Garvan Walshe was National and International Security Adviser for the Conservative Party until 2008

“Thank God for France.” Not words likely to emerge from the lips of veteran Republican senator, Lindsay Graham. He spoke them after a major diplomatic error, which could have gutted the West’s position in its negotiations with Iran, had been averted, with help coming from a place whence many have learned not to expect it. Paris.

As John Kerry promised, a deal was in the works, as the diplomatic choreographers prepared an ostentatious press conference in Geneva and, as the ever-panglossian William Hague intoned “There is now a real concentration on these negotiations, so we have to do everything we can to seize the moment and seize the opportunity to reach a deal,” it became clear to the Quai d’Orsay that a vital flank had been left uncovered.

There are two ways to make a fission bomb: with uranium, used in the device that obliterated Hiroshima, and plutonium, which formed the “fat man” dropped on Nagasaki.

The centrifuge plants, designs for which Tehran obtained from the proliferation network spun by the Pakistani nuclear scientist AQ Khan, and which fell victim to the technological marvel that was the ‘stuxnet’ cyber attack, are used for making the highly enriched uranium needed for the first type of bomb.

Bear with me: the physics is important. Uranium comes in two versions (called isotopes). Only one of them, U-238, can perform the chain reactions needed to generate nuclear power or explode a nuclear bomb.  It’s rare and has to be separated out from the innocuous U-235 in centrifuges that work a bit like giant sieves, sifting out the heavier U-238. When we say uranium has been enriched to the civilian standard of 20 per cent (sufficient for power generation) it means that enough of the lighter U-235 has been discarded, so what’s left is one fifth U-238.

A uranium weapon needs fuel that has been enriched to 85 per cent. So it looks like there’s a good margin of safety in Iran’s demand to only have fuel for civilian use. Yet because enrichment just separates out the “wrong” kind of uranium, rather then transforming one type into another, it takes an awful lot more work to create civilian fuel out of naturally occurring uranium (about 0.5 per cent U-238) than it does to turn civilian fuel into weapons grade material. If a single enrichment “cycle” doubled the concentration of U-238, it would take around six and three quarter cycles to make civilian fuel, but just slightly more than two further cycles to get to weapons grade.

It’s the same maths as in the episode from the great Persian epic the Shahnameh, or Book of Kings, where the inventor of chess asks that his reward be one grain of rice on the first square of a chessboard, two on the next, and so on. By just the seventeenth square, the emperor would have had to give him more than four billion grains of rice.

It means that should Iran possess enough 20 per cent enriched uranium, it could convert it into enough for a bomb very quickly (perhaps quickly enough to construct one before the ink dried on John Kerry’s Nobel peace prize). That’s why France was right to insist Tehran reduce its fuel stockpile.

More dangerous still is Iran’s heavy water reactor at Arak. Heavy water reactors do one thing well: produce material for plutonium bombs. They’re not much good for generating electricity.

They are also dangerous. Once the fuel rods have been introduced into a reactor, it requires great care to shut down. Even a small mistake could produce a horrific nuclear disaster. This matters because it makes bombing such a reactor once it’s got going almost inconceivable. Civilians would suffer terribly from the fallout, and the state that carried out the attack suffer a lot of blame for the consequences. No western country would have the stomach to inflict Chernobyl on the Middle East. It would remove the military option from President Obama’s, and probably Prime Minister Netanyahu’s, proverbial table.

And that’s why France was right to insist that an interim deal not allow Iran to fuel the reactor. Her tough stance on Iran has become part of an increasingly assertive pattern. Paris took speedy, tough and effective action to prevent a jihadist takeover of Mali. She was willing to step up in Syria when the House of Commons and US President were not. This goes back to the Iraq war, since which France, stung by the breach with the United States, has been determined to show that she is a responsible member of the Western alliance: determination that has now paid off.

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