Talk:Fissile material

Clean-up ?

The article needs clean-up?? It looks fine to me now, and especially since the main concern re: "fissile" is in stuff that you can manufacture in large quantities, like we can do for U-233, U-235, and Pu-239. Does anyone know of any more isotopes?? 74.249.92.92 (talk) 06:57, 28 August 2008 (UTC)

Yes, it does. It fails to list all fissile isotopes. Since there aren't that many of them, listing them all isn't hard. I could edit it if I would be reasonably sure I'm getting it right, but I don't. — Preceding unsigned comment added by 209.132.186.34 (talk) 12:48, 4 August 2013 (UTC)

How is 244 an odd number??? :-)

“Several … isotopes are known to be fissile, all of them having … odd atomic mass numbers. These include: … Curium-244[2]”

I do not know the subject enough to relate oddness to fissilitude and, therefore, to fix the article. Oddly, my degree is in physics; I recall no rule postulating that only odd-weighing isotopes be fissile (no implication that there is no such rule). The reference [2] mentioning 244Cu as a fissile material is but a safety instruction, and it is OK for such documents to err on the “safe” side, so this reference is not convincing enough to me for a blank-minded edit.

— [1] 11:57, 11 February 2006 (UTC)

“Several other transuranic isotopes are known to be fissile, all of them having… even atomic numbers…. These include: Neptunium-237[1] … Americium-241”

• The New Math at work again?---Dah31 02:50, 20 April 2006 (UTC)

None of the nuclei ²⁴¹Am, ²³⁷Np and ²⁴⁴Cm is fissile. All of them undergo fission after irradiation with fast neutrons, but not with thermal neutrons.

Furthermore, only ²⁴⁴Cm has an even mass number (244) and an even atomic number(96). Both ²³⁷Np and ²⁴¹Am have an odd atomic number (93 and 95 respectively) and an odd mass number.

--Pgrouse 11:04, 5 October 2006 (UTC)

Now, the article is stating that the "rule" for even N and odd # of neutrons is an "in general" rule - in other words, a "rule of thumb". I known as an engineer (M.S. Georgia Tech) that rules of thumb do tend to have exceptions - and sometimes, lots of them... 74.249.92.92 (talk) 06:57, 28 August 2008 (UTC)

Also, considerations of fissibility for higher-order or hard-to-make actinides are not too relevent here, because they are impossible to collect large amounts of for making nuclear reactors and bombs.74.249.92.92 (talk) 06:57, 28 August 2008 (UTC)

Heavier actinides do accumulate in nuclear reactors, many are long-term radioactive waste hazards, and fission is the only way of destroying them. Look at Curium#Nuclear fuel cycle.--JWB (talk) 07:41, 28 August 2008 (UTC)

The OE isotopes are unbalanced and less stable, and are fissable because they can be caused to fission by thermal neutrons. The EE isotopes are balanced and more stable (long halflived) and can only be fissioned with fast neutrons (>14Mev), and therefore are fissionable>WFPM (talk) 02:11, 21 April 2011 (UTC)

What does fissile actually mean?

The word fissile can be used to refer to any item which can be split. For example it is used in geology to refer to slates and shales. Should the definition be limited to the specific case of nuclear fission?? —Preceding unsigned comment added by 155.198.91.107 (talk) 16:06, 4 March 2008 (UTC)

Well, now, the article is saying now at the top that it is an article about nuclear physics and nuclear engineering. Thus, for some other meaning of "fissile", you need to write your own article. 74.249.92.92 (talk) 06:57, 28 August 2008 (UTC)

I have two comments on this point. First. this is about "fissile material" or "fissile isotopes." Perhaps the title could be changed accordingly. Second, there is another meaning of the term "fissile material" to mean material that can be used to make a nuclear weapons by supporting a fast fission chain reaction. In other words, the defining factor the ability to produce fission not with neutrons near zero energy but with neutrons as they emerge from a fission. I think it would be worth having a section reflecting this alternate definion. By this definition, any material with a bare critical mass is "fissile." That includes, for example, Np-237 and Pu-238.

Suggest to any nuclear engineer that any material with a bare critical mass is fissile and they'll probably laugh in your face. But it's a widely held belief, and Wikipedia has probably been part of the problem.

