Nuclear 101: How Nuclear Bombs Work Part 1/2

okay welcome and thanks for tuning in online we're be talking today about the basics of nuclear weapons before we get to that though I wanted to mention a couple of books for those who want to go into some more detail first of all if you really want to understand the physics of nuclear weapons the Los Alamos primer is an amazing document this is the lectures that were given to the scientists as they were arriving at Los Alamos for the Manhattan Project in about 1943 with lots of additional notes and information by the guy who gave the lectures it really gives you a good introduction to the physics of nuclear weapons as it was understood in 1943 with some updates in the notes if you want a wonderful page-turner historical account of how nuclear weapons were created in the Manhattan Project the making of the atomic bomb by Richard Rhodes is really a classic it's it's I think understandable by everyone and yet it gets the physics more or less right if you want a more technical account of the Manhattan Project critical assembly by hottes and Henriksen Mead and Westfall is much drier than the making of the atomic bomb but much more detailed on the technical front on the effects of nuclear weapons the US government's textbook on that subject by Gladstone and Dolan is called the effects of nuclear weapons and it's still available and in fact it's been put online in PDF form if you search for it and if you want a detailed account of the impact of the bombings of Hiroshima and Nagasaki there's a book called the physical medical and social effects of the atomic bombings that really is a nice job on that okay with that said let's get started first of all just to remind us what we're talking about this is an image of the Trinity test the first-ever nuclear explosion I was a huge ball of flame unlike anything that had been seen on earth before so we're going to talk today about how nuclear bombs were work fish infusion the different kinds of nuclear bombs we're going to talk about the nuclear materials that you can use to make nuclear bombs we're going to talk about what the hard parts are of making a nuclear bomb and we're gonna talk about nuclear weapons effects okay so it all starts with the splitting of atoms and there are a few large unwieldy types of atoms uranium-235 plutonium-239 are the main ones that if you hit them with a neutron will begin to oscillate and will oscillate with enough energy sometimes that they fall apart into two smaller atoms they split in half okay and when that happens the two smaller atoms are not stable with the number of neutrons that the original uranium or plutonium atom had and so it also releases typically two or maybe even three neutrons in this reaction now if an a big atom has a whole lot of protons which are positively charged and neutrons which have no charge held together in a big ball in the nucleus all right now there's protons since they're all positively charged are all repelling each other okay like charges repel now what's holding them together is the nuclear forces the strong and weak nuclear forces and including those neutrons are like a glue holding that nucleus together but once it splits you've got two halves that are each have a lot of positive charge and so they go flinging apart at extreme speeds and that's where a lot of the energy that is released in a nuclear reaction comes from is actually the kinetic energy of motion of those what are called fission products the products of fission when that atom splits and you get a lot of energy out of this fission reaction you get about 200 million electron volts that's a very rough number a little bit more for some a little bit less for others out of each of these fishin's now that's more than a million times more than you get out of each atom that takes part in a chemical reaction why chemical reactions only effect the electrons that are in a haze around the nucleus of the atom they're far away if this is the nucleus of an atom then the electrons are way over here someplace and over there someplace okay so they're much more loosely bound these nuclear forces are much stronger holding that atom together so when it splits it releases this huge amount of energy a million 10 million times more than the typical chemical reaction as in a conventional explosive and so that's why you can get so much energy out of one bomb that's why one bomb from one plane can destroy a city rather than hundreds of planes dropping thousands of bombs returning again and again and again as happened to other cities in world war two similarly it can also provide the energy for civilian power generation with a lot of energy coming from a small amount of material in a small space just to give you an idea if you have a 1 gigawatt coal plant you need an 80 car train full of coal every day to provide fuel if you have a 1 gigawatt nuclear plant you need about half of one of those train cars once a year huge huge difference so almost everything about nuclear weapons and about nuclear power arises from this basic technical fact you get a lot more energy out of each unit of material taking part in the reaction than you do in the case of conventional Thanks all right so but even though it's a lot of energy per atom it's still an atom is very small and it's still a very small amount of energy so you need to do gigantic numbers of atoms in order to get a significant amount of energy out of it so how could that happen how could you get huge numbers of atoms to to split all at once and the key is this notion that one Neutron starts the reaction one Neutron splits an atom and then that atom splitting releases two or maybe even three neutrons and that's what means you can have a chain reaction you can have the sufficient of one atom leading to the sufficient of a couple more atoms leading to the fission of more atoms and so on so here we have an image of fission happening in a group of atoms that's not enough to get that kind of chain reaction going it's not enough for a chain reaction to be sustaining it's called sub critical less than critical critical is when the nuclear reaction can sustain itself over a long time super critical is when it grows exponentially as you want in a nuclear bomb so here what's happening is a neutron