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Author Topic:   Spontaneous fission, decay rates, and critical mass
DWIII
Member (Idle past 553 days)
Posts: 72
From: United States
Joined: 06-30-2011


Message 1 of 29 (647147)
01-08-2012 6:28 AM


In "How did the Aborigines get to Australia?" (Message 57):

Zen Deist writes:


Fission is a type of decay process.

http://en.wikipedia.org/wiki/Spontaneous_fission

quote:
Spontaneous fission (SF) is a form of radioactive decay characteristic of very heavy isotopes. Because the nuclear binding energy reaches a maximum at a nuclear mass greater than about 60 atomic mass units (u), spontaneous breakdown into smaller nuclei and single particles becomes possible at heavier masses. ...

As the name suggests, spontaneous fission gives much the same result as induced nuclear fission. However, like other forms of radioactive decay, it occurs due to quantum tunneling, without the atom having been struck by a neutron or other particle as in induced nuclear fission.

Spontaneous fissions release neutrons as all fissions do, so if a critical mass is present, a spontaneous fission can initiate a self-sustaining chain reaction. ...


The process that results in alpha and beta decay is the same process for spontaneous breakdown into nuclei larger than a Helium nuclei (alpha particle).

You can't affect decay rates without affecting fission decay.

When you reduce the nuclear binding energy or lower the barrier for radioactive decay to occur, and reduce the decay rate, you would increase the occurrence of all forms of radioactive decay, including fission.

This means that the critical mass required to reach a sustained reaction is reduced.

Nonukes quite correctly points out the errors in the preceding, namely, that the phenomenon of induced fission, which proceeds extremely rapidly on the successful absorption of a free neutron, is very different from spontaneous fission, which is an extremely rare phenomenon compared to the typical natural decay rate of a radioactive substance.

The statements "You can't affect decay rates without affecting fission decay" and "When you reduce the nuclear binding energy or lower the barrier for radioactive decay to occur, and reduce the decay rate, you would increase the occurrence of all forms of radioactive decay, including fission" and "the critical mass required to reach a sustained reaction is reduced" collectively imply some sort of correlation between decay rates and critical mass. This is arguably not the case. Here is the raw data for fissionable isotopes of plutonium, extracted from Wikipedia (Critical mass and Isotopes of plutonium)


Isotope Critical mass Half-life Proportion of
(bare unbounded sphere) spontaneous fission events
-----------------------------------------------------------------------------
Pu 238 9 kg 87.7 y 1.9 * 10^ -7 %
Pu 239 10 kg 24100 y 3.1 * 10^-10 %
Pu 240 40 kg 6560 y 5.7 * 10^ -6 %
Pu 241 12 kg 14.3 y 2.4 * 10^-14 %
Pu 242 85 kg 375000 y 5.5 * 10^ -4 %
-----------------------------------------------------------------------------

Dividing the proportion of spontaneous fission events by the half-life (where half-life is inversely proportional to activity) would, in addition, give an indication of how many spontaneous fission events are to be expected per unit time per unit mass.

It turns out that, as far as I can see (at least for plutonium), there seems to be essentially no correlation between the following paired sets of data:

1) half-life and proportional spontaneous fission rate
2) half-life and critical mass
3) proportional spontaneous fission rate and critical mass
4) absolute rate of spontaneous fission and critical mass

(Feel free to make your own scatter-plots, of course.)

As Nonukes points out, the rate of spontaneous fission events has nothing to do with the size of a fissionable isotope's critical mass (all you really need is one fortuitous event in a critical mass to start a chain reaction), and even less to do with the overall decay rate of a fissionable isotope that is not undergoing a chain reaction.

An analogy would be what is known as the "percolation threshold" for feedback systems, such as a forest fire, or a field of upright dominoes. It's a given that the probability of a tree spontaneously catching fire is extremely low, but, if that probability isn't precisely zero, a spreading conflagration would be inevitable solely dependent on tree flammability, asymptotic density, and size of the forest.


DWIII

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AdminModulous
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Message 2 of 29 (647149)
01-08-2012 8:05 AM


Thread Copied from Proposed New Topics Forum
Thread copied here from the Spontaneous fission, decay rates, and critical mass thread in the Proposed New Topics forum.
    
RAZD
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Message 3 of 29 (647157)
01-08-2012 8:48 AM
Reply to: Message 1 by DWIII
01-08-2012 6:28 AM


Decay rates, change, and atomic stability
Hi DWIII, thanks -- you beat me to it.

Nonukes quite correctly points out the errors in the preceding, namely, that the phenomenon of induced fission, which proceeds extremely rapidly on the successful absorption of a free neutron, is very different from spontaneous fission, which is an extremely rare phenomenon compared to the typical natural decay rate of a radioactive substance.

