American Physical Society, Physics & Society, Jan 2002
Long a disputed issue, the disposal of excess weapons plutonium seems to be headed for technical resolution in U.S. national policy. In a report posted on the Internet, the National Academy of Sciences Committee on International Security and Arms Control (CISAC) has concluded that irradiating the plutonium in MOX (mixed oxides of uranium and plutonium) in a once-through nuclear reactor fuel cycle would meet its "spent-fuel" standard for resistance to theft and proliferation.
A close look at the report also indicates that previously considered methods of immobilization by underground burial are not now approved or available for weapons plutonium.
Because of the longstanding debate about plutonium disposition, it’s a little odd that these significant NAS results have not gotten more public attention. I’ve been following this issue for years, partly in connection with a book being prepared with some colleagues about Cold-War nuclear arsenals and legacies, yet it was only this year (2001) that I noticed the July 1999 "Interim Report." No final report is available, although it was scheduled for the Fall of 1999.
It seems strange that all three of their significant conclusions–regarding MOX burning, immobilization, and demilitarization–are only published in an unheralded interim report. It is hard not to wonder whether the apparently imminent final report was not issued because it was considered ideologically unacceptable in certain quarters.
CISAC adopted for their 1999 review a systematic methodology to compare options for plutonium disposal. Perhaps because it is only "interim," the 1999 report is difficult to read and understand, with many of its important findings rather obscure or relegated to footnotes. It is not a simple matter to sort through the jargon, but a careful reading indicates that today's state of the art does not permit the spent-fuel standard to be met through immobilization (either by vitrification or can-in-canister).
Those of us who have long been calling attention to the advantages of isotopic "denaturing" of weapons plutonium can find some satisfaction in a footnote (no. 13) in the CISAC report. For the first time in print, the official body explicitly acknowledges that any attempt to insert isotopically demilitarized plutonium in existing weapon configurations would be "likely" to run into an abundance of difficulties, such as "design modifications and . . . new nuclear-explosive tests . . . to confirm that the change in isotopic composition had not unacceptably degraded performance."
The chairperson of CISAC, John Holdren, explained his own views about reactor-grade plutonium in an article he wrote for The Bulletin of the Atomic Scientists in 1997:
. . .because the isotopics are different, weapons using this plutonium would have to be redesigned, which would require nuclear tests. That means the path to reuse of spent fuel would be more difficult technically and politically–as well as easier to detect–than reusing weapons plutonium extracted from glass.
Thus the Committee has definitively embraced the proposition that demilitarized plutonium is really not useful for making military-quality weapons. This has important implications in evaluating treaty-breakout scenarios for nuclear-weapons states after deep cuts in arsenals have taken place. The report now effectively supports the contention that isotopic demilitarization would make plutonium inherently unsuitable for rapid recovery into weapons. Demilitarized plutonium is more securely protected against reversal into high-quality weapons than is buried "immobilized" plutonium that has not been put through a reactor fuel cycle.
After deciphering the oblique vocabulary of the report, it looks as though the Academy is saying not only that MOX irradiation (which results in chemical, metallurgical, and isotopic demilitarization of plutonium) meets the disposition standard, but that irradiation in reactors is the only practical way currently available to dispose of weapons plutonium.
Retired from Argonne National Laboratory
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