Most Common Pu-239 Fission Chain Reaction?

[Mike_Fontenot]

Anyone on this forum know what the highest-probability fission chain reaction of plutonium-239 is?

Mike Fontenot

 

[QuantumPion]

Anyone on this forum know what the highest-probability fission chain reaction of plutonium-239 is?

Mike Fontenot

Are you looking for fission product yield? If so you can see a plot http://www.nndc.bnl.gov/sigma/getFissionYieldsPlot.jsp?submit=Update+Plot&ymin=3.737E-19&ymax=6.084E-2&yscale=lin&ylimits=auto&energy=0&zplot=24&zplot=25&zplot=26&zplot=27&zplot=28&zplot=29&zplot=30&zplot=31&zplot=32&zplot=33&zplot=34&zplot=35&zplot=36&zplot=37&zplot=38&zplot=39&zplot=40&zplot=41&zplot=42&zplot=43&zplot=44&zplot=45&zplot=46&zplot=47&zplot=48&zplot=49&zplot=50&zplot=51&zplot=52&zplot=53&zplot=54&zplot=55&zplot=56&zplot=57&zplot=58&zplot=59&zplot=60&zplot=61&zplot=62&zplot=63&zplot=64&zplot=65&zplot=66&zplot=67&zplot=68&zplot=69&zplot=70&evalid=4615&mf=8&mt=454" .

 

[Mike_Fontenot]

Are you looking for fission product yield? If so you can see a plot http://www.nndc.bnl.gov/sigma/getFissionYieldsPlot.jsp?submit=Update+Plot&ymin=3.737E-19&ymax=6.084E-2&yscale=lin&ylimits=auto&energy=0&zplot=24&zplot=25&zplot=26&zplot=27&zplot=28&zplot=29&zplot=30&zplot=31&zplot=32&zplot=33&zplot=34&zplot=35&zplot=36&zplot=37&zplot=38&zplot=39&zplot=40&zplot=41&zplot=42&zplot=43&zplot=44&zplot=45&zplot=46&zplot=47&zplot=48&zplot=49&zplot=50&zplot=51&zplot=52&zplot=53&zplot=54&zplot=55&zplot=56&zplot=57&zplot=58&zplot=59&zplot=60&zplot=61&zplot=62&zplot=63&zplot=64&zplot=65&zplot=66&zplot=67&zplot=68&zplot=69&zplot=70&evalid=4615&mf=8&mt=454" .

No. I want to know what the most likely IMMEDIATE fission products are. I.e., what are the two large nuclei that are immediately produced by the fission of a Pu-239 nucleus most likely to be? Surely someone somewhere knows the answer ... I'm just amazed that the answer isn't easy to find on the web.

Usually, those two initial large nuclei are very unstable, with half-lives in minutes, seconds, or even less. They will then decay into longer-lived nuclei, as time passes after the explosion.

I didn't find any of those "yield charts" on the web to be useful, because they seem to be mostly concerned with the eventual longer-lived products. That's probably because the issues that usually are motivating those charts are issues of the health effects of nuclear reactions, or issues of nuclear reactor design, or of issues of spent-fuel-rod disposal. The extremely short-lived initial products don't exist long enough to be important in those issues.

Mike Fontenot

 

[QuantumPion]

No. I want to know what the most likely IMMEDIATE fission products are. I.e., what are the two large nuclei that are immediately produced by the fission of a Pu-239 nucleus most likely to be? Surely someone somewhere knows the answer ... I'm just amazed that the answer isn't easy to find on the web.

Usually, those two initial large nuclei are very unstable, with half-lives in minutes, seconds, or even less. They will then decay into longer-lived nuclei, as time passes after the explosion.

I didn't find any of those "yield charts" on the web to be useful, because they seem to be mostly concerned with the eventual longer-lived products. That's probably because the issues that usually are motivating those charts are issues of the health effects of nuclear reactions, or issues of nuclear reactor design, or of issues of spent-fuel-rod disposal. The extremely short-lived initial products don't exist long enough to be important in those issues.

Mike Fontenot

The link I posted is direct fission product yields, not cumulative yields. For example, the highest yield isotope indicated is Zr-100 which has a half-life of 7 seconds.

 

[Mike_Fontenot]

The link I posted is direct fission product yields, not cumulative yields. For example, the highest yield isotope indicated is Zr-100 which has a half-life of 7 seconds.

Thanks ... I'll take another look at your chart.

 

[Mike_Fontenot]

The link I posted is direct fission product yields, not cumulative yields. For example, the highest yield isotope indicated is Zr-100 which has a half-life of 7 seconds.

I looked at your chart again, but I couldn't tell what the highest-probability REACTION was. The chart does show that the most likely smaller nucleus is 40_Zr_100, but what is the larger nucleus in that reaction? The chart says that the most likely larger nucleus is 52_Te_134, but I doubt that the most likely reaction produces those two nuclei (because I THINK that the initial reaction won't involve the conversion of any protons ... so Z will be conserved).

