In which I’m going to do my damndest to get the West’s security services interested in my blog. Hi guys!

Making a nuclear bomb is harder than you might think. When you stuff nuclear fuel into a reactor the fission chain reaction is carefully controlled via neutron moderators and absorbers so that it occurs in a stable, steady-state fashion. This nevertheless liberates a tremendous amount of energy every second, so much so that we need dedicated cooling systems just to deal with the aftermath of the chain reaction when the reactor core is shut down. However, if you took away all the control rods and failsafes and dumped all of the coolant out of the reactor and basically tried to make the thing explode, you’d run into two problems.

1)      The percentage of nuclear fuel that is fissile u-235 (3%) is far too small to create much of a bang.

2)      While nuclear reactors are deliberately built to be inefficient in terms of the rate of neutron reactions (after all, we don’t really want melted down reactor cores dotting the planet) just taking the neutrons off the leash isn’t particularly efficient either.

The efficiency (or rate) of your nuclear reaction largely on how densely your critical mass is packed together; if you have a lot of u-235 atoms crammed into a small volume you dramatically increase your chances of scoring a hit with a fast¹ neutron, as well as increasing the chances of child neutrons inducing subsequent fission events. A critical mass that is unintentionally triggered into undergoing a chain reaction goes on the big list of criticality accidents. Wikipedia says there’s been about sixty of these since nuclear power first became a thing, but you’ll notice there isn’t a corresponding big list of sixty accidental nuclear explosions. This is because criticality accidents are inherently self-correcting in the same way that the light water moderator I talked about in the PWR post is; inducing a chain reaction in a critical mass heats it up, which causes it to expand, which spreads out the fissile atoms, which reduces the critical mass below the density required to sustain a chain reaction.  As a result of this most criticality accidents are like the ones caused by the demon core; they release a burst of hard neutrons that usually kill whoever was fiddling with the critical mass, but they don’t proceed any further than that. In order to get our critical mass to actually explode we’re going to have to get creative.

The first step is in packing as much fissile material into the critical mass as possible. That 3% u-235 is far too weedy and the other 97% u-238 is effectively useless for our purposes; if we want a really big bang we’re going to have to enrich our uranium to the point where 90%+ of it is u-235. To do this we need industrial scale isotope separation facilities like the Manhattan Project’s three plants at Oak Ridge, which work in a sequential cascade process. The way the separation process works is that after you “enrich” raw uranium that you get out of the ground you end up with two separate collections of material: one with a slightly higher concentration of u-235, and one with a slightly lower concentration of u-235. The material with more u-235 is sent up to the next step in the chain which separates it again, producing another two sets of material with more and less u-235 respectively. The material with more u-235 is sent further up the production chain where it progressively has more and more of its u-238 content squeezed out of it. The leftover material with less u-235 isn’t wasted either; instead it’s sent back down the chain to start the process again and get as much pure u-235 out of the raw uranium as possible.

The separation process takes a lot of time and effort and is pretty much the reason why only nation-states have the resources to make nuclear weapons from scratch. Iran keeps trying to establish the industrial base from which they can perform sustained isotope separation, which is why the US and Israel are investing quite a lot of time and effort in fucking with the Iranian centrifuges. Assuming we can build the facilities and that the US doesn’t come in and drop a JDAM on it, though, we’ll eventually end up with a lump of weapons-grade uranium. Or rather, several small lumps none of which are large enough to constitute a critical mass. The next step is to find some way of making them go critical on demand.

The simplest way of doing this is the gun-type fission bomb, which is what was dropped on Hiroshima. Here you have two sub-critical pieces of fissile material separated by a short distance, and when you want your bomb to explode you detonate an explosive inside the bomb which “fires” one piece of material at the other, smooshing them together and making them go critical. Cue one big bang.  This design works, but it’s rather inefficient in terms of both the total amount of fissile material used to make the bang and the amount of that fissile material which is consumed before the bomb explodes. A bomb which uses 100% of its fissile material in the chain reaction before releasing its energy and exploding is one which is operating at maximum efficiency; however this is very much the ideal scenario, whereas in reality the chain reaction inside a simple critical mass doesn’t occur fast enough to ensure all of the atoms in the fissile material get to react with a neutron. If the reaction is so slow that the fissile material explodes before all of it has had a chance to react, then you’ve effectively wasted a certain percentage of your fissile material, and this is bad for two reasons. One: the bang the bomb will make isn’t as big as it could be. Two: given the trouble you went through to produce the fissile material in the first place, you really don’t want to be wasting it.

