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Page history last edited by Andrew Alder 3 months, 3 weeks ago

A page of energy issues




A nuclide is fissile if (and only if) it can support a fission chain reaction sufficiently to achieve criticality.


In other words, unless you have at least some fissile material, you simply can't have a nuclear_chain_reaction. None at all. Neither in a reactor nor in a bomb. You can and will have fission in some materials. But no chain reaction. So no working reactor, and no working bomb. None at all.


And this is important. It is the most important technical term in nuclear engineering bar none. If you don't understand it then you are simply and profoundly ignorant of nuclear engineering.


That is my definition. It is the one every nuclear physicist and engineer I have ever worked with has used... but that hasn't been a great many I admit, although I did work at AAEC for some years. It's the one my late father, who was one of Australia's most prominent nuclear engineers, used and taught me. I dearly wish he were here to write this.


So why the controversy? Why are there other, competing, confusing definitions? That is a puzzle indeed.  



Curiouser and curiouser 


But https://www.nrc.gov/reading-rm/basic-ref/glossary/fissile-material.html reads in part:


Fissile material: A nuclide that is capable of undergoing fission after capturing low-energy thermal (slow) neutrons.


(and it is archived at https://web.archive.org/web/20200824102305/https://www.nrc.gov/reading-rm/basic-ref/glossary/fissile-material.html if that above link doesn't work or if they've updated it.)


So who is right? Well, they both are. But one is a lot more helpful than the other.


The Problem part 1

I think it's worth citing the whole sorry NRC definition:


Fissile material A nuclide that is capable of undergoing fission after capturing low-energy thermal (slow) neutrons. 
Although sometimes used as a synonym for fissionable material, this term has acquired its more-restrictive interpretation 
with the limitation that the nuclide must be fissionable by thermal neutrons. With that interpretation, 
the three primary fissile materials are uranium-233, uranium-235, and plutonium-239. 
This definition excludes natural uranium and depleted uranium that have not been irradiated, 
or have only been irradiated in thermal reactors. Page Last Reviewed/Updated Monday, June 29, 2020


What's wrong with that? Where to start...!


What's not wrong with it is easier to say! A nuclide that is capable of undergoing fission after capturing low-energy thermal (slow) neutrons is technically accurate.


And that is just about all that's not wrong. But my definition is equally accurate, as is a third alternative based on A, Z and N numbers and binding energy. They all give the same results exactly.


The errors


The NRC page continues and later says: 


This definition excludes natural uranium and depleted uranium that have not been irradiated, or have only been irradiated in thermal reactors.


I'm sorry, but that is not even close. For a start, natural Uranium contains 0.7% fissile U-235, which is why CP-1 and the original CANDU reactors and the MAGNOX and the UNGG and the fateful Chernobyl #4 and Windscale #1 reactors (and #2 of course on which my father was working at the time of the fire) and all Plutonium production reactors of the Manhattan Project were able to go critical with natural Uranium fuel.


It is true that 99.3% of natural Uranium is not fissile, But to say or imply that natural Uranium is not fissile is highly confusing. How did CP-1 and the others go critical if that is true?


To say or imply that a reactor fuelled with natural Uranium can't go critical is just plain wrong. Basic error #1.


But it's a bit like saying that a mixture of sand and sugar is not soluble in water. Is it? Isn't it? What sort of idiot would try to say, either way? No wonder people get confused.


And this natural occurrence of a fissile material, U-235, in natural Uranium is the basis of all reactor programs to date. In theory you might be able to create enough fissile material using a particle accelerator to make a reactor go critical, but nobody ever has or is likely to. And that is very important, especially in understanding one of the obstacles that the Thorium fuel cycle faces. 


And while it's true that the U-238, which is the rest of natural Uranium and even more of depleted Uranium, can be turned into fissile Pu-239 when irradiated in a reactor, any reactor can do this to some extent, and almost any Uranium fuelled reactor does. It doesn't need to be other than "thermal" as this suggests. The MAGNOX and UNGG and Windscale and Manhattan Project reactors all produced perfectly fissile Pu-239, and they were all thermal reactors.


Yes, a fast reactor is in some ways an even better way of producing Plutonium, particularly if the Plutonium is intended for power reactor fuel not bombs. A fast reactor, because of its superior neutron economy, can breed nuclear fuel. A Uranium-fuelled thermal reactor can't; For every atom of Pu-239 that the natural Uranium fuelled Plutonium production reactors produced, they consumed at least one atom of fissile U-235.


(But a Thorium-fuelled thermal breeder is possible, and India hopes to develop one.)  


Ah! So why did the Manhattan Project, and the Brits and the French and the Russians and now the North Koreans, use these Plutonium production reactors, rather than just use the U-235 they burned? 


They used them, most successfully, to produce purified fissile material. A bomb requires highly concentrated fissile material, U-235 and/or Pu-239 (or there has been at least one one successful test, and probably two, using U-233). The Plutonium from a Plutonium production reactor is almost pure Pu-239, the reactor is carefully designed and operated specifically to achieve that, so the Pu-239 can be chemically separated and there's your bomb fuel. The 0.7% U-235 in natural Uranium must be separated from the U-238 to build a bomb, a process called Uranium enrichment. The Manhattan Project did both, but nearly all subsequent bomb programs have used Pu-239. Pure U-235 (called HEU for Highly Enriched Uranium) is just too difficult and expensive to produce.       