I have not yet discovered any authority for a good definition of fissile, despite asking many nuclear engineers (having worked for the AAEC in the past I still know some) and consulting every text they recommended. But both the term and the odd-even principle have been introduced in the very first chapter of most. It's a basic concept of both reactor and weapon design, and well understood but poorly defined. If that seems contradictory perhaps it's because we are speaking here about engineering rather than science! Andrewa (talk) 21:03, 3 May 2010 (UTC)

As well as I know, in the earliest years, it wasn't yet known that U235 would fission with prompt neutrons. Also, the number of prompt neutrons wasn't known. In most cases, more energy is better than less, but not always. In any case, as far as I know, anything that will fission with thermal neutrons will also fission with prompt neutrons. Prompt neutrons, even ones that start out with enough energy to fission U238, can quickly lose that energy. That is why U238 isn't fissile. On the other hand, any nuclide that will fission with thermal neutrons will also fission with prompt neutrons, though the cross section is lower. Gah4 (talk) 01:26, 25 January 2019 (UTC)

Prompt and delayed neutrons both start out as fast neutrons, and can be turned into thermal neutrons by a moderator. Prompt and thermal neutrons aren't opposites. Andrewa (talk) 13:59, 22 August 2020 (UTC)

In about 1943, the fission cross sections were not known so well. Fermi showed a moderated chain reaction, which didn't prove that a non-moderated bomb would work. Bombs work with prompt neutrons, reactors with moderated and delayed neutrons. Los Alamos had to do the measurements of prompt, or fast enough, neutron fission, and also the prompt neutron production value. Experiments like tickle the dragon's tail are much discussed, but not so much what they were measuring. In any case, the distinction I was trying to make is bomb vs. reactor. Gah4 (talk) 20:27, 22 August 2020 (UTC)

The distinction between a bomb and a reactor is simply that you want the bomb to go prompt-critical before it vaporises itself, otherwise you get a fizzle. You don't ever want a reactor to go prompt-critical. Yes, that has implications in the neutron spectrum you want. But there are fast reactors and thermal neutron reactors, and the PWR is only partly thermalised these days.

There's a lot going on. The Manhattan Project guys were brilliant, and made some brilliant guesses. My advice is, don't even try to understand their thought processes. They did make some mistakes, as did later programs with equally brilliant teams. Castle Bravo was the most spectacular, and there is still a smokescreen of secrecy and deliberate misinformation around it. Andrewa (talk) 00:53, 23 August 2020 (UTC)

I think I have resolved that confusion. See http://alderspace.pbworks.com/w/page/141055923/fissile%20and%20fissionable for my latest analysis. I confess I am not a nuclear engineer and have lost my best source of information with the death of my father who was, but I have some of his library! Andrewa (talk) 13:59, 22 August 2020 (UTC)

I've been surprised to see confusion about the definitions of fissile and fissionable recently. These terms are clearly defined and well understood in the field of nuclear engineering. The textbooks I used all use the same definition of fissile: a nuclide for which fission can be induced by a neutron with zero kinetic energy (Introduction to Nuclear Engineering, 2nd Edition, by John R. Lamarsh, page 64). Fissionable means that an atom can be fissioned, but a positive threshold energy exists (Lamarsh, page 65). I never saw any different definition used in college (NE major) or in my career as a nuclear engineer. Echawkes (talk) 19:51, 1 April 2023 (UTC)

Seems that the book is about 2001. As well as I know, the distinction between fissile and fissionable is relatively (to 1939) recent. There might be some years where both words were used, but not completely defined to be different. I haven't found a WP:RS with the exact date, though. From some sources, it looks like it was during writing of some treaties where it was specifically defined. Gah4 (talk) 06:57, 2 April 2023 (UTC)

The book I cited is the 2nd edition, published in 1983. You probably looked at the 3rd edition, published in 2001. The definitions are the same, but they are on page 77 in the 3rd edition.

The earliest source I have found where a clear distinction is made in the definitions is in 1961.

Blizard, E. P., Editorial, Nuclear Science and Engineering, Vol. 9, 1961, pg. i.

His definition is similar to Lamarsh.

Blizard's proposal seems to have caught on: apparently the American Nuclear Society was recommending those definitions by 1965

Physics of Nuclear Kinetics, G. Robert Keepin, Addison-Wesley Publishing Company, Inc., 1965, Footnote on p. 3.

which says:

“ *Regarding the use of “fissile” and “fissionable,” we shall adopt the convention recommended by the American Nuclear Society. Thus, “fissile” will herein refer to those heavy nuclides which can be fissioned by thermal neutrons (notably U235, Pu239, and U233), while “fissionable” refers generally to all heavy nuclides which can be fissioned by whatever means. Echawkes (talk) 23:08, 2 April 2023 (UTC)

Amazon makes it hard to find the right edition and year. They call it "subsequent edition" without saying the number. But yes, if you find the right one it is 1983. It still seems that even if the ANS agreed early, it took longer to get accepted by everyone, especially through physics. Might see that IUPAP says (said) about it. And even after it is officially accepted, many people use the wrong one out of habit. (That is, in general, not necessarily in this specific case.) Gah4 (talk) 01:05, 4 April 2023 (UTC)

