When international talks about Iran's nuclear program reconvene next month, a key test of progress will be whether U.N. inspectors get access to a bus-sized metal chamber, where specialists suspect Iranians might have tested a trigger for an atomic bomb.
The chamber likely was used to test a device called an "implosion system," which helps set off a nuclear weapon, according to Paul Brannan of the Institute for Science and International Security, a nonprofit nuclear-proliferation watchdog.
He said U.N. inspectors have requested access to the site at the Iranian military complex at Parchin, a few miles southeast of Tehran.
Allowing them in would be a "straightforward" way for Iran to demonstrate good faith and to allay international concerns about a possible military element to their nuclear program, he said.
"It has substantive value, and it could happen very quickly," Mr. Brannan said.
Earlier this month, Iranian officials said that there was no nuclear activity at Parchin, and that inspectors from the U.N.'s International Atomic Energy Agency already had been there.
After last weekend's talks between Iran and the five permanent members of the U.N. Security Council and Germany, the next opportunity to gauge progress will be when officials reconvene May 23 in Baghdad.
U.S. intelligence agencies believe that Iran is developing the capability to make a weapon, but that its supreme leader, Ayatollah Ali Khamenei, has yet to make a decision on building a nuclear bomb.
Iran insists its nuclear program is purely for civilian use.
Any evidence of a nuclear trigger test found at the Parchin chamber would show Iran lied because there are no civilian applications for such devices.
From the Iranian point of view, allowing an inspection is fraught with risk, said Michael Elleman of the International Institute for Strategic Studies.
"If the inspectors find anything, the burden of proof will totally shift to the Iranians. They will have to prove a negative," he said.
Iranian officials may also see Parchin as the thin end of the wedge.
"Once that door is opened, the Iranians will be asking themselves, 'Will [the inspectors] want to see more?'" he noted.
Western intelligence officials believe the testing of the implosion trigger probably occurred about 10 years ago, the German newspaper Sueddeutsche Zeitung reported over the weekend.
But Mr. Brannan said the fact that the testing happened so long ago is immaterial.
"They've never admitted to having any kind of a military nuclear program," he said.
"If they can't even acknowledge that they had one in the past, it will be hard to get a level of confidence among the international community that they are being transparent about their current activities."
However, Mr. Elleman said, if the Iranians acknowledge past weapons development activities, "there will always be people who won't be satisfied" with any explanation or declaration they make.
"As long as there's another facility to visit, they'll never get a clean bill of health," he said.
Nonetheless, he added, it still would be in Iran's interest to come clean.
"Transparency is key," he said. "Without hard data [about Iran's nuclear program], speculation about worst-case scenarios tends to rush in to fill the vacuum."
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