Nuclear power is presently a sustainable energy source, but could become completely renewable if the source of uranium changed from mined ore to seawater. Since U extracted is continuously replenished through geologic processes, nuclear would become as endless as solar.
But do renewable and sustainable mean the same thing in the energy world?
Not necessarily. As Professor Jason Donev from the University of Calgary puts it, “Not everything renewable is sustainable, and in turn not everything which is sustainable is necessarily renewable.”
HiCap Adsorbents from Oak Ridge National Laboratory and Hills Inc. for extracting uranium from... [+]
Renewable literally means 'to make new again'. Any resource that naturally replenishes with time, like the creation of wind or the growth of biological organisms for biomass or biofuels, is certainly renewable. Renewable energy means that the energy humans extract from nature will generally replace itself.
On the other hand, a sustainable energy source can be maintained for a definable period of time, one whose total amount will last for the period of human history that needs it, at the rate it is being used or expected to be used. It may or may not be renewed at some rate.
Human energy use is dominated by a small list of primary energy types, of which the following sources are considered to be renewable: Solar power, Wind power, Hydropower, Tidal power and Geothermal energy
Energy sources are considered non-renewable if they take a very long time to be created, like fossil fuels, or if their creation happened long ago and is not likely to happen again, like uranium.
However, we are not running out of coal, oil or natural gas anytime soon. Thanks to fracking, our oil and natural gas reserves keep growing. Just look at Donev’s data visualization to see what’s happened to the amount of economically-recoverable natural gas as a function of time. What was thought to be global gas reserves at the time went from 60,000 Mtoe in 1980 to 170,000 in 2013.
Natural gas is not renewable but current technology allows us to access such a staggering amount, that it may seem infinite by past standards. And future technologies will extract even more. Keeping humans from using so much accessible fossil fuel just to protect the environment will be very, very difficult.
But even a renewable energy resource becomes unsustainable whenever it's used faster than it regenerates. This is often seen for geothermal energy, where the heat is not renewed fast enough by the hot rocks and the temperature at the inlet decreases below the point that produces steam.
Conversely, a non-renewable resource can be sustainable if it's used at a slow enough rate that supplies last for thousands of years, and the environmental impacts don't cause huge problems. This is most obvious with respect to nuclear power. Like fossil fuel, the amount of uranium available from conventional sources has long been underestimated. Reserves are only calculated for economically recoverable sources using present-day technologies and pricing. Both keep getting better.
Better yet, the advanced nuclear reactors that will replace our existing reactors use much more of the fuel than present-day reactors. Small modular reactors can better tailor energy use to demand. Molten salt reactors obtain up to ten times the energy from the same amount of uranium because the fission products and reaction poisons are removed as it goes. Fast reactors obtain at least ten times the energy from existing nuclear fuel by burning everything, not just U-235.
Just using existing uranium from U-mine sites, as well as burning existing spent fuel in fast reactors in the near-future, provides sufficient uranium fuel to produce 10 trillion kWhs/year for thousands of years, making it presently sustainable by any measure.
But using U extracted from seawater, instead of mining uranium ore, makes nuclear truly renewable as well as sustainable. The amount of U in seawater is only 3.3 micrograms/liter (parts per billion), but that totals 4.5 billion tons of U in the billion cubic kilometers of seawater in the ocean.
But these numbers are not static. The uranium mined from normal uranium rock formations and burned in reactors is gone forever. But uranium extracted from seawater is replenished continuously.
Seawater concentrations of U, as well as all other dissolved constituents, are determined by steady-state chemical reactions between waters and rocks on the Earth, both in the ocean and from continuous weathering of continental rocks on land. Leaching and weathering of the crustal rocks continuously replenish all elements in seawater, and it is impossible for humans to extract enough U to lower the overall seawater concentrations of U faster than it is replenished.
As with any commercial commodity, the process of seawater extraction must become sufficiently economic to replace mining as the source of U. And new technologies of extracting U from seawater are fast becoming economic.
Research and development on extracting U from seawater has been ongoing since the 1960s, especially in Japan, including inorganic adsorbents, absorbent polymer fibers and uranium-specific nonwoven fabrics. In 2012, DOE announced development of a new adsorbent material called HiCap (pictured above). According to Dr. Per Peterson at UC Berkeley, these improvements have reduced seawater extraction production costs to between $100 and $300 per pound of yellowcake (U3O8). It is expected to fall further in the next ten years.
The cost of uranium is a small percentage of the cost of nuclear fuel, which is itself a small percentage of the cost of nuclear power. Over the last twenty years, uranium spot prices have varied between $10 and $120/lb of U3O8. The differences have mainly resulted from changes in the availability of weapons-grade uranium to blend down to make reactor fuel.
So when the cost of extracting U from seawater falls to below $100/lb, then it will become a commercially viable alternative to mining new uranium ore. And nuclear power will become completely renewable and sustainable for as long as humans need energy.