It seems to me the issue isn't "confusion" about the definition but different communities using different definitions. The term is used differently in the context of weapons and reactors. The arms control community uses "fissile material" to refer to nuclear material (typically not isotopically pure) that can be used in a fission-based nuclear weapon, i.e. material that can sustain a chain reaction using fast prompt neutrons. Nuclear engineers use the term in the context of reactor design to refer to the specific isotopes that fission readily with thermal neutrons. NPguy (talk) 17:07, 5 April 2023 (UTC)

One of the first things they had to learn at Los Alamos, was the effect of fast neutrons on U-235. Presumably it didn't seem likely, but it might be that U-235 wouldn't fission with fast neutrons. Though the cross section is a lot smaller. Note, for example, the Los Alamos use of 25 and 49 for describing the material, so there is no need for the words. In any case, it does seem to have taken longer than expected for the words to have unambiguous meanings. In addition to reactors and weapons, there is also the physics community. Gah4 (talk) 19:04, 5 April 2023 (UTC)

The definition of "fissile" is actually very rigorous, and very understandable at the same time. Whenever an atom nucleus absorb a free neutron, if the energy released by the capture (i.e. the neutron bond energy for the +1 isotope of the same nucleus) is higher than the fission barrier energy for the same nucleus, then that nucleus is said to be fissile, otherwise it is only fissionable. As you can see, the cross section is not even a parameter for the definition of "fissile", nor the ability of that nucleus to sustain chain nuclear fission. Given that simple definition, there are many isotopes that are fissile, even light ones, like He-3, Li-6, or B-10. These isotopes are not able to sustain a nucleare chain fission simply because they do not have enough neutrons to be released to continue the chain fissions, although their fission cross sections are huge, and fissionate releasing a very large amount of energy. For example, Li-6 fission releases about 80% of the energy released by the fission of U-235 (per weight unit). Other heavy isotopes are capable of nuclear chain fission even if their are not fissile, like U-234, Np-237, Pu-238, Pu-240, Pu-242, Pu-244, Am-241, and so on. Those nuclides are said to be "fissiBle", unofficially. MartinN2 (talk) 13:11, 25 July 2024 (UTC)

Fissile is supposed to mean fissions with zero energy (commonly 0.025eV) neutrons. The complication is that fission neutrons come out with significant energy. Some might have enough energy to fission another nucleus. For U238, though, even an infinite sphere is sub-critical. That depends on the fission cross section and elastic scattering cross section. For odd Z like Np, fissile is normally even mass, so odd N. I didn't look up the fission cross section curve for Np237, though. Gah4 (talk) 20:11, 25 July 2024 (UTC)

Np-237

Neptunium-237 can sustain a chain reaction with fast neutrons, but rarely fissions with slow neutrons. Does this mean it is fissile or not? This also contradicts a sentence in the article stating that all fissile materials can sustain a chain reaction with both fast and slow neutrons. --JWB (talk) 05:16, 12 March 2009 (UTC)

In the technical sense Np-237 is not fissile, since it does not have a large fission cross section for thermal neutrons. But "fissile" material is commonly understood to mean material you can make a bomb from - i.e. that supports a fast fission chain reaction. In that sense NP-237 is fissile. NPguy (talk) 19:23, 12 March 2009 (UTC)

Np-237 is NOT fissile, since the definition of fissile is very rigorous. Whenever an atom nucleus absorb a free neutron, if the energy released by the capture (i.e. the neutron bond energy for the +1 isotope of the same nucleus) is higher than the fission barrier energy for the same +1 isotope, then that nucleus is said to be fissile, otherwise it is only fissionable. Since for Np-238 (+1 isotope for Np-237) the neutron bond energy is lower than the fission barrier energy, Np-237 cannot be considered as fissile. In fact, it is said to be "fissiBle", unofficially. FissiBle means "an isotope that is NOT fissile, but with a critical mass". MartinN2 (talk) 13:20, 25 July 2024 (UTC)

What do we say in the articles then? And are there sources for either or both of the conflicting definitions? --JWB (talk) 03:27, 13 March 2009 (UTC)

I added a line to this article. See what you think. NPguy (talk) 09:08, 15 March 2009 (UTC)

Good, but we need to address the second sentence of the article, and get references for each legitimate definition of "fissile". Also, is there commonly used terminology disambiguating being able to sustain thermal and fast neutron chain reactions?

