, that you can use natural unenriched uranium as your reactor fuel instead of the more expensive enriched stuff. And because the fuel is so cheap, you can buy a lot of it and stuff it all together into a single reactor that produces loads of energy.

That’s one of the main draws of the RMBK design; it’s cheaper to run and you get more electricity out of it. This comes at the cost of making the reactor core absolutely massive. Seriously, look at that picture above. It’s huge. It’s so big, in fact, that building a proper containment structure around it would be prohibitively expensive, especially when the whole point of the thing is that it’s supposed to be nuclear power done on the cheap. Additionally the RMBK reactors had a secondary purpose, which was to produce plutonium-239 for nuclear weapons, and this necessitated being able to remove fuel rods from the reactor while it was still in operation. As a consequence the whole thing was sunk into a concrete pit on the floor and covered by a “biologic shield”, which is essentially a three-foot thick metal lid; this stopped immediate radiation escape while keeping the reactor core accessible to an overhead crane which removed and replaced the fuel rods, but it wasn’t even remotely capable of containing any potential explosive event that might occur in the core.

The RMBK reactor may not use water as a moderator but it does use it as a coolant. This means that the fuel rods and the moderator rods and the control rods are all constantly immersed in water. Again, water absorbs neutrons where graphite does not, meaning that the basic “safe” operation level of the reactor is one that automatically takes into account the presence of the water. If the water is removed for whatever reason so is its absorption effect, and the reactivity of the reactor will begin to increase exponentially as neutrons that would have been absorbed now interact with u-235 nucleii. This gives the RMBK design what is called a positive void coefficient, and it works like this: some of the coolant water boils into steam. Steam has bugger all neutron absorption ability due to having a far lower density than water, and so now you have an effective void inside the reactor that isn’t absorbing neutrons. This increases the reaction rate, which increases the reactor temperature, which boils more water, which increases the reaction rate, and so on. A positive void coefficient means that the reaction can spiral out of control very very quickly. Wikipedia says that the RMBK reactor design has the highest void coefficient of any commercial nuclear reactor in operation.

Finally there’s the issue of the control rods. These were stock neutron absorbers so there’s no problem with the rods themselves. The fault was with the way they were raised and lowered into the reactor core. Instead of electromagnets the initial RMBK design – and Chernobyl in particular – used a hydraulic system which could, in the event of the SCRAM button being pushed, lower the control rods into the reactor core at a whopping forty centimetres per second. Furthermore, the way the rods were structured was something like this:

Instead of having the control rod channels filled with cooling water (which would reduce the reactivity by absorbing neutrons) when they weren’t inside the reactor, each control rod instead had a graphite moderator tip that kept the reaction going at tip-top capacity.  Between the two was a telescoping section that was also filled with water, making the setup of the control rod channel one of a 4.25m long graphite moderator buttressed at either end by 1.25 m of water. When the control rod was inserted into the reactor, the graphite tip was displaced downwards, shunting that water at the bottom out of the way. However, that 1.25 metres of water was absorbing neutrons; when replaced with the graphite as the control rods moved downwards the reaction rate at the bottom of the reactor actually increased temporarily – not really something you want when you’re trying to shut the damn thing down.

The Chernobyl disaster itself was caused (ironically) in an attempt to improve the working of the SCRAM system. Once a reactor is SCRAMed the nuclear reaction doesn’t stop right away. Continuing decay of leftover fission fragments inside the fuel rods creates thermal energy equal to about 7-8% of the reactor’s normal output, and so the rods still need to be cooled once the reactor has been shut down. This is why the situation at Fukushima went totally gonzo; they successfully SCRAMed the reactor but their cooling system failed and they couldn’t dump the remaining thermal energy, resulting in partial meltdowns of the fuel rods. Anyway, there was a gap of about sixty seconds between the SCRAM being activated and the diesel backup which powered the cooling pumps coming fully online, and the Chernobyl staff were trying to reduce this time lag by seeing if they could substitute power from the steam turbines as they wound down after a shutdown. A series of unfortunate events resulted in the reactor being in a configuration where the control rods were fully extracted from the reactor core prior to this test, which was the last element needed for everything to go horribly, horribly wrong.

The cooling system was successfully connected to the steam turbines, as planned. The steam turbines were shut off and the diesel generator started to pick up the load to power the coolant pumps, with some of the shortfall being provided by the turbines as they wound down. However, the diesel didn’t pick up the slack fast enough, and as the power provided by the turbines decreased the coolant pumps slowed down. As a result the cooling water took longer to circulate around the reactor and absorbed more thermal energy than it should have, and some of it vaporised and started the reactor off on the positive feedback loop described above. The sudden increase in reactivity clearly made the planet operators nervous because somebody in the control room hit the SCRAM button. The rods started to lower into the core verrrrrry slowly (taking 18-20 seconds to traverse the full 7 metre height of the core) which increased reactivity at the bottom of the core still further past the point of thermal tolerance for the fuel rods. They fractured under the thermal stress and the control rods became wedged about a third of the way into the reactor.