Agreed, but that is not all of the picture. The physical constraints that result in the relative stability seen today affect not just decay rates, and any changes that result in an increase in decay rates have effects on other aspects of the stability of atoms.

From How did the Aborigines get to Australia? where this was off-topic:

Message 63 NoNukes: The mechanism for increasing the decay rate has not been specified. At this point the mechanism is PFM (pure freaking magic). I think it is reasonable (but perhaps not inevitable) that the mechanism will increase spontaneous fission in the same way it increases U238 decay rates, exactly as you have proposed, and I have assumed that such a thing will happen.

We are in agreement then, that keeping things working according to the scientific principles, increasing the decay rate results in less stable atomic materials, and that any claim otherwise invokes PFM (otherwise known as god/s-did-it), which means you can make up your own fantasies.

(ibid) I think our disagreement results from your belief that enrichment enables criticality by increasing the number of spontaneously generated neutrons.

That is not what I have argued.

I have argued (and provided evidence) that the level of enrichment in the past in natural ores was sufficient to cause a natural reactor to form. Several did in Oklo.

I have argued that changing the stability of atoms to increase the decay rate would mean that more such natural reactor events should have occurred, even for a small change in decay rate to be achieved.

For the purpose of continuing this debate I will stipulate that claiming a strict 1 to 1 correlation\relationship between decay rates and critical mass is incorrect.

It is a little more complicated than that. It involves the physics of atomic stability to change the decay rate, and this has larger effects than just changing the decay rate.

Message 60 Zen Deist: http://nuclearweaponarchive.org/Library/Fission.html

quote:
The stability of an atomic nucleus is determined by its binding energy - the amount of energy required to disrupt it. Any time a neutron or proton is captured by an atomic nucleus, the nucleus rearranges its structure. If energy is released by the rearrangement, the binding energy decreases. If energy is absorbed, the binding energy increases.

The isotopes important for the large scale release of energy through fission are uranium-235 (U-235), plutonium-239 (Pu-239), and uranium- 233 (U-233). The binding energy of these three isotopes is so low that when a neutron is captured, the energy released by rearrangement exceeds it. The nucleus is then no longer stable and must either shed the excess energy, or split into two pieces. Since fission occurs regardless of the neutron's kinetic energy (i.e. no extra energy from its motion is needed to disrupt the nucleus), this is called "slow fission".

By contrast, when the abundant isotope uranium-238 captures a neutron it still has a binding energy deficit of 1 MeV after internal rearrangement. If it captures a neutron with a kinetic energy exceeding 1 MeV, then this energy plus the energy released by rearrangement can over come the binding energy and cause fission. Since a fast neutron with a large kinetic energy is required, this is called "fast fission".


In nuclear reactions today some neutrons are lost from the chain reaction due to neutron capture without fission, due to the binding energy level of the various isotopes.

Curiously, the binding energy also affects the decay rate, and increased decay rate means that the effective binding energy of the atom\isotope is reduced.

With lower binding energy, neutron capture is more likely to exceed the (lower) binding energy limit for fission to occur, with the result that induced fission would occur more often: less critical mass is needed.

In addition, the numbers of neutrons resulting from fission would also increase:

quote:
(ibid) The nuclei of these isotopes are just barely stable and the addition of a small amount of energy to one by an outside neutron will cause it to promptly split into two roughly equal pieces, ... and several new neutrons (an average of 2.52 for U-235, and 2.95 for Pu-239).

Amusingly, neutrons exist in integer quantities, not fractions. There is variation in the number of neutrons produced from individual events.

The number of neutrons produced is also related to the binding energy that controls decay rates. Faster decay = more neutrons produced by induced fission = less critical mass.

Because the atoms are less stable (to allow the increased decay rate) they are more susceptible to fission, and have a lower threshold to energy increases that result in induced fission.

It is just not logical (without invoking PFM) that any change that allows for atoms with less hold on decay particles (to increase the rate of decay) would not also have less hold on neutrons etc in the nucleus, and on holding themselves together. These are due to the same atomic bonding forces.

Thus lower energy neutrons would induce fission rather than just be absorbed (as often happens today), AND induced fission would release more neutrons than now (an "average of 2.52 for U-235, and 2.95 for Pu-239" today) ... neglecting for now that this could result in 238U and other elements being able to support a chain reaction, the inevitable result is that smaller critical mass would be the case.

You just can't invoke an increase in the rate of decay without getting a reduction in the stability of atoms across the board. Unless you want to invoke PFM.

Enjoy

Edited by Zen Deist, : wrding

Edited by Zen Deist, : more wrding


we are limited in our ability to understand
by our ability to understand
Rebel American Zen Deist
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NoNukes
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Posts: 9322
From: Central NC USA
Joined: 08-13-2010
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Message 4 of 29 (647196)
01-08-2012 1:09 PM
Reply to: Message 1 by DWIII
01-08-2012 6:28 AM


That's just about right...
Thanks for doing the work for starting this thread.