Is there a way to tell from that chart which initial REACTION is the most probable? That's what I'm looking for.

Mike Fontenot

 

[Astronuc]

I looked at your chart again, but I couldn't tell what the highest-probability REACTION was. The chart does show that the most likely smaller nucleus is 40_Zr_100, but what is the larger nucleus in that reaction? The chart says that the most likely larger nucleus is 52_Te_134, but I doubt that the most likely reaction produces those two nuclei (because I THINK that the initial reaction won't involve the conversion of any protons ... so Z will be conserved).

Is there a way to tell from that chart which initial REACTION is the most probable? That's what I'm looking for.

Mike Fontenot

If one goes to the chart of nuclides (as indicated by QuantumPion), and selects 239Pu FY and uses Zoom 1, then one will see the yields. The highest yield is ₄₀Zr¹⁰⁰ (at 0.048) and its complement ₅₂Te¹³⁴ (at 0.044). Two fission (prompt) neutrons would also be emitted.

The proton number Z is preserved 40 + 52 = 92 and 100 + 134 + 2n = 236 = 235 + 1n

 

[QuantumPion]

The proton number Z is preserved 40 + 52 = 92 and 100 + 134 + 2n = 236 = 235 + 1n

Remember we are starting with Z=94 and A=239 (240 with incident neutron) not U-235. So Te-134 cannot be the compliment (unless there is a third fission product consisting of an He-6 nucleus, which does happen to have a positive Q value although I don't know if that reaction actually can occur).

According to the http://www.nndc.bnl.gov/qcalc/" the complimenting fragment of Zr-100 is Xe-140 and the compliment of Te-134 is Mo-106 (Type in Pu139 for the target, n for the projectile, and anything for the ejectile).

 

[Mike_Fontenot]

If one goes to the chart of nuclides (as indicated by QuantumPion), and selects 239Pu FY and uses Zoom 1, then one will see the yields.

I couldn't figure out what you are describing above. I went again to QuantumPion's link, but what comes up is "the sigma chart", whatever that is. I never got to a screen that had the choice "zoom" that you described. And I couldn't make any sense of your thumbnail images.

But thanks for trying to help.

 

[Mike_Fontenot]

[...] which does happen to have a positive Q value although I don't know if that reaction actually can occur).

According to the http://www.nndc.bnl.gov/qcalc/" the complimenting fragment of Zr-100 is Xe-140

That "Q calculator" IS helpful. I assume that "Q" is the energy released (when Q > 0) for the indicated reaction. But I'm confused about your comment about whether a reaction "can actually occur".

It STILL seems to me that the question "What is the most probable immediate reaction when Pu-239 is hit with a low-energy neutron?" is a well defined question, that should have a simple and straightforward answer. I'm surprised that the answer doesn't seem to be widely known.

 

[QuantumPion]

That "Q calculator" IS helpful. I assume that "Q" is the energy released (when Q > 0) for the indicated reaction. But I'm confused about your comment about whether a reaction "can actually occur".

It STILL seems to me that the question "What is the most probable immediate reaction when Pu-239 is hit with a low-energy neutron?" is a well defined question, that should have a simple and straightforward answer. I'm surprised that the answer doesn't seem to be widely known.

It's not widely known because the question is meaningless qualitatively. Fission has a wide range of possible products. This fission product yield distribution IS well known as you can see from the linked plot. While one particular isotope might technically have the highest yield probability, it is still less than 5% so it is kind of pointless to characterize it as "the most likely reaction".

 

[Mike_Fontenot]

Before asking my question on this forum, I tried to come up with what I thought might be a very likely reaction when Pu-239 absorbs a neutron, and breaks up into two large nuclei, plus a few neutrons that are capable of causing further Pu-239 fissions.

I started with a reaction that I had found somewhere on the web, for U-235 fission:

92_U_235 + 1n -> 56_Ba_141 + 36_Kr_92 + 3n.

I looked for a similar reaction involving Pu-239, and picked the one that gave the largest amount of released energy. Here's what I got:

94_Pu_239 + 1n -> 58_Ce_148 + 36_Kr_89 + 3n.

But in arriving at the above, I didn't know how to determine the energies of the emitted neutrons. From my limited understanding, in order for the emitted neurons to cause further fission of Pu-239, they must have a relatively low energy. So my above reaction may or may not be capable of producing a chain reaction. How do I determine the energies of the emitted neutrons in any given proposed reaction, and what is the range of acceptable neutron energies that will produce a chain reaction? I think that is the missing link for me.

Mike Fontenot

 

[QuantumPion]

Before asking my question on this forum, I tried to come up with what I thought might be a very likely reaction when Pu-239 absorbs a neutron, and breaks up into two large nuclei, plus a few neutrons that are capable of causing further Pu-239 fissions.

