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A Plutonium potpourri

Challenges in plutoniumphysics and chemistry

Seaborg Institute Director David Clark kicked off an approximately four-hour plutonium tutorial designed to "stimulate the next generation of scientists and students." He framed his discussion in the most interesting way possible, that is, in terms of the metal's chemical and metallurgical peculiarities. From the uniqueness of its oxidation-state behavior (whereby, under certain conditions, plutonium solutions can simultaneously exhibit the presence of three or four different oxidation states) to the different crystal structures and physical properties of its allotropes (physical phases), the discussion accurately reflected the reason why so many scientists worldwide are engaged in the study of a single chemical element and its compounds.

"I'm going to walk you through some of the interesting scientific challenges that will be addressed at the conference," Clark offered, and he quickly delivered on the promise, covering the areas of stockpile, materials, and environmental stewardship. His focus was the phenomenon of nuclear materials aging, and he used an example of an aged tanker that had recently broken apart off the coast of Spain to generally frame the issue's importance.

Clark did not ignore the fact that, as science often does, a seemingly completely negative characteristic has also been harnessed for positive applications. Most notable was the use of plutonium-238 as a heat and power source in space missions, particularly the Mars Rover projects in which Los Alamos plutonium scientists have been intimately involved. Additionally, Clark predicted that because of self-irradiation-based changes occurring in plutonium-based superconductors over time, "plutonium will teach us quite a bit about superconductivity mechanisms."

IAEA activities in plutoniumnonproliferation and security

The tutorial's second presentation, given by Graham Andrew of the International Atomic Energy Agency (IAEA), proved to be a pertinent segue, particularly in light of recent international events precipitating concern over nuclear terrorism. He reviewed how the agency functions and interacts with national states and emphasized the ongoing challenge implicit in "checking whether what you measure can possibly be what you're told."

Of particular interest were new analytical tools developed to assay plutonium samples for isotope ratio, as well as to determine the content of minor actinides such as neptunium, americium, and curium. Also of interest was the revelation that, in addition to its central offices in Vienna, the agency maintains laboratories at certain large nuclear-fuel reprocessing plants such as the one at Rokkashamura, Japan. This need was highlighted by the statistic that mixed-oxide (MOX) fuel requirements for commercial light-water reactors comprise about 190 metric tons per year, compared with only 24 tons per year currently being produced by reprocessing.

Andrew also discussed the limitations of traditional safeguards and the efforts to expand safeguards by getting nations to "sign on to more wide-ranging access and forensic environmental sampling . . . from mines to nuclear waste" and via expanded inspector training to help enhance each individual inspector's recognition capabilities. This concern extended to the proliferation resistance of future energy systems, in the sense of "making that nuclear material less attractive to a proliferator." This IAEA initiative is, of course, particularly relevant since the events of 9/11/01, with the subsequent increased concerns about sabotage and radiological terrorism ("dirty bombs").

The nuclear fuel cycle and new initiatives

In comparing the broad outlines of so-called "once through,""European/Japanese," and "advanced proliferation-resistant" nuclear fuel cycles, Edward Arthur's presentation was eminently pragmatic. Arthur, former Los Alamos associate director for Strategic Research, cited the requirement for reprocessing of spent fuel as the first step in any of the cycles. He then made it clear that the cost of spent-fuel reprocessing did not currently compare very favorably with the reprocessed fuel's electricity value.

He stressed that "front end" operations such as decladding, storage, and separations, and back-end waste-management components collectively accounted "for 75 percent of the cost of a fuel reprocessing facility." But he was also unequivocal in his view that economic issues had to be "looked at from an overall system perspective."

Regardless of the obstacles, the U.S. Senate's FY04 budget includes a funding target of $78 million for the multi-laboratory Advanced Fuel Cycle Initiative and authorizes a demonstration hydrogen-fueled electricity plant. Ultimately however, to achieve the goals of Power for the Twenty-First Century, reprocessing must handle tens of thousands of tons of spent nuclear fuel. It must also deal with such factors as radiation, criticality, and chemical hazards such as acids and solvents-what Arthur characterized as "major technical issues still facing reprocessing."

Colloid-facilitated transport of plutonium

The afternoon's final presenter was Annie Kersting of Lawrence Livermore National Laboratory. "What we really want to understand is the life cycle of a colloid," Kersting said, and she proceeded to explain why this was a vital goal to chemists trying to sort out the complexities of plutonium's interaction with the natural environment.

Mobile particulates generally smaller than 1 micron, colloids originate from a diversity of environmental sources; and both organic (carbon-based) and inorganic colloids are ubiquitous in soil and water. Understanding colloids and their significance in environmental transport of plutonium and other actinides thus entails a combination of geology, hydrology, and chemistry, as Kersting's presentation illuminated at every turn.

As such, she discussed the need for field studies to firmly foot laboratory experiments and theory in the local geology and hydrology of sites such as Rocky Flats and Savannah River. Such local geochemistry determines if colloids will or will not facilitate the transport of plutonium or other actinides through the environment.

Kersting explained that these studies have so far painted an incredibly complex landscape for actinide transport, since both adsorption and desorption had to be assessed and because "measuring the concentration of colloids in water is very difficult." For example, "you may see plutonium sorb very strongly to say silica and iron oxide, and then you go down a few meters, and it may desorb off that silica but still stay on the iron oxide."

Kersting summarized her talk by framing the overall issue as an imperative to understand the fundamental chemistry of actinide nanoparticles.

ÑVin LoPresti<


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