is coming in and hitting an atom and that releases a couple of neutrons and one of those neutrons hits an atom but the other one goes flying out of the group of atoms and so they if that keeps happening the reaction will come to a halt because you'll run out of neutrons and you won't have this continuing and accelerating chain reaction so the question is if I want to make a nuclear bomb how do I fix that problem how do I deal with the fact that my neutrons are leaking out of the system and stopping reaction well there really several ways that people have thought of this how to fix that problem really three ways so the solution one is to add more material okay here I have a larger group of add-ons and you can see that in most cases the neutrons are hitting other atoms before still some of them fly out into the surrounding countryside but generally they're hitting other atoms first and so that will lead to a continuing nuclear chain reaction so one basic concept that's worth remembering is what's called a critical mass a critical mass is enough nuclear material to keep this kind of chain reaction going okay another idea is to reflect the neutrons back in so here I've I pictured the atoms in a box and the neutron and with it is a perfect reflector of neutrons there isn't any such a thing it's a perfect reflector of neutrons okay but there are things that will reflect neutrons more or less well generally things that are among the the atoms that have a lot of protons and neutrons in their nuclei the heavy atoms tend to reflect neutrons better but some of the lighter things can reflect neutrons some as well so obviously if fewer of the neutrons go out because they're being reflected back you need less material before the reaction can be self-sustaining so you can reduce the amount needed for critical mass by having a neutron reflector another idea is to take this group of atoms and crush it so that it's more dense okay so the atoms are closer together the atoms are closer together that means that when one of them releases a couple of neutrons it's much more likely that those neutrons will run into another atom before they leak out of the system now these three solutions enough material reflecting the neutrons and crushing the neutron the crushing the atoms so they're closer together or often use together rather than just separately so as you might imagine what you want ideally is for if you want to have a critical mass with the least amount of material and the material is expensive to produce and difficult to produce as we'll talk about so you do want to use as little material as you can and so usually having it in a sphere is a good idea because if it's in a flat pancake then there's lots of opportunities for the neutrons to leak out if it's in a sphere it's the minimum surface area for those neutrons to come out on so you will often see in describing particular kinds of nuclear material a phrase that's the bare sphere critical mass so that's how much you need to have a critical mass in if it's in a sphere and if the sphere hasn't got any neutron reflector around it if it's bare okay so remember the idea of a bare sphere of critical mass okay yeah what's gonna happen when this chain reaction starts happening what happens is this energy is getting released and it gets released incredibly fast these reactions take place in x measured in billionths of a second in fact during the Manhattan Project the scientists invented a new time unit a shake you know the expression two shakes of a lamb's tail referring to some very short period of time a shake is ten billionths of a second and that's sort of the timeframe for for fishing okay so what happens is it's releasing all this energy in a pretty small ball of material and that means that material is going to heat up it's going to turn to gas and it's gonna start blowing itself apart well if bringing the atoms closer together by crushing them on increasing their density reduces the amount you need for a critical mass having them expand out increases the amount and it causes the neutrons to start flying out into the surrounding countryside again and it shuts down the chain reaction so the key difficult part of making a nuclear bomb is figuring out how do I get my material together into the shape and that I want it to be in to get a good explosive yield out of it before it starts blowing itself apart because one of the key things to understand about these materials is that sometimes they efficient by themselves what's called spontaneous fission and they release neutrons so there are always some neutrons around so you can't just have your nuclear material sitting in a supercritical form because it's going to start going off so the key problem is how do I get the material together into the form I want it in a form that's really supercritical that's way beyond what's needed for just a self-sustaining chain reaction will give me a growing explosively growing chain reaction before it starts blowing itself apart okay that's the key problem getting the material together fast enough alright so there are two ways people have thought of to do this basically one is what's called a gun type bomb and that is literally slamming two pieces of nuclear material together at high speed yeah if I had for example a two-thirds of a critical mass of highly enriched uranium in this end let's pretend I'm strong because that would be quite a bit of material and two thirds of a critical mass of highly enriched uranium in this hand and I brought them together there with my hands as fast as I can bring them together with my hands that would not give me a good nuclear bomb why because as they got maybe somewhere around here they would be close enough together that together they would critical mass and as soon as a loose Neutron started a reaction going it would start going and that uranium would turn would melt and turn to gas and it would blow itself apart before I got any significant nuclear yield out of it everybody in this room would be dead I'd certainly be dead we'd have a tragic accident but we wouldn't have you know many blocks of a city vaporized in a nuclear blast so the way that a gun type bomb works is to slam those pieces together much faster than I could do it with my hands the bomb that obliterated