N. Korean nuclear weapons: How real is the threat?

If getting international attention is North Korea's goal, then there is nothing quite like detonating a nuclear device to make your adversaries sit up and take notice. But experts say North Korea probably has a long way to go before it will be able to actually deploy a nuclear weapon.

While North Korea is adept at getting political mileage out of showy military displays, Pyongyang's attempts to show off its strength are, just as often, reminders of its weaknesses — and a nuclear test would likely fit that pattern.

Fears that such a test may be imminent were heightened last month, when North Korea marked an important anniversary with a long-range rocket launch. Its two previous tests came soon after such launches. Satellite imagery also suggested stepped-up activity at the North's Punggye-ri nuclear testing site.

Little progress at the site has been reported since, which could mean the activity was a ruse or the device is simply not ready yet. It also could mean that the new regime headed by Kim Jong Un, who assumed power after the death of his father in December, is having second thoughts about whether to risk international sanctions by forging ahead.

Sooner or later, however, a test is highly likely.

"The North Koreans clearly value the demonstration effect of nuclear and missile tests, even if the test is only partially successful," said Jeffrey Lewis, of the Center for Nonproliferation Studies at the Monterey Institute of International Studies. "North Korea gets a tremendous amount of leverage from our fear that these weapons might work someday."

But he noted that Pyongyang failed miserably in its attempt to launch an ICBM-style rocket last month, then capped off a lavish military parade with the unveiling of a half-dozen ominous-looking new missiles that analysts now believe were low-quality mockups of a design that could never fly.

"They are trying to run before they can walk, with the predictable outcome of tripping," Lewis said.

A test could have two practical goals.

North Korea may be developing devices that use highly enriched uranium, instead of the harder-to-obtain plutonium it has relied on in the past. If so, it needs to try one out and see if it works. Either way, the North has to shrink its warheads down to fit them on a missile — so it needs to test that capability as well.

"There can be a huge difference between a nuclear explosive 'device' and a weapon," said Ivan Oelrich, a nuclear weapons consultant and former head of strategic weapons at the Federation of American Scientists. "We have no idea how large North Korea's bombs are, or even whether they have anything that would be described as a 'bomb.'"

North Korea's devices are likely along the lines of the first plutonium bomb the U.S. built — Fat Man, which was dropped on Nagasaki in 1945. That bomb was 3 meters (10 feet) long and weighed more than 4.5 tons.

Such a bomb could be loaded on a ship or an airplane, but without significant "miniaturization," which requires difficult technological redesigning, it would be useless as a missile payload.

"A weapon has to be light and compact, a more or less self-contained package," said Oelrich. "To fit on a missile, they would have to be less than a few hundred kilograms (about 600 pounds) and smaller than a cubic meter or two."

Though estimates vary, outside experts say the North has enough plutonium for about four to eight "simple" bombs, more if it can employ uranium. But, so far, North Korea's attempts to demonstrate it has mastered the technology — with tests in 2006 and 2009 — have not been entirely successful.

The first produced a yield of less than 1 kiloton of TNT, and the second was equivalent to only 4 kilotons, both quite small by nuclear standards, though some experts believe North Korea in the second test may have been trying out the smallest device it could put on a missile.

Success or failure, the tests provide an opportunity for North Korea's nuclear scientists to learn valuable lessons. That's why the international community has imposed harsh sanctions after each of its previous underground blasts. But turning those lessons into a viable weapon is no easy task.

"Testing a device underground is relatively easy, as one can initiate the test once everything is in order and verified to be ready," said Michael Elleman, of the International Institute for Strategic Studies. "A military or strategic nuclear weapon must be able to detonate on demand, with little forewarning."

Then there is the other problem — how to deliver it to a target.

South Korea and Japan — and the more than 70,000 U.S. troops based in those countries — are already within range of the North's Nodong weapon, which was test fired in 1993 and can travel up to 1,300 kilometers (800 miles) with a 1,200-kilogram (2,600-pound) payload.

If tipped with nuclear weapons, they would put millions of lives at risk.

But all of North Korea's long-range rocket launches have ended in failure, meaning it is 0-4 since 1998. That has led some experts to doubt whether North Korea, lacking in resources and expertise and hamstrung by stringent international trade sanctions, will ever succeed in fashioning an ICBM of its own.