Risky Recycling?
In “Rethinking Nuclear Fuel Recycling,” Frank N. von Hippel describes why he would like nuclear reprocessing to go away, but it won’t. Nuclear power is resurging, both globally and domestically. Continuing to discard as “waste” 99 percent of the energy in uranium ore is clearly unsustainable.

The technology is spreading inexorably, increasing its potential to be subverted for weapons production. To minimize that risk, fuel processing must be done under international auspices—with ironclad guarantees that nations will have uninterrupted access to fuel if they forgo their own enrichment and reprocessing facilities.

Von Hippel is correct that using MOX (plutonium oxide mixed with uranium oxide) to cycle plutonium back into today’s “thermal” reactors is expensive, is only marginally useful and produces plutonium of weapons-quality chemical purity. But recycling methods for advanced fast reactors are different. Such methods address resource utilization, waste and proliferation concerns (see our piece, “Smarter Use of Nuclear Waste,” in the December 2005 Scientific American).

Technology alone cannot remove the proliferation threat. The U.S. Department of Energy’s Global Nuclear Energy Partnership (GNEP) is a useful step toward sensible management, and some 21 nations have signed on so far. But without continued U.S. leadership, the GNEP will fade away. Coordination will be lost, and the technology for producing nuclear weapons materials will spread uncontrolled.

--William H. Hannum, Gerald E. Marsh and George S. Stanford
Argonne National Laboratory (retired)

VON HIPPEL REPLIES: Nuclear power could cut the growth of greenhouse emissions by up to 15 percent. Reprocessing makes nuclear power more expensive, however, and breaks down the barrier between it and nuclear weapons.

Hannum, Marsh, Stanford and I agree that reprocessing and recycling plutonium in water-cooled reactors make neither technical nor economic sense. A dozen countries have not renewed their reprocessing contracts with France, Russia and the U.K. Having lost virtually all its foreign customers, Areva, France’s reprocessing company, has not yet been able to agree on more than a one-year extension of its contract with France’s nuclear power utility. And the U.K. is giving up on reprocessing altogether.

Liquid-sodium-cooled, fast-neutron reactors utilizing recycling could fission plutonium almost completely but are so expensive that no private utility will pay for one. If costs change and proliferation concerns can be dealt with, the potential energy resource in the plutonium and uranium in spent fuel will still be there. In the meantime, we must dispose of hundreds of tons of already separated plutonium that is a legacy of the cold war and premature expectations of breeder reactors. For the foreseeable future, there will be no need to separate more.

Moving Line
The Genesis of Planets,” by Douglas N. C. Lin, describes how, in the leading planet formation theory, planets form within a disk of gas rotating around a star. At a certain distance from the star is a “snow line” beyond which water stays frozen. I wonder about the stability of the snow line. It seems that it should move as the disk progresses. Could this be why Earth has an ocean?

--Tom Brown
Gainesville, Fla.

LIN REPLIES: The snow line does evolve. Because of intense irradiation by central stars and friction heating within the disk, our solar system’s snow line was initially located well outside the orbit of Jupiter. It gradually propagated inward as the mass flux through the disk declined and the gas dissipated. Eventually the relocation of the snow line was more or less stalled, although the ice-vapor demarcation face may have moved back and forth over about 1 to 2 AU. This essentially covered a substantial fraction of the region between Mars and Jupiter. The parent bodies of meteorites in the asteroid region formed over several million years. During that epoch, the snow line may have intruded on regions fairly close to Mars. Consequently, the water content in the meteorites gradually increased with the distance of their parent bodies from the sun. This evolution may have promoted the acquisition of Earth’s ocean.

Cosmic Credit
David Appell errs in attributing the discovery of dark energy so completely to Saul Perlmutter’s Supernova Cosmology Project (SCP) team in “Dark Forces at Work” [Insights]. The SCP made real contributions to the discovery of dark energy, but other groups had solved some of these problems earlier.

In 1988 a Danish team searched for distant supernovae using methods anticipating those of the SCP. And the program of supernova discovery for nearby objects at Cerro Tololo Inter-American Observatory in Chile formed the basis for using supernovae as distance indicators, not the robotic search Perlmutter worked on. The SCP did publish a result in July 1997 that claimed supernova observations were unlikely to be consistent with dark energy, but our HighZ Supernova Search Team developed superior methods for dealing with dust, published in 1996. With careful observation of supernovae, we were confident that we saw cosmic acceleration, which we announced in February 1998. A paper detailing our work was submitted to the Astronomical Journal in March 1998 and appeared in print before the SCP paper was submitted.

Everybody has a lot to be proud of, but credit should be given where it is due.

--Robert P. Kirshner
Harvard University

APPELL REPlIES: Kirshner is not entirely correct and, as a member of the High-Z team, perhaps not entirely objective. The Danish team did perform consequential early measurements, but only on one supernova and too late to obtain its peak brightness. Both the SCP and High-Z teams did important work and exchanged vital data and insights in both directions. But it is undisputed that the SCP announced its discovery first, on January 9, 1998, at a meeting of the American Astronomical Society. In Kirshner’s book The Extravagant Universe (Princeton University Press, 2002), he describes the two teams’ relationship as “getting it first” versus “getting it right.”

I am sure that history will acknowledge the contributions of both teams in the final analysis.