VIENNA (Reuters) - The outside world may never find out what type of fissile material North Korea used in its nuclear test four months ago, leaving a key question about the explosion unanswered, officials and experts said on Tuesday.
The isolated northeast Asian state is believed to have tested plutonium bombs in two previous such blasts, in 2006 and 2009. Any switch to uranium would increase international alarm as it could enable Pyongyang to greatly expand its arsenal.
A global nuclear test monitoring agency said in April it had detected radioactive xenon gases that could have come from the February 12 underground explosion. But the measurement gave no indication of which material was in the bomb.
"We would very much like to know whether it is plutonium or highly enriched uranium," U.S. nuclear scientist Siegfried Hecker, who has visited North Korea, told a news conference.
"But in the end - unless the xenon people get very lucky, very soon - we just don't know. There is no other way to tell," he said, referring to the analysts of such radioactive traces.
Large amounts of xenon gases are produced in fission, an atomic reaction occurring both in nuclear arms and reactors.
To distinguish between plutonium and uranium, it helps if the detection is made soon after the test and the amount of gases released is large, experts say.
"The sooner, the better," said Mika Nikkinen of the Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO), the Vienna-based monitoring body which registered the February blast virtually instantaneously via seismic signals around the world.
Speaking at the same event, he suggested the fissile source in the device detonated by the North would not be known "until somebody is able to get" to the test site and see what is there.
Anders Ringbom, deputy research director at the Swedish Defence Research Agency, said it was not possible to determine the material on the basis of the gases picked up two months ago.
"If we look at the (isotope) ratios you cannot distinguish in this case because the release was so late," he said.
North Korea abandoned plutonium production six years ago following international pressure but later acknowledged that it had built facilities to produce enriched uranium, which can also be used in bombs if refined to a high degree.
Experts say plutonium, a by-product of nuclear reactors, can be difficult to use as bomb material because specifications have to be precise. It could be relatively easy for North Korea to make large quantities of highly enriched uranium.
Hecker, former head of the U.S. Los Alamos National Laboratory, estimated the North had enough plutonium left for four to eight weapons. It was possible that it had "mastered both the plutonium bomb and the highly enriched uranium bomb".
The February test yielded a stronger blast than the North's previous explosion four years ago and Pyongyang said it had made progress in miniaturising an atomic weapon, essential to fitting it into the cone of a missile.
But Hecker of Stanford University said: "I don't believe it can reliably mount a nuclear warhead on a missile yet."
The test ban treaty was negotiated in the 1990s but has not taken effect because some holders of nuclear technology have not yet ratified it, including the United States and China.
However, the CTBTO already monitors possible breaches, deploying about 290 stations worldwide to look out for signs of atomic tests, including seismic waves and radioactive traces.
Editing by Gareth Jones