I have a nit to pick with one statement in the OP.

The statements "You can't affect decay rates without affecting fission decay" and "When you reduce the nuclear binding energy or lower the barrier for radioactive decay to occur, and reduce the decay rate, you would increase the occurrence of all forms of radioactive decay, including fission" and "the critical mass required to reach a sustained reaction is reduced" collectively imply some sort of correlation between decay rates and critical mass.

I don't think the statements have the implication stated above. I am willing to accept that lowering the barrier for decay will have an affect on the spontaneous fission rate. But accepting that hypothesis does not imply a correlation between decay rates and critical mass.

I'd also question the relevance of your plutonium evidence. A Pu 238 nucleus behaves very differently in nuclear reactions from a Pu 239 nucleus. Comparing isotopes may not be a good indicator of what would happen if the binding energy for a particular nucleus were to be supernaturally reduced.

But you have accurately capturing my position on the matter.


Under a government which imprisons any unjustly, the true place for a just man is also in prison. The proper place to-day, the only place which Massachusetts has provided for her freer and less desponding spirits, is in her prisons, to be put out and locked out of the State by her own act, as they have already put themselves out by their principles. Thoreau: Civil Disobedience (1846)

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NoNukes
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Joined: 08-13-2010
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Message 5 of 29 (647200)
01-08-2012 1:51 PM
Reply to: Message 3 by RAZD
01-08-2012 8:48 AM


Re: Decay rates, change, and atomic stability
Because the atoms are less stable (to allow the increased decay rate) they are more susceptible to fission, and have a lower threshold to energy increases that result in induced fission.

No one disputes that the spontaneous fission rate would increase.

But essentially every low energy neutron (upwards of 96 per cent) captured by a U235 atom already produces an induced fission. There simply isn't much room for improvement in this area. In the six factor formula, this ratio is wrapped up into "eta" which is the neutron yield per captured neutron.

Thus lower energy neutrons would induce fission rather than just be absorbed (as often happens today), AND induced fission would release more neutrons than now (an "average of 2.52 for U-235, and 2.95 for Pu-239" today)

I note that this is the first time you have made this argument. Most (if not all) of your previous arguments haven't dealt with chain reactions. We are at last on the same page.

As I argued above, it is not true that absorption of a thermal neutron in U235 often fails to produce fission. Fast neutrons, on the other hand have a lowered probability of being captured by a U235 nucleus. I'm curious to see what use you make of this information.

I am aware of no good reason why the products from fission would necessarily include more neutrons simply because the binding energy changes.

A typical split of a U235 atom yields two big roughly equal sized fragments, with some alpha particles and some neutrons and a bunch of energy in the form of gamma rays and kinetic energy of the fragments. I will accept the values you provide as average values for the produced neutrons. But who knows what the mix might be if we introduce supernatural meddling? More alphas? More neutrons? Slightly bigger fragments and fewer neutrons? Slightly more energy per fission than before with the same fission products? A lower percentage of neutron pre-cursors among the fission products reducing the number of delayed neutrons produced so that keff is actually lowered rather than increased?

I'll admit that I don't know enough atomic physics to speculate. But I do know enough to be skeptical. I can suggest that it is not inevitable that more neutrons would be produced per fission. But I'd welcome your explanation of why there is likely to be more neutrons produced.

... neglecting for now that this could result in 238U and other elements being able to support a chain reaction, the inevitable result is that smaller critical mass would be the case.

You haven't established an inevitable result for at least the reasons I've discussed above. I don't dispute that changing the nuclear binding energy might make other nuclei capable of sustaining a chain reaction, but I don't see the link to your conclusion of more neutrons from U235.

Finally I ask, how do we change properties of atomic nuclei without PFM?

Edited by NoNukes, : Change fusion to fission. Minor corrections.

Edited by NoNukes, : Fusion ain't fission, sigh.


Under a government which imprisons any unjustly, the true place for a just man is also in prison. The proper place to-day, the only place which Massachusetts has provided for her freer and less desponding spirits, is in her prisons, to be put out and locked out of the State by her own act, as they have already put themselves out by their principles. Thoreau: Civil Disobedience (1846)

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foreveryoung
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Posts: 879
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Message 6 of 29 (647205)
01-08-2012 3:39 PM


Does anybody know what factors determine how fast or slow any particular unstable isotope decays?
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DWIII
Member (Idle past 553 days)
Posts: 72
From: United States
Joined: 06-30-2011


Message 7 of 29 (647214)
01-08-2012 4:51 PM
Reply to: Message 4 by NoNukes
01-08-2012 1:09 PM


Re: That's just about right...
NoNukes writes:


I have a nit to pick with one statement in the OP.