Almost all transuranics seem to have a critical mass - see Critical mass#Critical mass of a bare sphere.--JWB (talk) 12:44, 15 March 2009 (UTC)

The definition of "fissile" is actually very rigorous, and very understandable at the same time. Whenever an atom nucleus absorb a free neutron, if the energy released by the capture (i.e. the neutron bond energy for the +1 isotope of the same nucleus) is higher than the fission barrier energy for the same nucleus, then that nucleus is said to be fissile, otherwise it is only fissionable. As you can see, the cross section is not even a parameter for the definition of "fissile", nor the ability of that nucleus to sustain chain nuclear fission. Given that simple definition, there are many isotopes that are fissile, even light ones, like He-3, Li-6, or B-10. These isotopes are not able to sustain a nucleare chain fission simply because they do not have enough neutrons to be released to continue the chain fissions, although their fission cross sections are huge, and fissionate releasing a very large amount of energy. For example, Li-6 fission releases about 80% of the energy released by the fission of U-235 (per weight unit). Other heavy isotopes are capable of nuclear chain fission even if their are not fissile, like U-234, Np-237, Pu-238, Pu-240, Pu-242, Pu-244, Am-241, and so on. Those nuclides are said to be "fissiBle", unofficially. MartinN2 (talk) 13:15, 25 July 2024 (UTC)

[2], one of the references in Critical mass, uses "thermal/fast spectrum fissile actinide nuclides". --JWB (talk) 13:12, 15 March 2009 (UTC)

The whole "odd Vs. Even" section

hello all,

Did anyone notice that Jeopardy had a question last week relating the most important criteria in selecting U-235 was that it had an odd number of neutrons???

That whole section of the article is CONFUSING and, like someone said above, sounds way to much like a rule of thumb.

I am doing some research to try and clarify this point. Right now, from reading "Introduction to Nuclear Engineering" By J.R. Lamarsh (Prentice hall 3rd Edition) I can find no equivalent explanation for fissile behaviour and odd number of neutrons. He relates it solely to the number of neutrons per fission relased in a fuel mixture and the ratio of absorption to fission cross sections (See equation 3.51). of course, the underlying physics could still deal with # of nuetrons which is the point I am still researching.

In any case one line that DEFINITELY should be changed is the line about " More generally, elements with an even number of protons and an even number of neutrons, and located near a well-known curve in nuclear physics of atomic number vs. atomic mass number are more stable than others - and hence, less likely to undergo fission"

The well known line in nuclear physics is not a even to even neutron/ proton line... the line is # neutrons = # protons. As the Mass of the nuclei increases the "stable line" deviates further and further away from this line... regardless of if you have even or odd number of neutrons. what the line indicates is that as mass increases you need much more neutrons per proton in order to maintain stability. (i.e. you need the nuclear force to overcome the electromagnetic repulsion of the positeveloy charged protons). This includes isotopes with both even and odd numbers of neutrons thus it is not clear to me what point the article is trying to make. Do you mean closer to the line relative to isotopes of the same element?

Anyway, thank you. Sorry for the spelling... A.L. Daley —Preceding unsigned comment added by 192.75.48.150 (talk) 12:30, 25 May 2009 (UTC)

Let's just hypothesize that An E isotope starts out with even number of proton/neutron pairs, so it can be balanced or not, depending on the number of extra neutrons. With an even number of extra extra neutrons you then have a relatively balanced EE type isotope which is the least unstable. However with an odd number of extra neutrons, you then have an unbalanced and therefor more unstable EO type isotope. In the case of an O type isotope, you start out with an odd number of proton/neutron pairs, which is unbalanced, and which cannot be completely balanced with extra neutrons, but can be less unbalanced with an odd number of extra neutrons (making an OE isotope) than it can with an even number of extra neutrons, (making an OO isotope). Of the 255 stable isotopes, approximately 150 are EE's, approximately 55 are OE's, and approximately 50 are EO's. So it's significant.WFPM (talk) 02:29, 21 April 2011 (UTC)

Selecting would have been from available isotopes in quantity. Even Z actinides have longer half life, which is why we have them. Odd Z odd N actinides are also fissile, but not available in quantity. So, only EO isotopes are reasonable for use in reactors or bombs. Gah4 (talk) 01:34, 25 January 2019 (UTC)

The whole "odd Vs. Even" section PART 2

Further research has told me that the odd-even concept in the article has some validity.

In the Lamarsh reference above see section 2.12 Nuclear Models, "Liquid Drop Model".

However, I beleive the concept as it relates to Fission (and thus fissile materials) is incomplete. I beleive a discussion of the formation of a Compound Nucleus, Binding Energy, and Fission is required... See Section 3.7 of Lamarsh "Fission". particularily useful is the table of "Critical Energies for Fission" is very useful. In this table, the change in Critical Energy as one alternates between even and odd numbers and the effect that this has on an isotope being fissile or fissionable is much mroe relavant than the generalized disucssion on stability that is now included in the article.

Thoughts?

