Read more: “Instant Expert 32: Nuclear energy“
With the world’s population set to rise to 9 billion in 2050, humanity will soon consume more energy than the combined total used in all of history. Even today, we produce greenhouse gases at the rate of 1000 tonnes per second.
Nuclear energy could fulfil our needs without generating any greenhouse gases, but we will need to find novel ways to use it that will reduce its risks
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Thorium: The superfuel?
Why use uranium as the nuclear fuel of choice when another fuel offers the same emissions-free energy without the danger? That’s the argument made by proponents of thorium reactors. They claim that thorium can provide nearly unlimited clean energy without generating long-lived waste or reprocessing dangers. Perhaps because of this promise, India’s future nuclear programme will rely heavily on thorium, and recently China has also joined this race.
Thorium is two places away from uranium on the periodic table, and occurs naturally as a single stable isotope, thorium-232. It is not fissile – its nucleus does not readily split. But it can supply fuel for a reactor if it is converted to uranium-233 by firing neutrons at it (see diagram). It is the same process that bumps uranium-238 to plutonium in existing reactors.
The technology has not yet been fully developed, but claims that it would be immune to the problems that led to the disasters at Chernobyl and Fukushima are unfounded. It is true that thorium technology would produce a fraction of the long-lived radioactive waste generated by standard uranium reactors. However, thorium breeders must derive some of their power from uranium fission. It remains to be seen how the claims made for thorium will stand up.
Nuclear fuel from the ocean
A typical nuclear reactor requires about 200 tonnes of uranium a year. The costs of mining it are rising, because we are rapidly depleting uranium-rich sources. This has created interest in extracting it from the sea. The metal is present in seawater at a concentration of about 3.4 parts per billion, so Earth’s oceans contain about 4000 million tonnes of the stuff. Japanese researchers have developed a kind of “artificial kelp” that preferentially absorbs uranium. Extracting the metal in this way will cost perhaps $1000 per kilogram, and recent innovations promise to make the kelp twice as efficient. It looks as if uranium from seawater is now an affordable source of fuel for reactors, even though it still costs more than the mined material. Environmental aspects of seawater uranium have yet to be analysed.
A new breed of reactor
Who would want a 20-year-old computer? Some likewise baulk at the notion that reactors now being built are based on designs that were new when colour television was introduced. But nuclear reactors don’t evolve at the same rate as computers; decades of testing is necessary to ensure they are safe.
Several new designs are now under consideration (see diagrams). Before they create the next generation of reactors, however, designers must incorporate important lessons from the accident at the Fukushima Daiichi plant in Japan. One lesson is that using water as a coolant poses some dangers because it can vent radioactive steam in the event of a meltdown.
One class of the new designs, known as a breeder reactor, does not use water as a coolant. One example of a breeder, the , is instead cooled by a liquid mixture of lead and bismuth, which allows a higher operating temperature. The molten metals also cannot react violently with air or water, preventing explosions in the unlikely event of a leak.
Breeder reactors would use less fuel than current designs – as little as one-hundredth – instead “breeding” their own fuel. They would produce no “heavy metal” waste; instead recycling and consuming their plutonium, uranium, and heavier elements and isotopes. However, the fission product waste is the same as that from an ordinary reactor.
China expects breeder reactors to supplant light water reactors after 2050, but the future is uncertain; experience with previous breeder designs has been almost uniformly bad. Japan’s Monju and France’s Superphenix have spent more time shut down than operational.
“Why not burn all the world’s nuclear waste in the sun: the ultimate nuclear furnace?”
Nuclear dustbin
The most suitable place to put the world’s growing stockpile of nuclear waste is in repositories deep underground. Sweden and Finland are now building these, but not every country has the right combination of geological features. One possible solution is to create common repositories, commercial ventures that would accept any country’s spent nuclear fuel or other nuclear waste, under the firm regulation of the International Atomic Energy Agency. The European Union has recently authorised member states to use common repositories, even outside the EU.
Over the past 50 years, many speculative alternatives for disposing of nuclear waste have been suggested, some more plausible than others.
Sunshot
Why not send the world’s tens of thousands of tonnes of spent nuclear fuel to a distant nuclear furnace that could easily absorb it: the sun? The answer lies in the tragedies of Columbia, Challenger and every other failed spacecraft. Should a spacecraft carrying used reactor fuel disintegrate over a populated continent, the waste would stay airborne for months, with substantial health consequences.
Transmutation
Exposing nuclear waste to neutrons can convert or “transmute” it into isotopes that decay more rapidly, thus shortening the lifetime of its radioactivity. Although this has long been touted as a solution to the nuclear waste problem, an extensive study by the US National Academy of Sciences in 1996, funded (and since ignored) by the Department of Energy, . Like reprocessing, separation and transmutation add totally new failure modes to the nuclear fuel cycle.
Burial at sea
In a subduction zone, one section of Earth’s crust is dragged beneath another by the slowly flowing mantle. Where this happens at sea, the region where one section sinks below another is marked by an offshore trench. Some people suggest disposing of nuclear waste in such trenches, where it would be drawn deep into the Earth. One problem with the idea is that international agreements prohibit disposal of nuclear waste at sea. Since subduction occurs at the rate of perhaps a centimetre per year, it could take 100,000 years for waste to sink to a depth of 1 kilometre below the seabed.
Boreholes
Another approach is to put nuclear waste in boreholes so deep that the material will never reach the surface again. Putting it in a highly saline environment will stop it from contaminating freshwater aquifers, and suitable sites for holes 5 kilometres deep seem easier to identify than those for mined geological repositories. However, the cost could prove prohibitive, and any failure during placement could prove catastrophic.
This article appeared in print under the headline “Nuclear Frontiers”
