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Tuesday, October 17, 2006

Airborne Debris Could Yield North Korean Nuclear Fingerprint

By Jon Fox, Global Security Newswire

WASHINGTON � Radioactive particles detected by U.S. sensors might provide insight into the weapon design and material that North Korea used in its nuclear test last week (see GSN, Oct. 16).

Radionuclides leaking from the mine shaft where North Korean scientists detonated a device with a subkiloton yield on Oct. 9 could also offer the U.S. a valuable signature of North Korean plutonium � a sort of nuclear fingerprint.

�In that sense, the North Koreans may have done us one kind of favor,� said Christopher Chyba, a professor of astrophysical sciences and international affairs at Princeton University.

Air sampling early last week initially showed no radioactive particles drifting from North Korean territory, but by Wednesday aircraft had detected a radioactive signature consistent with a nuclear blast, congressional officials told the Associated Press over the weekend.� On Monday, U.S. officials confirmed that the blast was plutonium device with a yield below one kiloton.

The United States still has little reason to fear that North Korea will be able to arm an ICBM with a nuclear warhead any time soon, Chyba said during a conference here last week.

Based on Pyongyang�s abortive test of the Taepodong 2 long-range ballistic missile in July � it reportedly flew for just 40 seconds � and doubts about North Korean ability to build a nuclear device small enough to fit on top of a missile, a nuclear ICBM attack on the United States is just not possible, he said.

Even if North Korea possessed the technological capabilities to produce a nuclear warhead that could be fitted inside a nose cone, and that missile could reach U.S. shores, an ICBM attack is immediately attributable, he added.� There would be no doubt where the missile came from.

�One has to worry about the possibility of other means of delivery that would be less attributable,� he said.� That could include nuclear material being transferred to a terrorist group willing to detonate a weapon inside another nation.

�In this sense the ability to do post-attack forensics and the ability to determine the origin of a nuclear weapon based on the debris from the explosion is extremely important,� Chyba said.

By examining particles from the North Korean test, scientist might be able to determine the amount of time the plutonium spent in a nuclear a reactor and how long it has been since it has been reprocessed, according to experts.� Such information could provide a unique nuclear fingerprint that could in the future identify nuclear material as North Korean in origin.

The day after the world learned of the North Korean test, U.S. President George W. Bush warned that �the transfer of nuclear weapons or material by North Korea to states or nonstate entities would be considered a grave threat to the United States and we would hold North Korea fully accountable for the consequences of such action.�

Experts, however, have raised doubts about the capabilities of the U.S. nuclear forensics program.� If an act of nuclear terrorism were to occur in a U.S. city, scientists could be unable to conclusively link the nuclear material to its origin, Chyba said.

�I think there is likely to be a certain credibility problem,� he said.� The success of nuclear forensics depends, in part, on a catalogue of nuclear signatures and data.� Possible sources could be identified and others eliminated, but it remains unclear how complete any such U.S. databank is, Chyba said.

Open source information about the technological challenges and status of U.S. nuclear attribution capabilities is remarkably sparse.� In July, at a hearing before the Senate Judiciary Terrorism, Technology and Homeland Security Subcommittee, nuclear terrorism experts identified lagging attribution abilities as a serious issue (see GSN, July 28).

Fred Ikle, a scholar with the Center for Strategic and International Studies and former defense undersecretary under President Ronald Reagan, called it �a deep problem� but declined to elaborate.� He suggested a closed hearing to discuss the sensitive issue.

In testimony to the same committee, Deputy Energy Secretary Steven Aoki described a credible U.S. attribution ability as a core element in deterring nuclear terrorism.� Despite its importance, �a lot of hard work remains in fleshing out both the technical and policy dimensions of attribution,� Aoki wrote in a prepared statement.

As U.S. nuclear scientists sift through the data provided by airborne radio isotopes, they might be able to garner a significant amount of information about the North Korean device beyond the simple confirmation that it was nuclear.

�To be cautious here, it would amaze everybody � except for the people that have served in government what we will be able to tell if we�re able to pick up certain things, like radioisotopes, and able to calibrate some of the information,� Jon Wolfsthal, a nuclear expert with the Center for Strategic and International Studies, said last week.

A recent article in Arms Control Today examined a hypothetical case of nuclear terrorism in Moscow and suggested scientists examining debris or airborne particles could infer a significant amount about a device�s design and fuel.

Among the first questions answered would be whether the bomb was fueled by highly enriched uranium or plutonium, wrote William Dunlap, former head of arms control programs at the Los Alamos National Laboratory, and Harold Smith, former assistant to the defense secretary on nuclear defense programs during the Clinton administration.

In the case of a plutonium device, scientists could determine how much time the fuel had spent in a nuclear reactor, the length of time since the plutonium was separated from the spent fuel and possible clues as to methods of separation, they wrote.� Based on the efficiency of fission, analysts might also be able to make inferences about the sophistication of the weapon and its design.

The dynamics of the surface explosion considered by Dunlap and Smith are significantly different than an underground test.� North Korea apparently detonated its device deep underground, which could limit the amount of material that escapes into the atmosphere.

�What little gets up is not is not very rich,� said William Happer, a physics professor at Princeton University and former director of energy research at the Energy Department.� That is in stark contrast to the era of atmospheric tests, where everything was floating around for the sampling.� �It was amazing what kind of detail you could get from that,� he said.

The small nature of the explosion might actually lead to more nuclear evidence � radionuclides � reaching the atmosphere than would occur with a larger underground detonation.� The larger the explosion, the more likely it is to crush and melt surrounding rock into a barrier against the leak of radioactive material.

�It�s well known from experience at the Nevada Test Site that it was the small nuclear explosions that had the tendency to leak,� said Paul Richards, a seismologist at Columbia University�s Lamont-Doherty Earth Observatory in Palisades, N.Y.� It is important to catch leaking material within a few days of the event, Richards said, �because the most diagnostic isotopes have a not very long half life.�

Depending on the quality of the samples, the radionuclide data could provide information about the yield of the weapon as well something about the construction, said Philip Coyle, the Pentagon�s former head of weapons testing and former head of nuclear testing at Lawrence Livermore National Laboratory.� �If we get good samples, yes, it will tell us quite a bit,� said Coyle, now a senior adviser to the Center for Defense Information.

While the scientific data might provide insight into the weapon�s composition and general design goals, what exactly North Korea intended to achieve with the detonation will, however, remain cloudy, Wolfsthal said.

�We won�t be able to tell anything about the overall makeup, the size of it, whether it was intended for warhead or a simple demonstration � I think that may be beyond our capacity,� he said.
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