You may have seen yesterday that Iran has agreed to scale back its nuclear program for six months after two years of economic sanctions in an effort to halt, or at least alter the course of, its nuclear program.

By Martin Sevior, University of Melbourne

Iran insists its aims are for the peaceful use of nuclear technology, such as providing nuclear power – but its sceptics believe that Iran’s ambitions are to produce nuclear weapons.

The heart of the issue is that the uranium isotope U-235 (which has three fewer neutrons per atom than the most common uranium isotope U-238) is necessary for both.

The hardest part of making a nuclear weapon is to produce a “critical mass” of either U-235 or the plutonium isotope Pu-239 in the right ratios:

• in the case of uranium, U-235 must be enriched to ratio of 80% or more
• in the case of plutonium, the weapon must contain 97% or more Pu-239 compared to the contaminant isotope Pu-241.

Uranium enriched to 80% in U-235 or plutonium enriched to 97% in Pu-239 is called “weapons-grade” material.

Different reactors for different uses

All uranium reactors produce plutonium as a waste product, but the longer the nuclear fuel is present in the reactor, the more the contaminant isotope Pu-241 builds in the remaining fuel.

Nuclear fuel is typically inserted into a reactor in fuel assemblies – groups of fuel rods filled with pellets usually made from uranium dioxide.

At the end of a nuclear burn the remaining fuel is removed and becomes nuclear waste. The nuclear waste contains both Pu-239 and Pu-241 along with many other isotopes, many of which are extremely radioactive.

Reactors operate with a cycle time, which means the reactor is started, operated for some time, spent fuel is removed and replaced with new fuel.

Typically fuel assemblies must be removed from a reactor three months after commencing operation in order for the plutonium in the spent fuel to be present at the 97% concentration required for weapons.

Commercial reactors generally operate for one or two years before replacing their fuel. This long cycle time of a commercial power reactor means that the Pu-239 concentration is 83% or less, rendering the plutonium useless for weapons.

Light water nuclear power reactors, used to generate electricity, require uranium enriched to 5% abundance.

There is no way that the fuel used in a light water power reactor can be made to explode (and is therefore no threat in terms of nuclear weaponry), but the technology required to enrich uranium to 5% U-235 is exactly the same as that required to enrich it it to weapons-grade 80% or more – all that is required is to continue to feed previously enriched uranium through the centrifuge system until the desired concentration is reached.

Iran has already enriched uranium to 20% which it says is necessary for the proper operation of its research reactor. Research reactors, like the Opal reactor employed by the Australian Nuclear Science and Technology Organisation (ANSTO) at the Lucas Heights facility near Sydney, are used for a wide range of scientific experiments.

They are also used to produce radioactive isotopes employed in modern medical facilities to diagnose and fight cancer. However, modern reactors can comfortably fulfil their functions with a U-235 enrichment of 5%. Certainly Opal works very well with its enrichment level.

The nuclear reactor at MIT, Massachusetts. spanginator

While research reactors have many scientific, commercial and lifesaving uses, it is far easier to change their fuel assemblies than those in a commercial light water power reactor, making it possible extract weapons-grade plutonium from spent fuel.

Another sticking point in yesterday’s agreement was that Iran is constructing a heavy water reactor. Heavy water reactors use heavy water (deuterium oxide) as coolant (opposed to light water reactors, which use normal water) and employ natural unenriched uranium as fuel and produce plutonium as a waste product.

Unlike light water reactors which typically take one to three months to shut down and restart, heavy water reactors do not need to be shut down in order to change fuel assemblies. Thus heavy water reactors are much better suited for weapons-grade plutonium production.

So how will we know?

Going the plutonium route to nuclear weapons is more difficult than using highly enriched uranium. The plutonium must be extracted from spent fuel assemblies, which as we have seen via the Fukushima disaster, are extremely radioactive.

The Iranians would have to build a sophisticated reprocessing plant which would be very hard to conceal while constructing, and requires even greater skill to conceal while operating.

The agreement reached with Iran is they will limit enrichment to 5% U-235 and allow International Atomic Energy Agency (IAEA) inspectors regular visits (even daily) to their facilities.

The inspectors can easily determine the ratios of U-235 and Pu-239 in the input fuel and waste streams via the characteristic radiation signatures of the isotopes involved. These stand out like a sore thumb to their instruments.

In addition, the IAEA will measure the amount of U-235 employed at each facility to determine if any of the uranium is diverted to undisclosed locations.

While this arrangement is operating it is highly unlikely that Iran will be able to build nuclear weapons.

Martin Sevior does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.