the Japanese city of Hiroshima was literally a cannon that fired one piece of highly enriched uranium into another piece of highly enriched uranium so these are simple and reliable types of nuclear weapons the United States never bothered to test the Hiroshima bomb before using it because it was so obvious it would work and the basic idea of the Hiroshima bomb was worked out by one professor and two of his graduate students in the summer before they all arrived at Los Alamos so this is not an incredibly complicated thing unfortunately you can with a gun type bomb you can only get a good yield with highly enriched uranium you can't get a good yield with the other nuclear material we'll talk about which is plutonium why because plutonium falls apart much more than highly enriched uranium does so there are a huge number of neutrons flying around when you have plutonium even if you have quite good plutonium and therefore as the two pieces are coming together in a gun type bomb even fired from a cannon they'll start fishing before they come together in the full configuration that you want so you won't get hiroshima scale nuclear bomb trying to do a gun type bomb with plutonium so you can only get high-yield with highly enriched uranium okay so if what you have plutonium or if what you have is not enough highly enriched uranium for a gun type bomb because remember those gun type bombs are very inefficient then what you use is an implosion type bomb so this is one where you are trying to crush that nuclear material down to a higher density these are much more efficient and therefore need less nuclear material so just as to give you some ballpark numbers the hiroshima bomb which was the inefficient gun type bomb had about a little over 60 kilograms of 80% enriched material in it that it means 80% of the material was uranium-235 as opposed to uranium 238 which has three more neutrons in it which is the other kind of uranium that is in the Iranian you dig up out of the ground we'll talk next time about how you produce highly enriched uranium and plutonium and we'll talk a little bit more about the different kinds of elements which are called isotopes which are things that have different numbers of neutrons in the nucleus an element is defined by the number of protons it has in the nucleus and the isotopes of an element like uranium 235 or uranium 238 is the numbers 2 35 to 38 are the number of protons plus neutrons in the atom ok so back to the to the implosion type bomb so here we have a picture that shows the nuclear material surrounded with some other stuff that I'll talk about in a moment and then surrounded by explosives and a lot of the explosives are cut away here so that you can see what the assembly looks like all right now what you need to do in this situation is you need to Det the explosives at exactly the same time all around this bomb because if the explosives on this side go off first before the explosives on the sides go on what you'll have is not a crushed ball but a pancake and remember in a pancake the neutrons are leaking out all over the place okay so what you want is a crushed ball which means you need the explosive shockwave to be a spherical shockwave going inward all right so that's significantly more complex to design and build and would be much more difficult for terrorists although it's still conceivable especially if they got knowledgeable help and there are some approaches that are significantly less complex than the Nagasaki bomb was I can't say much more than that in this unclassified talk so most of the approaches require explosive lenses they required millisecond timing of multiple detonations it is a tricky thing but as I say there are some approaches that are less complex okay let me talk a little bit about the evolution of designs for implosion bombs because they have changed over the years okay so the Nagasaki bomb was a solid ball of nuclear material okay Nagasaki bomb was about unlike the Hiroshima bomb that was about 60 kilograms of highly enriched uranium Nagasaki bomb was about six kilograms of plutonium now plutonium is about three times more reactive than highly enriched uranium so even if they were identical bombs you would have needed three times less plutonium the reason you needed ten times less is because you had the implosion device rather than the gun type device and the implosion is much more efficient so you have this solid ball of about six kilograms of material a ball that fits in my hand I'll show you a picture in a minute and around that you had some heavy stuff for what's called tamper so the tamper does two things first of all it's the neutron reflector reflecting those neutrons back and the way I talked about before but secondly if you think about it you would like to figure out a way to keep that material from blowing itself apart for at least a little bit longer and if you think about it even a tiny amount of time might help a lot why because in one of these exponentially growing chain reactions let's say you had so let's say it was doubling every neutron generation so an atom splits releases two neutrons each of those split two more atoms releases two more and so on then the last Neutron generation in that case would be giving you a big chunk of all the energy you were getting so if you can keep it from blowing itself apart long enough for just one or two more generations of neutrons to happen you can get a substantial amount of yield now so one idea would be let you know let's put our steel cage around the thing hold it in turns out there's no way you can hold this in you've got a ball that is roughly this size where as much energy as 10 or 20,000 tons of conventional explosives has just been released okay and the ball has turned to gas and that gas is that unbelievable temperature and pressure billions of degrees incredible pressure okay and that ball is gonna go flying out no matter what you do about it okay there is no steel cage or whatever material titanium whatever that's gonna hold that ball from blowing apart black if you just have some fun too heavy then the inertia it has to move that heavy stuff in order to blow out and that might slow it down again we're talking about billions of seconds here it might slow it down enough to give you enough extra growth