That may be what has misled the author of the NRC page, and will similarly mislead its unfortunate readers. The truth is that a thermal reactor will do fine for converting U-238 to Pu-239, or Th-232 to U-233. Just ask the good citizens of Nagasaki whether the thermal reactor produced Plutonium dropped on them in 1945 was fissile.


So to say or imply that natural or depleted or Uranium irradiated in a thermal reactor is not fissile is just plain wrong. Basic error #2.  


And that last bit about "thermal reactors" on the NRC page is pure rubbish. And again, no wonder people get confused.


Food for thought? More to follow!  


The Problem part 2

So that NRC page is riddled with errors that no competent nuclear engineer would make.


But that's not the main problem, or the reason for this page. If it were we would just need to correct those errors, not change the definition.


For a start, we need a term to mean what I (and they) are calling fissile. It's the most important technical term in the whole nuclear literature. And this is the NRC glossary. That's an important place to define such a term.


No, they say Although sometimes used as a synonym for fissionable material... Absolutely gutless! No, it is sometimes wrongly used to mean fissionable. If they wished to promote confusion and stifle understanding and suppress rational discussion, they are already doing very well. See fissile and fissionable.


But that's just an indication of the problem. Think about it. The target audience of the NRC page is the general public, not nuclear scientists and engineers. (And they don't read it anyway, as the failure to laugh it clear off the WWW clearly shows. Its errors have been there for a long time.)


Now, what is it reasonable and important for these people to understand? It's that fissile material can go critical, and other stuff can't. Fissile material can support a fission_chain_reaction and other stuff can't. You can use fissile material... any fissile material... to make a bomb or a reactor core. It just needs to be pure enough... very pure for a bomb, far less so for a reactor (less than 1% will do as many reactors have shown, starting with CP-1). But without it, neither will work. Neither can achieve criticality without fissile material. Nothing else will do. Simple enough? Fair enough?


And that's why fissile material is so important. Fair enough? Simple enough? Are you starting to understand? Does the NRC definition start to make sense? Because at its core it is accurate... the errors are in the irrelevant details. It's just a rather stupid way of trying to explain it. So stupid that if they are trying to confuse people, they are doing very well indeed.


Fissile material can go critical with fast or slow neutrons, prompt or delayed neutrons, all sorts of combinations. And there are reasons for this and that is where the odd/even stuff and binding energy curve come into it. But that is all irrelevant to a sound and basic understanding of what is going on. 


And that's not the only reason that the NRC definition would be far better not to even mention fast or thermal neutrons. The other is, to even talk about fast or slow (thermal) neutrons and what they do to what they hit is in some ways oversimplified. The modern PWR, for example, gets a significant part of its energy from fission produced by faster than thermal neutrons. Neutron capture and its results is not just a matter of is it fast or is it thermal. There is a spectrum of neutron energies present in any reactor (or bomb). Less so in a bomb or unmoderated ("fast") reactor but still significant. There are "resonances", neutron energies at which particular results are particularly likely.


There's a lot going on. Nuclear engineers earn their degrees! 


But the basics are relatively easily understood. And will be more so if the NRC and others who should know better can ever be motivated to talk plain. Or even to check their facts.


The puzzle

The thing that I (and I'm not a nuclear engineer) found hardest to grasp in all this is this: Why is there only one term fissile?


Is it really the same stuff that is needed to make a fast (unmoderated) reactor, or a moderated reactor (thermal or at least partly thermalised), or a bomb, go critical? They are three very different criticalities. And that's where we get into fast and thermal neutrons, and prompt and delayed neutrons. And the more I dig into this the more brilliant Leo_Szilard and Enrico_Fermi and the others appear.


It is. And I think most of us just need to take their word for that. Welcome to the modern world.


The solution

While I believe strongly in nuclear power, I have far stronger faith in education and knowledge. See my page The Wikipedia Creed for more on this. 


If informed public discussion condemns nuclear power, so be it. But I do not think it will. The anti-nuclear arguments do not stand up to even a little homework. See also energy reality.


The NRC, the WNA and probably others have as part of their mission to inform the public. It's about time they took it a bit more seriously! 



Whoever wrote that NRC page thought they knew far more than they did. Whoever chose them to write the page was similarly ignorant. I'm sorry of that's harsh.


Actually it is giving them the benefit of the doubt. The other possibility is that it was deliberately written in order to confuse people. I'm not suggesting that is the case, although others have and it would explain a lot, I'd prefer to believe that Hanlon's_razor is the explanation. But that is bad enough.  


The NRC needs to do better. And so do the writers of the also pathetic (but not nearly so bad) Wikipedia article at Fissile_material. Watch this space!


See also


  • fissile and fissionable
  • the pathetic PR record of nuclear power 
  • https://www.quora.com/Why-is-uranium-238-non-fissionable/answer/Andrew-Alder-8 for a very spirited defence of the NRC definition, (The NRC definition is correct. Yours in not.) by someone who thought that U-238 could sustain criticality without any (other) fissile material being present. They later realised that this is not true, and described the need for (other) fissile material as a detail that they had forgotten. But I don't really blame them for their unflattering assessment of my understanding. It's reasonable to assume that the NRC would do a reasonable job. After all, that's what they're being paid for. The good citizens of the USA should ask for their money back. 




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