Iran nuclear deal seen reducing bomb risk despite Israeli criticism

* Iran nuclear accord seen lengthening any breakout timeline

* Halts Iran's most sensitive atomic activity

* But still leaves large stockpile, many centrifuges

* Nuclear expert warns of verification "landmines"

By Fredrik Dahl

GENEVA, Nov 24 (Reuters) - Denounced by Israel as a "bad deal", a breakthrough agreement between Iran and six world powers to restrain its nuclear programme should nevertheless make it significantly harder for the Islamic state to build any atomic bomb.

By halting Iran's most sensitive enrichment of uranium, Sunday's interim accord is designed to stop the expansion of Iranian atomic activities and buy time for negotiations on a final settlement of the decade-old nuclear dispute.

However, Iran will for now retain thousands of centrifuges refining uranium - albeit only to concentrations far below that needed for nuclear weapons - and a stockpile that could potentially be used for bombs if processed much more.

"The short-term deal accomplishes a great deal," nuclear proliferation expert David Albright of the U.S. Institute for Science and International Security (ISIS) said.

For example, he said, it would eliminate Iran's stock of uranium gas refined to a fissile purity of 20 percent, a source of deep concern for the West as it represents a relatively short technical step away from bomb-grade material.

Under the agreement, Iran must halt this higher-grade enrichment and also dilute or convert its existing reserve of such uranium to a form that is not suitable for further enrichment, according to a U.S. fact sheet.

Once this is done, the breakout time - how long it would take Iran to produce sufficient highly-enriched uranium for one atomic bomb - would lengthen from at least 1-1.6 months to at least 1.9-2.2 months if the Iranians used all their installed centrifuges, Albright said in an e-mail.

"This may seem a small increase, but with the IAEA daily checking the camera film at Natanz and Fordow, this increase in breakout times would be significant," he said, referring to the U.N. nuclear watchdog, the International Atomic Energy Agency.

Iran has committed to grant IAEA inspectors daily access to its underground enrichment facilities at Natanz and Fordow, its most controversial nuclear sites, the U.S. fact sheet said.

"This access will provide even greater transparency into enrichment at these sites and shorten detection time for any non-compliance," it said.

The U.N. watchdog, tasked with ensuring that no nuclear material is diverted for military purposes in member states, is currently believed to visit these plants about once a week.