Axil wrote:I think where delayed fission really matters is in the precise timing that is to be found in the design of a nuclear weapon.
There is no such thing as "delayed fission" -- only delayed neutrons from fission products
that decay by neutron emission.
Anyway, neither has ANY role in nuke weapons.
What *is* important in nuke weapons is that some nuclides don't fission by thermal neutrons, only fast ones: in that case, you don't have to worry about pre-detonation criticality (or high sub-critical multiplication creating large neutron background).
One good example is Np237.
Typically, thermal-fissile nuclide critical mass is far lower than fast-fission critical mass -- hence the problem !
In the case of reactors, this is also a problem, since delayed neutrons are NOT fast, so any core that has a significant amount of that type of fissile nuclide will be less responsive to delayed neutrons -- in the extreme, not responsive AT ALL ! (i.e. uncontrollable)
Thanks, jaro you are very generous with your time and patience with us beginners. I do like to get into the esoteric details. IMHO, to fully appreciate the Lftr, one needs to understand its fuel, U233 in some detail.
For one thing, when U233 absorbs a neutron, it briefly becomes U234 before U234 fissions. Most of the time, (92%) this excited U234 nucleus will fission, but sometimes it won’t (8 %).
Traditionally, neutrons in reactors have been categorized by the reaction that produces them. Thus, all neutrons from neutron induced fission are called "fission" neutrons, with two subsets, prompt fission neutrons and delayed fission neutrons.
When U234 fissions in the prompt fission process, it produces two or three prompt neutrons. I still think that there is some variable delayed time period described by some Gaussian time distribution that U234 takes to break apart in the fission process. Why would U234 be any different in this regard from other delayed neutron precursors?
This means the mean delta time that U234 takes to fission is a major time component in the mean prompt neutron lifetime that defines the rate of multiplication of prompt neutrons.
I looked at this study "Benchmark Critical Experiments of Uranium-233 Spheres Surrounded by Uranium-235
”. The multiplication of U233 prompt neutrons is very low at only 6.31. When you add some U235 to it, it goes way up to the mid 80’s.
There might be some sort of U233 anti-proliferation argument to be made from the teapot experience as follows:
If you need U235 or Plutonium to make U233 critical, why not eliminate U233 from the mix altogether as a redundant and a needlessly complicating factor that can cause unpredictable performance?
In conclusion, U233 is confusing to me. The more I look into U233, the less I understand it, and the more questions that I have about it. But at the end of the day, it is still very fascinating.
The old Zenith slogan: The quality goes in before the name goes on.