in the neutron population to have a significant amount of extra energy release so pretty much every nuclear bomb you're gonna see is gonna have this stuff the tamper around it okay then around that you would have the conventional explosives okay in these explosive lenses all right yeah so things have evolved since that first generation thing and one thing people thought of is let's not have a solid ball let's have a hollow ball okay hollow ball of nuclear material why because when the explosives crush it it starts off since this part is nowhere near that part it starts off quite far from being a critical mass when the explosives crush it it doesn't become a critical mass until it's all crushed down in the middle okay and it's much easier for the explosives to crush it at high speed because there's this big empty gap in the middle okay alright so that was step one in the evolution another good step in the evolution was creating an air gap in between the explosives and the ball in the middle you still got tamper but you have an air gap between the explosives and the ball so sometimes referred to as a levitated pit this ball of nuclear material in the middle is called the pit of a nuclear weapon now why would you want an air gap all right think about this when you're pounding on a nail would you want to have the hair right in contact with the top of the nail and just press on the nail no you're gonna get almost nowhere with that what you want to do is pull the hammer way back and weighing that now give the hammer enough time to accelerate before it hits the nail so here the explosives accelerate through the air gap before they hit the nuclear Mottola it turns out it crushes the ball much more efficiently if you do that okay yeah then came the really big evolution okay so we had hollow balls we had levitated balls okay now we're gonna put something else in the ball okay put a little tube in the ball and you put some tritium it's a hydrogen atom with two neutrons and a proton okay referred to as tritium if it has only one Neutron with the proton it's referred to as deuterium okay so what happens is when the fission reaction goes and you have this tiny ball heated up to billions of degrees okay that will cause a fusion reaction in this tritium okay and that fusion reaction will do two things it will release a bunch of energy itself and it will release a bunch of really fast neutrons that will fish in more of the nuclear material okay so you get this is what's called a boosted weapon this is not yet a real hydrogen bomb but it's what's called a boosted weapon the fission is being boosted by the fusion okay so every modern thermonuclear weapon in the US nuclear arsenal in the Russian nuclear arsenal is a boosted weapon like that okay all right yeah let's I'm I apologize in advance now we're gonna get really complicated for a minute because we're gonna talk about modern thermonuclear weapon and you know nuclear weapons designers have been creative over the years and things have gotten complicated but let me just try to go through it for a minute here all right so you start off with the primary also also known as the pet okay that's the same thing we were just talking about so that's a hollow shell of nuclear material it's got some tritium inside it okay so the explosives go off they crush that ball the fission reaction starts going you also have what's called a neutron generator in the middle of that ball that's set off a shower of neutrons at the right moment so that your nuclear reaction goes at the time that you want it to okay so your fish and reaction goes off it starts some fusion with the tritium that's in the middle of the pit okay the tritium causes some more efficient to happen so few sufficient causes fusion then fusion causes more efficient okay so then you have a huge amount of energy released by that pit that energy is focused on what's called the secondary the secondary will have lithium deuteride typically so that is the fusion fuel the deuteride means it's got deuterium which is in a chemical bond with this lithium the lithium actually splits from all this energy and provides the provides tritium that the deuterium can fuse with okay so you have this energy from the fission and fusion of the boosted pet that is then focused on the secondary and it actually crushes the secondary with radiation pressure believe it or not it's the pressure of very high frequency light gamma rays x-rays that actually is like a hammer crushing the secondary and heating it up so you have in these incredible pressures and temperatures in the Andheri that then lead to fusion in the secondary which releases a huge amount of energy and also releases a big shower of neutrons and so the designers thought well if we've got another big shower of neutrons let's put in some nuclear material and have some more efficient as a result of those neutrons so what do you have you know this is this just stay with me for a minute you have fishin that starts the whole thing fusion that boosts that fish and more all of that then causes fusion which then causes more fishing so you have a fission fusion fission fusion fission reaction in the end you end up with a typical bomb you might have of order half of the energy coming from fusion half coming from fishing the designer can change those proportions considerably nobody has ever managed to come up with a fusion bomb that doesn't have a fission bomb ought to set it off okay now most of the nasty radioactive fission products that create fallout and things like that are coming from the fishing they're coming from the splitting of uranium or plutonium so the fusion part is in a certain sense a cleaner bomb then the other part which is why people thought gee couldn't we figure out how to make a fusion bomb without the fishing part nobody's ever figured out how to do that okay all right onward so to make nuclear bombs there are basically two key potential bomb materials next time we'll talk more about how to make these the first is highly enriched uranium alright when you dig uranium up out of the ground less than 1% of it is the kind that's easy to split uranium 235 0.7% is uranium 235 and 99.3% roughly is uranium 238 wait uranium 238 is almost useless for fishing it will split under certain circumstances but