With everyone worried about carbon emissions and sustainability, the idea of harnessing the energy from nuclear fission comes up for discussion now and again. It makes sense: there's an abundance of an otherwise currently useless substance with an incredibly high energy density, and this one doesn't put more carbon in the atmosphere. It does create nuclear waste, the comparative danger of which remains to be seen; and the threat of terrorists with access to market plutonium has everyone scared shoeless.
Some people think Thorium is much better than either the Uranium-235 we use now for nuclear power or the Uranium-238 that we use to make Plutonium. Could it be the answer?
Thorium is more abundant than uranium and, although not fissile, it can be used as nuclear fuel because when it is irradiated with slow neutrons fissile uranium-233 is produced. This is typically achieved by using a 'blanket' of thorium around a 'seed' of uranium fuel. A thorium fuel cycle leads to the production of only very small amounts of plutonium, and is seen as being more proliferation-resistant than the uranium fuel cycle. It also has advantages over uranium fuel in terms of the radiotoxicity of the spent fuel. Thorium fuel has been studied in various countries for at least 30 years, but abundant uranium supplies have in the past lessened the need for its full development for commercial use.
If there were an energy currency made of Thorium, how much energy would we get from a single coin? The answer requires some research and math.
A UIC Nuclear Issues Briefing Paper states parenthetically that "the French self-breeding variant claims 50kg of thorium and 50kg U-238 per billion kWh". Many thorium-positive sites state, and the web-at-large repeats verbatim, that "The energy contained in one kilogram of Thorium equals that of four thousand tons of coal. The CDC repeats a similar metric, that "one kilogram is equivalent to about 22 million kilowatt-hours of heat energy".
For a sanity check, I did the math myself. Wikipedia reports the total decay energy of the decay chains for U-233, which naturally decays down to Thallium-205, releasing 43 MeV (mega-electron-volts are the units of energy on the scale of a single nuclear decay). Multiply by Avogadro's number and that should be the energy released from one mole of Thorium-232/Uranium-233 (ignoring the cost of breeding), which would be about 232 grams.
To compare in standardized form:
| source | total energy (terajoules/kilogram) |
|---|---|
| UIC | 36 |
| web-factoid | 80 |
| CDC | 79 |
| my physics | 18 |
I have no idea why my calculation of the available energy is so much lower than other people's. My physics is rusty and probably wrong, but it's in the same ballpark, and it's lower than even the low and reasonable-sounding claims of the French. So I think it serves as a somewhat credible lower bound, if you trust our knowledge of physics and math (and have hopefully verified both above).
Now for the fun part. A US silver dollar is 26.5mm in diameter and 2mm thick. Thorium has a density of .011725 g/mm^3 (for comparison, Uranium is .019 g/mm^3). The volume of one coin is 1100 mm^3. Therefore the mass of the coin is about 13 grams. So about 75 "thorium dollars" per kilogram.
Therefore, a coin the size of our silver dollar made of thorium would contain, by the most conservative estimate of my research and calculations, 200 gigajoules, or one fifth of a terajoule. If you are willing to believe factoids, one coin fully burnt would release one terajoule of energy.
One terajoule, by the way, is an unbelievable amount of energy: over 6,000 gallons (25,000 liters) of gasoline; 250 tons of TNT; enough energy to power your average American home (10 kW) for over 3 years.
Of course, to get that energy in 20 years instead of 20 billion years, you have to have at least one billion-dollar breeder reactor and a $50m/year staff of scientists and technicians to run it. But with enough of those, our known stores of thorium would cover our entire global energy consumption for thousands of years. That's an order of magnitude more than our most optimistic estimates for the reserves of all of our fossil fuels combined.
The ultimate problem, however, is not solved. How are Thorium and Uranium made? Are these renewable resources? Is a nuclear energy economy sustainable?
All heavier elements must have been created in a supernova from before our solar system existed. So burnt Thorium will never regenerate in the lifetimespace of our solar system. Every other known fuel either gives a fixed amount of energy and is expected to last for millions if not billions of years (the tide and geothermal energy); or regenerates eventually with energy input from the sun (even coal and oil only take a few million years to produce on a lifeful planet). But Thorium and Uranium, for all their power, are a fixed resource that can never be replaced.
Now at this point in the discussion some optimist usually disagrees, saying that there's a thousand or more times as much of nuclear fuel or any material we want on other planets and asteroids. Okay, that is probably true; but it takes a tremendous amount of energy to cross such vast distances with the payload and crew required to mine it and bring it back (which of course is not socially sustainable--eventually they won't and Earth will perpetually be at the mercy of the United Asteroid Emirates). What's the most compact and reasonable fuel we've found so far? No one cares if you leak radioactive waste in space.
Transportation on this planet only exploded once we got a compact and efficient fuel and the internal combustion engine. We've already discovered the next compact and efficient fuel; transportation between planets will explode once we have the internal fission engine. We need to continue research, and thus we will need nuclear power plants of many different varieties. But we don't know everything yet, and we need to take it slow. We can't afford to go whole hog on nuclear power, tempting though it be; we need to save our nuclear material to use as fuel for the coming space era, and sustain ourselves in the meantime with the tides and the wind and the colossal amounts of energy coming to us every day from that greatest nuclear power plant, the Sun.