In the Persian calendar, 22 Bahman has a revered place as the anniversary of the overthrow of the Western-backed monarchy by the world's first Islamic revolution. Tomorrow marks 31 years since that event and in Tehran the preparations are underway. Bunting and flags are strung across the streets, and loudspeakers have been fixed to lamp posts to relay speeches which, according to tradition, should be met by mass roars of "Death to America" from crowds gathered beneath the city's Azadi monument.
This year, however, as Iran's nuclear standoff with the West escalates to crisis levels, the opposition is planning to hijack the annual "celebrations" for its biggest show of strength in months. Pro-government forces will also be out in force – as they were yesterday, protesting outside European embassies – and the pieces are in place for a volatile confrontation. No one doubts that the security forces have been readying their weapons and the prisons, clearing space for "rioters", and the supreme leader Ayatollah Ali Khamenei has threatened they will be given a "punch in the mouth" if they dare to protest.
This internal political drama helps to explain Iran's latest nuclear challenge to the world, analysts believe. Yesterday, Tehran said that scientists at its Natanz nuclear enrichment plant had begun, in the presence of inspectors from the IAEA, upgrading stocks of uranium from a current level of purity of 3.5 per cent to 20 per cent.
Reformist Iranians, intellectuals and young urban middle classes have long stayed away from the commemorative events. Revolution day has become a ritual which even the faithful find a bore.
But this year the revolution itself is under attack, from within. After months of a crackdown that has included the execution of several leading reformists, many – including some in the clerical establishment – believe the values on which the Islamic republic were founded have been betrayed by a ruthless and militarised cabal motivated by wealth and power. They have even begun to question the doctrine which vests supreme authority in Ayatollah Khamenei.
Faced with this internal threat, Tehran sees value in raising the stakes on the nuclear front. The government insists the higher grade uranium that yesterday's step purportedly brings closer is for use by oncologists in the treatment of cancer, which is not illegal under UN rules. But the US, Israel and other Western governments say the move is an alarming provocation because Iran does not have the technology to turn enriched uranium into fuel rods for medical uses. Once it reaches 20 per cent purity however, it could easily continue to enriching to the 90 per cent required for a nuclear bomb.
Either way, President Mahmoud Ahmadinejad has walked away from a proposed uranium export deal which would have given Iran a supply of French-made fuel rods for its medical research reactor in return for ceding two-thirds of its existing stockpile of low-enriched uranium.
US President Barack Obama responded to Iran's move yesterday by saying he wanted a new push on sanctions within weeks. Mr Obama said it was now clear to him that Iran was pursuing a path to "nuclear weaponisation". Russia also indicated it might support tougher UN sanctions. Bernard Kouchner, the French foreign minister accused Iran of engaging in "blackmail" saying a negotiation track was now "impossible". China remains unconvinced, but yesterday urged all sides to return to discussing the fuel exchange deal. Israeli Prime Minister Benjamin Netanyahu has demanded immediate and "crippling" sanctions.
Experts doubt whether Iran is even equipped to enrich to 20 per cent on its own. But the prospect of a political crisis with the West is something President Ahmadinejad may gamble on so that he can profit domestically, according to Western diplomats.
The Iranian regime has been suffering a legitimacy crisis since the June elections. Having sidelined political moderates, the hardliners that support Mr Ahmadinejad and the Supreme Leader have less interest than ever in a rapprochement with the West, which could erode the international isolation that entrenches their grip on power.
Tackling Iran on the nuclear issue is fraught with difficulty for Western governments because even those Iranians who detest Mr Ahmadinejad's policies defend the right to a nuclear programme. For them the nuclear issue has another purpose; it is an important badge of patriotism with which to defend themselves against charges that they are puppets of foreign powers.
The West also has a dilemma on sanctions. Even if China were to go along with such measures, any move to apply the kind of harsh squeeze that would satisfy Iran's harshest critics may lead ordinary Iranians to blame the West, rather than the embattled regime. New sanctions would target Iran's central bank, blacklist shipping companies and expand lists of Iranians facing travel bans and asset freezes. Iran is facing an economic crisis with oil money drying up. But the Revolutionary Guard Corps, which controls much of the economy, and would be the key to any nuclear weapons programme, has found ways around three rounds of sanctions, Western intelligence sources say. They have established front companies and banks in Belarus, Turkey and Syria.
Uranium enrichment: A costly process
At the moment, Iran is able to refine its uranium to 3.5 per cent purity – but it now intends to use a chain of 164 centrifuges to raise that to 20 per cent. To put that in context, a nuclear bomb needs uranium of 90 per cent purity. While that is still some way off, the move from 20 per cent to 90 is considerably more straightforward than the complex and costly process of enrichment required for the earlier steps.
The centrifuges needed to separate the potent U-235 isotope from the stable U-238 that makes up most of mined uranium need to be able to spin at about 100,000 rpm to take advantage of the two variants' differing weights, throwing the U-238 atoms to the walls of the tube and leaving the more useful molecules at the centre. They also need to be extremely sturdy and well-balanced, an expensive combination.
If Iran reached 20 per cent, it would indicate that it had a command of the process that would trouble the West. Despite the stated explanation – that the enrichment is for a medical reactor – Tehran currently lacks the technological capacity to convert the uranium to the fuel rods that such a reactor would need. And while the medical reactor needs 1.5kg a month, Iran says the new process produces 3-5kg in the same period.