I was shown around the Bushehr reactor on Iran's Gulf coast four years ago, as part of a magical mystery tour of nuclear sites laid on by Tehran for a small group of western journalists.

Back then in 2007, construction work was already many years behind schedule and the relationship between the Iranians and the Russian engineers building Bushehr was clearly tense. On the side of the reactor wall, someone had scrawled, in Russian: "Everything passes and we will get past this."

Eventually perhaps, but the Russian technicians are going to be there for some time yet. Today, Iran confirmed a UN report that it will have to remove the uranium fuel rod assemblies from the reactor core, a significant setback.

The reactor has been under construction since the Shah's era in the 1970's. The fuel assemblies were finally introduced into the reactor four months ago and it was meant to start producing electricity in January, but that deadline was missed.

Ali Asghar Soltanieh, Tehran's envoy to the International Atomic Energy Agency, told the ISNA news agency that the fuel was being removed on Russian advice:

Based on Russia's request to run tests and technical measures, the fuel will be unloaded from the core of the reactor and will be returned to it after completion of the tests

Reuters quotes a source "close to the project" as saying the fuel may have been contaminated by the old and deteriorating metal equipment used in the construction of the reactor core.

We're talking about particles of about 3 millimeters in size. Some of the equipment used in the construction has been sitting around for 30 years. The fuel has to be unloaded and examined to make sure no metal particles have gotten into it.

The problem was first reported on Friday in the latest quarterly report on Iran's nuclear programme by the International Atomic Energy Agency (downloadable here courtesy of the Institute for Science and International Security, ISIS)

In the 'Other Matters' section towards the end, it briefly notes:

On 23 February 2011, Iran informed the Agency that it would have to unload fuel assemblies from the core, and the Agency and Iran have agreed on the necessary safeguards measures.

The Iranians have been on record as saying that the Bushehr plant was affected by the Stuxnet computer worm, which is widely believed to have been a joint venture in cyber warfare by the US and Israel. But the initial signs are that Stuxnet has not played a part in this setback. Before Soltanieh spoke, ISIS provided an instant analysis of the IAEA report, saying it had learned that:

the unloading may be motivated by concerns about the possible sabotage of the fuel assemblies or defective fuel assemblies.

David Albright, who runs ISIS, told the New York Times:

It raises questions of whether Iran can operate a modern nuclear reactor safely...The stakes are very high. You can have a Chernobyl-style accident with this kind of reactor, and there's lots of questions about that possibility in the region.

The other interesting line from the IAEA report comes in the "Possible Military Dimensions" section, which is quite toughly worded and suggests that the agency has been given new evidence of weaponisation. The relevant passage reads:

Moreover, based on the Agency's analysis of additional information which has come to its attention since August 2008, including new information recently received, there are further concerns which the Agency also needs to clarify with Iran

.