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fissile and fissionable

Page history last edited by Andrew Alder 1 year, 6 months ago

A page of energy issues

 


Fissile materials

The most important materials in all of nuclear engineering are fissile. That means that they can support a nuclear chain reaction in which a neutron causes the nucleus to split, and in the process release more than one neutron to cause a further fission. In this way the reaction can accelerate, and produce significant heat and a significant output of neutrons for other purposes. When this is achieved it's called criticality. Without fissile materials, there would be no criticality, and no nuclear engineering.

 

There is only one naturally occurring fissile nuclide, Uranium-235 (aka U-235). So again, without U-235 being fissile and available naturally, there would be no nuclear engineering.

 

There are two other important fissile materials, Plutonium-239 and Uranium-233. But both are produced by irradiating a naturally occurring nuclide with neutrons produced in a reactor, Pu-239 from U-238 and U-233 from Thorium-232.

 

So fissile is a very important term, perhaps the most important in all of nuclear engineering.

 

Alternative definitions of fissile

Confusion number one: You will often hear fissile defined as the ability to fission after capturing a thermal neutron. And that's a perfectly acceptable alternative definition, and there are others, and they all end up meaning the same thing for reasons I won't go into here.

 

But to the layperson it is confusing. It doesn't do the term justice unless you know quite a lot about neutron spectrums and capture cross-sections for a start. So it's not in my view nearly as good a definition. It leads to confusion and to failure to understand processes which really are not as complex as some people seem to think.

 

Fissionable materials

While U-238 is not fissile, it can fission after capturing a neutron if the neutron energy is right, and release more neutrons that can cause more U-238 nucleii to themselves similarly fission. In that sense it can achieve a chain reaction of sorts. But it cannot of itself achieve criticality. Ever. It is not fissile.

 

And similarly, Th-232 is fissionable but not fissile. As well as being fissionable, these two materials are also referred to as being fertile, which means that they can capture a neutron and it can turn them into a fissile material, Pu-239 in the case of U-238 and U-233 in the case of Th-232, as already stated above.

 

Reactor design 101

Yes, there is quite a lot going on. Because a reactor is not fuelled with pure U-235, for example. A PWR, the most common and successful power reactor to date, is fuelled with a mixture of U-235 and U-238, sometimes with some recycled Plutonium added (in which case the fuel is called mixed oxide or MOX). Let us just assume pure Uranium fuel for the moment. In a PWR this is oxide of enriched Uranium, which is Uranium in which the percentage of U-235 has been increased to be greater than the 0.7% present in natural Uranium. It is possible to achieve criticality with natural Uranium, but a lot more difficult both to achieve it and to control it, and a PWR doesn't try and would not succeed if it did.

 

In general, the more fissile material in the fuel, the better. (Showing again just how important fissile material is.) So the PWR uses enriched Uranium. And so now do the remaining RBMKs, to prevent instabilities of the sort that blew one up at Chernobyl, and the AGR always has, it was designed to use natural Uranium but that didn't work. And so do the latest CANDU designs despite the "N" originally standing for "natural" reflecting the original design decision to use natural Uranium fuel. It's good stuff.   

 

Anyway, our PWR starts off burning pure Uranium fuel. But not for long. As soon as criticality is achieved, U-238 in the fuel begins to be converted to Pu-239 and it starts to fission itself. By the time the fuel is replaced, up to 70% of the power the fuel produces is being produced by fission of Plutonium rather than Uranium. Pu-239 is fissile, so no problem.

 

And it's not just these two that are fissioning. U-238 is itself fissionable, remember? It requires a faster neutron then U-235 or Pu-239 for that to be likely, but that fast neutron is exactly what fission produces. In a PWR these fast neutrons are slowed down by the water moderator that is there in the core for exactly that purpose (and which also provides the cooling and is used to generate steam to produce power), and when these neutrons reach the same temperature (which is just a measure of the speed of the particles) as their surroundings they are then called thermal and have a much better chance of producing fission in fissile material. That's what a moderator does. But if one hits a U-238 nucleus while still fast and produces fission there, that's fine too. So some of the power produced by a modern PWR comes from fast fission of U-238 too (and also fast, or faster than thermal anyway, fission of U-235 and Pu-239). We say that the reactor is not fully thermalised because a significant amount of its fission is being produced by faster then thermal neutrons.

 

And it's not just U-238 that is absorbing neutrons without fissioning. Almost everything is to some extent! An important example is Pu-239, which doesn't always fission when it absorbs a neutron. Sometimes it becomes Pu-240, which is not fissile, in fact it's a real nuisance. (Well, it has one big plus. Just a little of it makes the Plutonium practically useless for military purposes.)

 

Yes, there is quite a lot going on.

 

So what is the problem

Ah, yes, why this page?

 

Well, you see, some people say that fissile materials are also fissionable. That sounds OK and it's not too bad. It would mean that fissile material was a subset of fissionable material. And that sounds sensible enough. It just gives fissionable its natural and obvious meaning of being able to fission, rather than its alternative technical meaning of being able to fission but not to achieve criticality.

 

I guess that's just a matter of definition. It's a little bit clumsy, we'd now need to say that U-238 was fissionable but not fissile, rather than just say that it's fissionable, because fissionable could by that definition mean that it was also fissile. And of course it's not. But that sort of definition of fissionable is not too bad. It robs us of a very useful technical term, but that's about all.

 

But others say that fissile and fissionable are synonyms. Really! And that is disaster! Because for example U-238 is not fissile, and that is very important, and I think everyone agrees on that. But it is fissionable. And everyone agrees on that too. Or as far as I know they do. So, how can the two terms be synonyms? Unless of course you are speaking pure rubbish. 

 

Ah yes. Politics. Spin. There's a lot of rubbish around.

 

Anyway, I hope you now understand the terms, and the problem, and will be better able to understand what the many people who do use the term fissionable in various ways are saying.

 

See also

 

 

 

 

 

 

 

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