Uranium, plutonium, heavy water … why Iran’s nuclear deal matters

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 percent U-235 is exactly the same as that required to enrich it to weapons-grade 80 percent 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 percent 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 Organization (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 fulfill their functions with a U-235 enrichment of 5 percent. Certainly Opal works very well with its enrichment level.

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 the weekend’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 percent 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 is Associate Professor of Physics at University of Melbourne.This story is published courtesy of The Conversation (under Creative Commons-Attribution/No derivatives).