Thorium vehicle will run 100 years on 8 grams of fuel

[PamW]

35. Actually it is NOT a huge assumption...

johnd83,

Actually it is NOT a huge assumption. It's one of the very first things taught in a "reactor physics" course for nuclear engineers.

Here is an excerpt from a text. From page 2 of:

http://www.eolss.net/Sample-Chapters/C08/E3-06-01-04.pdf

The reactor design also separates the fuel from the neutron moderator so that during the moderating or slowing process the neutrons are away from the fuel and less likely to be absorbed by the strong resonance absorption peaks of the non-fissile fuel (U-238).

When you do this calculation, you can calculate what is called the "k-infinity" of the material. The "k-infinity" would be the neutron multiplication constant for a reactor which was infinitely big. If the "k-infinity" of a material is less than 1; then even if you had an infinitely large reactor; it would not go critical.

This is what happens if you mix low-enriched uranium fuel, such as used in LWR reactors, with the appropriate fractions of water, structural steel, control poison.... That would be the composition of your melted reactor core. One can calculate the "k-infinity" of this mixture and it will be less than unity (1). That means that even if you had an infinitely large mass of this material it can't go critical.

If an infinitely large mass of material can't go critical; then neither can a finite-sized mass because the finite sized mass has an addition neutron loss mechanism which is neutrons escaping from the surface of the mass.

The reason the "k-infinity" is less than 1 for the above mixture is due to the parasitic absorption of neutrons by U-238 as they are slowing down in the resonance region of energy.

If you have a heterogeneous lattice with fuel rods immersed in water, when the neutrons slow down passed the resonance region of energy, they are in the water, and there's NO U-238 in the water to absorb them.

PamW