From this point on the reaction became unstoppable; reactivity and power output in the reactor started to spiral and the entirety of the coolant inside flashed to steam, causing a steam explosion which ripped the lid off of the core.  With the loss of the remaining coolant there was nothing to stop the reactor undergoing runaway criticality. Now, this was not quite the same thing as what happens in nuclear weapons, which are designed to go critical and have lots of cunning features built in to improve their yield. Here the increase in reactivity was comparatively slow, which is why the fuel material in the reactor itself released that energy by exploding like a block of TNT rather than a fission bomb. With the critical mass now dispersed over a wide area the reaction finally ceased, but Chernobyl’s problems didn’t stop there; the graphite moderator blocks inside the reactor flashed into flame upon contact with the atmosphere and started spewing thick, highly radioactive smoke into the atmosphere.  The steam explosion is what contaminated Chernobyl itself, but the graphite fire spread radioactivity into the western Soviet Union and eastern and northern Europe (the first inkling the West had that something bad had happened was when workers at a Swedish nuclear plant found radioactive particles on their clothing but couldn’t locate any leaks in their own reactor.)

And that’s how you fuck up building a nuclear power plant. In fairness to the Soviets, it’s easy to say it was a bad design in hindsight when so much of modern nuclear reactor design is based on avoiding another Chernobyl, and the reactor was working fine for nine years before the disaster. It was only the combination of a bad design and a significant element of human stupidity that led to the reactor exploding. In the case of Fukushima, it was the combination of an old design, the most powerful tsunami for a century and endemic corruption and inefficiency within the Japanese power industry. Nuclear power isn’t inherently unsafe; it’s only unsafe if you do it wrong. However, if we are going to build these things we should probably do it right, and next time I’m going to talk about how.

The post How Not To Build A Nuclear Reactor. appeared first on The Scientific Gamer.

Nuclear reactors, then. A good idea with pretty much the worst PR in the history of the planet, since when people hear the word “nuclear” they think of this or this or this or this, with little idea of what nuclear power generation is actually about. In light of the recent screw-up at Fukushima the politics surrounding nuclear power are fairly muddied and the future for it is uncertain, which is a bit of a shame because nuclear reactors are – if done correctly – actually a fairly neat way of fulfilling your country’s energy requirements.

Everyone should have learned how electricity is generated back in secondary school/high school/whatever their national equivalent is. There is a fuel material which generates heat, which vaporises water, which produces steam, which drives a turbine, which creates electricity by dint of providing the mechanical work for a electromagnetic dynamo/generator of some kind. There’s several subtypes of power plant but this is the setup everyone will be familiar with and all power plants work by roughly the same principle anyway: the conversion of mechanical work, whether generated by wind or tides or steam, into electrical energy. The only major difference is in what you use to do the mechanical work. In the case of steam-driven power plants, it’s the fuel you use to generate the heat. Fossil fuel power plants burn their fuels directly and create steam that way; however, nuclear power generation works very differently.

A nuclear fuel rod starts its life as raw uranium ore mined from the ground. The raw ore is overwhelmingly (99.3%) made up of an isotope of uranium called uranium-238. The remaining 0.7% is uranium-235. The difference between these two isotopes is that uranium-235 is fissile – that is, it’s possible to initiate a fissile chain reaction in u-235 by bombarding it with neutrons of any energy – whereas the 238 isotope is fertile. Fertile material is not immediately fissile, but if you hit it with high-energy (or “fast”) neutrons then it too will start to undergo radioactive fission and may eventually become something immediately fissile (in the case of u-238, plutonium-239) a couple of steps down the decay chain.

Now, why is the distinction between fertile and fissile material important? It’s all to do with the chain reaction. Say you’ve got your u-235 nucleus, and you hit it with a neutron. This splits the atom into two fission fragments (child atoms with a total mass of just less than the parent u-235 atom, but whose exact atomic type is impossible to predict), an average of 2.4 free neutrons and some now-unneeded binding energy, which manifests itself as heat. Obviously you can’t have 2.4 neutrons, so what we’re saying here is that the majority of u-235 fission reactions will produce either two or three free neutrons. These two or three neutrons spin off into the atomic ether, and if one of them encounters another u-235 nucleus then the fission reaction will happen all over again, releasing another 2.4 neutrons along with some thermal energy and so on.

This sort of fissile reaction is potentially very bad news if you have a lot of fissile material jammed together into what’s called a critical mass. The neutron multiplication factor k is basically how many child reactions your byproduct neutrons cause in the next step in the chain. For example, if two neutrons span out of a u-235 fission reaction, and each of them hit another u-235 nucleus, then the neutron multiplication factor would be 2 since the first reaction begat two more reactions. Having a k of 1 means your reaction is just barely self-sustaining – each reaction leads to one more reaction, and then one more reaction, and so on – and any mass of fissile material with a k of at least 1 is referred to as a critical mass. Having a k of more than 1 can be catastrophic; this is a supercritical mass and can lead to a runaway chain reaction that increases exponentially with every step unless it either self-corrects somehow or else is moderated by an outside source.

