Monday, January 02, 2006

A New Breed of Nuclear Reactors?: BLOG: SciAm Observations

A New Breed of Nuclear Reactors?: BLOG: SciAm Observations

A New Breed of Nuclear Reactors?
Does the world need a new kind of nuclear reactor? Does it want one? Those are the questions Matthew Wald addressed in his New York Times article (Dec. 27) about the proposal by some scientists to resuscitate the idea of breeder reactors, ones based on an electrorefining process for stripping the degraded fission products from nuclear waste and leaving the still-usable uranium and plutonium in a more concentrated form. Readers may recognize this as the technology that William H. Hannum, Gerald E. Marsh and George S. Stanford of Argonne National Laboratory described in their December Scientific American article, "Smarter Use of Nuclear Waste."
That article focused primarily on the "how it would work" aspects of the technology, while also laying out the rationale for it. Briefly, here's the pitch: If we're to produce adequate energy for the future while curbing global warming, we might need to rely more on nuclear fission. But conventional fission plants have two liabilities. First, they leave behind 95 percent of the fissionable energy in their fuel. Second, as a consequence of the first, their voluminous wastes are highly radioactive for thousands of years. A new type of breeder reactor, however, could make more efficient use of the fuel and reduce the waste stream to a more manageable level. Moreover, unlike older breeder reactors, the new ones would not be attractive to terrorists or rogue states seeking plutonium for bombs.
Here's one description from Hannum et al. of the new reactors:
A safer, more sustainable nuclear power cycle could be based on the advanced liquid metal reactor (ALMR) design developed in the 1980s by researchers at Argonne National Laboratory. Like all atomic power plants, an ALMR-based system would use nuclear chain reactions in the core to produce the heat needed to generate electricity.
Current commercial nuclear plants feature thermal reactors, which rely on relatively slow moving neutrons to propagate chain reactions in uranium and plutonium fuel. An ALMR-based system, in contrast, would use fast-moving (energetic) neutrons, which permits all the uranium and heavier atoms to be consumed, thereby allowing vastly more of the fuel's energy to be captured. In the near term, the new reactor would burn fuel made by recycling spent fuel from thermal reactors.
In most thermal reactor designs, water floods the core to slow (moderate) neutrons and keep it cool. The ALMR, however, employs a pool of circulating liquid sodium as the coolant. Engineers chose sodium because it does not slow down fast neutrons substantially and because it conducts heat very well, which improves the efficiency of heat delivery to the electric generation facility.
A fast reactor would work like this: Nuclear fire burning in the core would heat the radioactive liquid sodium running through it. Some of the heated sodium would be pumped into an intermediate heat exchanger, where it would transfer its thermal energy to nonradioactive liquid sodium running through adjacent but separate pipes and into a secondary sodium loop. The nonradioactive sodium would in turn bring heat to a final heat exchanger/steam generator, where steam would be created in adjacent water-filled pipes. The hot, high-pressure steam would then be used to turn steam turbines that would drive electricity-producing generators.
As they noted, the electrorefining method associated with fueling these reactors could also be used to process some of the existing mountains of radioactive fission waste.
The key to pyrometallurgical recycling of nuclear fuel is the electrorefining procedure. This process removes the true waste, the fission products, from the uranium, plutonium and the other actinides (heavy radioactive elements) in the spent fuel. The actinides are kept mixed with the plutonium so it cannot be used directly in weapons.
Spent fuel from today's thermal reactors (uranium and plutonium oxide) would first undergo oxide reduction to convert it to metal, whereas spent metallic uranium and plutonium fuel from fast reactors would go straight to the electrorefiner. Electrorefining resembles electroplating: spent fuel attached to an anode would be suspended in a chemical bath; then electric current would plate out uranium and other actinides on the cathode. The extracted elements would next be sent to the cathode processor to remove residual salts and cadmium from refining. Finally, the remaining uranium and actinides would be cast into fresh fuel rods, and the salts and cadmium would be recycled.
Sounds nifty, no? Hannum et al. expound on the virtues of the system at greater length in the rest of their article, which I recommend for a fuller sense of the argument.
Yet there are counterarguments to all this, as Wald reports. He cites physicist Frank von Hippel as saying that "a new generation of reactors would cost tens of billions of dollars and that it would be a long time before it was clear that reprocessed fuel was needed." (Von Hippel has an article in the upcoming February Scientific American about the need to better safeguard civilian reactors from would-be nuclear terrorists.) And then there's this:
Ivan Oelrich, vice president of the Federation of American Scientists, said that building scores of new reactors, with a reprocessing plant adjacent, was unlikely, and that while opening Yucca would be hard, switching to this kind of reprocessing was "trading one difficult political problem for an impossible problem."
The most damning points for the technology's prospects might be the ones at the very end of Wald's article:
Further, the companies that run reactors are showing no interest in new kinds of reactors and little interest in plutonium.
When the Energy Department decided to get rid of some surplus weapons-type plutonium by turning it into nuclear fuel, no utilities would take it, even at no charge.
Still, the idea has its proponents--in particular, Rep. David Hobson (R-Ohio), the chairman of the House's Energy and Water Development Appropriations Subcommittee. He led the push for Congress's allocation of $50 million to investigate the new reactors and reprocessing facilities.
Many months ago, when Hannum et al. proposed their article to Scientific American, the other editors and I debated whether to invite it. Was it realistic to suggest that a new nuclear fission technology might contribute significantly to the world's energy future when the technical and economic forces freighted against it seemed so daunting? Might this not be just another in a long series of attempts by nuclear power enthusiasts to draw attention to their favorite technology? The argument that eventually prevailed was that, for better or worse, these fast-neutron reactors and electrorefining ideas did seem to be getting taken seriously by some physicists and policymakers, and as such, it was worth making sure that our readers were fully aware of them.
Where this technology goes from here is anyone's guess.


Blogger yuvakuran said...

4:27 AM  

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