It is well known now that Iran has not only pursued an enrichment program to develop highly enriched Uranium (the only purpose for which is a nuclear bomb), but a heavy water reactor as well, the purpose for which includes the production of Plutonium.  The Middle East Media Research Institute reports on why Iran says it wants to produce heavy water (quoting Iran nuclear chief Mohammad Sa’idi):

“One of the products of heavy water is depleted deuterium. As you know, in an environment with depleted deuterium, the reception of cancer cells and of the AIDS viruses is disrupted. Since this reception is disrupted, the cells are gradually expelled from the body. Obviously, one glass of depleted deuterium will not expel or cure the cancer or eliminate the AIDS. We are talking about a certain period of time. In many countries that deal with these diseases, patients use this kind of water instead of regular water, and consume it daily in order to heal their diseases.

“In other words, the issue of heavy water has to do with matters of life and death, in many cases. One of the reasons that led us to produce heavy water was to use it for agricultural… medical purposes, and especially for industrial purposes in our country.” 

Sa’idi is lying.  The use of heavy water for the treatment of disease is so costly and unproven that it would be absurd to use it on patients in lieu of chemotherapy.  The consumption of heavy water by biological organisms inhibits cell reproduction, but is also too toxic and too costly to be considered useful for the treatment of cancer or any other disease.  It simply makes more sense to use chemotherapy and other modern treatments.

On the other hand, heavy water has two strategic military advantages to Iran.  First, rather than worry with trying to achieve Uranium enrichment on the order of that necessary to sustain criticality in conventional light water reactor designs, natural Uranium can be used in heavy water reactors.  The design of Iran’s reactor is similar to the Canadian CANDU reactors, which can be studied here, here, and here.  The consequent Plutonium produced could then be reprocessed and purified to produce a nuclear bomb that requires less mass than its counterpart Uranium bomb.  This makes such a warhead able to be delivered with the rockets that Iran currently has in its arsenal.

This is about the miniturization of nuclear weapons, and it shows that Iran is pursuing two distinct paths towards the holy grail.  The first is highly enriched Uranium, in itself capable of being a weapon, and the second is Plutonium, produced without the aid of enriched Uranium by the use of natural Uranium in heavy water plants.

It is noteworthy that Jane’s Intelligence Review in 2003 said the following of Iran’s heavy water reactor:

The IR-40 heavy water research reactor is significant because it produces high quality plutonium, the most important component for a compact, nuclear device. If Iran wishes to develop a nuclear weapon small enough to launch on top of its Shahab 3 or 4 missiles, it will most probably be an implosion device with a plutonium (Pu) core.  The only way to acquire that is through reprocessing irradiated fuel. Bushehr is a light water reactor that has received much international attention and most probably will continue to be closely scrutinised, making it difficult to clandestinely remove its spent fuel for reprocessing. Even if the IR-40 has just as much attention, the Iranians would have a better chance of removing irradiated fuel or irradiating natural uranium targets for Pu production in this reactor.

Indeed, a heavy water reactor is among the most dangerous in existence from a proliferation perspective.  One reason is that the low neutron cross section of heavy water facilitates a high number of U238 (uranium-238 isotope) atoms to absorb neutrons, resulting in the production of a greater quantity and better quality of plutonium product.

According to David Albright, Director of the Institute for Science and International Security, the IR-40 will be able to produce 8-10kg of plutonium per year – approximately one to two bombs’ worth of nuclear material.  The IAEA holds that 8kg of plutonium constitutes a “significant quantity