Electronic Design

Can Greens And Nukes Coexist?

The future probably inolves a mix of energy-generating technologies.

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.

Dirty Pebbles
On the radioactive-waste side, the pebble-bed reactor is slightly better than conventional reactors, but the picture still isn't pretty. A reactor of the anticipated scale would generate about 19 tons of spent fuel pebbles per year. That's mostly the weight of the graphite. Less than one ton would be depleted uranium.

Spent pebbles are easier to store than fuel rods, because the silicon-carbide coating around the fuel particles will keep the radioactive decay products isolated for approximately a million years—if they're kept safe somewhere. The real problem lies in the virtue of pebble-bed reactors: They're small-scale power generators that lend themselves to a geographically distributed power-production model. That adds up to a lot of trucks hauling fresh and depleted carbon spheres full of uranium hither and thither across the landscape.

Pebbles Worldwide
The first effort to create a commercial pebble-bed reactor, the South African PBMR (pebble bed modular reactor), is a project of Eskom, the South Africa electrical utility, and some partners. As of 1998, construction of a demonstration plant was supposed to begin in 1999 with completion by 2003. Commercial orders would follow. At that time, Eskom projected a worldwide market of about 30 units per year.

This year, the May 3 Business Day reported that "The PBMR Company has set itself the deadline of 2011 for the construction of a pilot pebble bed nuclear reactor in Cape Town and a pilot fuel plant at Pelindaba outside Pretoria. Commercial production is scheduled to commence in 2012." To be fair, the causes of the schedule slippage appear to have been legal challenges more than technical problems. But that's part of the environment we live in, and engineers and entrepreneurs need to take it into account.

Alternative designs include the gas-turbine modular helium reactor (GTMHR) (Fig. 3). The most fascinating thing about the GT-MHR is that it was developed in Russia under a joint U.S.-Russia agreement to cooperate on the development of systems for the disposition of surplus weapons plutonium. Apparently, pebbles can contain plutonium, uranium, thorium, or spent fuel from conventional reactors.

The GT-MHR's ability to function as a commercial power generator is frosting on the cake. Partners in the GT-MHR include General Atomics in the U.S., the Russian Federation Ministry for Atomic Energy (MINATOM), Framatome in France, and Fuji Electric in Japan.

The earliest pebble-bed research, on which the South African design is based, was carried out in Germany. The latest pebble-bed reactor to be fired up is the HTR-10, which uses steam rather than helium as its working fluid. It is located at the Institute of Nuclear Energy Technology (INET), a unit of Tsinghua University, near Beijing.

The Chinese State Council approved the project in March 1992. Ground was broken in 1994, and construction was completed in 2000, with criticality achieved on December 1, 2000. It began full operation in 2003. In 2006, HTR-10 operated for a total of 97 days and reactor power reached 458.2 Megawatt Days (MWD). That provided 660 MWh to the grid as well as steam heating for the INET campus.

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