The Thorium Story

What is thorium?

It is a basic chemical element, atomic number 90.  (Uranium has an atomic number of 92).  Thorium is about four times more abundant than Uranium.  It is also more abundant in the earth’s crust than gold, silver, mercury, tungsten, and tin.  As Thorium is found in nature, it is not radioactive and more than 99.99% of it is a single isotope, ²³²Th. This means that it is almost ‘ready to go’.  No expensive and dangerous enrichment process.

Where can thorium be found? 

All over the US, but larger concentrations exist in the western US.  The US is estimated to have between 15% - 19 % of the world’s supply.  Most people in the field think that no one has really tried hard to find all the Thorium yet.  Where is a good place to look?  Uranium mine tailings.  It is a by-product.  This means that we won’t have to send platoons of grizzled old prospectors on donkeys out to find it or bulldoze huge tracts of land.  Another potential easily accessible source might be coal ash piles.  The US also has a stockpile of 3200 metric tonnes sequestered in Nevada.  In a ramped up development program, this could meet all of the US’s electricity needs for at least the first five years.

How powerful is it? 

You could get enough power for your entire life from a thorium ball smaller than that used for golf.  But to unleash the power, you have to bombard it with neutrons.  After absorbing two, a thorium atom morphs to fissionable ²³³U.  This is a relatively well-behaved form of uranium.  The worst of its byproducts has a half life of about 350 years compared to today’s 350,000 years for those from ²³⁵U or ²³⁹Pu (plutonium).  Thorium provides a one-thousand times advantage in this alone. 

If Thorium is so good, why haven’t we heard more about it? 

Good question.  The US built a reactor cooled by molten salt in the sixties.  The project was headed by Alvin Weinberg, the father of the light-water uranium reactor.  They ran it everyday for four years turning it off every night by turning off the fan on the freeze-plug.  The salt in the pipe melted and the fuel emptied into a drain tank whose geometry did not support continued fission.  In the morning they would turn on the fan and pump the fuel back into the reactor.  But Dr. Weinberg ran afoul of the Atomic Energy Commission for claiming that molten-salt reactors were inherently safer than light water reactors.  They asked him to leave and the project was shut down.  Mixed in the discussion was the fact that they had determined that thorium was not very good for making nuclear bombs and the military favored ²³⁵U and ²³⁸U for its plutonium byproduct potential. When the cold war ended, no one reintroduced the issue of what the best approach to energy production should be.

Reactor design has a lot of do with efficiency and safety. By using molten salt (thorium fluoride) you avoid a lot of traditional problems.  If you have a spill, the fuel cools and turns solid; no ground water contamination.  With the fuel in liquid you can recycle it continually; 100% utilization versus 1% with solid fuels. With no water and hydrogen in the reactor there is no chance of explosion; no need for an expensive, high-pressure containment facility. This allows for standard, manufacture-able designs with systematic quality control. Unlike today’s 104 plants, with 104 safety interpretations.  And today’s nuclear waste can also be used and consumed with the waste half-life lowered to thorium proportions.  A Thorium fluoride molten salt reactor provides passive safety features, like the freeze plug, that automatically move the reactor to a safer state during disasters without manual intervention.