it can't sustain a fish and chain reaction so you've got to separate this you've got to figure out how do I get from 0.7% up to very high concentrations of uranium-235 almost all and that process is called enrichment the more u-235 percentage you have in your uranium the more enriched uranium is so when people say highly enriched they mean 20% or more uranium-235 that's the International definition of highly enriched uranium and almost all the techniques that people have thought of for how do i how do I get those u-235 atoms separate from the u-238 atoms have to do with the very slight difference in mass so the mass basically goes as the number of protons and neutrons in the nucleus so it's you know 238 rather than 235 so it's you know only a little more than 1% difference in the mass and that slight difference is what most of the most of the approaches play on and we'll talk about a couple of the approaches that have been next time but there's gaseous diffusion approaches that use huge amounts of energy there's centrifuges there's a variety of others okay plutonium is what they figured out to do with the uranium 238 if you put uranium into a reactor and there's a instead of an explosively growing chain reaction like an a-bomb you have a steady state chain reaction you still have neutrons all over the place okay and some of those neutrons when they hit uranium 238 items they get absorbed and so you have uranium 239 but uranium 239 isn't stable and it kicks out some things and decays to plutonium 239 okay and plutonium 239 it turns out is a nice weapons material the way you're in m23 five is and so you can take this you know useless uranium 238 turn it into plutonium-239 once it but you only end up with something like 1% plutonium in the spent fuel from a reactor and so you have to chemically separate out the plutonium that you want from all the other stuff the remaining uranium the fission products which are intensely radioactive and nasty you want to get the purified plutonium and that chemical process of separating out the plutonium is called reprocessing ok we'll talk about that more next time there are a few other isotopes that have this property that they are big and unwieldy enough that when a neutron hits them they'll split and they'll release more than one Neutron so you can have a chain reaction but there aren't any others that have ever been used for stockpiled nuclear weapons in a state now the key point is none of these materials occur in nature there isn't a rock you can turn over that will have highly enriched uranium in the rock or plutonium in the rock all of them are extraordinarily difficult to produce so in a certain sense Mother Nature has been both kind and cruel to us in the way she set up the laws of nuclear physics kind in the sense that these materials don't exist in nature and they're hard to produce and so our species didn't invent them until we were had developed civilization to a reasonably extensive degree but cruel in that once you have these materials it's actually not as hard to make a nuclear bomb as it would be preferable for it to be now as I mentioned the amounts of material required are not very big so this hand has a glass ball not a plutonium ball that is the same size as the ball for the Nagasaki bomb so that is just not a big amount of material for the gun type bomb you need more as I mentioned but the amount that you have because uranium is very dense metal would fit roughly in two 2-liter bottles okay and again as I mentioned it's only six kilograms for Nagasaki of order 60 at Hiroshima because of the greater efficiency of the implosion bomb what's more it's not particularly hard to smuggle this material this is a guy by the name of sergeant her Blair he's carrying in one hand a box that has the plutonium core for the Trinity nuclear test the first-ever nuclear bomb you can see he's got no special equipment he's wearing a t-shirt and remarkably dirty chinos and he's carrying it in one hand so once nuclear material gets out of the place where it's supposed to be if it's stolen or something like that it could be anywhere and trying to find it and recover it is a very difficult job it is radioactive any material that you can make a nuclear bomb out of is an atom that is unstable and therefore is radioactive and you can detect that radioactivity but turns out highly enriched uranium especially and also to a lesser degree plutonium are not so radioactive that either they require special equipment to carry them around or to make them very easy to detect years ago I was talking to a guy who had done a sort of international survey of the radiation detection equipment that was available for people to install at borders and so on and one of the things you'll see at airports and things like that that are international you'll often see that the customs guy is wearing a little pager on his belt well that's a radiation detector so I said to this guy who've done this survey of the equipment let's suppose I'm a customs guy I've got one of those things on my belt the bag directly in front of me that I'm looking at has enough finally remaining for a bomb in it what is the probability that that detector will go off he said zero okay now the bigger detectors that's not true the bigger detectors have some chance of detecting highly enriched uranium but it's relatively easy to shield some of the radiation it emits is similar to things that are normal radioactivity you'd be surprised how much radioactivity is just around us in in nature you get a certain radiation dose when you eat a banana for example and kitty litter is one of the things that routinely sets off radiation detectors at borders so detection is another issue but it's just not that hard to smuggle new clean the toilets the point I'm trying to make alright so some somewhat misleading terms that you should nevertheless remember first of all highly enriched uranium I mentioned that's 20% or more uranium-235 natural uranium is what you dig up out of the ground that's 0.7% uranium-235 low enriched uranium is when it has more than 0.7 percent but less than 20 percent typically for a power reactor you're talking somewhere in the range of 4 to 5 percent uranium-235 depleted uranium is what you get when you've stripped out some of that u-235 