Iran has tripled its amount of enriched uranium. (Courtesy Creative Commons)

The United Nation’s latest report on Iran’s nuclear program released Friday said Iran has tripled its production of enriched uranium, ignoring international concern about its possible atomic bomb plans.

The International Atomic Agency (IAEA) report said Iran failed to give a convincing explanation about a quantity of uranium metal that is missing amid its 20 percent enrichment of uranium. While a natural concentration of uranium-235 is less than 1 percent, nuclear power requires 3 to 5 percent and weapons-grade uranium requires about 90 percent.

Daily newspapers in Iran previously reported the Tehran facility where the radioisotopes are being produced to treat cancer patients. This is of global concern because uranium at 20 percent enrichment can be more quickly converted into material for a nuclear warhead than at a lower percent enrichment.

CNN said Iran has been trying to keep inspectors from visiting enrichment sites while accelerating production of enriched uranium:

The IAEA found that between the November report and its current one, Iran added 3,000 centrifuges for a total of 9,000, according to Paul Brannan, a senior analyst at the Institute for Science and Security. More centrifuges mean more enrichment capability.

"That's a significant jump," he said.

Brannan described Iran's growing stockpile of the enriched uranium as a "major concern" and questioned why so much is needed.

"They stockpiled so much of it they can operate for over a decade," Brannan told CNN.

The 20% level is significant because nuclear experts believe further enrichment needed to create a material for a possible weapon is easy to achieve once one is capable of 20%.

But Brannan said Iran is likely to be deterred from increasing the enrichment because of international attention, noting that it would have a hard time not arousing suspicions of inspectors if it tried to keep them from the enrichment sites in the time it would take to further enrich.

Iran's representative to the IAEA, Ambassador Ali Asghar Soltanieh, denied Friday that officials blocked inspectors from visiting Parchin, saying inspectors left a day early.

He told CNN that access to the complex would be granted once Iranian officials and the agency can agree on the conditions under which such a visit would take place.

"We are trying to be cooperative," he said.

According to Wired.com, Tehran has a long, possibly difficult road to travel before developing a missile that can hit American soil:

For one thing, Iran needs to master what’s called “clustering” of the engines needed to power its missile. Picture a box with an engine — probably from a North Korean Nodong-2, the paterfamilias of Iran’s missiles — on each corner. Iran in fact unveiled precisely such a design in 2010.

There’s a long way between design and a working set of thrusters, however. Basically, in order to keep the missile on track as it streaks through the heavens, each engine has to provide precisely the same amount of thrust. If not, the pulses of acoustic energy from one engine might destroy another. “That’s not an easy thing, to make sure they fire simultaneously and don’t shake themselves to death in process,” says Greg Thielmann, a former missile analyst at the State Department’s intelligence wing.

Then there are additional technical obstacles Iran isn’t believed to have overcome. Guidance systems need to be able to withstand the pressures of atmospheric reentry to keep the missile on course. “Then the warhead itself has to function at such extreme physical conditions,” says Hans Kristensen, director of the nuclear studies program at the Federation of American Scientists. “There are several really complicated steps they have to go through to do this.”

The IAEA report said it requested access to records and personnel involved in the experiments that made the metal, but Iran said the information was no longer available.