North Korea's vow to restart its mothballed nuclear facilities raises fears about assembly lines churning out fuel for a fearsome arsenal of nuclear missiles. But it may actually be a sign that Pyongyang needs a lot more bomb fuel to back up its nuclear threats.

Despite the bluster, even if Pyongyang started work today on its reactor, it could be years before it completes the laborious process of creating more weaponized fuel. North Korea's announcement, experts say, is also likely an effort to boost fears meant to keep its leadership safe while trying to extract concessions from the U.S. and its allies.

North Korea has declared itself a nuclear power and threatened to expand its atomic arsenal after its third nuclear test in February sparked the recent rise in hostility on the Korean Peninsula. But that arsenal is estimated to be only a handful of crude devices.

To assemble a cache of weapons that would make it a true nuclear power, and to back up its threats, North Korean scientists need more bomb fuel -- both for the weapons they hope to build and for the repeated tests required to perfect those weapons.

``Despite its recent threats, North Korea does not yet have much of a nuclear arsenal because it lacks fissile materials and has limited nuclear testing experience,'' Siegfried Hecker, a nuclear scientist who has been regularly granted unusual access to the North's nuclear facilities, said this week in answers posted to website of Stanford University's Center for International Security and Cooperation.

North Korea nuclear capabilities are something of a mystery.

What is known is that it possesses the ability to produce both the fuels that can be used to make atomic bombs -- plutonium and uranium.

This causes serious long-term worries following North Korea's announcement Tuesday that it is ``readjusting and restarting'' all facilities at its main Yongbyon nuclear complex, including a plutonium reactor shut down six years ago as part of now-failed nuclear negotiations, and a uranium enrichment plant.

It may also be a sign of frustration from Pyongyang that weeks of posturing and threats haven't driven U.S. and South Korean negotiators back to nuclear disarmament-for-aid talks.

``What they really want is a safety blanket and a blackmail tool,'' Ralph Cossa, president of the Pacific Forum CSIS think tank in Hawaii, said in an email.

The announcement ``is primarily political, designed to signal strength and intimidate. It should not necessarily be seen as a revelation about North Korea's capabilities and true intent,'' Greg Thielmann, a senior fellow at the Arms Control Association in Washington, said in an email.

A reactor at the main Yongbyon nuclear complex could eventually make, in one year, enough plutonium to power one bomb. It was shuttered as part of international disarmament talks in 2007, its cooling tower blown up in a dramatic show of commitment to a now-scrapped nuclear deal. North Korea shocked many when in 2010 it unveiled an industrial-scale uranium enrichment facility, which gives it an alternative route to create bombs.

Estimates on restarting the vital facilities at the plutonium reactor vary from three months to a year, depending on the expert.

North Korea has already begun construction at the reactor and it could be back in operation sooner than expected, according to a U.S. research institute that analyzed recent commercial satellite imagery of Yongbyon. Rebuilding the cooling tower would take six months, but a March 27 photo shows building work may have started for an alternative cooling system that could take just weeks, the U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies said Wednesday.

But even if the reactor is now up and running, Cossa estimates it would be two to three years before scientists could obtain more plutonium for bombs.

There are other challenges to restarting the reactor.

North Korean scientists need to clean, check for any leaks, test components and replace ones that no longer work, according to No Hee-cheon, a nuclear expert at the Korea Advanced Institute of Science and Technology in Daejeon, South Korea.

``Nuclear material can be very corrosive. Cleaning the chemical equipment for reprocessing plutonium can be an overwhelming task,'' No said.

North Korea isn't thought to have nuclear armed missiles that can hit the United States and is extremely unlikely to launch a direct attack on Seoul or its U.S. ally, knowing that military retaliation would threaten the leadership's survival.

Experts estimate it has enough plutonium for between four to eight crude plutonium-based weapons. But North Korea has yet to show that it has mastered the technology needed to shrink down warheads so they can be placed on missiles, although Pyongyang has bragged -- as recently as Thursday -- that it has ``smaller, lighter'' nuclear weapons ready to strike the U.S.

To back up that boast, however, Pyongyang needs more tests, which would deplete its limited supply of nuclear fuel. This motivation may partially explain the vow to restart Yongbyon.

Two other larger plutonium reactors had construction halted because of a past nuclear disarmament deal; Hecker said the North Koreans claim both are unsalvageable. North Korea is also thought to be making progress on building a small experimental light-water reactor.

The North also suggested this week that it was boosting uranium enrichment efforts.

North Korea's uranium program worries Washington because the centrifuges that enrich the fuel into bomb-grade material are much easier to conceal than bulky plutonium reactors, which produce large amounts of heat that can easily be seen by satellites. A crude uranium bomb is also easier to produce than one made with plutonium, and North Korea has large natural uranium deposits.

Hecker was shown 2,000 uranium centrifuges at Nyongbyon in 2010, but it's not clear whether the centrifuges have been reconfigured to make highly enriched uranium. It's also unknown what fuel North Korea used in its Feb. 12 test, its third since 2006; a confirmed uranium-based nuclear test would show that North Korea has centrifuges producing highly enriched uranium.

North Korea built its secret uranium program at its main nuclear facility without the knowledge of the U.S. intelligence community, Bruce Klingner, a former U.S. intelligence officer and now an analyst at The Heritage Foundation think tank in Washington, said in an email. ``As such, we do not know how many covert uranium enrichment sites North Korea has nor how many uranium weapons they can produce per year.''

Still, scientists can't make a uranium bomb overnight.

Even if the North's 2,000 centrifuges were configured properly and spinning 24 hours a day, every day for a year, they could only make one or two uranium bombs, said Kune Y. Suh, a nuclear expert at Seoul National University.

The North's plan to restart the plutonium reactor looked to some like an admission that Pyongyang hasn't made much progress in its uranium enrichment program.

``Why else would it go to the trouble of a time-consuming and expensive restart to plutonium production at a known and vulnerable facility?'' Thielmann asked. (AP)