I started with a reaction that I had found somewhere on the web, for U-235 fission:

92_U_235 + 1n -> 56_Ba_141 + 36_Kr_92 + 3n.

I looked for a similar reaction involving Pu-239, and picked the one that gave the largest amount of released energy. Here's what I got:

94_Pu_239 + 1n -> 58_Ce_148 + 36_Kr_89 + 3n.

But in arriving at the above, I didn't know how to determine the energies of the emitted neutrons. From my limited understanding, in order for the emitted neurons to cause further fission of Pu-239, they must have a relatively low energy. So my above reaction may or may not be capable of producing a chain reaction. How do I determine the energies of the emitted neutrons in any given proposed reaction, and what is the range of acceptable neutron energies that will produce a chain reaction? I think that is the missing link for me.

Mike Fontenot

Prompt fission neutrons have an energy distribution ranging from 1 to 10 MeV with a peak around 2 MeV (plot http://www.nndc.bnl.gov/sigma/getMF...max=0.011453942787843738&yscale=lin&endf-6=0"). Fast neutrons may cause fission however thermal neutrons have a much higher probability. To have a self-sustaining chain reaction it is merely a requirement that at least 1 neutron per fission event goes on to cause 1 or more new fissions, it does not matter whether they are from fast neutrons (as in a bomb or fast reactor) or thermal neutrons (as in a moderated reactor).


Oh I made a mistake in my previous post, the compliment to Zr100 would be Xe138 + 2n or Xe137 + 3n (I forgot to add the extra neutrons in).

 

[Mike_Fontenot]

Prompt fission neutrons have an energy distribution ranging from 1 to 10 MeV with a peak around 2 MeV (plot http://www.nndc.bnl.gov/sigma/getMF...max=0.011453942787843738&yscale=lin&endf-6=0"). Fast neutrons may cause fission however thermal neutrons have a much higher probability.

I finally found a wiki webpage (on neutron moderators) that clears up a lot of my misconceptions. Although it IS true that a high-energy neutron, for a GIVEN encounter with a Pu-239 nucleus, is MUCH less likely (than a low-energy neutron) to be absorbed (and thereby cause a fission), it is NOT true that high-energy neutrons are useless as producers of fission. Early in the Manhattan project, it was concluded that in bombs, high-energy neutrons actually result in a faster (and thus more complete) chain reaction. The plutonium pit is surrounded by beryllium metal, which reflects the high-energy neutrons. So the neutrons bounce back and forth through the pit, and even though the probability that they cause fission on ANY GIVEN encounter with a Pu-239 nucleus is very low, they have a huge number of encounters in a very short time. The result is that the chain reaction goes much faster than it would with low-energy neutrons.

It was also clear from that webpage that essentially ALL neutrons that are emitted by a fissioning Pu-239 nucleus are high-energy neutrons (as you already pointed out), no matter what the particular reaction happens to be. So I don't NEED to determine the energy of the emitted neutrons in my prospective reaction, in order to determine if that reaction is capable of a chain reaction.

Mike Fontenot

 

[Mike_Fontenot]

It's not widely known because the question is meaningless qualitatively. Fission has a wide range of possible products. This fission product yield distribution IS well known as you can see from the linked plot. While one particular isotope might technically have the highest yield probability, it is still less than 5% so it is kind of pointless to characterize it as "the most likely reaction".

I still maintain that it IS a perfectly reasonable and well-defined question to ask. I'm sure that there IS a definite answer. Even if the probability of the most likely reactions is low, I'd still like to see a list of, say, the three highest-probability reactions.

Here's a posting of mine (that I previously posted to a physics newsgroup), in response to a responder's comments about "yield curves". I think it is pertinent to some of the issues that have been raised in this current thread:> From the graph in wikipedia Nuclear_fission_products, it looks
> like the peaks are at A=103 and 134. The curves are complicated
> by the preference for even Z and N.
>

But I don't think that actually implies that, for a particular common REACTION, that the two large nuclei produced have mass numbers of 103 and 134. And even if it DID imply that, knowledge of the two mass numbers doesn't uniquely specify the particular isotopes (even for the case when there are no immediate proton conversions (conservation of Z)). That's the problem: the yield curves don't specify REACTIONS ... there is no direct correlation information between the two peaks ... no direct information about the two specific products produced in a given reaction.

If the curve really concerns ONLY immediate products, then I suppose that it may indicate that the two products in the most likely reactions DO usually have mass numbers in the vicinity of 103 and 134. If so, that means that my prospective reaction is probably NOT very likely ... i.e., that there are other factors that determine the likelihood of a given reaction, beyond just the amount of energy produced.

Some of the yield curves I've seen were labeled as "after 1 year", or "after 2 years", etc. I HAVE seen the term "prompt products" used as a label for a yield curve, but I'm still not sure that "prompt" really means "immediate".

Mike Fontenot