The statements "You can't affect decay rates without affecting fission decay" and "When you reduce the nuclear binding energy or lower the barrier for radioactive decay to occur, and reduce the decay rate, you would increase the occurrence of all forms of radioactive decay, including fission" and "the critical mass required to reach a sustained reaction is reduced" collectively imply some sort of correlation between decay rates and critical mass.

I don't think the statements have the implication stated above. I am willing to accept that lowering the barrier for decay will have an affect on the spontaneous fission rate. But accepting that hypothesis does not imply a correlation between decay rates and critical mass.

And yet Zen Deist was apparently arguing for lower critical masses on the basis of lower nuclear stability by causing more induced fission events due to more spontaneous fission events, something which both you and I disagree with. Anyway, he seems to have since changed gears somewhat, claiming that enrichment due to lower nuclear stability would also play a factor. Well, we will see, I hope...


I'd also question the relevance of your plutonium evidence. A Pu 238 nucleus behaves very differently in nuclear reactions from a Pu 239 nucleus. Comparing isotopes may not be a good indicator of what would happen if the binding energy for a particular nucleus were to be supernaturally reduced.

I agree that it's not a particularly great example, given that the numbers are all over the board just for those five closely-similar isotopes. You are quite right in that a difference of just one neutron can have profound consequences for many aspects of nuclear behavior; but in the absence of magically increasing overall nuclear decay rates (or equivalently having a complete theory of nuclear stability), perhaps it could be seen as the next best thing for the sake of testing for any such correlations with respect to critical mass.


DWIII

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NoNukes
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Message 8 of 29 (647233)
01-08-2012 6:59 PM
Reply to: Message 6 by foreveryoung
01-08-2012 3:39 PM


Does anybody know what factors determine how fast or slow any particular unstable isotope decays?

I'd like to clarify what types of comparisons you are asking about.

Are you referring to nuclear factors that determine why one isotope of oxygen is stable or long lived while another isotope of oxygen has a short decay half life, and a third isotope has a long decay half-life ?

Or are you referring to factors that might affect the decay rate of a particular isotope such as K40 or Carbon 14?


Under a government which imprisons any unjustly, the true place for a just man is also in prison. The proper place to-day, the only place which Massachusetts has provided for her freer and less desponding spirits, is in her prisons, to be put out and locked out of the State by her own act, as they have already put themselves out by their principles. Thoreau: Civil Disobedience (1846)

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foreveryoung
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Posts: 879
Joined: 12-26-2011


Message 9 of 29 (647248)
01-08-2012 8:37 PM
Reply to: Message 8 by NoNukes
01-08-2012 6:59 PM


Or are you referring to factors that might affect the decay rate of a particular isotope such as K40 or Carbon 14?

Those kind


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RAZD
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Posts: 18241
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Message 10 of 29 (647255)
01-08-2012 9:12 PM
Reply to: Message 5 by NoNukes
01-08-2012 1:51 PM


Re: Decay rates, change, and atomic stability
Hi NoNukes and DWIII (and anyone else reading)

No one disputes that the spontaneous fission rate would increase.

So it appears that we are all in agreement, as I said in Message 3:

quote:
We are in agreement then, that keeping things working according to the scientific principles, increasing the decay rate results in less stable atomic materials, and that any claim otherwise invokes PFM ...

Would you not also agree that this change in stability necessarily affects the behavior of other particles, nuclei, etc, operating under the same laws and forces that regulate decay rates? That this is why the occurrence of spontaneous fission would increase, not just decay events, yes?

There should be no special pleading for one set of particle\behavior compared to others operating by the same laws and forces.

Thus lower energy neutrons would induce fission rather than just be absorbed (as often happens today), AND induced fission would release more neutrons than now (an "average of 2.52 for U-235, and 2.95 for Pu-239" today)

I note that this is the first time you have made this argument. Most (if not all) of your previous arguments haven't dealt with chain reactions. ...

You can consider this a more detailed explanation than before (or that I am "changing gears", etc., if you want). I sometimes get ahead of myself.

But essentially every low energy neutron (upwards of 96 per cent) captured by a U235 atom already produces an induced fission. There simply isn't much room for improvement in this area. In the six factor formula, this ratio is wrapped up into "eta" which is the neutron yield per captured neutron.

But even a 1% increase would cause more induced fission than now would it not? This would translate to less critical mass needed to achieve a sustained chain reaction would it not?

I am aware of no good reason why the products from fission would necessarily include more neutrons simply because the binding energy changes.

Are they not operating under the same laws and forces that hold the decay particles in the nucleus until decay occurs? Can I affect one without affecting the other(s)? Is there any good reason to think that they would NOT be affected? See next.

