FACTBOX: Iran's relations with U.N. nuclear watchdog

By Mark Heinrich

VIENNA (Reuters) - Western leaders have accused Iran of hiding a second uranium enrichment plant from U.N. nuclear inspectors for years before admitting its existence in a letter to the International Atomic Energy Agency this week.

Iran denied concealing anything, with President Mahmoud Ahmadinejad telling a New York news conference, "It's not a secret site. If it was, why would we have informed the IAEA about it a year ahead of time?" Iranian officials said the plant was being built according to IAEA regulations.

Here are some relevant points about the dispute.

NOTIFICATION RULES

* The United States and major Western allies believe Iran violated its nuclear non-proliferation safeguards agreement with the IAEA about when it should inform the agency of a new nuclear facility or plans for one.

* Under the original IAEA statute governing member states, such a declaration was not required until six months before nuclear materials were introduced into a new atomic facility.

* But in 1992, the 35-nation IAEA Board of Governors decided this not was not sufficient time to arrange requisite inspections. They amended the rule to say notification has to be made "as soon as the decision to construct or to authorize construction has been taken, whichever is earlier."

* This amendment is known in the safeguards community as "Code 3.1." Iran and many other countries signed up to it.

* Western diplomats with access to intelligence said Iran started building the undeclared second enrichment plant in 2006. "Iran was then observing the modified Code 3.1, so (legally) it should have notified the IAEA then," one diplomat said.

* In 2007 Iran reverted to the previous arrangement -- a six-month notice period -- in protest after the IAEA governors referred its case to the U.N. Security Council in 2006, a move that led to three sets of punitive sanctions.

Since then, Iran generally has refused to provide advance design information on planned nuclear sites to the IAEA.

* The IAEA's stance is that countries cannot unilaterally go back to the old system, but rather are bound to the new one.

PREVIOUS PATTERN OF SECRECY

* Iran set up a nuclear program based on proliferation-prone uranium enrichment with the help of Pakistani-led traffickers in atom bomb know-how and concealed it for 18 years until Iranian exiles blew the whistle in 2002.

* Iran admitted the existence of a nascent enrichment facility at Natanz and submitted to regular IAEA inspections meant to verify the dual-use technology used there would be limited to refining uranium for electricity, not atom bombs.