A. Daley —Preceding unsigned comment added by 192.75.48.150 (talk) 13:21, 25 May 2009 (UTC)

http://www.tpub.com/doenuclearphys/nuclearphysics26.htm

Seems to be an appropriate discussion of the phenomenon... I think it is a lot less confusing to disuss energy first, then bring in the odd-even concept. —Preceding unsigned comment added by 192.75.48.150 (talk) 13:30, 25 May 2009 (UTC)

Quantum mechanics likes paired fermions. More stable molecules have an even number of electrons. And more stable nuclei have (separately) even numbers of protons and neutrons. So, there is extra binding energy released when an odd N nuclide gets the next neutron. Also, odd N nuclides tend to have a large cross section for slow neutrons. Gah4 (talk) 07:07, 2 April 2023 (UTC)

Requested move 01 February 2014

The following discussion is an archived discussion of a requested move. Please do not modify it. Subsequent comments should be made in a new section on the talk page. Editors desiring to contest the closing decision should consider a move review. No further edits should be made to this section.

The result of the move request was: Move. Cúchullain ^t/c 23:31, 10 February 2014 (UTC)

---

Fissile → Fissile material – (1) "Fissile" is an adjective, not a noun or noun phrase; (2) the article's subject is material suitable for atomic fission, not cell division, secession, etc. Sardanaphalus (talk) 16:24, 1 February 2014 (UTC)

Note a related previous discussion here. —  AjaxSmack  18:07, 2 February 2014 (UTC)

Survey

Feel free to state your position on the renaming proposal by beginning a new line in this section with *'''Support''' or *'''Oppose''', then sign your comment with ~~~~. Since polling is not a substitute for discussion, please explain your reasons, taking into account Wikipedia's policy on article titles.

• Support the addition of some noun. The current title is far too enigmatic. —  AjaxSmack  18:07, 2 February 2014 (UTC)

• Support the proposed redirect to "fissile material." It would be useful to link this to an updated version of article Nuclear material to include such IAEA terms as indirect use material, direct use material, and the irradiated and unirradiated forms of all such materials. Some distinction should be made in that article between "fissile" and "fissionable" material. NPguy (talk) 18:12, 2 February 2014 (UTC)
• Support move as proposed. - WPGA2345 - ☛ 03:39, 5 February 2014 (UTC)
• Support. The addition of the noun is better grammar, and more clearly describes the scope of the article. The two words are already used together in the lede. --BrownHairedGirl (talk) • (contribs) 14:57, 9 February 2014 (UTC)

Discussion

Any additional comments:

Note that the target has a significant edit history, as the material from it was merged to this article. This history should be preserved for copyleft reasons. Andrewa (talk) 17:59, 9 February 2014 (UTC)

The above discussion is preserved as an archive of a requested move. Please do not modify it. Subsequent comments should be made in a new section on this talk page or in a move review. No further edits should be made to this section.

Brief discussion of radioactivity would be helpful here

I had a look at this article, and tried to wrap my head around the meaning of "fissile" and its relationship to "radioactive". I understand that they are very different (if related) events, but after making an attempt to write it down, realized that I just don't have the vocabulary at my fingertips to correctly explain the difference. Am putting in this section here on the talk page to request that someone more eloquent in nuclear physics than I give it a go. I think it might go well under the heading "Fissile vs. fusionable" (perhaps as "Fissile vs. fusionable vs. radioactive"?). Not that it needs this, of course, but for the the lay person, things just get confusing rather quickly at the quantum level. Please consider the request. Keep it simple. Thanks! KDS4444^Talk 14:58, 28 November 2015 (UTC)

U-235 half life is about 700 million years, so isn't all that radioactive. The physics of fission is different enough that it isn't impossible that there would be non-radioactive nuclides that would fission with low energy neutrons. They have to be close enough to unstable, though, so that the math comes out that they are radioactive. Gah4 (talk) 01:13, 21 October 2020 (UTC)

Two

Release two or more neutrons on average per neutron capture On average, it just needs to be enough above one to sustain the chain reaction. It doesn't seem that two is so special, though. At least some have to release two or more for the average to be above one. Gah4 (talk) 14:56, 3 June 2016 (UTC)

NRC fissionable

From a recent edit, referencing the NRC, fissionable is: A nuclide that is capable of undergoing fission after capturing either high-energy (fast) neutrons or low-energy thermal (slow) neutrons. Seems to me that this is one of those you know what they meant, but not what they said. U238 is not capable of fissioning with slow neutrons, but is with fast neutrons, and that is the most confusing way to say it. As far as I know, there are no nuclides that will fission with thermal neutrons, but not with fast (prompt) neutrons, so there is no need to make the distinction what this quote seems to be making. Gah4 (talk) 01:43, 25 January 2019 (UTC)

OK, I put in a comment on the NRC web site comment page. We will see what they say about it. Gah4 (talk) 03:16, 25 January 2019 (UTC)

And again, you are confusing the speed of a neutron with the timing of its release. Andrewa (talk) 00:37, 23 August 2020 (UTC)

Well, first, I am quoting NRC, so are you saying that I am confusing them, or the NRC? In any case, not confusing, just ignoring the difference. Bombs need prompt because it will be gone before delayed arrive. Bombs need fast because slow is too slow. In the early days, Los Alamos had to verify that U235 would fission with fast neutrons, just to be sure. As we know now, it does. Gah4 (talk) 01:06, 23 August 2020 (UTC)

Gah4, have a look at http://alderspace.pbworks.com/w/page/141078537/fissile and comments welcome.

