N Korea's next task -- a small, missile-ready bomb

LONDON, Oct 9 (Reuters) North Korea's next challenge after testing a nuclear warhead is to make one small enough to fit to a missile -- a tricky undertaking, experts say, but one that might not take long if it has the requisite know-how.

''They've got to get the weight down to about 500 to 700 kg (1,102 to 1,543 lbs), which I would have believed is perfectly possible with knowledge that is available today,'' said Duncan Lennox, editor of Jane's Strategic Weapons Systems.

''It would be about 1.5 metres tall, a cone, with a base diameter of 0.8 to one metre.'' Such a device could be delivered to its target by one or more missile types among North Korea's extensive arsenal.

The Scud C, with a range of up to 500 km would be a viable choice ''if they wanted to be particularly nasty to South Korea'', Lennox said.

The Rodong, with a range of up to 1,400 km is capable of hitting all of South Korea and most of Japan.

Another choice would be the Taepodong-2, a type tested by North Korea in July which flew for about 40 seconds before it destructed.

South Korea said the test was a failure, but some experts suggest the North may have aborted it deliberately after finding out the capability of the engine.

A nuclear bomb can also be dropped from a plane, but defence analysts assume that North Korea would prefer to fit it to a missile, which reaches its target faster and is harder to shoot down.

''MATING'' PROCESS However, the process of ''mating'' a nuclear warhead to a conventional missile is complex, said Lee Willett, defence analyst at the Royal United Services Institute in London.

''Nuclear warheads have a different weight, there'll be a difference guidance requirement than you have with a conventional missile,'' he said.

''It's not just a case of getting a conventional missile and sticking a nuke on top of it. It's a fundamentally different weapon.'' One unknown in the equation is the extent of the help obtained by North Korea from disgraced Pakistani scientist A Q Khan, the father of his country's own successful drive to acquire the nuclear bomb.

The key requirements are ''design, know-how and having the right materials'', Lennox said.

''But in my view that knowledge is available on the black market.

I think A. Q. Khan provided them with what they needed ... I think the data and the help provided by Pakistan would indicate that they could put a warhead on a missile fairly quickly.'' Willett said the unconfirmed information that Monday's device was a neutron bomb -- as reported by Reuters from a source close to the North Korean government -- was an intriguing possibility.

Such a weapons differs from standard nuclear bombs by producing a smaller blast but dispersing more radiation.

''It doesn't do that much damage (to infrastructure) but sends out massive radiation which kills people. It would send out big, big waves of radiation,'' he said.

If North Korea were working on such a device, it could be as part of a battlefield nuclear weapons programme in order to deploy it against South Korean or US troops in the event of a North-South war, he added.

REUTERS PDM PM1939