A typical split of a U235 atom yields two big roughly equal sized fragments, with some alpha particles and some neutrons and a bunch of energy in the form of gamma rays and kinetic energy of the fragments. ...

With increased decay rates and atomic stability it would seem highly likely that the numbers of alpha particles (bound by the same laws and forces as the ones involved in alpha decay events, after all) would increase in number in these events as well. Would not whatever binds one also bind the other to the same degree, whether effectively increasing the binding energy or decreasing it.

More alpha particles lost in fission would mean that the main members, the "two big roughly equal sized fragments" would be slightly smaller than the ones we see today.

Stable atoms generally have decreasing proportions of neutrons to protons as they get smaller ...

See link to periodic chart to compare atomic number (number of protons) to atomic mass (number of protons, neutrons, et.)
(from http://wikis.lawrence.edu/...iodic+Table+%28Ashley+Vokral%29)

... so this would mean the smaller "big roughly equal sized fragments" produced compared to the ones we see today would, on average contain fewer neutrons, thus indicating that the number of neutrons would also increase, yes?

Even with no other considerations it seems we should see an increase in the number of neutrons produced.

... I will accept the values you provide as average values for the produced neutrons. ...

It seems to me that the variation is likely due to the variation in energy of the neutrons that cause the induced fission and the variation in energy levels within the nuclei being struck. Higher energy combinations leading to the larger production of neutrons and lower energy combinations leading to the lesser production of neutrons.

If nothing else we have a balance between events resulting in 2 neutrons and events resulting in 3 neutrons (and possibly rarer events resulting in 1 or 4 neutrons). Likely there is a (skewed) probability distribution in the numbers of neutrons produced.

The reduced stability of the atoms necessary to achieve a reduction in decay rate would affect this proportion and logically result in more neutrons than we see today.

Would you not agree that a slight shift in the proportions of these events, that raised the average number, say by 1% (+0.03 neutrons on average), would cause more induced fission than now, yes? And this would translate to less critical mass needed to achieve a sustained chain reaction would it not?

... But who knows what the mix might be if we introduce supernatural meddling? More alphas? More neutrons? ...

Indeed, even (perhaps) resulting in all radioactive elements falling apart, or engaging in run-away fission reactions, especially when you get to the level of change required to turn 4.55 billion years into a YEC age (ie several orders of magnitude of change in the decay rates), yes?

... Slightly bigger fragments and fewer neutrons?

Not likely imhysao, as that would be a more stable, lower energy, condition.

... Fast neutrons, on the other hand have a lowered probability of being captured by a U235 nucleus. I'm curious to see what use you make of this information.

And logically they also would have more energy (same laws and forces). The whole energy spectrum should shift to a slightly higher level, which would be in keeping with compressing radioactive decay behavior into shorter time periods.

I also look at the energy required to cause fission in 238U:

http://nuclearweaponarchive.org/Library/Fission.html

quote:
By contrast, when the abundant isotope uranium-238 captures a neutron it still has a binding energy deficit of 1 MeV after internal rearrangement. If it captures a neutron with a kinetic energy exceeding 1 MeV, then this energy plus the energy released by rearrangement can over come the binding energy and cause fission. Since a fast neutron with a large kinetic energy is required, this is called "fast fission".

Thus we should see a point where the increased particle energy coupled with the increased instability of the nuclei would result in a chain reaction in 238U. It would not take much of a change to reach this point.

... I don't dispute that changing the nuclear binding energy might make other nuclei capable of sustaining a chain reaction, ...

At that point it would be ball-game over for any reduced decay rate hypothesis would it not?

Enjoy.


we are limited in our ability to understand
by our ability to understand
Rebel American Zen Deist
... to learn ... to think ... to live ... to laugh ...
to share.


Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click)

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NoNukes
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Posts: 9322
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(1)
Message 11 of 29 (647284)
01-08-2012 11:54 PM
Reply to: Message 10 by RAZD
01-08-2012 9:12 PM


Re: Decay rates, change, and atomic stability
But even a 1% increase would cause more induced fission than now would it not? This would translate to less critical mass needed to achieve a sustained chain reaction would it not?

I agree that such a change might have the effect you describe, but maybe not. When a U235 nucleus absorbs a neutron and forms U236, the U236 nucleus may either fission or emit a gamma ray. If we lowered the binding energy, why wouldn't the gamma ray emission probability be increased?

What we can say for sure is that an enormous increase in the decay rate could correspond to only a tiny effect on criticality through this mechanism. Surely this effect cannot be used to demonstrate that no rapid decay occurred in the past because of the lack of more natural reactors.