Now the problem of course is that I'm not a "reliable source" and the NRC is. But they have stuffed up on this occasion in a couple of ways.

If you'd like to read and comment here on what I say there, I'd appreciate it.

In particular, https://www.nrc.gov/reading-rm/basic-ref/glossary/fissile-material.html (archived at https://web.archive.org/web/20200824102305/https://www.nrc.gov/reading-rm/basic-ref/glossary/fissile-material.html if they've changed it) says that natural Uranium is not fissile. That's a confusing oversimplification. The U-235 in natural Uranium is fissile, and this is why natural Uranium can be used to fuel a nuclear reactor and was at Chicago Pile-1 and the B-reactor and many others that I list.

It also implies that a thermal reactor cannot be used to produce fissile Plutonium. That's just not true. It can't breed Plutonium, you always end up with less fissile material than you started out with if you count the U-235 that is consumed. You do need a fast reactor with its superior neutron economy to breed fissile material, but you can produce Plutonium in a thermal reactor, and the Manhattan Project and other reactors (that I also list) did exactly that.

Why did they do that, if they were consuming more fissile material than they were producing? Simply because the Plutonium could be purified by far simpler means than the U-235 could be. The bomb of whatever design requires fairly pure fissile material. Andrewa (talk) 13:25, 24 August 2020 (UTC)

I suppose I agree with LA-UR-04-6514 that it isn't well defined. The two terms have been used in different ways by different people (organizations). But the whole point of this discussion is that the article said something similar. I don't think we can say whether NRC or DOE (Los Alamos) are a more WP:RS. It seems to me that most often the distinction is made in terms of bomb making ability, so that agrees with NRC. (You can't make bombs from natural uranium.) Then there is politics. I almost remember when it was decided that the US would avoid reprocessing, and so also fast breeder reactors, which was mostly for political reasons. Well, also, originally it was thought that uranium was rare, and needed to not be wasted. After some time, it was found to be not so rare, though the highest quality ores are rare. There is enough uranium around to support not reprocessing for many years. (Besides that someone could go extract it from old reactor waste.) Gah4 (talk) 22:04, 24 August 2020 (UTC)

The definition that I've most commonly seen in the nuclear engineering community (as a nuclear engineering PhD student) is the one used by Duderstadt and Hamilton (p. 55). It uses a physics definition—that a fissile nucleus is one where the binding energy released by the absorption of a neutron is greater than the activation energy. More broadly, any nucleus that may undergo fission is fissionable. Using this definition has the nice benefit that it relies on the math, and so there is no ambiguity as to what is or isn't fissile/fissionable. Mnegus01 (talk) 23:19, 20 October 2020 (UTC)

The source you cited (D+H) says, "... nuclides that can be induced to fission with neutrons of essentially zero kinetic energy ... are referred to as fissile nuclides." Echawkes (talk) 01:24, 16 April 2023 (UTC)

Fissile and fissionable... formerly synonyms?

Article currently currently reads Although the terms were formerly synonymous... but no reference is given.

I'm dubious. The term fissile has always been a technical term, coined to distinguish materials that can sustain a fission chain reaction from those that can't.

The two terms have often been confused by people who think they know more than they do, and still are. That doesn't make them synonyms! But to state that they are or have been synonyms does serve to conceal the ignorance of these people, and even encourage such ignorance. Is that the idea? Andrewa (talk) 23:40, 18 August 2020 (UTC)?

I have added a dubious template. I intended to add a a simple unsourced or citation needed template but then noticed that the dubious template is OK in cases such as this. Template:Dubious/doc#Incorrect uses reads in part to flag unsourced statements, unless you think they are incorrect. (my emphasis)

That is exactly the point here. The statement is not only unsourced, it is subtly politically charged, and likely to prove to be complete rubbish. Andrewa (talk) 23:55, 18 August 2020 (UTC)

If you go back to the 1930's and 1940's when this was being figured out, they didn't have separate names yet. I am not sure when the specific names started being widely used, but it seems worth finding out. Gah4 (talk) 00:18, 19 August 2020 (UTC)

So, you think that fissile was originally used as a synonym for fissionable? But why then would they invent a new and rather strange term such as fissile? Particularly as the difference between fissile material (U-235 and Pu-239 in particular) and non-fissile material (U-238 and Pu-240 in particular) was so vital, and so relevant to the work going on at the time? Andrewa (talk) 02:02, 19 August 2020 (UTC)

OK, this one from the NRC explains exactly that. That they were once synonyms, sometimes are still used that way, but changed meaning. It doesn't say when. Gah4 (talk) 00:21, 19 August 2020 (UTC)

Great source! A bit enigmatic, but should be added to the article as a ref for the statement I have flagged. For now anyway.

