Neutron causing Nuclear Fission how?

[pfalk]

If anyone can help me with this question I'd be greatly appreciative...

Here's what I think I understand:
Nuclear fission is the breaking of an atomic core of an atom. Like with U-235.
It absorbs a slow moving neutron which yields the unstable U-236. Because of the instability the atomic core fissions. Yielding Kr and some other element (can't remember which one) along with 2 neutrons (or more). These 2 neutrons hit other U-235 atoms, amplifying the nuclear fission with each generation of more neutrons.

So, where does the stray neutron come from?
I kind of understand radioactive decay... but I haven't read any mention of a neutron coming out.
Radioactive decay releases alpha particles, beta particles and gamma rays.
Alpha particles are Helium atoms without the electrons, so they are positive and would be repelled from the atomic core of U-236. Also, they have a lot of mass, travel slowly and have short range.
Beta particles are electrons, they move faster, have a longer range but would get repelled from the electron cloud (I think), and wouldn't be able to make an atomic core unstable.
Gamma rays aren't particles but are waves and have no mass. So they wouldn't be able to make an atomic core unstable.

What am I missing?
Everywhere I read the article just assumes this neutron floating around approaching a some heavy elements atomic core. But where does this neutron come from? Is it a 4 type of radioactive decay? Along with alpha, beta and gamma... a neutron can get shot out?

Thanks for any help.

 

[malawi_glenn]

Neutrons can come from sponateous fission, from cosmic rays (high energy muons/protons hitting atmospheric nuclei and producing neutron amongs other particles)
.
And yeah, some nuclei undergoes neutron emission, but those are very very few (if there is any natural occurring nuclei which does so)

 

[Astronuc]

Neutrons may also come from (p,n) reactions, but that would be very inefficient,

from (d+t => alpha + n, or d+d => t + n, and others) fusion reactions,

from photoneutron reactions, with d or Be-9, and

from alpha + Be9 spallation reactions. The source of alpha particles could be Ra, Po, or Pu, or other alpha emitters.

Nuclear reactors use a photoneutron source for startup, and continue to use the sources until they have accumulated fuel with sufficiently high burnup, which then has sufficient amount of spontaneous fissions from transuranic radionuclides which can be used instead of the startup source.


See page 108 of this document - http://hss.energy.gov/NuclearSafety/techstds/standard/hdbk1019/h1019v1.pdf

Examples of intrinsic neutron sources are listed below:

• Spontaneous fission of heavy nuclides in fuel, such as uranium-238, uranium-235, and plutonium-239, results in fission fragments and free neutrons.
• Boron-11 mixed with the fuel undergoes an alpha-neutron reaction and becomes nitrogen-14.
• Deuterium present in the reactor coolant undergoes a gamma-neutron reaction and becomes hydrogen-1.

Examples of installed neutron sources are listed below:

• Spontaneous fission of californium-252 results in fission fragments and free neutrons.
• Beryllium-9 undergoes an alpha-neutron reaction (alpha from the decay of plutonium, polonium, or radium) and becomes carbon-12.
• Beryllium-9 undergoes a gamma-neutron reaction (high energy gamma from decay of antimony-124) and becomes beryllium-8.

 

[pfalk]

Thanks for the answers.

Making a bit more sense now.