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* No radioactive traces detected a month after test
* May be hard to determine what fissile material was used
VIENNA, March 12 (Reuters) - A month after North Korea's nuclear test, a monitoring agency said on Tuesday it was highly unlikely to find any "smoking gun" radioactive traces from the blast, potentially leaving key questions about the device unresolved.
The lack of this kind of scientific evidence may make it difficult to determine what fissile material was used in the isolated Asian state's third nuclear test, which was detected by seismic monitors.
The Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), which has a global network of monitoring stations designed to pick up radioactive traces emitted from tests, said it had yet to find any such signs.
"It is very unlikely that we will register anything at this point ... at this late stage," CTBTO spokeswoman Annika Thunborg said.
Thunborg did not give details, but the failure to detect radioactive traces could indicate that North Korea managed to prevent any such release from the Feb. 12 underground explosion.
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.
But the organisation already monitors possible breaches, deploying more than 270 stations worldwide to look out for signs of atomic tests, including seismic waves and radioactive traces.
It can take weeks to pick up radioactive so-called noble gases, depending on the weather.
"We are confident that the system works very well," Thunborg said. A senior CTBTO official last month said the "smoking gun" of any nuclear test would be thel detection of radionuclide traces.
One critical question is what kind of fissile material North Korea used in the latest test. In 2006 and 2009, it is believed to have used plutonium as the fissile core.
North Korea abandoned plutonium production in 2007, 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.
Without the trace evidence, however, U.S. and allied officials have said it would be very difficult for outsiders to determine whether the latest test involved a plutonium or uranium core. (Reporting by Fredrik Dahl; editing by Andrew Roche)