By Francois Murphy

VIENNA, Nov 18 (Reuters) – Iran has breached another limit in its nuclear deal with major powers by accumulating slightly more than 130 tonnes of heavy water, a substance used in a type of reactor it is developing, a U.N. nuclear watchdog report showed on Monday.

The restriction is the latest Iran has exceeded in protest at the United States for withdrawing from the deal last year and imposing punishing economic sanctions on Tehran.

Washington says its “maximum pressure” will force Iran to negotiate a broader deal that will also include its role in Middle Eastern conflicts. Tehran says it will not negotiate until sanctions are lifted.

As the deal is eroded, France, Britain and Germany are torn between trying to save it and responding to Iran‘s breaches.

Heavy water is not as sensitive as uranium, which Iran is enriching in a quantity and to a level of purity beyond limits set by the pact. But the 2015 agreement says Iran should not have more heavy water than it needs, an amount the deal estimates to be 130 metric tonnes.

The International Atomic Energy Agency said in a report to member states, obtained by Reuters, that it had been informed by Iran on Nov. 16 that “its stock of heavy water had exceeded 130 metric tonnes”.

The IAEA said that on Nov. 17 the agency had “verified that the Heavy Water Production Plant (HWPP) was in operation and that Iran‘s stock of heavy water was 131.5 metric tonnes”.

After the Reuters report, the IAEA issued a statement confirming the findings.

Heavy water is, among other things, used as a moderator to slow down reactions in the core of nuclear reactors like one Iran has been developing at Arak.

Since that reactor could eventually have produced plutonium, which can also be used in atom bombs, the deal required Iran to remove its core and fill it with concrete. The reactor is now being redesigned with a view to reducing the proliferation risk.

Iran has breached the heavy water cap before. It first exceeded the limit in 2016 https://www.reuters.com/article/us-iran-nuclear/iran-once-again-exceeds-a-nuclear-deal-limit-iaea-report-idUSKBN1342T1, soon after the deal went into force and well before the U.S. withdrawal in 2018. Major powers then agreed Iran could store its excess heavy water outside the country while it sought a buyer.

Since July, the IAEA has confirmed that Iran has surpassed the deal’s limits on its stock of enriched uranium and the purity to which it enriches uranium, as well as breaching its bans on enriching at sites other than its main plant at Natanz. It is also using models of centrifuges for enrichment that were not approved for that purpose in the nuclear pact.

(Reporting by Francois Murphy; Editing by Catherine Evans and Edmund Blair)