Say, in a gun-type atomic bomb. I know that once an atom of U-235 fissions, it splits into radioactive elements, energy and neutrons, and those neutrons go on to split other nearby uranium atoms. But when the U-235 mass is fired into a U-235 core, how is the neutron required to split the first U-235 atoms produced?
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You make a critical mass of the fissile material. U-235 (and plutonium, the other commonly used bomb material) are emitting neutrons all the time, but they’re not usually in high enough quantities for problems to occur. Once you get to a critical mass, though, you get a chain reaction where the number of neutrons being emitted is sufficient to be self-sustaining–e.g. neutrons already emitted hit other atoms in the mass and break them apart, causing more neutrons to be emitted.
This is actually a problem that they had to solve working on the manhattan project. You can get some neutrons from natural decay of your isotopes, but that may not be reliable enough for your needs. The result is a neutron source.
The one I could find information on used something which radiates alpha particles next to some beryllium, which produces neutrons after being hit with alpha particles.
The oil and gas industry uses pulsed neutron generators to scan the rock around exploration wells.
The basics are explained here :
https://glossary.oilfield.slb.com/en/terms/n/neutron_generator
Could someone confirm if the same tech is used to start chain reactions in boms or nuclear reactors ?
Uranium naturally will undergo decay and sometimes these decays will emit neutrons. In most situations the U will be in a sub critical configuration. That is each neutron released leads to <1 additional fission on average. In a nuclear reactor the environment is manipulated such that the configuration is critical (each fission leads to 1 additional fission on average).
To create a nuclear explosion, you have to have your configuration reach a supercritical state (each fission leads to >1 additional fission). This is done through 2 primary methods in modern 1 stage weapons*. First in increasing the density. This can be done in both a gun type or implosion type weapon. The premise here is simple, put more fissionable atoms closer together to increase the chance any given neutron will induce fission. Second is using a neutron booster. This is usually a mixture of lithium and deuterium in the center of an implosion type weapon that when compressed will temporarily undergo fusion releasing extra neutrons. This fusion reaction itself is likely a net loss of energy and not self-sustaining and only used to increase the criticality.
*Whether or not you consider a boosted weapon a 1 or 2 stage device is somewhat semantical.
So in any actual nuclear weapon you usually have some sort of neutron source — a way of quickly generating some neutrons to make sure the reaction starts are the right time. The ones in World War II were just little balls of polonium and beryllium that, when crushed together, produced neutrons (polonium emits alpha particles, and when beryllium absorbs an alpha particle it ejects a neutron). They only released a 100 or so neutrons; modern neutron generators (which are sort of like tiny particle accelerators) can make a lot more.
But even without that, you would expect there to be some free neutrons just floating around in fissile material. This is because the nuclear fuel, and impurities in it, undergo spontaneous fission, which releases neutrons. So U-235 has a spontaneous fission rate of 0.0056 neutrons per second per kilogram. So for the Little Boy bomb, which had 51 kg of U-235 in it, that’s basically one fission every 3 seconds, and each fission releases 1-3 neutrons. Pretty low, but it’s something. Better off is the U-238 in the material (it was 80% enriched, so 20%, or 13 kg, was U-238), which has a spontaneous fission rate of 5.51 fissions per second per kg. So that’s 72 fissions per second — probably enough to start your reaction. But a neutron source helps with both the timing and making sure the reaction gets a little “jump start” to really get going.
For the Fat Man bomb, its core was 94% Pu-239 (10.1 fissions/second/kg, times 5.8 kg = 59 fissions per second) and 6% Pu-240 (478,000 F/s/kg, times 3.7 kg = 1,768,600 fissions per second), so it’s got neutrons galore by comparison. (A million fissions per second sounds like a lot — and it is — but the reactions that create explosions are on on par with a trillion trillion fissions in a millisecond!)
Physically, the first neutron can come from spontaneous decay or other processes.
Really though, we want there to be a definitive criticality event, so we can monitor and control it. So reactors usually have neutron emitters in them like californium 242.
Once a reactor has had sufficient runtime, the waste products produce neutrons, and you can remove the startup sources and allow the nuclear waste to provide the required neutrons.
The goal of having source neutrons in the reactor is to make it capable of monitoring and control. If the count was too low, you could go critical without seeing it and have a very rapid/runaway system. So we require a minimum neutron count before commencing a reactor startup.
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