The fuelling of the Bushehr reactor represents a milestone in the development of Iran's nuclear programme.
It has been something of a marathon challenge.
"Once up and running, some 35 years after the project first began, Bushehr will be the first nuclear power reactor to generate electricity in the Middle East," says Mark Hibbs, a veteran nuclear analyst with the Carnegie Endowment for International Peace.
Technically this could be within a matter of months, though US sources believe that it may be up to a year before the reactor is supplying power to the Iranian grid.
"The project has survived international sanctions, aerial attacks by Iraq, and decisions by Iran's revolutionary leaders first to scrap the programme, and then to restart it with new partners in Russia," says Mr Hibbs.
Given all the controversy surrounding Iran's nuclear activities and the widespread concerns in the US and Europe that it is pursuing a clandestine nuclear weapons programme, many people may wonder why alarm bells are not sounding at the news that the Bushehr reactor is taking another important step towards becoming operational.
The reason is that Iran's power-generating programme, which for now rests upon this single Russian-supplied reactor, and its uranium enrichment programme are, up to a point, two different things.
Even many of Iran's critics do not deny it the right to generate power by nuclear means.
The fuel for the Bushehr reactor will be provided by Russia and it will be returned there for re-processing.
And for all its disagreements with the UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), the Russian-supplied fuel will be subject to a wide array of safeguards.
Indeed experienced arms control experts like Mark Fitzpatrick at the International Institute for Strategic Studies in London (IISS) believe that a working Iranian reactor does not pose a proliferation risk as long as it is run solely to produce power for electricity generation.
"It would be a risk if Iran operated it differently, i.e. for short periods at low burn-up in order to produce weapons-usable plutonium. But in this case, the IAEA would know," Mr Fitzpatrick says.
"Likewise, the IAEA would know if Iran tried to divert the spent fuel, before it is cooled sufficiently to send it back to Russia."
'Not a target'
Some pundits, not least Washington's former Ambassador to the UN in the Bush years, John Bolton, have warned that the loading of the reactor changes western calculations.
It means that it will now be impossible for Israel or the US to attack the facility from the air, since bombing the reactor after the fuel rods are loaded would spread radiation.
But Mr Fitzpatrick wonders why Bushehr would be a target at all.
"If and when Israel does carry out air strikes, I don't think Bushehr would be in their target set," he says.
"With limited sorties available, they would go after Natanz, Esfahan, Fardo and targets associated with Iran's enrichment effort."
Mr Hibbs, too, believes that the proliferation risks from Bushehr are minimal.
"Theoretically, Iran could take the high risk of diverting IAEA-inspected fuel from Bushehr to make fuel for a nuclear weapon," he says
"But Iran's centrifuges are far more tailor-made for that purpose. Any attempt by Iran to divert Bushehr fuel would become known."
Nonetheless, Mr Hibbs argues that the eventual start-up of Bushehr does represent a setback to Western diplomatic efforts.
"Western governments have sought to put Bushehr under international sanctions. But the sanctions policy failed on two accounts: they failed to prevent Iran from operating Bushehr, and they also probably made the reactor less safe," he says.
When Bushehr starts up, it will be the only power reactor in the world operating without its national government belonging to the international Nuclear Safety Convention.
Mr Hibbs worries that because Iran's co-operation with the IAEA is limited, "without Iran's participation in this convention, the outside world has no credible assurances from Iran that it is operating Bushehr in a manner reflecting internationally acceptable safety standards."
As the first electricity-generating reactor in the region, the Iranian project is being watched closely by its neighbours.
Many other countries in the Gulf and beyond - the UAE and Jordan, for example - are eager to pursue nuclear power.
Any safety setback in Iran could seriously damage the image of nuclear power generation in the region as a whole.
Certainly the Iranian authorities will seek to present the fuelling of the reactor as a significant national step forward.
In general terms, it will help Iranian technicians to gain more experience in the nuclear field.
But it still leaves open a fundamental question: What is Iran's uranium enrichment programme for?
Iran claims that it is developing this technology to supply fuel for a future series of locally-built reactors to fulfil the country's long-term energy needs.
But Mr Fitzpatrick wonders how Iran will build reactors on its own.
"They certainly would not be safe", he told me.
With or without Bushehr operating, the basic issues surrounding Iran's enrichment effort remain. In the absence of negotiations, Washington and its allies look set to try to pull the sanctions net even tighter.