Sometimes what is left unsaid is more important than what is actually written down in the text of an agreement. In the case of the deal between Iran and the five permanent members of the UN Security Council plus Germany (P5+1), the interim agreement struck on Sunday allows Iran to retain all the important elements of a nuclear fuel cycle, including uranium enrichment.

It also seems to envisage Iran will retain most elements of the fuel cycle under a final agreement, even if the P5+1 declined explicitly to acknowledge Iran's "right" to enrich uranium under the Nuclear Non-Proliferation Treaty (NPT).

THE RIGHT TO ENRICH?

The joint plan of action commits negotiators in the next phase to "reach a mutually-agreed long-term comprehensive solution that would ensure Iran's nuclear programme will be exclusively peaceful."

It explains that the comprehensive solution "would enable Iran to fully enjoy its right to nuclear energy for peaceful purposes under the relevant articles of the NPT in conformity with its obligations therein. This comprehensive solution would involve a mutually defined enrichment programme with practical limits and transparency measures to ensure the peaceful nature of the programme."

It further explains the mutually defined enrichment programme would involve "mutually agreed parameters consistent with practical needs, with agreed limits on scope and level of enrichment activities, capacity, where it is carried out, and stocks of enriched uranium, for a period to be agreed upon."

There are some safeguards. Iran has agreed not to enrich uranium beyond the 5 percent level for the next six months.

Half the stock of uranium that has already been enriched to 20 percent will be blended back down to 5 percent. But Iran will be allowed to keep the other half "as working stock" in the form of uranium oxide for the fabrication of fuel for the Tehran Research Reactor.

For the next six months, Iran will not start enriching at any new locations. It will not start feeding uranium into any new centrifuges. But it will be allowed to replace damaged centrifuges with new ones of the same type.

The International Atomic Energy Agency (IAEA) will intensify its surveillance of Iran's nuclear programme. Iran has agreed to provide more detailed information on the design and operations of all its nuclear sites.

IAEA inspectors will have daily access to nuclear sites, and the right to conduct unannounced inspections at the enrichment facilities at Fordow and Natanz.

But it is all a far cry from previous rounds of negotiations, where the P5+1 pressed Iran to ship its entire stock of 20 percent-enriched uranium out of the country and dismantle its enrichment facilities, in exchange for access to uranium enriched overseas.

No wonder Iranian officials have appeared jubilant, while hawks in Israel, Saudi Arabia and the US Congress have appeared downcast, describing it as a bad deal or a "historic mistake."

TROUBLESOME ISOTOPES

Naturally occurring uranium is found as two main isotopes: uranium-235 (U-235) and uranium-238 (U-238). U-235 can be made to split (fissioned) when bombarded with neutrons in a nuclear reactor, releasing heat as well as more neutrons to start a chain reaction. U-235 is also the isotope needed to make a bomb, where the chain reaction is much faster and more intense.

By contrast, U-238 is not fissionable and is useless for either a nuclear reactor or bomb-making. It can however be transmuted into plutonium in a nuclear reactor, which produces additional power, but is also suitable for making a bomb if the spent nuclear fuel is reprocessed.

In its natural state, uranium consists of 99.28 percent U-238 and just 0.711 percent U-235.

Britain's first generation of Magnox nuclear power reactors used fuel made from unenriched uranium with less than 1 percent U-235. Canada's CANDU reactors also use uranium in its natural state as fuel.

But using unenriched uranium is not efficient. Among other problems, the reactor core and containment vessel have to be very large because so little of the fuel being fed into them can actually be fissioned to release energy.

So modern reactors use fuel fabricated from enriched uranium, where the proportion of U-235 has been artificially raised to between 3 percent and 5 percent, allowing the reactor to be more compact and refuelled less frequently.

GAS CENTRIFUGES

The enrichment process exploits the minute weight difference between the (lighter) U-235 and (heavier) U-238 atoms.

Modern enrichment processes employ gas centrifuges. Uranium is turned into a gas, uranium hexafluoride (UF6) which is fed into the centrifuges.

As the centrifuges rotate at high speed, the slightly heavier molecules containing U-238 are flung towards the outside of the centrifuge, while the slightly lighter molecules with U-235 are concentrated near the centre.

The degree of enrichment at each stage is very small. So centrifuges are arranged in cascades: the slightly enriched output of Centrifuge 1 becomes the input to Centrifuge 2 and so on.

By running UF6 through enough centrifuges for long enough, any desired degree of enrichment can be achieved.

Power reactors require fuel enriched to between 3 and 5 percent U-235. To make an atomic bomb, uranium needs to be enriched to around 90 percent. But the difference is purely one of degree: bomb-making requires running UF6 through more centrifuges for longer.

CONSTRUCTIVE AMBIGUITY

Iran's strongest opponents have insisted that Iran should not be allowed to have any enrichment capability at all, insisting all enrichment should take place outside the country under international supervision.

The interim agreement fudges the issue. The work plan recognises Iran will continue to enrich uranium to the 5 percent level using its existing cascades for the next 6 months. In the long term Iran will be allowed a "mutually defined enrichment programme" within "mutually agreed parameters" including "where it is carried out."

In theory, the final deal could still insist that all enrichment be conducted outside Iran, but that seems unlikely.

Iran insisted the deal recognise its "right" to enrich. The P5+1 declined to go that far. But the deal tacitly recognises Iran will continue to enrich in the short term, and probably in the long term, whether it has a formal right to do so under the NPT or not.

That was the big concession the P5+1 made to get a deal.

In exchange, the agreement insists Iran will not construct any facility capable of reprocessing spent nuclear fuel, which would prevent the country from recovering any plutonium produced in the reactors. This is part of the interim deal, but the work plan stipulates it will be part of any final agreement too.

By limiting Iran to its current number and type of centrifuges, the interim deal will also stop the country from deploying more and newer machines that could speed up the separation process, allowing to enrich fuel to higher levels quickly.

BREAKOUT CAPABILITY

The intentions of Iran's nuclear programme remain unclear. Iran insists its objective is "exclusively peaceful" and that it will "under no circumstances ... ever seek or develop nuclear weapons," points reiterated in the agreement.

Iranian officials insist the country must be able to enrich its own fuel to avoid dependence on hostile powers, which it fears want regime change.

But the United States and its allies fear the civilian nuclear power programme is a cover for building atomic weapons.

The quality of the intelligence on which the US assessment is based remains (necessarily) uncertain. It is unknown how many and what quality of sources US and other intelligence agencies have inside Iran's nuclear programme.

Many experts, however, believe Iran's aim is to have the capability to build a weapon quickly, if necessary, in response to an external threat, rather than build an actual bomb straight away.

The interim agreement strikes a compromise which gives everyone, except Iran's most ardent opponents, some of what they want.

For Iran, it leaves intact the centrifuges and enrichment programme. If the Islamic Republic felt threatened in future, it could breach the agreement and start enriching up to the 90 percent level needed to create a bomb. In the meantime, the country's civilian nuclear power programme would be assured of its own fuel supply.

For the P5+1, the restrictions on the number of centrifuges, plus Iran's agreement to cap new enrichment at 5 percent, and dispose of half of its stock of 20 percent enriched fuel, coupled with more intensive inspections, means the length of time needed to break out will increase and the decision to break out should be easier to spot.

There would be more time for the US or other countries to respond to any Iranian decision to race for a weapon.

Whether that is a good deal depends on what the alternatives are. But for the Obama administration and the Iranian leadership, it appears to be good enough for now. - Reuters

* The opinions expressed here are those of the author, a columnist for Reuters.