I'm going to assume that any non-dystopian future will involve a great deal of large- and small-scale wind, solar, geothermal, and tidal power generation. But I'm not sure we're simply going to transition easily from smokestacks to sunshine. The nuclear option is going to play a part too. For example, pebble-bed reactors are set to power petroleum recovery from Canadian tar sands.
Hard-Core Approach Many advocates of nuclear technology propose a "more of the same, only better" strategy. Bernard L. Cohen, Professor Emeritus of Physics at the University of Pittsburgh, may be its most credible and passionate spokesman. For details, see www.phyast.pitt.edu/~blc/.
Physics Today published one of Cohen's letters in its November 2004 issue. Cohen was responding with his usual enthusiasm to an earlier article in which another physicist said that breeder reactors could "provide the world's energy needs for hundreds of years."
"The world's energy needs could be provided by uranium-fueled breeder reactors for the full billion years that life on Earth will be sustainable, without the price of electricity increasing by more than a small fraction of 1% due to raw fuel costs," Cohen wrote, citing his own 1983 article from the American Journal of Physics.
Cohen also pointed out that the author of the previous article erred in his calculations by referring to uranium at its then current price of $10 to $20 per pound. "But in breeder reactors, 100 times as much energy is derived from a pound of uranium as in present-day light water reactors, so we could afford to use uranium that is 100 times as expensive," he continued.
"The cost of extracting uranium from its most plentiful source, seawater, is about $250 per pound—the energy equivalent of gasoline at 0.13 cent per gallon! The uranium now in the oceans could provide the world's current electricity usage for 7 million years," he explained.
"But seawater uranium levels are constantly being replenished, by rivers that carry uranium dissolved out of rock, at a rate sufficient to provide 20 times the world's current total electricity usage. In view of the geological cycles of erosion, subduction, and land uplift, this process could continue for a billion years with no appreciable reduction of the uranium concentration in seawater and hence no increase in extraction costs."
Nuclear Waste I'm reluctant to embrace the "more of the same, only better" approach. My concerns boil down to waste disposal and the production of weapons-grade plutonium in fast breeders. The February 2007 IEEE Spectrum featured a very good article on waste disposal, which included a sidebar on Greenpeace's unhappiness with the transport of nuclear waste in France. (see "Nuclear Wasteland").
"Greenpeace found a dramatic means of spotlighting the vulnerability of the supposedly top-secret plutonium shipments between La Hague and Areva's mixed-oxide (MOX) fuel plant at Marcoule: it intercepted a convoy carrying more than 138 kg of plutonium in the center of Chalonsur-Saône, a small city in Burgundy, and invited the French media along for the show," the article explained.
At least the French haul the stuff somewhere. In the U.S., waste stays put. When the cooling tower at the decommissioned Trojan nuclear plant at Rainier, Ore., was demolished back in 1999, the old reactor core was barged up the Columbia River to Hanford, Wash. However, "thirty-four spent nuclear fuel rods remain at Trojan in 17-foot tall steel-lined casks. The rods will stay put until they can be moved to Nevada's Yucca Mountain nuclear-waste dump—when it opens," wrote the Seattle Times.
Pretty Pebbles The term "pebble-bed" refers to helium-cooled, graphite-moderated, high-temperature reactors in which graphite-coated uranium spheres are continually cycled through a funnel-shaped vessel that surrounds a tube through which flows the helium working fluid for a turbine generator (Fig. 1). The reactor is contained in a vertical steel pressure vessel lined with graphite bricks that reflect neutrons generated by the nuclear reaction back into the core and transfer heat away from the core.
Vertical holes in the graphite lining contain control elements. Where the spheres are densely packed in the neck of the vessel, they generate heat, which is passed to the working fluid. The continuous fueling scheme ensures that there's never a large amount of excess reactivity within the core.
Typically, the spherical pebbles measure 60 mm (2.36-in.) in diameter and weigh 210 g (Fig. 2). Each contains 9 g of uranium. The helium working fluid enters the reactor at about 500°C and 80 atmospheres. It flows down through the pebble bed, is heated to about 900°C, and is sent to a three-stage turbine/generator. Compared to steam, helium is a much better working fluid. It won't change phase, and it's chemically inert.
Power output is in is the 10-MW range, about 10% of the output of conventional nuclear power reactors. The idea is to distribute power production to preclude single-point-of-failure problems and to locate generation sources close to loads, much like natural-gas generating stations today.
The pebble-bed approach's supporters cite a number of advantages, not least of which is safety. Negative feedback places an upper limit on fuel temperature. It rises to a designed "idle" temperature and stays there. Also, there's no physical process that can cause an induced radiation hazard outside a radius of about 400 m.
Finally, the silicon-carbide coatings surrounding the uranium fuel particles in each pebble essentially constitute a pressure vessel, which serves as a barrier against the release of fission products that would contaminate the primary circuit. Specifically, the graphite shell around the uranium remains stable up to 2800°C, much hotter than the normal-operation maximum temperature (1200°C) or the 1600°C maximum temperature designers anticipate if all forced cooling breaks down. Basically, the core can't melt down.
Looking at that last point a little more closely, the power density in proposed pebble-bed reactors is low enough so given the high thermal conductivity of the graphite and the thermal inertia of the core, the core temperature is calculated not to exceed 1600°C, even under worst-case conditions. Empirically, experimental fuel pebbles have been shown to operate at 1600°C without releasing fission products.
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