Compared with the other effects that are postulated to have resulted in the natural reactors at Oklo going critical, the theoretical maximum contribution to keff from increasing the probability that a captured thermal neutron will cause fission is quite small, making it impossible to say that we would have seen more such natural reactors if the creationists were right.

For example this effect could not overcome the changes in enrichment that make it impossible for a natural critical reactor to form today. Consider that immersing a mass of U235 in ordinary water produces a substantial increase in reactivity.

That this is why the occurrence of spontaneous fission would increase, not just decay events, yes?

Spontaneous fission is very like decay. I am supposing that the rate of spontaneous fission increases for reasons very similar to the reasons why K40 decay would increase. But many of the other events that effect a chain reaction are quite dissimilar to decay. I believe it is necessary to make an independent case for each effect that dialing down the binding energy of nuclei results in the effect in a helpful direction. One method for doing this would be to look at the effect on each of the six factors in the keff formula. (Where keff is the ration between neutrons populatin in a generation and the neutron population in the previous generation. keff=1 means a critical reactor). For example, what would you expect would be the effect on the probability of absorption of a neutron in some material other that U235. If that probability were to increase then the margin below criticality (or negative reactivity) would increase.

With increased decay rates and atomic stability it would seem highly likely that the numbers of alpha particles (bound by the same laws and forces as the ones involved in alpha decay events, after all) would increase in number in these events as well.

Give me an argument for the above.

Coincidentally, I had to tutor a high school student on nuclear reactions including fission this afternoon. It turns out that fission fairly rarely produces alpha particles directly. However, I think this is actually a side issue. With respect to sustaining a chain reaction, the important fission products are neutrons, and the production of those particular fission fragments that beta decay to produce neutron emitters.

The reduced stability of the atoms necessary to achieve a reduction in decay rate would affect this proportion and logically result in more neutrons than we see today.

Maybe. But perhaps fewer neutrons might be produced than we see today. My gut feeling is that there would be no significant effect, but I'm not professing to know the answer.

Would you not agree that a slight shift in the proportions of these events, that raised the average number, say by 1% (+0.03 neutrons on average), would cause more induced fission than now, yes? And this would translate to less critical mass needed to achieve a sustained chain reaction would it not?

Yes, if this were the only effect. And maybe the effect is even larger than you postulate. But you need to give me a reason to believe that the effect would occur, and be in the direction you say. I still maintain that I haven't yet seen an argument that a higher average number of neutrons would be produced from fission.

Edited by NoNukes, : Discuss 236


Under a government which imprisons any unjustly, the true place for a just man is also in prison. The proper place to-day, the only place which Massachusetts has provided for her freer and less desponding spirits, is in her prisons, to be put out and locked out of the State by her own act, as they have already put themselves out by their principles. Thoreau: Civil Disobedience (1846)

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 Message 10 by RAZD, posted 01-08-2012 9:12 PM RAZD has responded

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 Message 15 by RAZD, posted 01-09-2012 1:57 PM NoNukes has responded

    
NoNukes
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Posts: 9322
From: Central NC USA
Joined: 08-13-2010
Member Rating: 2.6


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Message 12 of 29 (647285)
01-09-2012 12:33 AM
Reply to: Message 9 by foreveryoung
01-08-2012 8:37 PM


Off topic.
I'll join you in that radioactive decay thread.

Under a government which imprisons any unjustly, the true place for a just man is also in prison. The proper place to-day, the only place which Massachusetts has provided for her freer and less desponding spirits, is in her prisons, to be put out and locked out of the State by her own act, as they have already put themselves out by their principles. Thoreau: Civil Disobedience (1846)

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DWIII
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Posts: 72
From: United States
Joined: 06-30-2011


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Message 13 of 29 (647339)
01-09-2012 10:28 AM
Reply to: Message 10 by RAZD
01-08-2012 9:12 PM


Re: Decay rates, change, and atomic stability
Zen Deist writes:


No one disputes that the spontaneous fission rate would increase.

So it appears that we are all in agreement, as I said in Message 3:

quote:
We are in agreement then, that keeping things working according to the scientific principles, increasing the decay rate results in less stable atomic materials, and that any claim otherwise invokes PFM ...

Well, that goes without saying; it's practically a tautology. A decay rate is (by definition) inversely proportional to stability.


Would you not also agree that this change in stability necessarily affects the behavior of other particles, nuclei, etc, operating under the same laws and forces that regulate decay rates? That this is why the occurrence of spontaneous fission would increase, not just decay events, yes?

There should be no special pleading for one set of particle\behavior compared to others operating by the same laws and forces.

You may be right, but how could we directly test that? Given that (1) spontaneous fission decay events tend to be an extremely small proportion of all decay events for a typical isotope in question, and (2) even among the slightly different isotopes which I put forth as a test sampling, the range of variation of the absolute rate of spontaneous fissions varies by a whopping 6 orders of magnitude, even though the overall decay rates vary by no more than 4 orders of magnitude, the available evidence seems to be inconclusive at best. In fact, the half-lives and the absolute rates of spontaneous fission exhibit less correlation than any of the other four pairings of data which I alluded to earlier.

