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- Why does U-238 tamper increase nuclear yield of nuclear bomb? I mean it is not fissile.
Why does U-238 tamper increase nuclear yield of nuclear bomb? I mean it is not fissile.
That would also be true in case of a lead tamper.DBO said:The tamper has sufficient inertia that it holds the core together for a tiny greater fraction of time allowing more neutron multiplication thus increasing yield.
Not that much. Remember that these neutrons come from fission reactions, releasing ~80 MeV per neutron. If 1% of them induce another fission reaction you increase your yield by ~2 MeV per neutron leaving the core, or ~3% overall.snorkack said:Even a small fraction, like 1 %, of nuclei undergoing fission with fast neutrons would add a lot of energy.
Interesting, but misses the point. Neutron cross section is determined by neutron energy. U235 (Th232, U233, Pu239, ...) only has a large cross section for thermal or very low energy neutrons. A fission weapon has no moderator to lower neutron energy so literally ALL fissions occur due to fast neutrons. Fissions also release atomic fragments at high energy enough to fragment or fuse with other nuclei. AND you neglect the contribution of fusion (tampers are in thermonuclear weapons) gamma rays. The energy necessary to split an atom does not only come from particles. And heavy elements are good gamma absorbers. The gamma from fusion can be 40 MeV. Rem fusion nuclei weigh 1/20 U235 so 20x particles for same weight.willem2 said:I looked up the fusion cross section for U238 here: https://wwwndc.jaea.go.jp and I get 1.136 barn.
A square meter of a 1 cm layer of U238 would weigh, 191 Kg and contain 8.0* 102 moles, so would contain 4.8 * 1026 atoms/m2, the total cross section of those atoms is 0.054 m2. This would mean there would be a 5.4% chance of a fusion for a 14Mev Neutron crossing a 1 cm layer. There will be more because of elastic collisions , and (n,2n) and other reactions.
The cross section for neutrons with a fission spectrum is 0.31 barn, so I don't think the neutrons produced by fissions will add much, but I think a 1 cm layer already does enough, considering that a fusion produces 18 MeV and a fission about 200MeV
Where would such a high energy come from?shjacks45 said:The gamma from fusion can be 40 MeV.
Tamper is used in thermonuclear weapon ((US Ivy Mike and Russian Tsar bombs) hence small fission trigger big fusion bomb. Per unit weight Lithium Deuteride (fusion fuel) generates 20x neutrons and very high energy gamma rays than U235. The energy to force split atoms can come from momentum or photon not just from absorbing a neutron. In Wikipedia one test was for theory 50 megaton but with tamper gave 85 megaton.mfb said:It is not fissile - it cannot sustain a chain reaction - but it is still fissionable: It can split when hit by a fast neutron. The inner nuclear explosion produces a lot of high energy neutrons, they can split additional uranium-238 nuclei in the tamper.
Wikipedia on ‘thermonuclear weapon’ gives 17Mev. Still more than 6 MeV causing nuclear instability in U238.mfb said:Where would such a high energy come from?
α binding energy: 28,30 MeVmfb said:And where would 24 MeV come from? The reaction doesn't release that much energy in total!
Your intuition is misleading you here. Even after the box fails, it takes a fair number of microseconds for the cloud of vaporized bomb internals inside of it to disperse and end the reaction. With most devices there's no practical difference between "a fair number of microseconds" and "instantly", but in a fission explosion one additional microsecond is time for another few hundred doublings... and ##2^{100}## is a very big number indeed.le berger des photon said:Once it's lost its integrity, any chain reaction has to stop instantly.
le berger des photon said:but none of you have ever observed a fission chain reaction
Vanadium 50 said:I most certainly have.
le berger des photon said:was it a controlled chain reaction in a reactor?
le berger des photon said:If it was, tell me how you measured the output?
prompt neutrons are released when the uranium nucleus splits. as opposed to the delayed neutrons, which are released when the fission products decay.le berger des photon said:are the prompt neutrons I was just reading about being liberated by collisions? or created out of thin air?
U-238 is used as a tamper in nuclear bombs because it is a dense and heavy element, making it effective in reflecting and trapping neutrons, which are necessary for sustaining a nuclear chain reaction. This results in a higher yield of the explosion.
The presence of U-238 as a tamper in a nuclear bomb increases the yield of the explosion by reflecting and trapping neutrons, which leads to a more efficient and sustained nuclear chain reaction. This results in a larger and more powerful explosion.
Yes, other elements such as lead, uranium, and plutonium can also be used as a tamper in nuclear bombs. However, U-238 is the most commonly used due to its availability and effectiveness in reflecting and trapping neutrons.
Yes, the amount of U-238 used as a tamper in a nuclear bomb is significant. The more U-238 present, the more neutrons will be reflected and trapped, leading to a higher yield of the explosion. However, the amount used is carefully calculated to avoid an uncontrolled chain reaction.
Yes, other elements can be used as a tamper in nuclear bombs, but they may not be as effective as U-238. Scientists are constantly researching and experimenting with different elements to find more efficient ways to increase the yield of nuclear explosions.