N.Korea nuke test relatively small: scientists
North Korea's nuclear test on Monday might have been a "mini-nuke" explosion possibly as low as one kiloton, comparable to some small tests by India and Pakistan in 1998, scientists said on Monday.
The U.S. Geological Survey said it had detected a 4.2 magnitude quake in North Korea at 10:35 local time (0135 GMT) on Monday, confirming a similar report from South Korea.
Gary Gibson, senior seismologist at Australia's Seismology Research Center, said a 4.2 magnitude quake would be the result of a one kiloton explosion.
"It depends on how the thing is set off. There is not a perfect correlation between magnitude and the yield and depends to some extent on the rock type they set it off in," he said.
"It is a relatively small nuclear test."
A U.S. intelligence source agreed that a preliminary examination of the data did not indicate a large blast or a series of explosions. But the source stressed that analysts were still working toward a definitive evaluation.
"It's premature because they're still evaluating the data," the source said.
The RIA news agency quoted on Monday Russian Defense Minister Sergei Ivanov as saying that the nuclear device tested by North Korea ranged between five and 15 kilotons.
The nuclear weapon the United States exploded over Hiroshima in 1945 produced a 12.5-kiloton yield.
In 1998, India carried out five underground nuclear tests at Pokharan in the western desert state of Rajasthan and declared itself a nuclear weapons state.
The total yield of the first round of blasts measured near 60 kilotons. Two days later, it exploded sub-kiloton devices that scientists said made it capable of conducting computer tests not covered by the Comprehensive Test Ban Treaty.
"Our biggest one was in the vicinity of 45 kilotons. That was thermo-nuclear," said S.K. Malhotra, head of the public awareness division of the Department of Atomic Energy.
Nuclear analyst Andrew Davies, from the Australian Strategic Policy Institute, said if the North Korean test yield was only a kiloton, Pyongyang may be disappointed.
"A kiloton is a very low yield and would tend to suggest, I would have thought, that the device was not all they hoped it would be," Davies told Reuters.
"If a nuclear, plutonium bomb fizzles, you can still get one or two kilotons quite easily. You still get a significant energy release. But an efficient device will give you more like 20 (kilotons)."
The United States has said it is interested in developing so-called "mini nukes" -- nuclear weapons with a yield of less of than five kilotons.
Indian nuclear scientists said they thought the North Korean test was an indication that the country was going for smaller devices to ensure effective delivery.
"They are concentrating more on the small devices because of reliability of launching and effectiveness," said A.N. Prasad, former chief of the Bhabha Atomic Research Center, India's top nuclear research facility.
"If the situation demands, they can scale it up, which is not difficult. This is the best approach now."
After India exploded its devices, arch foe and neighbor Pakistan rushed to conduct its own tests.
Within days it had conducted six tests, although claims related to the number, strength and yield of the tests were doubted by scientists in India and the United States.
Pakistan said two nuclear tests had a total yield of between 34-48 kilotons, while three others were sub-kiloton. It said a sixth test yielded 10-15 kilotons.
The Southern Arizona Seismic Observatory said the two major tests yielded 9-12 kilotons, while the sixth yielded only 4-6 kilotons.
A source in Beijing who is close to the North Korean regime said Pyongyang had detonated a neutron bomb, designed to release larger amounts of deadly radiation than other nuclear weapons. There was no immediate confirmation of the claim.
(Additional reporting by Krittivas Mukherjee in MUMBAI, Simon Cameron-Moore in ISLAMABAD, James Grubel in CANBERRA and David Morgan in WASHINGTON)
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