For all we know, the spontaneous fission decay rates themselves may be extremely sensitive to changes in nuclear stability, and not always in the same direction.


But even a 1% increase would cause more induced fission than now would it not? This would translate to less critical mass needed to achieve a sustained chain reaction would it not?

To initially achieve a chain reaction, perhaps. If the spontaneous fission rate was absolutely zero, it would never get started in the first place (barring an external source of thermal neutrons). But to sustain a chain reaction is another thing entirely, and that's what k=1 criticality (as opposed to k<1 subcriticality) is all about.

If a k=1 chain reaction has already started, you could subsequently PFM-supress all forms of spontaneous decay, including the already-comparatively-rare spontaneous fission events while leaving the specifics of induced fission unchanged, and the chain reaction process itself would be (to an accuracy of many decimal places) completely unaffected.


I am aware of no good reason why the products from fission would necessarily include more neutrons simply because the binding energy changes.

Are they not operating under the same laws and forces that hold the decay particles in the nucleus until decay occurs? Can I affect one without affecting the other(s)? Is there any good reason to think that they would NOT be affected?

Again, for all we know, the specifics of neutron emission rates via fission (such as average number of neutrons released, and/or the energy spectrum of those neutrons, not even to mention rates of subsequent neutron absorption and/or rates of subsequent induced fissioning) may very well be extremely sensitive to small changes in overall nuclear stability, and not always in the same directions.

Edited by DWIII, : re: insufficient evidence

Edited by DWIII, : fixed html-character bug


DWIII

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 Message 10 by RAZD, posted 01-08-2012 9:12 PM RAZD has acknowledged this reply

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NoNukes
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Posts: 9322
From: Central NC USA
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Message 14 of 29 (647348)
01-09-2012 11:21 AM
Reply to: Message 13 by DWIII
01-09-2012 10:28 AM


Re: Decay rates, change, and atomic stability
Again, for all we know, the specifics of neutron emission rates via fission (such as average number of neutrons released, and/or the energy spectrum of those neutrons, not even to mention rates of subsequent neutron absorption and/or rates of subsequent induced fissioning) may very well be extremely sensitive to small changes in overall nuclear stability, and not always in the same directions.

Another way to frame this idea.

Fission of U235 can be thought of as not a single specific reaction but as a large set of competing reactions with products varying in both the number of neutrons released, and in the exact fission fragments produced. The distribution of fission fragments for fission of U235 looks like the illustration below:

Even if we accept that decreasing the binding energy makes fission of U235 easier, how can we say that a particular fission product result is favored since the different possible outcomes compete with one another? And if we cannot answer that question, then we cannot say whether on average more or fewer neutrons would be released per fission.

We also know that certain fission fragments (such as Xe135) are strong neutron absorbing nuclei which compete with U235 in absorbing neutrons, while other fission products release delayed neutrons after beta-decay. How can we predict whether reducing U235 binding energy will favor or disfavor producing chain reaction poisoning or neutron creating fission products?

Edited by NoNukes, : No reason given.


Under a government which imprisons any unjustly, the true place for a just man is also in prison. The proper place to-day, the only place which Massachusetts has provided for her freer and less desponding spirits, is in her prisons, to be put out and locked out of the State by her own act, as they have already put themselves out by their principles. Thoreau: Civil Disobedience (1846)

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 Message 13 by DWIII, posted 01-09-2012 10:28 AM DWIII has acknowledged this reply

    
RAZD
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Posts: 18241
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Message 15 of 29 (647384)
01-09-2012 1:57 PM
Reply to: Message 11 by NoNukes
01-08-2012 11:54 PM


Re: Decay rates, change, and atomic stability
Hi NoNukes, thanks.

I appreciate the work you put into these posts.

I agree that such a change might have the effect you describe, but maybe not. When a U235 nucleus absorbs a neutron and forms U236, the U236 nucleus may either fission or emit a gamma ray. If we lowered the binding energy, why wouldn't the gamma ray emission probability be increased?

I don't think it would be the case of one or the other, but both would be affected. If the number of absorptions increases then both more fission and more gamma ray emission could occur, and quite possibly in the same ratio.

What we can say for sure is that an enormous increase in the decay rate ...

Curiously, when we are talking about changing the age of the earth from 4.55 billion years to 10,000 years we are talking about an enormous increase in the decay rate, yes?

... could correspond to only a tiny effect on criticality through this mechanism. ...

Or it could correspond to a significant effect. You need to show why you think there would only be a small effect, yes?

