Continuing a discussion from yesterday, I want to explore why Iran is insisting on constructing a heavy water reactor at Arak:

Iran says Arak, due for completion in 2009, will produce radio-isotopes for medical care and agriculture. Western officials fear it could yield plutonium, one of the two prime components -- besides highly-enriched uranium -- in atom bombs.

There are various types of nuclear weapons , ranging from crude fission bombs like "Little Boy" to sophisticated fusion (hydrogen) bombs. Obviously even the crude devises can do enormous damage. These days the technological know-how is difficult to control (although there are almost certainly ongoing efforts by the US to provide incorrect information to sabotage development efforts) and so efforts to prevent a nation from acquiring nuclear weapons tend to focus on controlling access to the ingredients that could make up a bomb. Those ingredients can be produced as byproducts of a reactor, as mentioned above, or through repeated enrichment with centrifuges or other techniques.

Here's where it gets interesting. There are two common types of thermal nuclear reactors, "heavy" water (deuterium oxide) moderated and "light" (normal) water moderated. Because heavy water reactors do a better job of thermalizing neutrons they can run on unenriched uranium, while light water reactors require mildly enriched (a few percent) uranium. Note that weapons grade uranium is 90%+ enriched. Heavy water reactors are more difficult to manufacture but it's easy to obtain fuel for them; they also produce more plutonium and hence are a better choice if the goal is to make a bomb.

A summary of how this relates to Iran:

RFE/RL: Iran is building a heavy-water reactor at Arak, which it says will be used for research and other civilian purposes. Is that a plausible explanation, in your view?

Barnaby: It's a good size for research purposes, maybe a bit on the large side, but a good size. But it does produce plutonium, which could be used for nuclear weapons. So it's dual-purpose, really.

RFE/RL: Why would Iran choose to build a heavy-water plant, if it were truly only interested in research and producing electricity? Is there a rational explanation?

Barnaby: The advantage of a heavy-water reactor from the Iranian point of view is that it can be fueled with natural uranium [which Iran has]. You don't have to enrich the uranium to fuel it. So that is an advantage from the Iranian point of view.

The thing is, Iran also already has a deal with Russia to manufacture and supply fuel for a light water reactor at Bushehr. The US is still concerned about this because they worry that the fuel will not be tightly controlled. In theory Russia will supply the enriched uranium and Iran will ship back the spent fuel, and because Russia knows exactly how much went in they can calculate how much should come out and Iran will not be able to use the material towards a bomb. This is an ideal way to provide a country with nuclear power while denying them nuclear weapons -- give them a LWR, sell them the fuel, and dispose of the byproducts. I don't think the US is thrilled about Russia running the accounting, however. The plant has been delayed, according to Russia because Iran hasn't paid, and Iran claims this justifies them pursuing enrichment despite the UN's repeated calls for them to cease such activities.

So, to conclude, if I ran a country and wanted nuclear power but not nuclear weapons, and I wanted to convince the world of my good intentions, I would:

• Contract with a first-world nation to obtain a LWR and fuel
• Not build a heavy water reactor
• Not pursue uranium enrichment
• Provide the IAEA with complete access

This is not what Iran has done. Recent events (such as permitting the IAEA to tour Arak) give cause for optimism and we can certainly hope that Iran will provide full access and account for all nuclear material, but it must be emphasized that, whatever one thinks of the US' tone in negotiations, it is the actions of Iran that have raised legitimate skepticism as to their claims that they only want nuclear power and are not pursuing weapons.