Who Or What Caused The Explosion At Iran's Arak Heavy Water Plant?

Last week an explosion rocked the heavy water plant being built in Arak, Iran.  It is not known whether it set back the Iranian Nuclear Program back a while or if it was just another hiccup to that program.  Nor is it known whether it was sabotage or just an industrial accident.  But it did happen.

From Debka:

Tehran did its utmost to conceal the mystery blast which last week struck the heavy water reactor under construction at Arak in western Iran. It is revealed here for the first time by debkafile’s intelligence and Iranian sources. The explosion, whether accidental or not, will delay for a second time the reactor’s first test with real fuel. Tehran informed the International Atomic Energy Agency in August of a previous holdup.

The cause of the blast and the extent of the damage it caused have not yet been established. According to the partial information initially reaching our sources, it occurred inside the reactor building when preparations were underway for a test scheduled for this month with artificial fuel and light water. The site of the explosion may have been the large coolant containers and the pressure gauges attached to the reactor’s core.

Iranian Atomic Council experts and intelligence agents are testing four possible causes of the explosion:

1. Sabotage.
2. A virus planted in the computers that control the systems administering the test.
3. An error in engineering calculations in the design of the coolant containers which underestimated their strength for standing up to the required level of pressure.
4. The deliberate sale to Iran of inferior steel materials that were not strong enough to withstand such pressure.

Last August, Iran informed the nuclear watchdog that the test with real fuel would be the final one before the reactor entered its running-in stage. The damage caused by the explosion will have postponed that stage indefinitely.

The Arak reactor, known as IR-40 and designed for a capacity of 40 megawatt, is the cause of deep concern in Israel because it is capable of producing plutonium for use in nuclear bombs as an alternative to enriched uranium.

It has been known that there are men and women in Iran who disagree with Iran's nuclear program and have sabotaged it before.  Acts that have cost them dearly.  And it is known that in their rush to build their program they have used inferior materials in the buildings.  One good earthquake and they will crash to the ground.  And in the past computer viruses like Stuxnet have caused great harm to the Iranian program.

Any of the above scenarios could be the reason for this explosion.  Just was it big enough to stop the Iranians from getting a nuclear weapon soon?