to make highly enriched uranium or to make low enriched uranium and then the waste that you've got that's that's almost all UNM 238 is what's called depleted uranium so it has less than 0.7 percent u-235 now here's where things get misleading weapon-grade uranium is typically a little bit different in different countries but it's typically uranium certainly with 90% or more in the United States that's officially ninety-three percent or more uranium-235 but bombs can be made with material that is much less than weapon grid it's an irony of history that the hiroshima bomb the first-ever nuclear weapon used at war was not made out of weapons-grade material it was made out of 80 percent enriched highly enriched uranium alright so similarly weapons-grade plutonium again a little bit different depending on where your are but typically sort of 93 percent or more plutonium-239 which is the isotope you want why is plutonium-239 the isotope you want it turns out if the if the material absorbs one more Neutron then it turns into plutonium 240 and plutonium-240 has a huge spontaneous fission rate it falls apart and releases neutrons all the time and so you end up with lots of neutrons flying around if you have a lot of plutonium 240 and makes it more difficult to make your nuclear bomb plutonium 238 which is produced in are somewhat more complicated way but comes up a little bit as you were radiate that uranium longer and longer generates a huge amount of heat plutonium 241 isn't as bad as 240 or 238 but it's still not great and so on so plutonium 239 is is the thing that if you're a bomb designer you prefer to have so in a power reactor what you do is you leave the rhenium in for the uranium fuel in the reactor for a long time in order to efficiently produce power in a plutonium production reactor for weapons you leave the material in for a relatively short time in order to maximize the plutonium 239 and minimize the degree to which it absorbs more neutrons to build up these undesirable isotopes of plutonium so the stuff you get out from a power reactor is referred to as reactor-grade plutonium has much less plutonium 239 maybe only 60% or 70% plutonium 239 and weapon makers much prefer the weapon grade plutonium but just as with uranium it is possible to make reliable effective nuclear weapons from reactor-grade plutonium once that plutonium has been separated out from the spent fuel by this process known as reprocessing so this is just more on the point that you can make bombs with reactor plutonium it has a higher neutron emission rate it has higher heat emission that's higher radiation all of those can be addressed by bond designers and this is just a long disquisition on that topic officially from the US government this is again going back to the point that highly enriched uranium well below weapon grade is weapons-usable this is how much material you need if you do have a neutron reflector around a sphere as a function of how what percentage of uranium-235 it is and what I want you to see is that the graph is pretty flat for a long time until you get down to sort of 50 60 % uranium-235 and that's when it really starts going up now there as I mentioned there are other isotopes that could conceivably be used in nuclear explosives there aren't they're very difficult to produce and separate out only a couple of countries have kilogram quantities of separated materials of these other isotopes so it's I include them only for completeness all right so what are the hard parts of making a nuclear bomb first making the nuclear material that's overwhelmingly the most difficult part more than 90% of the effort in the Manhattan Project was devoted to making the nuclear material rather than to the design the manufacturing etc of the bomb in fact in the Manhattan Project as I'll talk a little bit about next time they built in just a few years an industrial floor space that was bigger than the entire u.s. automobile industry that existed at that time they were using I think something like 5 or 10% of all of the electricity generated in the United States for enriching uranium it was a gigantic industrial enterprise to make that weapons-usable nuclear material alright second hard part is if you want to make the kind of bomb that I they would want to have something that's efficient that's safe that's reliable that's predictable that you can put on a combat aircraft or better yet a lot of ballistic missile it is far harder to make that kind of a bomb than it is to make a terrorist nuclear bomb that would be crude unreliable maybe put in the back of a truck unfortunately that's much much easier to do if you have the nuclear material so that's why security for nuclear material is important enough to keep it out of terrorists hands to be the subject of these global nuclear security summits that have been taking place since 2010 all right the third hard really hard part is designing and making a hydrogen bomb that complex thermonuclear weapon I talked about thermonuclear hydrogen fusion all of those are that's referring to the same thing that is extremely complex and I think very difficult to do without some degree of testing there is no central secret to making nuclear weapons but there are a variety of very real engineering and manufacturing challenges all right so suppose I'm a terrorist group and I have dramatically less resources than a state has I don't have anything like the money that a state has I don't have anything like the organizational capacity that a state has I don't have anything like the technical expertise that a state has how am I going to put together even a crude nuclear bomb so there's hard first hard part again getting hold of the nuclear material once they have that there is unfortunately a significant risk that they might be able to make a crude nuclear bomb and that's been the conclusion of repeated studies by governments not only in the United States but in a number of countries elsewhere around the world so even once I have the nuclear material though it remains somewhat difficult it would be I think the most technically challenging attack that any terrorist group has ever accomplished if it happened they have to process the material into an appropriate form so they might get instead of metal they might get for example