Getting to the Good Stuff (Uranium Enrichment)

you it turns out that uranium has two isotopes uranium 238 represents ninety nine point three percent of all uranium uranium 235 is only 0.7% but it's this uranium 235 that is the useful thing for fission it's what we call fizz aisle and Physiol means any speed of neutron can make it fission thermal neutrons the ones you slow down with a moderator make it fission at a very high rate so this is what's useful as a fuel or this is what's useful to make a nuclear bomb uranium 238 is pretty boring it has a long half-life the half-life of this is four and a half billion years guy ten to the ninth right half-life of uranium 235 is only 0.7 billion years compared to short half-life materials these are fairly not radioactive because of their long half-lives but it also means that the plentifulness of uranium 235 over history has clearly changed but on our time scales of human activity this is our ratio 99.3% to 0.7% the thing is that this is how it occurs naturally if you want to run a reactor you need three percent enriched uranium 235 which will mean that uranium 238 is at 97% this is reactor fuel the process by which you get from the natural abundance to the reactor fuel is called enrichment and I should mention because it's in the news if you wanted to make a nuclear bomb you actually have to get up to 90 percent uranium 235 which of course leaves the rest of it the 10% as uranium 238 the same enrichment scheme you use to take naturally-occurring or to reactor fuel you could keep running many many more months or maybe years and reach nuclear bomb grade material the thing about this is that uranium as we've talked about is a plentiful natural substance so the key to making a bomb is being able to get it to this level of enrichment once you have it the knowledge of how to make a nuclear bomb is actually quite widespread gatekeeper to people or really countries having nuclear weapons is the ability to enrich it so let's go back in time to where it was first discovered that you could make a nuclear chain reaction and then of course that perhaps this could be something actually used in wartime World War two it was the genesis for creating the Oak Ridge National Laboratory enrichment is difficult because uranium 238 and 235 are the same chemical remember every one of these uranium's has 92 protons that's what makes it uranium and 92 electron the difference is that the 238 has a hundred and forty six neutrons versus 143 neutrons for the uranium 235 the only difference is this less than 1% master and we somehow have to utilize that mass difference to be able to enrich it enrichment is a huge effort in fact in the 1940s the u.s. created in part Oak Ridge National Lab for this very purpose now this is a modern picture but still the National Lab was nestled in the middle of the Tennessee mountains about an hour away from Knoxville being a place where they could enrich uranium there's another aerial picture of the lab showing you it's kind of deserted around there it's also in some beautiful hilly area and the convenient power feed away from massive amounts of electricity the Tennessee Valley Authority hydroelectric dams created to power the gaseous diffusion separation plant at Oak Ridge you see the idea to use this giant plant was somehow to utilize the mass difference between the two isotopes of uranium first to do that you have to turn it into a gas so the uranium fuel was made into the gaseous uranium hexafluoride and this dilutes the mass difference even further because you've got the fluorine molecules right so now our mass difference is exceptionally tiny but u-238 f6 is slightly heavier then uranium-235 f6 very slightly if I use diffusion diffusion happens proportional to one over the square root of the mass so a lighter substance will diffuse slightly faster than a heavy substance this was the thought of the gaseous separation plant at Oak Ridge basically take a filter and put something through it and the heavier stuff won't go through quite as fast so you need to build a lot of machines because the separation factor using gaseous diffusion between the two different masses is only one point zero zero four three four thousandth of 1% increase in separation and after all we're trying to get from point seven percent if we're making a bomb all the way to 90 percent and every stage only improves you pipe this much here's a picture of a gaseous diffusion system at Oakridge these are large large containers and there aren't just four of them there are thousands let's look at a diagram to see how this works so the high-pressure gas that uf6 comes in and these are all your little coffee filters your diffusion membranes and of course what's going to diffuse faster something that's slightly less massive the u-235 so here would be the enriched stream and here would be the depleted stream not by some huge factor remember this 1.0 4 4 3 that means if it starts at 0.7 right the depletion and the enrichment fraction goes to something like well multiply that times 0.7 we've got something like 0.7 oh oh three and point six nine nine seven through one stage then you would take this enriched stuff put it through another system like this and it would go up a little bit further and a little down further you have to take the stuff that was D enriched and put it back into the first one and eventually through massive numbers of these machines and years of doing this you could get up to the desired enrichment this was 1940s technology what about today today we go to something much better called a gaseous centrifuge here are pictures of centrifuges and as you might imagine and how this demo shows if you spin something the heavier part goes out to the side if the heavier part goes out to the side the lighter part stays in the middle these centrifuges spin like tops here's another picture and you need many of them of course but their volume is nowhere near as massive as the volume of the types of gaseous diffusion a diagram that shows how they work is again a very similar principle that you feed the gas in and that as this spins in a circle the heavier substance comes to the outside of the cylinder that would be the u-238 the depleted uranium and from the center of the column you get the lighter gas that is the enriched u-235 the separation factor for a gaseous centrifuge can be as high as 1.5 that's great you go from 0.72 all the way up to something that's like 1 right and down to something like 0.4 or so all in one step many fewer steps are needed to be able to get up to the enrichment level that you would like that also brings us to a very timely subject and that's the subject of Iran Iran has centrifuges to enrich fuel according to Iran they want to have a domestic nuclear power program and they need to make their 3% level reactor fuel great the worry of the West is that they will continue running the centrifuges and they will get up to the 90% level needed to make a nuclear bomb they say no of course we're not going to do that the West says ok we'll let us inspect and even if there's an agreement there's a worry that is there some diffusion plant one doesn't know about is there some centrifuges you don't know about are they really being truthful have they really already perhaps made the 90% enrichment material I'm not a political scientist I'm not teaching a course on politics I don't know what the answers are but I want you to understand technical things that are involved making a gas centrifuge that can get this type of separation factor is not trivial at all it's extremely high precision device that needs to run extremely tight tolerances but if you can enrich your fuel from the point seven percent to the three percent reactor with enough time and money and the very same machine you can take it from the 3% to the 90% that's what you need to know about enrichment [Music] you [Music]

Channel: Illinois EnergyProf

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Length: 12min 31sec (751 seconds)

Published: Tue May 14 2019