One of the problems is that there are two possible and equivalent definitions of fissile. Fissile nuclides, and only fissile nuclides, are capable of sustaining a chain reaction. That's their important property. And these are exactly the same nuclides that have a high probability of fission after capturing a thermal neutron, for very good reasons to do with the structure of the nucleus.

This high probability is the reason that they can sustain a chain reaction. But the trap here is, it's not just thermal neutrons that produce this chain reaction... in the case of prompt-criticality in a weapon, they're not even terribly important. It's basically a fast neutron criticality. Even a modern PWR is not fully thermalised, a significant amount of the reactivity is due to fast fission and faster than thermal fission. I think that is confusing for many.

It would be good to get a much clearer explanation in our article, IMO. Andrewa (talk) 02:02, 19 August 2020 (UTC)

U238 fission is a large part of the yield of H-bombs, so it had to be named. Physicists would probably say that they are all fissionable, and give the cross section vs. energy graph. It is politics where the actual naming is important, which I suspect is why the NRC answered it. Gah4 (talk) 03:23, 19 August 2020 (UTC)

The contribution of U-238 to the Castle Bravo test was only recognised after the event. That's a few years after the Manhattan Project.

The term fissile was in regular use for U-235 and Pu-239 well before Castle Bravo. Andrewa (talk) 04:15, 19 August 2020 (UTC)

Reminds me that early on, at least to the beginning of Los Alamos, the verb form was to fish instead of to fission. Gah4 (talk) 07:15, 19 August 2020 (UTC)

Quite possibly it was... that's a natural piece of English slang.

But the term fissile is in a different class entirely. It is used to distinguish those materials that can fission and thereby sustain the sort of chain reaction predicted by Leo Szilard in 1933. At the time he made several guesses as to what materials might support such a reaction, all of them wrong. It was with the discovery of neutron multiplication in Uranium in 1939 that this material was suggested, and it was quickly realised that only the U-235 isotope would support this chain reaction. That's when its property of being fissile became of great interest to say the least! Then in 1940, Egon Bretscher and Norman Feather at Cavendish Laboratory predicted that the newly-discovered Pu-239 would also be fissile, and things really heated up.

I do not know when the term fissile was first used for this property (or for anything else), and it would be very interesting to know. But it seems to me most unlikely that any competent person has ever described U-238 as fissile. Not in good faith, in any case. Not to you?

But there has never been any doubt that U-238 is fissionable. Even if the designers of SHRIMP didn't realise the significance of this until the thing went off at Castle Bravo.

Which makes the NRC glossary you cite a very interesting document indeed. It is indeed a jungle, and DOE have not always been immune to politics; Perhaps the same goes for the NRC. Perhaps that is the explanation. Its wording seems a bit bizarre to me, frankly. Not to you? Andrewa (talk) 16:47, 19 August 2020 (UTC)

As far as I know, physicists were never confused having only one word. Everyone knew pretty soon which one was which. I suspect that separate words were needed for political purposes. It is slightly complicated to trace down, with fissile being a subset of fissionable. The statement U235 is fissionable is not wrong, and doesn't show that the word fissile wasn't in use at the time. Gah4 (talk) 17:36, 19 August 2020 (UTC)

As far as I know, all nuclear engineers have always thought that the difference between fissile and fissionable materials was extremely important. My father was one, and I worked with several others at the Australian Atomic Energy Commission. Having read the history above, you still don't agree? Andrewa (talk) 19:39, 19 August 2020 (UTC)

I was thinking about Los Alamos physicists. Nuclear engineers are a different question. Gah4 (talk) 21:22, 19 August 2020 (UTC)

Interesting to read Los_Alamos_Primer which tells what was known in 1943. There is no fissile, only fissionable. There is a book that you can buy, which has extra annotation, or a free PDF of the original, without the extras. Gah4 (talk) 21:52, 19 August 2020 (UTC)

Great find. See https://ncsp.llnl.gov/docs/LA_UR_04_6514.pdf for mine. Andrewa (talk) 03:36, 21 August 2020 (UTC)

with fissile being a subset of fissionable... I think we may be onto something interesting here. Watch this space! Andrewa (talk) 20:48, 19 August 2020 (UTC)

