How well do we understand plutonium?

Researchers debate the issue at a collaborative workshop

"Plutonium is an element at odds with itself..."

As alluded to in Sig Hecker's opening presentation, the keynote of a recent collaborative workshop between Nuclear Materials Technology (NMT) and Theoretical (T) divisions was the need to understand the processing-structure-properties relationships for plutonium, its alloys, and its compounds.

Because the ability to perform underground testing gave researchers a "shortcut from processing to performance," they never really developed a goo" picture of material microstructure, "flying half-blind because we could test," according to the former Laboratory director.

With the current mandate for stockpile stewardship without underground testing, the microstructure of the different allotropes and alloys of plutonium has become an issue of major importance, and the workshop provided a diversity of perspectives on that broad topic.

The complexity of the issue underscored the irony that an element whose nuclear properties make-as the "swing element" in the actinide series‹physico-chemical properties that render it an enigma to empiricists and theoreticians alike.

In turn, this lack of clarity doubly underscores the crucial need for the exchange of information, for collaboration‹and therefore, for such a workshop.  The workshop was organized by Shao-Ping Chen of the Theoretical Division Office (T-DO) and Jeremy Mitchell of Nuclear Materials Science (NMT-16), who also served as morning and afternoon session chairs.

Presenters came from T, Materials Science and Technology (MST), NMT, and Applied Physics (X) divisions, and from the Los Alamos Neutron Science Center (LANSCE), and there was enough interest in the morning unclassified session to fill Materials Science Laboratory (MSL) Auditorium beyond capacity, necessitating the setup of a closed-circuit feed to a room across the hall.

The afternoon classified session attracted at least as much interest and was relocated to the Administration Building auditorium to accommodate what one presenter referred to as a group containing a significant number of "fresh faces." Those new faces were one indicator of the breadth of Laboratory interest in this intriguing area, another reason why the sharing of research information is so vital to genuine progress.

The workshop's organizers were pleased with the variety of approaches in the talks, and likewise, with the diversity of presenter backgrounds. They felt that this combination helped strike a good balance in the range of topics, although they also admitted to being unable to accommodate all potential presentations because of the high response rate and limited time availability‹the aim being to keep the workshop only a day in duration.

And despite the fact that all presenters adhered to the stringent time limits effectively and diplomatically enforced by the session chairs, discussion time was at a premium. Both the organizers and the presenters concurred that more time for informal discussion and questions would have been desirable.

 The electronic structure of plutonium seemed to crop up in just about every talk, frequently as a central issue. The key question, whether the metal's 5f electrons were best described as "localized" or "itinerant" (delocalized), was broached in presentations as diverse as Roland Schulze¹s plasmon resonance spectroscopy and John Wills's electronic structure calculations in the context of Density Functional Theory.

While there was a modicum of agreement that s, p, and d electrons might best be seen as delocalized with f electrons viewed as localized, one sensed more skepticism than complacency. John Joyce introduced a related issue in raising the possibility of f-electron­conduction-band electron hybridization.

Another important topic was what one presenter termed "phase diagramania." With the largest number of allotropes known for any element, phase diagrams for plutonium‹and particularly those for plutonium-gallium alloys‹bear some resemblance to a white rabbit's maze.

Marius Stan reexamined the controversial question of a eutectoid point in the Russian version of that latter phase diagram, and several other presenters, including Michael Baskes, Andrew Lawson, and Frank Cherne, discussed relevant aspects of plutonium-gallium interactions.

Crystal-structure quandaries were also in evidence, as presenters considered possible space-group cells for plutonium, the surface chemistry of its hydrides and oxides, and the crucial stockpile-aging issue of structural disordering resulting from helium-cluster formation, a consequence of plutonium's self-irradiation during radioactive decay. Nor did this workshop lack an out-of-the-ordinary component. While simultaneously characterizing the finding as "rather odd," John Sarrao presented what he described as "as good as it gets" evidence for a plutonium-based superconductor (see story on page 1).

Reflecting on what the workshop achieved, co-organizers Chen and Mitchell were liberal in spreading praise for its success, first crediting the many volunteers who assisted in the various aspects of arranging the details. They noted an openness to collaboration among the presenters and were especially grateful to division leaders Tim George (NMT) and Alan Bishop (T) for fostering that collaborative mindset.

According to theoretician Chen, one of his incentives for proceeding with the project was his six-month stay in NMT-16, which helped him appreciate that conducting experiments on the idiosyncratic metal was "much harder than I thought," particularly in the face of the many regulations that make "good data precious."

But good data becomes even more useful when subjected to constructive scrutiny. And  although other forums for plutonium-related discussions certainly exist (NMT seminars, the Plutonium Futures conference, and Seaborg Institute seminars, for example), Chen and Mitchell envision the possibility of smaller, more-focused mini-workshops on topics that emerged from this more diverse meeting, perhaps even as frequently as on a monthly basis.

For those at Los Alamos working in the field as well as for those who are merely intrigued by the possibilities implicit in plutonium's physics, chemistry, and metallurgy, such a possibility is welcome news. Meanwhile, for anyone who was unable to attend this workshop, the proceedings will soon be available on videotape.

This article was contributed by Vin LoPresti (IM-1)

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