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 events should have occurred, even for a small change in decay rate to be achieved.
For the purpose of continuing this debate I will concede that claiming a strict 1 to 1 correlation\relationship between decay rates and critical mass is wrong.
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.
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, : ...
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