Updates

WASHINGTON — The Obama administration’s claim that the Iran nuclear accord provides for airtight verification procedures is coming under challenge from nuclear experts with long experience in monitoring Tehran’s program.

Energy Secretary Ernest J. Moniz has insisted that Iran would not be able to hide traces of any illicit nuclear work before inspectors gain access to a suspicious site. But several experts, including a former high-ranking official at the International Atomic Energy Agency, said a provision that gives Iran up to 24 days to grant access to inspectors might enable it to escape detection.

Olli Heinonen, a former deputy director of the agency, said in an interview that while “it is clear that a facility of sizable scale cannot simply be erased in three weeks’ time without leaving traces,” the more likely risk is that the Iranians would pursue smaller-scale but still important nuclear work, such as manufacturing uranium components for a nuclear weapon.

“A 24-day adjudicated timeline reduces detection probabilities exactly where the system is weakest: detecting undeclared facilities and materials,” he said.

David Albright, the president of the Institute for Science and International Security and a former weapons inspector in Iraq, also said that three weeks might be ample time for the Iranians to dispose of any evidence of prohibited nuclear work. Among the possibilities, he said, were experiments with high explosives that could be used to trigger a nuclear weapon, or the construction of a small plant to make centrifuges.

“If it is on a small scale, they may be able to clear it out in 24 days,” Mr. Albright said in a telephone interview. “They are practiced at cheating. You can’t count on them to make a mistake.”

Mr. Moniz, Secretary of State John Kerry and Treasury Secretary Jacob J. Lew were at the Capitol on Wednesday afternoon for separate classified briefings to the entire House and entire Senate, with public appearances scheduled for Thursday before the Senate Foreign Relations Committee.

They are likely to be questioned about a recent statement by Iran’s supreme leader, Ayatollah Ali Khamenei, that Tehran would not discuss regional issues with an “arrogant” United States. Mr. Kerry called those remarks “very disturbing” in an interview on Monday on the news channel Al Arabiya.

But Senator Bob Corker of Tennessee, the Foreign Relations Committee chairman, said the cabinet secretaries would be grilled on the nuclear issues, including on how the accord would be monitored.

Inspectors’ access to sites in Iran was one of the hardest-fought issues in the closed nuclear talks. In Tehran, Iranian officials initially declared that military sites could not be inspected. And inside the negotiating room in Vienna, Iranian officials insisted that if the international inspectors wanted to visit a suspicious site, the atomic energy agency should provide months of notice.

Administration officials said the procedure that they worked out in Vienna ensured that Iran could not endlessly drag out the process and made it clear that military sites were not off limits.

“The 24-day review process provides an extra layer of assurance that Iran cannot stonewall the agency’s access to suspect sites,” said Kelsey Davenport, who monitored the negotiations for the Arms Control Association.

But the debate among experts, including some who support the accord, is whether 24 days is too long.

“ ‘No notice’ inspections were clearly not achievable, but a limit shorter than 24 days would have been desirable,” said Robert J. Einhorn, a former State Department official who served on the American delegation to the Iran nuclear talks from 2009 to 2013.

“While evidence of some illicit activity — construction of a covert enrichment facility or work with nuclear materials — would be difficult or impossible to hide or remove in 24 days, incriminating evidence of lesser activities probably could be removed,” Mr. Einhorn said. “But it is probably the case that the greater the significance of a covert activity, the more difficult it will be to remove evidence of it in 24 days.”

Under the terms of the accord, Iran would have 14 days to either grant access to international inspectors who request access to a suspicious site or find another way to satisfy the atomic energy agency’s concerns.

If Iran and the agency could not come to terms, the matter would be referred to an eight-member commission made up of the United States, Britain, France, Germany, Russia, China, Iran and a representative of the European Union. That panel would have another week to decide whether access should be granted.

The makeup of the committee makes it very likely that the United States could come up with four additional votes to form a majority supporting the demand for access. If Iran relented, it would have three more days to provide access to the inspectors, for a total of 24 days. If it refused, the matter would be referred to the United Nations Security Council, where economic sanctions could be reimposed.

Mr. Corker said his concerns went beyond those expressed by the weapons inspectors. The 24-day notice may actually understate the time Iran would have to prepare for inspections, he said, because under the accord, site visits would be announced in two stages, with the first being notice to Iran detailing why inspectors were suspicious of a site.

“So you’re signaling to Iran what you’re looking for,” he said. “It’s not just 24 days.”

Further complicating the picture, Mr. Corker said, only countries with diplomatic relations with Iran would be able to send in inspectors, “so all this great expertise we have in this country will not be usable.”

“You’re going to have a lot of questions about the verification process,” Mr. Corker said.

Energy Department officials assert that 24 days is not too long to wait for an inspection, because Iran would not be able to scrub away all signs of prohibited work.

“If they actually tested any centrifuges or did uranium conversion or did actual production of uranium metals, these types of activities generate contamination,” said an Energy Department official, who spoke on the condition of anonymity to discuss the negotiations. “That is a very difficult thing to clean up. It is like trying to get all the grease off your car and your car has 100,000 miles on it.”

And while Iran might initially be able to conceal its work on high-explosive triggers for a nuclear weapon, the official said it would be hard for the Iranians to hide their efforts as that work progressed and they started using uranium in the tests. The official said that there would be “uranium spread all over the place.”

Mr. Heinonen, however, said there had been cases in which Iran had successfully hidden evidence of illicit nuclear work even when nuclear enrichment was involved.

When the atomic energy agency sought to inspect the Kalaye Electric Company site in Iran in 2003 to check whether the Iranians were using centrifuges that they had obtained from Pakistan, the Iranians kept inspectors at bay while they spent weeks removing the equipment and renovating the building where it had been kept.

“Certain parts of the installation were renovated, leaving no trace of enrichment activities that had taken place,” Mr. Heinonen said. “However, nonrenovated parts had uranium in the 2003 contamination, which raised concerns.”

As impressive as the Iranians’ efforts at concealment were then, Mr. Heinonen said they would be better prepared to remove the evidence of illicit work if they decided to cheat on the accord.

“There will likely be plans to be executed promptly to avoid getting caught,” he said.