research reactor fuel that's typically a mix of aluminum and uranium and they'd have to actually chemically separate out the uranium from the aluminum and then they'd have to convert it to metal okay so some degree of chemical processing may well be required depending on the kind of material that they manage to get hold on now on the other hint and you might say well tariffs couldn't do that but then it turns out that there's a lot of things that we know terrorists and other adversaries do routinely that involve a fair amount of chemical processing going from a poppy plant to heroin actually involves a substantial and fairly sophisticated set of chemical processing all right casting and machining once you have the metal you have to cast it into the right shape for the components of your bomb then you have to machine it to make sure it's exactly the right shape you have to build your explosives the reflector etc and get them to work if you wanted to do an implosion weapon on the standard type then you'd have to have precisely shaped explosives with very precise timing and so on be it's really quite difficult and all that requires you to be able to recruit train skilled people raise money and sustain an organizational effort over a substantial period of time which is something terrorist groups in most cases are not good at so some scenarios might allow you to bypass some of these steps just to give you an idea in the US Department of Energy there are some facilities where the security rules require them to have a security plan that prevents the terrorists even from getting to the nuclear material let alone leaving the building with the nuclear material because of concern that terrorists might be a set off a nuclear blast while they're still in the building if they manage to get there I won't say any more than that on that topic so here are a few technologies you might want to remember because they're sometimes in the news cry Tron's these are devices for delivering a very powerful electrical signal a big spark with very precise timing and they do have civilian applications they're used for example in licit ripsi for smashing kidney stones subject dear to my heart since I've had kidney stones multiple times but they're used for detonating explosives in implosion systems there are various alternative approaches that can also be used for detonating the explosives Neutron generators this is another one of our heart one of the hard parts it's tricky to get do you want a shower of neutrons set off at exactly the right moment but it's a moment when the conventional explosion is already crushing down the the nuclear material so figuring out how to make something that's going to generate the neutrons you want right at the right moment in the right way and not set off a whole bunch of neutrons ahead of time is a little bit tricky it's not necessarily required for gun type bombs because they can rely on stray neutrons that are always around but it's it's needed for implosion type phones x-ray flash photography is another thing that's often used especially in state programs because you want to be able to take a sort of strobe picture of the imploding system to see if it's imploding properly all of these things are subject to export controls they also have civilian uses all right let's talk for a minute about the effects of nuclear weapons and just to give you an idea this is what Nagasaki looked like a month after the bomb was dropped you can see it's an image of almost total devastation and destruction so the first thing that happens is the nuclear fireball all right so back to our ball of nuclear material so in less than a millionth of a second we have released the equivalent of say 15,000 tonnes of chemical explosives in a ball this pick the ball hasn't had a chance to expand yet so that energy is all contained in this tiny little space so we're talking literally billions of degrees much hotter than the center of the Sun in the center of this ball okay and unbelievable pressures okay so that ball is going to be radiating like mad it's going to be sending out gamma rays x-rays etc because of that incredible temperature and pressure and that pressure will cause it to fly outward so that ball will grow at incredible pace sending out these gamma rays x-rays etc and as it gets bigger and at least slightly cooler it will still be a fireball but it will be sending out visible light infrared light and so on so this is an image of the fireball from the Trinity nuclear test you can see this one was sent off in a tower a little bit above the ground but the fireball was big enough that it came into contact with the ground and in fact melted you can still go and visit the site it melted the sand and rock underneath and you can go to that site and pick up little pieces of what's called Trinitite which is the sort of molten glass that resulted from that friend of mine actually took a piece of Trinitite into the lab and managed to detect the that the presence of an isotope created when the barium in the explosives absorbed neutrons from the ongoing reaction and there in so decades later just from a piece of stuff picked up out of the ground he was able to confirm that barium was used in the explosives of the of the Trinity test which is in fact true all right so at the at the edge of this expanding thing there's incredible pressure still at the sort of outer edge as it's moving out that's called the shock front and that's labeled here and you can see there's sort of a dirt cloud coming out as it kicks up dirt all right so this fireball is hot right and in the middle it no longer has high pressure so what happens with really hot air is it rises so the fireball incredibly hot the blast is going out like this the fireball is starting to rise because it's hot and it's starting to spread and it's sucking up air that's coming up into this column okay because it's lifting up and so in order to avoid creating a vacuum underneath where it was air comes in from the sides and comes up in this column and then it's going out and it creates the classic mushroom cloud okay all right so the question is how far do various effects occur and the answer depends on how big your bomb is as you might expect one of the surprising things is that the different effects