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

By Martin Sevior

Published 26 November 2013

The agreement reached with Iran 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.

You may have seen over the weekend 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.

Iran insists its aims are for the peaceful use of nuclear technology, such as providing nuclear power — but its skeptics 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 percent or more
  • in the case of plutonium, the weapon must contain 97 percent or more Pu-239 compared to the contaminant isotope Pu-241.

Uranium enriched to 80 percent in U-235 or plutonium enriched to 97 percent 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 percent 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 percent or less, rendering the plutonium useless for weapons.

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