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Kirk Sorensen Contributor

nuclear and aerospace engineer full bio →

Opinions expressed by Forbes Contributors are their own.

Founder and Chief Technologist, Flibe Energy, Inc., May 2011-present Chief Nuclear Technologist, Teledyne Brown Engineering, May 2010-May 2011 Aerospace Engineer, NASA Marshall Space Flight Center, Mar 2000-May 2010 MS, aerospace engineering, Georgia Tech, Dec 1999 BS, mechanical engineering, Utah State, Jun 1998

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Fissile Material and the Threshold of Two

Fissile material is worth six times its weight in gold. Fissile material is worth more than that because it can create new fissile material from nearly worthless uranium-238 and thorium. This gives fissile material the ability to keep creating more fissile material even as it is releasing the incredibly valuable energy of fission.

The key to this amazing ability is the threshold of two.

A fission event consumes a fissile nucleus and the neutron that caused the fission. At least one more neutron must be created from the fission event to continue the fission reaction. Another neutron is necessary to create a new fissile nucleus from fertile material like uranium-238 or thorium. That’s the reason why two is the important number in a fission reaction.

At least two neutrons must be created. One to continue the fission reaction, one to create a fissile nucleus to replace the one that was consumed. If less than two neutrons are created in fission, then the Midas touch of fissile material is lost and a nuclear reactor will simply burn away its fissile inventory (the amount of fissile material it contains). The reactor will make money, but it will do so burning up naturally fissile uranium-235, a very valuable and rare resource. It will not access the far greater riches that await in burning up cheap and abundant uranium-238 or thorium.

If you click on this graph you’ll see the performance of the three main fissile fuels, uranium-233, uranium-235, and plutonium-239 at a range of energies. Uranium-233 comes from thorium, uranium-235 is found naturally, and plutonium-239 comes from uranium-238, which is common.

The number of neutrons generated in a fission is plotted on the vertical axis, and the energy of the neutron that caused the fission is plotted on the horizontal axis. The horizontal axis is a logarithmic scale where each increment is a power of ten. This allows the graph to show all the data from 0.001 electron-volt all the way to 1 million electron volts. The energies that correspond to slowed-down, thermal neutrons are shaded in blue, and the energies that correspond to fast neutrons are shaded in magenta.

The lines on the graph are pretty flat. This means that they don’t depend very much on the energy of the neutron that caused fission. Slowed-down neutrons work just about as well as fast neutrons, at least when it comes to producing neutrons from fission.

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