Iran stands at the threshold of a nuclear weapons capability as the world watches in indecision.

Sanctions, covert action, and diplomacy have failed to alter Iran’s nuclear policy. Nor have they had a visible effect Iran´s the enrichment program – including Tehran´s growing stockpile of 19.75% low-enriched uranium (LEU).

Obtaining weapons-grade high-enriched uranium (HEU) is the most difficult and technically challenging obstacle to acquiring a nuclear weapon.

Assessing the “breakout” time – the time required to convert LEU to weapons-grade HEU – is therefore a critical component of determining progress toward a nuclear weapons capability.

Iran’s bank of rapidly spinning centrifuges has produced a growing stockpile of low-enriched uranium, able to fuel nuclear reactors, but able also to fuel nuclear weapons if further enriched. Enrichment raises the concentration of the uranium isotope U-235, which fissions in first-generation nuclear weapons.

As Iran increases its stockpile of low-enriched uranium, and its stockpile of uranium is enriched to 20 percent U-235, it will consolidate its status as a "virtual" nuclear weapon state.

Iran´s enrichment activities occur at its facilities in Natanz and Fordow. The Natanz facility is above ground and – despite Iran´s attempts to protect it with anti-aircraft defenses and a fighter screen – remains vulnerable to attack.

As a result, Iran has accelerated its uranium enrichment activities at the Fordow facility. The site – once covert and grossly mischaracterized by US officials as a façade – is buried in the side of a small mountain outside Qom.

Considered a "hard target" by military analysts, Fordow is the focus of intense scrutiny by the International Atomic Energy Agency (IAEA) and the subject of the nuclear watchdog’s detailed analysis of Tehran´s weaponization work.

According the IAEA, Fordow began producing uranium enriched to 20 percent earlier this year and has recently seen an expansion of its advanced centrifuges – the key and difficult-to-obtain component in enrichment activities.

These developments reduce the time Iran needs to produce fuel for a nuclear weapon and accelerate the stockpiling of weapons grade uranium. Should Iran choose to make a dash for a nuclear weapon, the world will be faced with a narrow window in which both to discover the move and take action to stop it.

The most recent IAEA report published earlier this month predicts Iran will possess enough 19.75% LEU for a 15 kiloton nuclear bomb – sufficiently large to be strategically useful – by 1 June 2012.

The worst case scenario is that Iran could reach the 90% HEU threshold for weapons grade uranium within one month of beginning its proliferation sprint. However, this scenario is considered highly unlikely and relies on contested technical assumptions about Iran´s enrichment capabilities.

Proliferation experts say the most likely scenario would be Iran´s reaching 90% HEU within 2.5 to 3 months of beginning its break-out.

A second concern is Iran´s attempts to render its critical centrifuge operations both more diffuse and impenetrable, which would take Iran into Defense Minister Ehud Barak´s "immunity zone."

At present the destruction of the Fordow and Natanz sites could set Tehran´s enrichment program back years, giving sanctions time to have their desired effect. While the Natanz site is vulnerable to attack, US officials have recently said neither Washington nor Jerusalem have the ability to penetrate the Fordow facility.

Simply destroying the Natanz facility while Fordow remains operational would only extend the window for an Iranian nuclear break out – to perhaps one year – rather than stopping it. According to Air Force officials, its current 20.5 foot-long Massive Ordnance Penetrator (MOP) carries over 5,300 pounds of explosive material and is designed to penetrate up to 200 feet underground before exploding.

The mountain above the Iranian enrichment site at Fordow is estimated to be at least 200 feet tall, which has raised doubts about the MOPs ability to effectively destroy Fordow. Those doubts have prompted Pentagon officials this month to secretly submit a request to Congress for funding to enhance the bomb´s ability to penetrate deeper into rock, concrete and steel before exploding.

The push to boost the power of the MOP is part of stepped-up contingency planning for a possible strike against Iran´s nuclear program, say U.S. officials. US Secretary of Defense Leon Panetta has said the current generation of MOPs could cause "a lot of damage" to the Fordow facility, but wouldn´t necessarily destroy it outright.

"We´re developing it. I think we´re pretty close, let´s put it that way. But we´re still working at it because these things are not easy to be able to make sure that they will do what we want them to." he said. Panetta added: "But I´m confident, frankly, that we´re going to have that capability and have it soon."

As a result, Tehran finds itself facing a ticking clock of its own and will have to time its nuclear sprint – should it choose to make one – to beat Washington´s own rush for a bigger and better bunker- buster. The Air Force has so far contracted to buy 20 of the new bombs and more deliveries are expected in early 2013.

Israel has large bunker-buster bombs, but the US hasn´t provided the MOP to Jerusalem. Nor is Washington likely to provide Israel with its replacement in 2013. Analysts believe it is highly unlikely repeated strikes with Israel´s current bunker-busters would prove effective in destroying Fordow. Those doubts render an Israeli strike on Iran fraught with difficulty and potential failure.

This stark reality that Israel´s leaders must confront is rendered even more complicated and dangerous by the Obama administration´s diffident posture vis-a-vis taking direct military action against Iran. Washington has declared an Iranian nuclear bomb is "unacceptable," but refuses to commit to a strike on Natanz and Fordow should Iran choose to make a nuclear sprint.

That leaves leaders in all three capitals – Jerusalem, Tehran, and Washington – watching the clock and waiting for the starter´s gun to fire. (IsraelNationalNews © 2012 02/26/12)