In previous ‘numbers’ pages, we started with the easier estimates. The basic number on which all of our estimations are based is that a single metric tonne of either U or Th, is enough to produce the electricity needed to power a western city for a year.
In short 1 tonne = 1GWe-yr.
This was a rough calculation (you actually need only about 800 kgs), but there was no guessing involved.
In our next step, we had to start guessing. How many people will live on this planet, how much power will they use? We guessed: 10 billion people, they will want to live according to the British standard of living of the year 2009 (because we have those numbers, provided by David MacKay).
With the next step, we want to estimate how long our supplies of uranium and thorium will last, if we would have to satisfy these different levels of demand, with power produced with molten salt reactors.
Here, things may get a little confusing. On the internet, and in the press for that matter, you find quotes varying from ‘we only have the uranium for a couple of decades’ to ‘we have enough uranium to last us for millions of years’. Who are you going to believe? Short answer: nobody. Just find reliable sources, start calculating and start understanding.
Uranium for 150 years?
Ask a geologist how much uranium we have and he won’t give you an easy answer. Or maybe he will, but then the answer is not of much use. The simple answer is: the earth’s crust contains 2,8 parts per million (ppm). That’s enough uranium to serve us until the time the sun turns into a red giant, more than a billion years from now. But it would mean ploughing over the planet and most people would want to avoid that – so let’s get practical.
Uranium is literally everywhere, in rocks and in oceans. How much of it we can use, depends on how hard we look for it and on what we are willing to pay for it. Let’s start with a moderate estimate of available resources of uranium. On world-nuclear, we see the known supplies of the world: 5.327.000 tonnes. In our extreme scenario, using 70.000 tonnes per year, this would last us 76 years. Not really a an impressive number. Even if we add the known supplies of thorium (3.385.000 tonnes worldwide), we would only roughly double this number to, say, 150 years. (To make things appear even worse, the number of tonnes per year in our extreme scenario is almost the same as the amount our present nuclear power plants use: 68.000 tonnes annually. That’s mainly because conventional nuclear reactors use only about 0,5% of the energy content of the uranium)
Let’s have a better look. The quantity of thorium quoted above (5.327.000 tonnes) is the thorium that can be sold for the market price of 80$ per kg (and hence, must be produced cheaper). What if we are willing to pay more? How much more uranium and/or thorium does that make available? For instance for Thorium, the Atomic Energy Commission has studied the available resources in 1969 (section 4.2, table 4.2). Of this thorium, we’ve hardly used anything since those days. The report raises the question how much thorium is recoverable at a price of 500$/kg in 1969 dollars, perhaps 3000$/kg today. The answer is 3 billion short tonnes or 2.700.0000.000 metric tonnes, enough to last us 40.000 years in our extreme scenario. For uranium, the figures will be not much different. (And no, 3000$/kg is not a ridiculous price. At this price, we’d need to pay $3.000.000 for the fuel to produce 1GWe-yr. And 1 GWe-yr equals 8.760.000.000 kWh, which means a fuel cost of $0,0004 per kWh.) This means that even in our extreme scenario, the combined uranium and thorium of the United States would be enough to power the world for about 100.000 years.
How sustainable do you want it to be?
If that is not enough to be called ‘sustainable’, consider yet another option: seawater. Uranium forms soluble salts and the seas contain 0.003 ppm Uranium. Again, that doesn’t sound like much, but according to Masao Tamada of the Japanese Atomic Energy Agency it adds up to about 4.5 billion tons, adding another 64.000 years of sustaining our extreme scenario. The technique of winning this sea-uranium is still in its infancy, but Japanese researchers have succeeded in winning it at a cost of $240/kg. And here’s an article that describes the technique of extractring the uranium. The production speed is still very low and not nearly enough for the yearly refill of a single molten salt reactor, but we have all the time in the world to improve our technique… Still not satisfied on the sustainability? The concentration of the uranium in the sea is an equilibrium. Meaning: if we take some out, nature will refill the store through rivers and rock-weathering – it already does: rivers carry uranium to the sea all the time.
How are we going to finish that?
Charles Barton – a respected blogger on the subject of molten salt reactors (he grew up as the son of Oak Ridge researcher Charles J. Barton, Sr., who worked on molten salt reactors) – estimates that dissolved natural uranium from terrestrial sources, that rivers continually carry to the seas, amounts to about 32,000 tons per year*. [*=tons yellow cake, still have to recalculate for uranium, but that won’t change the argument-GZ]. Finally, uranium in seawater is in equilibrium solution. Charles Barton: ‘Added dissolved uranium causes other dissolved uranium to precipitate out of sea water. The uranium precipitation is deposited on the sea bottom, but may re-dissolve at some future time.’ In short: even in our extreme use scenario, we won’t run out of uranium.
And remember, our extreme scenario was pretty extreme: energy produced by solar and wind, and saved by energy conservation, were all discarded. While in reality, these will fill in a substantial part of our energy demands. These sources combined will provide us with all the energy we need.
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