VIENNA (AP) — Iranian scientists have run computer simulations for a nuclear weapon that would produce more than triple the explosive force of the World War II bomb that destroyed Hiroshima, according to a diagram obtained by The Associated Press.

The diagram was leaked by officials from a country critical of Iran's atomic program to bolster their arguments that Iran's nuclear program must be halted before it produces a weapon. The officials provided the diagram only on condition that they and their country not be named.

The International Atomic Energy Agency — the Vienna-based U.N. nuclear watchdog — reported last year that it had obtained diagrams indicating that Iran was calculating the "nuclear explosive yield" of potential weapons. A senior diplomat who is considered neutral on the issue confirmed that the graph obtained by the AP was indeed one of those cited by the IAEA in that report. He spoke only on condition of anonymity because he was not authorized to discuss the issue.

The IAEA report mentioning the diagrams last year did not give details of what they showed. But the diagram seen by the AP shows a bell curve — with variables of time in micro-seconds, and power and energy both in kilotons — the traditional measurement of the energy output, and hence the destructive power of nuclear weapons. The curve peaks at just above 50 kilotons at around 2 microseconds, reflecting the full force of the weapon being modeled.

The bomb that the United States dropped on Hiroshima in Japan during World War II, in comparison, had a force of about 15 kilotons. Modern nuclear weapons have yields hundreds of times higher than that.

The diagram has a caption in Farsi: "Changes in output and in energy released as a function of time through power pulse." The number "5'' is part of the title, suggesting it is part of a series.

David Albright, whose Institute for Science and International Security is used by the U.S. government as a go-to source on Iran's nuclear program, said the diagram looks genuine but seems to be designed more "to understand the process" than as part of a blueprint for an actual weapon in the making.

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"The yield is too big," Albright said, noting that North Korea's first tests of a nuclear weapon were only a few kilotons. Because the graph appears to be only one in a series, others might show lower yields, closer to what a test explosion might produce, he said.

The senior diplomat said the diagram was part of a series of Iranian computer-generated models provided to the IAEA by the intelligences services of member nations for use in its investigations of suspicions that Iran is trying to produce a nuclear weapon. Iran denies any interest in such a weapon and has accused the United States and Israel of fabricating evidence that suggests it is trying to build a bomb.

Asked about the project, Iran's chief IAEA delegate, Ali Asghar Soltanieh, said he had not heard of it. IAEA spokeswoman Gill Tudor said the agency had no comment.

Iran has refused to halt uranium enrichment, despite offers of reactor fuel from abroad, saying it is producing nuclear fuel for civilian uses. It has refused for years to cooperate with the U.N. nuclear agency's efforts to investigate its program.

Iran's critics fear it could use the enriched uranium for military purposes. Such concerns grew this month when the IAEA said Iran is poised to double its output of higher-enriched uranium at its fortified underground facility — a development that could put Tehran within months of being able to make the core of a nuclear warhead.

In reporting on the existence of the diagrams last year, the IAEA said it had obtained them from two member nations that it did not identify. Other diplomats have said that Israel and the United States — the countries most concerned about Iran's nuclear program — have supplied the bulk of intelligence being used by the IAEA in its investigation.

"The application of such studies to anything other than a nuclear explosive is unclear to the agency," the IAEA said at the time.

The models were allegedly created in 2008 and 2009 — well after 2003, the year that the United States said Tehran had suspended such work in any meaningful way. That date has been questioned by Britain, France, Germany and Israel, and the IAEA now believes that — while Iran shut down some of its work back then — other tests and experiments continue today.

With both the IAEA probe and international attempts to engage Iran stalled, there are fears that Israel may opt to strike at Tehran's nuclear program. The Jewish state insists it will not tolerate an Iran armed with nuclear arms.

An intelligence summary provided with the drawing linked it to other alleged nuclear weapons work — significant because it would indicate that Iran is working not on isolated experiments, but rather on a single program aimed at mastering all aspects of nuclear arms development.

The IAEA suspects that Iran has conducted live tests of conventional explosives that could be used to detonate a nuclear weapon at Parchin, a sprawling military base southeast of Tehran. The intelligence summary provided to the AP said data gained from those tests fed the model plotted in the diagram. Iran has repeatedly turned down IAEA requests to visit the site, which the agency fears is undergoing a major cleanup meant to eliminate any traces of such experiments.

The intelligence summary named nuclear scientists Mohsen Fakhrizadeh, Majid Shahriari and Fereidoun Abbasi as key players in developing the computer diagrams, adding that Shahriari and Abbasi were also involved in the Parchin testing.

Iran has for years rebuffed IAEA attempts to question Fakhrizadeh for his suspected involvement in secret programs. Shahriari was assassinated in 2010 by what Iran says were Israeli agents. Abbasi, now the head of Iran's nuclear agency, was wounded in a separate assassination attempt the same day that Shahriari was killed.

The senior diplomat, who is familiar with the Iran probe, said the agency has not yet determined any connection between Parchin and the computer models. But Olli Heinonen, who headed the IAEA's Iran investigation until 2010, said using the results of the alleged Parchin tests would "make sense as part of the design and testing of a (computer) model."