scale differently with the size of the bomb some of them scale roughly by the 1/3 power like the blast of the bomb scales roughly by the 1/3 power some of them scale by the 0.4 power roughly the thermal the radius at which the thermal effects will be enough to set things on fire or to cause third-degree burns or what-have-you scales is roughly the 0.4 power but the radiation radius scales at a much smaller number so that means that for small bombs the radiation is more important than it is for big bombs for big bombs other things become more important as you see here so the prompt radiation in the case of a 1 kiloton bomb a small nuclear bomb actually extends further as a potentially lethal effect than any of the other effects whereas for a hundred kiloton bomb it's the thermal that extends the farthest and even the blast extends further than the prompt radiation so I should say there are really two kinds of radiation from a nuclear blast one is what's called the prompt radiation so you have as I mentioned gamma rays x-rays released essentially instantly and you have neutrons that are released essentially instantly but then you also have the delayed radiation that's coming mainly from the fission products from fission which are extremely radioactive and what happens especially if you there are two ways you can detonate a bomb you can detonate it up in the air which turns out to be best for creating a big blast over a wide area or you can detonate it right on the ground which is called a ground burst that's best if you want to destroy hardened things right near where that blast is like a underground missile silo or an underground command bunker or something like that either on the ground or close to the ground is best for that purpose but if you're on the ground or close to the ground what happens is the fireball comes in contact with the ground it vaporizes a whole bunch of rocks and whatever's on the ground there and that stuff gets sucked up into the mushroom cloud it mixes with those nasty fission products that's really radioactive fission products while it's in the mushroom cloud and then it starts falling down downwind from the reaction that's what's called fallout okay so if you have an airburst to maximize the area that you're destroying buildings over you don't have fallout there the fish and products go way up into the upper atmosphere dispersed over the whole earth you don't have stuff raining down just downwind of the blast but if you have it close enough to the grounds that it comes in contact the fireball comes in contact with rock dust etc and sucks it up into the cloud then you're gonna have fallout okay so this is this here is only referring to the prompt radiation the neutrons and the gamma rays and so on so the fireball there's the blast let me talk about the blast for a little bit the blast is basically a big shockwave like you would have from a conventional bomb only it's much bigger over a much broader area and with the combination of effects it is usually thought that most people that are in the area if it's a fairly large bomb like a you know 10 20 30 kilotons and up most people who are in the area that has at least 5 pounds per square inch of blast pressure what's called overpressure will probably die most of the people who are outside the 5 PSI radius will probably live now that's a very rough very very rough approximation and there are much more detailed models and effects calculators and so on but there are calculations that have been done over the years that use what's called a cookie cutter model where they just assume everybody dies within 5 psi and nobody dies outside 5 psi and the the mistake you make by assuming everybody dies within 5 psi is similar in size to the mistake by assuming everybody survives outside the five psi and so but it's it is a gross oversimplification it doesn't really work if the population isn't sort of homogeneous over a broad area on its own so this just gives you the areas that go along with those those effects so this is a somewhat more detailed calculation about who would die so this is where the overpressure is greater than 12 psi then almost everybody dies in the 5 to 12 psi range of order half the people die and almost everybody is seriously injured in some way then you have in the outer ranges you have some people dying substantial number of people getting injured as you get out toward one to two psi you have a minority of people who are injured so what kinds of injuries are we talking about so first of all from the blast what usually happens is people being crushed buildings falling down etc from the heat we're talking about people being burned from the prompt radiation we're talking about radiation sickness that might take you a couple of weeks to kill you that as if the dose is big enough to kill you so we're talking about in the case of both Hiroshima and Nagasaki tens of thousands of people being really horribly killed and tens of thousands more being really terribly terribly injured so what we've talked about today is the basics of nuclear weapons some takeaway points you need highly enriched uranium or plutonium to a nuclear bomb those are hard to make the amount of material you need however is small the hardest part of making a nuclear bomb is making the nuclear material there are other difficult parts however and the effects are overwhelming a single nuclear bomb can incinerate the heart of a major city so next time we will talk about how is it that you make plutonium and highly enriched uranium thanks again for tuning in

Channel: Belfer Center

Views: 1,616,716

Rating: 4.7894235 out of 5

Keywords: matthew bunn, Nuclear Weapon (Invention), Harvard University (Organization), managing the atom, bombs, making bombs, belfer center, nuclear terrorism, terrorism, nuclear energy, energy, Atomic, Destruction, Weapons, Nuclear Power (Industry)

Id: zVhQOhxb1Mc

Length: 65min 28sec (3928 seconds)

Published: Tue Sep 10 2013

Reddit Comments

[u/WeCameAsBromans]

The first thing you need to know about nuclear energy is how to use it to make a bomb.

[u/prof_rattigan]

Thank you very much for posting, highly informative and interesting.

[u/enferex]

If I wasn't on a list before, I certainly am now.

[u/OceanFury]

Excellent lecture. Thanks for posting.