The back end

‘Sustainable’ fission?

11 December 2009

How great are the worldwide supplies of uranium? Do we have only a few decades’ worth of uranium, or do we have enough for millennia? To estimate a sustainable power from uranium, I took the total recoverable uranium in the ground and in seawater, divided it fairly between six billion humans, and asked: “How fast can we use this if it has to last 1000 years?”

Almost all the recoverable uranium is in the oceans, not in the ground: seawater contains 3.3mg of uranium per cubic metre of water, which amounts to 4.5 billion tons worldwide. I called the uranium in the ocean ‘recoverable,’ but this is a bit inaccurate – most ocean waters are quite inaccessible, and the ocean conveyor belt rolls round only once every 1000 years or so; and no-one has yet demonstrated uranium-extraction from seawater on an industrial scale. So we’ll make separate estimates for two cases: first using only mined uranium, and second using ocean uranium too.

The uranium ore in the ground that is extractable at prices below $130/kgU is about one thousandth of the amount in seawater (4.5 million tons). If prices went above $130/kg, phosphate deposits that contain uranium at low concentrations would become economic to mine. (Recovery of uranium from phosphates is perfectly possible, and was done in the USA and Belgium before1998). For the estimate of mined uranium, I will add both the conventional uranium ore and the phosphates, to give a total resource of 27 million tons of uranium.

A once-through 1000MW nuclear power station uses 162tU/y. So the known mineable resources of uranium, shared between six billion people, would last for 1000 years if we produced nuclear power at a rate of 0.55kWh per person per day. This sustainable rate is the output of just 136 nuclear power stations, and is half of today’s nuclear power production. The average European energy consumption is 125kWh per day per person. It’s very possible this is an underestimate of uranium’s potential, since, as there is not yet a uranium shortage, there is no incentive for exploration and little uranium exploration has been undertaken since the 1980s; so maybe more mineable uranium will be discovered.

But could our current once-through use of mined uranium be sustainable? It is hard to say, since there is such uncertainty about the result of future exploration. Certainly at today’s rate of consumption, once-through reactors could keep going for hundreds of years. But if we wanted to crank up nuclear power 40-fold worldwide, in order to get off fossil fuels, we might worry that once-through reactors are not a sustainable technology.

Uranium can be used 60 times more efficiently in fast breeder reactors, which burn up all the uranium – both the U-238 and the U-235. As long as we do not throw away the spent fuel that is spat out by once-through reactors, this source of depleted uranium could be used too. If we used all the mineable uranium (plus the depleted uranium stockpiles) in fast breeder reactors, the power would be 33kWh per day per person.

“Known mineable resources of uranium, shared
between six billion people, would last for 1000 years,
but only at a rate of 0.55kWh per person per day”

The oceans’ uranium, if completely extracted and used in once-through reactors, corresponds to a total energy of 28 million GW-years per planet.

How fast could uranium be extracted from the oceans? The oceans circulate slowly: half of the water is in the Pacific Ocean, and deep Pacific waters circulate to the surface on the great ocean conveyor only every 1600 years. Let’s imagine that 10% of the uranium is extracted over such a 1600-year period. That’s an extraction rate of 280,000t/y. In once-through reactors, this would deliver power at a rate of 1750GW (around four times the current installed nuclear capacity), which, shared between 6 billion people, is 7kWh per day per person. I conclude that ocean extraction of uranium could turn today’s once-through reactors into a “sustainable” option – assuming that the uranium reactors can cover the energy cost of the ocean extraction process.

If fast reactors are 60 times more efficient, the same extraction of ocean uranium could deliver 420kWh per day per person. At last, a sustainable figure that beats current consumption! – but only with the joint help of two technologies that are respectively scarcely-developed and unfashionable: ocean extraction of uranium, and fast breeder reactors.

So is the potential contribution of ocean-uranium-based power economically plausible? Japanese researchers have found a to extract uranium from seawater at a cost of $100–300/kgU, in comparison with a current cost of about $20/kgU from ore.

We are not home yet: does the Japanese technique scale up? In the Japanese experiment, three cages full of adsorbent uranium-attracting material weighing 350kg collected “more than 1kg of yellow cake in 240 days;” this figure corresponds to about 1.6kg/y. The cages had a cross-sectional area of 48m2. To power a once-through 1GW nuclear power station, we need a production rate 100,000 times greater than the Japanese experiment’s. If we simply scaled up the Japanese technique, which accumulated uranium passively from the sea, a power of 1GW would thus need cages having a collecting area of 4.8km2 and containing a weight of 350,000t of adsorbent material – more than the weight of the steel in the 1000MW reactor itself.

Author Info:

An extract from Sustainable Energy–without hot air by David JC MacKay. David MacKay was appointed to be Chief Scientific Advisor of the UK’s Department of Energy and Climate Change in September 2009.

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