To make the bomb as efficient as possible involves taking advantage of a few of the quirks of the critical mass. The critical mass isn’t set in stone; it’s simply the mass of fissile material required to sustain a chain reaction, and so it can change depending on the presence of various conditions that make things more or less favourable for the neutron pinball going on inside the bomb core. The three main ones we’re concerned with are:

Density. We’ve saturated our bomb core with as many fissile atoms as possible by enriching the uranium to 90% u-235, and this makes the core a target-rich environment for any neutrons looking to split an atomic nucleus. However, we can go further than this. The density of u-235 at room temperature is comparatively low. If we were to somehow artificially compress the bomb core we could jam the u-235 atoms into a much smaller space, increasing the efficiency of the chain reaction yet further and ensuring most of them are used to power the explosion.

Presence of a neutron reflector. The most efficient shape for a critical mass is a sphere, as this has the lowest surface area to volume ratio of all possible shapes and ensures the majority of u-235 atoms are surrounded on all sides by other u-235 atoms. This makes child neutrons produced as part of the chain reaction far more likely to strike and react with other u-235 atoms. However, a sphere of fissile material will still lose neutrons from the outer layers of the sphere, decreasing the efficiency of the reaction. This can be solved by surrounding the bomb core with a neutron reflecting material such as beryllium, which bounces escaping neutrons back into the core and ensures as many of them react with u-235 atoms as possible.

Presence of a tamper. When a critical mass undergoes a chain reaction, it will get hot. When things get hot, they expand. When things expand, their density decreases. This is undesirable when we’re trying to make the bomb core as dense as possible during the detonation process, so bomb cores are also surrounded by a very dense material called a tamper that contains this expansion to a degree. It cannot stop the expansion because no material can withstand that stresses and forces contained with an exploding nuclear weapon, but it will slow it down. Early fission weapon designs combined the tamper and the neutron reflector into one component – usually u-238 since this had the added bonus of occasionally reacting with the fast neutrons emitted by the chain reaction and boosting the yield of the explosion even more.

This is why the second type of nuclear weapon design – the implosion design – looks like this:

It’s a core of subcritical fissile material surrounded by a tamper/reflector and a series of high-explosive lenses. When you want to detonate the bomb you trigger the high explosive. This creates a shockwave focused inwards towards the core which compresses it very very quickly, increasing its density and causing it to go critical. The chain reaction starts, and escaping neutrons are reflected back into the core by the tamper. The tamper itself only delays the explosion itself by a fraction of a millisecond, but this is enough time for several additional steps in the chain reaction to take place. Given the exponential nature of the chain reaction (1, 2, 4, 8, 16, 32 etc.) that additional fraction of a millisecond is enough time for the energy of the explosion to increase tenfold².

Implosion weapons are more complex and advanced than gun-type nuclear bombs, but they’re also far more efficient. The Little Boy bomb dropped on Hiroshima was a gun-type fission weapon containing 141 pounds of 80% enriched uranium, and had a yield of 16 kilotons. The Fat Man bomb dropped on Nagasaki had a 13.6 pound core of plutonium, and yet the nuclear reaction catalysts described above increased its yield to 21 kilotons, producing a bigger bang with less than 10% of the fissile material. If you want economy in your fission weapon, then the implosion design is the way to go.

Fission weapons aren’t the whole story, though. In fact, they’re only the first page. There will be a sequel to this post at some point, and it will talk about the current cutting edge in nuclear weapon design: the thermonuclear bomb.

1. Note: there is no moderator present inside a nuclear weapon to produce low energy thermal neutrons, so we have to rely on fast neutrons to sustain the chain reactions
2. I can’t find a source for the exact number so I’ve made a guesstimate based on the prompt lifetime of a neutron in a fast fission reaction (10^-7 seconds). If anything I’ve lowballed it, though.

The post Our Words Are Backed By Nuclear Weapons! 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.

The post NUKULAR REEAKTOR FIZZICS. appeared first on The Scientific Gamer.