A nuclear fission chain reaction of U-235 starts with a slow neutron. What produces this first neutron?

Answer 1

Uranium is naturally radioactive; it's unstable. Somewhere in a sample of uranium, spontaneous fission occurs. All the time. Neutrons are released in this reaction. It cannot be stopped. If critical mass is assembled, a couple of neutrons will appear from a spontaneous fission somewhere within the sample, and a chain will immediately begin to build. There is no way to stop it except by separating the mass into subcritical quantities. But the reaction will do that. It happens in the twinkling of an eye. Always. Note that a so-called fast neutron, a neutron with a lot of kinetic energy, can cause fission. But it has a lot lower probability of doing that than a thermal neutron. Slowing down or "thermalizing" of neutrons increases the chance that they will be captured, and neutron capture will build a chain reaction. This was included because the question stated that a chain starts with a slow neutron, and this might not be the case. It is the slow neutrons that drive the fission chain in nuclear reactors. They have moderators to slow the neutrons down. But the fast neutrons are the chain builders in nuclear weapons. In a nuclear weapon, we don't put moderator material in the thing. We might incorporate some neutron mirrors or lenses in the geometry of the device, but we rely on the a lot of fast neutrons to carry out the mission of burning the fissile material very rapidly to get a big yield. The proof is in the pudding.

Answer 2

A neutron source. Examples are:

• Polonium alpha source, beryllium absorbs alpha and emits neutron.
• Radium alpha source, beryllium absorbs alpha and emits neutron.
• Electrically activated tritium fusion neutron source.
• etc.

For a bomb the source must emit roughly 10 neutrons during the nanosecond of maximum supercriticality (10 billion neutrons per second) to prevent a fizzle and maximize yield.

For a reactor in startup a much less intense source is needed but it must be continuous (the source in the bomb can be a short pulse).