The human race has come up with several ingenious methods of converting subcritical masses of fissile material into supercritical masses very very quickly since it’s a key part of nuclear weapon design, and I may do a post on that later. For now, though, we’re focusing on the chain reaction as it pertains to power generation, and this is where the fertile vs. fissile thing comes into play. A fission reaction can be induced in a fissile material by hitting it with any neutron; the reaction produces more neutrons, which produces more reactions, and so on. Getting energy out of fissile material is quite an easy thing to do – indeed, the hard part is getting it to stop. However, only 0.7% of mined uranium is the fissile u-235 isotope. The rest is the fertile u-238 isotope which can only be induced to split by bombarding it with fast neutrons. This presents a problem if we want to do anything useful with it because after you ram a fast neutron into a u-238 nucleus it loses most of its energy due to inelastic scattering, and so any child neutrons will be slow neutrons that can’t fission any subsequent u-238 nucleii they encounter.

So the problems we face if we want to use uranium as a prospective fuel for generating heat are twofold:

1)      Using u-235 would generate plenty of heat, but it makes up a very small percentage of the overall amount of uranium mined and it would be very easy for the chain reaction to go out of control, necessitating some way of moderating the reaction.

2)      There’s far more u-238 around that we can use, but the neutrons it emits after a fission reaction generally don’t have enough energy to induce another one, stopping the chain reaction in its tracks.

U-235 appears to be our best bet for heat generation on the basis that it actually, you know, generates heat. Unfortunately the tiny quantity of uranium that is naturally occurring u-235 isn’t enough to sustain a chain reaction for any significant length of time anyway – this, incidentally, is why there aren’t any natural nuclear reactors scattered around the planet apart from the one at Oklo. In order to get enough fissile material to make a critical mass in the first place we need to increase the percentage of the uranium which is made up of the u-235 isotope, and this is called enrichment. Enrichment is achieved by a process called isotope separation, which can be summed up as “dumping some of the u-238”. The quantity of u-238 removed from the uranium (and hence the increased percentage of u-235) dictates what the enriched fuel is good for; nuclear fuel rods are about 3% u-235 which is enough to sustain the reaction. Weapons-grade uranium has a much higher percentage of u-235 (80%+) but is made using exactly the same process, which is why the West gets so uppity every time Iran says it’s totally just making some harmless nuclear fuel, honest. The removed u-238 is what’s known as depleted uranium; not much good for power generation except in very specific circumstances, but its incredibly high density makes it very good for tank armour and cannon shells — if you’re willing to overlook the fact that firing a DU slug into something hard is going to shear bits off, and then when people (including your own soldiers) inhale this radioactive dust everyone gets cancer.

Anyway, we now have a quantity of fissile material that can sustain a chain reaction. The next step is controlling this chain reaction so that it doesn’t go out of control, and this is done by surrounding the fuel rods in a medium that functions as a neutron moderator, slowing down and blocking neutrons released from the fission process, consequently reducing the percentage of neutrons that initiate further fission reactions. With u-235 this is something of a double-edged sword; fast, high energy neutrons released from fission reactions are less likely to set off a chain reaction, but low-energy neutrons that have been slowed by a moderator that do make it through to a u-235 nucleus are almost certain to. Paradoxically, then, surrounding the u-235 with a neutron moderator like water makes the reaction more efficient, not less. To properly control the fission process you need something that can outright block or absorb neutrons very very efficiently, and which can be fine-tuned to block some, none or all of the neutrons emitted by the chain reaction.

This is where the control rods come in. Control rods are made up of a material that’s very good at absorbing the type of neutron which induces a reaction in the fissile material you’re trying to control; in the case of u-235 we want to block pretty much all neutrons so a mix of materials which have different neutron absorption cross sections are used in order to cover the widest range of neutron energies possible. These control rods are lowered into and raised from the nuclear pile in order to moderate the nuclear reaction as required. The rods are lowered and raised by an electromagnet, so if there’s a serious incident where the plant staff lose control all they have to do is cut the power to the electromagnet and the rods will automatically fall all the way into the reactor in a failsafe position, shutting it down. This emergency shutdown procedure is known as a SCRAM, which comes from the possibly apocryphal story of the control rods for the very first nuclear reactor – Chicago Pile One, famously built on top of a university squash court – being held up by a rope. If there was an emergency, a man would quickly cut the rope with an axe, hence the acronym SCRAM standing for Safety Control Rod Axe Man.

This is how light water reactors function; water is used as the moderator and control rods are used to control the reaction, and the energy liberated from this chain reaction inside the fissile u-235 heats water which provides the mechanical work to drive the turbine. There are other reactors which use different moderators but all u-235 reactors work by the same principle of moderators and blockers. What about all that u-238 going to waste, though? Isn’t there some way we could use that to generate energy? As a matter of fact there is, and the next sciencey post I make will deal with fast breeder reactors, which is where the real potential for nuclear power lies. Also more safety features. I like safety features.

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