... Surely this effect cannot be used to demonstrate that no rapid decay occurred in the past because of the lack of more natural reactors.

We have evidence of several natural reactors in Oklo, so the question is not whether natural reactors could form, but the number of reactors that could form and the number that should form under reduced binding energy that would allow faster decay to occur. We may even be able to go further and see if we can parse out some evidence that should occur and not occur under rapid decay physics and then test to see if they are present anywhere. For instance, should the behavior at Oklo have been different under rapid decay?

Give me an argument for the above.

Coincidentally, I had to tutor a high school student on nuclear reactions including fission this afternoon. It turns out that fission fairly rarely produces alpha particles directly. However, I think this is actually a side issue. With respect to sustaining a chain reaction, the important fission products are neutrons, and the production of those particular fission fragments that beta decay to produce neutron emitters.

If the binding energy holding alpha particles is reduced to allow more rapid decay, then it is also reduced for holding alpha particles within a nucleus, and they are more likely to be released under impact.

Consider a billiard table with magnetic balls: smack a group of balls with the cue ball and a number of different results can occur:

  1. The cue ball is absorbed into the group of balls, which is otherwise virtually unaffected
  2. The cue ball is absorbed into the group and one or more balls break away from the group
  3. The cue ball is absorbed into the group, which then splits into to two smaller groups and one or more balls break away from the group
  4. The cue ball is absorbed into the group, which then splits into to several smaller groups and several balls break away from the group
  5. The cue ball is absorbed into the group, which then splits up into many smaller groups and individual balls
  6. The cue ball shatters the group into individual balls.

Would you agree, for the sake of the argument, that these scenarios are roughly sorted by the initial energy of the cue ball? Would you not agree that 1 occurs with significantly less cue ball energy than number 6 but that 2 only needs a little bit more energy than 1?

If we take just 1 and 2 above, for example, we should be able to determine a threshold cue ball energy level below which 1 takes place and above which 2 takes place, for a constant input of magnetic force between the balls, yes?

Next we reduce the force of the magnets and repeat. With less binding energy I would expect there to be a shift towards the more energetic responses, wouldn't you?

The threshold between 1 and 2 would be shifted to occur with less cue ball energy than before, yes?

Similar threshold shifts should happen for all the other possibilities as well, yes?

Maybe. But perhaps fewer neutrons might be produced than we see today. My gut feeling is that there would be no significant effect, but I'm not professing to know the answer.

Can you see a scenario in the billiard analogy where this would be the case as you decrease the magnetic forces?

Compared with the other effects that are postulated to have resulted in the natural reactors at Oklo going critical, the theoretical maximum contribution to keff from increasing the probability that a captured thermal neutron will cause fission is quite small, making it impossible to say that we would have seen more such natural reactors if the creationists were right.

Yes, if this were the only effect. And maybe the effect is even larger than you postulate. But you need to give me a reason to believe that the effect would occur, and be in the direction you say. I still maintain that I haven't yet seen an argument that a higher average number of neutrons would be produced from fission.

So maybe we need to take it step by step and review the Keff equation and the individual factors.

(from http://en.wikipedia.org/wiki/Six_factor_formula)

k = η•f•p•ε•PFNL•PTNL

    where
  • η = the production factor (typical values* 1.65, 2.02)
  • f = the thermal utilization factor (typical values 0.71, 0.799)
  • p = the resonance escape probability (typical values 0.87, 0.80)
  • ε = the fast fission factor (typical values 1.02, 1.04)
  • PFNL = the fast non-leakage probability (typical values 0.97, 0.865)
  • PTNL = the thermal non-leakage probability (typical values 0.99, 0,861)

* - first value from wiki table, second value from diagram below

Also shown diagrammatically (in a different order) by:


From http://nuclearpowertraining.tpub.com/...2/css/h1019v2_35.htm

I note from the formulas for each factor that several of them are inter-related, and four of the formulas are approximations.

In addition I note that the probability factors, f, p, PFNL, and PTNL, would have maximum values of 1.0

The values for the factors in the diagram multiplied together = 1.00.

The values for the factors in the wiki table multiplied together = 0.998.

The next question then is which of these factors are affected by reducing the binding energy of the nucleus.

We can start with η, the production factor

η = υ•σFf/σFa

    where
  • υ = the average number of neutrons produced per fission in the medium
  • σFf = the microscopic fission cross section
  • σFa = the microscopic absorption cross section

Interestingly, the wiki table here lists 2.43 for the average number of neutrons produced per fission in Uranium-235, where previously we had 2.52.

Enjoy


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by our ability to understand
Rebel American Zen Deist
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This message is a reply to:
 Message 11 by NoNukes, posted 01-08-2012 11:54 PM NoNukes has responded

Replies to this message:
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