See my new essay fissile and fissionable for more on this. As I now say there, that isn't inconsistent, but it is clumsy. To say they are synonyms is however inconsistent. Andrewa (talk) 03:47, 21 August 2020 (UTC)

It's a dubious claim, but also unnecessary to the article, so I took the obvious step and deleted it. NPguy (talk) 03:27, 21 August 2020 (UTC)

Thank you. I was reluctant to do that myself for reasons stated above. Andrewa (talk) 03:47, 21 August 2020 (UTC)

The importance of being fissile

No nuclear bomb, fission or fusion, has ever been built or even AFAIK seriously proposed without a critical mass of fissile material. U-235 and Pu-239 and combinations of them have been used to date. U-233 has been used in combination with other fissile material, and in at least one very low yield but successful test.

No nuclear fission reactor, whether fast, moderated, or partly thermalised, can go critical without fissile material in the fuel. The only natural fissile material of any importance is U-235. All fission reactor programs to date have started with this material. Some have used enriched Uranium and some natural, but in either case it's the U-235 that is the fuel.

Our article does not make any of that clear at all, in my opinion. It has some political ramifications, and as a committed advocate of nuclear power I am unwilling to make any change to the article that is controversial. Andrewa (talk) 17:03, 19 August 2020 (UTC)

It seems that some reactors use MOX_fuel, a mix of reprocessed (or previously bomb grade) plutonium and U238. As far as I know, there has been interest in making a fuel with U233 (bred from Th232) and U238, but I don't know that it was ever done for a production reactor. Gah4 (talk) 17:32, 19 August 2020 (UTC)

Yes, MOX is a mixture of Uranium and Plutonium (oxides actually), and is used in some PWRs and BWRs in countries such as Japan which reprocess spent fuel from similar reactors to recycle the Plutonium.

And yes, there is interest in using U-233 and both China and India are committing significant funds to developing the technology to do it. The Th/U fuel cycle is far more difficult for many reasons than the U/Pu one, but Thorium is more abundant and India has lots of it.

Exactly the same principles apply. U-233 is fissile, like Pu-239 and U-235. Th-232 is not, like U-238. And that is of first importance.

One significant difference between Th/U and U/Pu (there are many) is that Th/U needs U/Pu to get started. As I said before, the only natural fissile material is U-235. So India for example is using the U/Pu fuel cycle to build up its inventory of fissile material before switching to Th/U. Or that is the plan.

Th-232 is in fact fissionable, like U-238. But that is not nearly so important. In fact it's even less important than with U-238, for reasons I will not go into here. The important thing is that it's not fissile. Andrewa (talk) 15:11, 21 August 2020 (UTC)

The NRC on fissile

I read at the Nuclear Regulatory Commission glossary (currently at least):

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

This is I think at least part of our problem with the Fissile material article. Watch this space! Andrewa (talk) 21:58, 24 August 2020 (UTC)

As I might have said before, I do agree that this is a strange way to define it. However, since all materials that fission with thermal neutrons can also make bombs, it makes some sense. They could have just said that, though. The cross sections are smaller, and it had to be measured by Los Alamos in the early years, but it is true. Gah4 (talk) 22:10, 24 August 2020 (UTC)

Some of this is true. But let's start with that very last statement This definition excludes natural uranium and depleted uranium that have not been irradiated, or have only been irradiated in thermal reactors. Does that make any sense at all?

Presumably the not been irradiated means, if it has been irradiated and turned into Pu-239, then it's fissile after all. So far so good. But what's that about only been irradiated in thermal reactors? The stuff that destroyed Nagasaki had only been irradiated in thermal reactors. I think the good citizens of that burg might be of the opinion that it was fissile enough! Andrewa (talk) 23:51, 24 August 2020 (UTC)

Gah4, I have just done a complete rewrite of Alderspace:fissile to make it more intelligible to the target reader. There is a lot going on but I think a sound basic understanding is achievable by the average reader, and that is my aim at Alderspace, and in theory it's what Wikipedia aims at too (and what the NRC should be doing too).

Comments (by anyone) welcome. If they are relevant to improving Wikipedia, here is the place for them. Otherwise an email to me is more appropriate (see the link on my talk and user page left sidebar), or there is a contact link on every page of Alderspace, or see Alderspace:contact me and thanks in advance. Andrewa (talk) 08:19, 25 August 2020 (UTC)

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Ronen Rule

Is the Ronen fissile rule actually useful? Looking at Uranium, for example, it tells me U-225 and U-229 are fissile but those are too light and not even included in the discussion of fissile isotopes. It gives us isotopes that are too light and unstable to be useful. 98.183.98.6 (talk) 19:47, 11 May 2023 (UTC)