400px-HEUraniumC

Douglas Birch and R. Jeffrey Smith published three articles last week on South Africa’s Pelindaba nuclear complex (Washington Post, Center for Public Integrity, Daily Beast). In all three, they refer to South Africa’s “nuclear explosives.”

South Africa built six nuclear weapons, and almost a seventh, in the 1980s and then revealed its program to the world in 1993, signed the Nuclear Nonproliferation Treaty, and accepted International Atomic Energy Agency inspections to verify that the weapons were no more. However, South Africa has retained the highly enriched uranium (HEU) from those weapons.

In the Washington Post article, Birch and Smith recount an unsuccessful attack on Pelindaba in November 2007, the object of which is still unknown. Another recent news story suggested that the goal of the attack was to obtain the design of a new type of reactor for China.

Birch and Smith identify their sources as

an alarming report that has never been released — or even acknowledged — in South Africa but was obtained by foreign intelligence agencies and described to the Center for Public Integrity by multiple people familiar with its contents.

According to these sources, the object of the raid was the “nuclear explosives.”

The CPI article discusses efforts by the United States to urge South Africa to secure those “nuclear explosives” more tightly, preferably by blending the HEU down below a bomb-usable level.

To me, the term “nuclear explosive” means fissile material kitted out with chemical explosives and detonators that can cause the fissile material to come together quickly in a critical mass, causing a nuclear explosion. Fissile material (HEU or plutonium) by itself can’t explode the way chemical explosives can if they are handled incorrectly. The fact that the IAEA has inspected South Africa’s stocks indicates that South Africa’s HEU is not stored ready-to-explode. Birch and Smith admit that the HEU was melted down into ingots.

Reporters devise their own names for the components of nuclear weapons, apparently thinking that this makes their descriptions more accessible to the general public. In this way, they have used the word “trigger” for primaries, neutron sources, the chemical explosives, and the detonators in nuclear weapons. Frequently they confuse themselves. I take reporter terminology with a grain of salt, particularly when it breaks new ground, as these articles did with “nuclear explosive.” Reporters, including Birch and Smith in the CPI article, refer to the fissile material in nuclear weapons as “nuclear fuel,” which, since it is what provides the energy of the explosion, makes some sense. I would prefer that they refer to the fissile material more specifically, as plutonium or HEU.

So what are Birch and Smith’s “nuclear explosives”? The term suggests that South Africa is violating the NPT, which we would have heard about from the IAEA if it were true.

I tweeted about this a few days back, and got a number of replies. Smith “gently” suggested that I was wrong and that I research further, but it should not be discussed in public. It didn’t seem to occur to him that the use of that term prompts speculation and that if there is a genuine danger, he should say what that danger is.

The problem seems to be the form in which the HEU is stored.  From the CPI article,

Technicians extracted the highly-enriched uranium from the apartheid regime’s nuclear weapons in 1990, then melted the fuel down and cast it into ingots. Over the years some of the cache has been used to make medical isotopes, but roughly 485 pounds remains, and South Africa is keeping a tight grip on it.

Presumably this means that the HEU is in the form of metal ingots, although the material that has been used to produce medical isotopes has been processed in some way – in solution, for example – which could mean that the stock of HEU was no longer in the form of ingots. But the article does not describe the material’s current chemical-physical form.

Other Twitter discussants were less reticent than Smith and spoke of metallic ingots and even hemispheres. I have a hard time believing that the IAEA tolerates HEU hemispheres as the storage form, and Birch and Smith say it was melted down into ingots. The metallic form is the only form that could be used in a weapon, so one might infer that the fact that some of South Africa’s stock of HEU is in the form of ingots is the reason for using the term “nuclear explosive.”

Luis Alvarez’s famous quote about HEU was cited on Twitter:

With modern weapons-grade uranium, the background neutron rate is so low that terrorists, if they have such material, would have a good chance of setting off a high-yield explosion simply by dropping one half of the material onto the other half. [quoted in this report]

What constitutes “half of the material”? A critical mass depends on many things. Wikipedia gives the critical mass of a naked sphere of uranium as 52 kilograms. Fissile material is usually stored in shapes and sizes that make it unlikely a critical mass will be assembled accidentally. That means much less than half a critical mass per ingot, in shapes that are not much like spheres.

The photo at top is a typical ingot of HEU (source: DOE, via Wikipedia). Maybe 12 centimeters across, and probably a centimeter or so thick. That comes to a volume of 113 cubic centimeters, or a little over 2 kilograms. If it’s two centimeters thick, then it’s 4 kilograms. Drop one of these on another, and you won’t get a nuclear explosion. And that’s if Alvarez wasn’t exaggerating, as scientists sometimes do.

3013 can

Ingots of fissile material are also packaged so that they don’t come together accidentally, in big cans with lots of extra space, pictured here. If the ingots came together, they would release neutrons and other radiation, but not an explosion (also pointed out here). Birch and Smith are not incorrect that enough for a bomb could be carried in a backpack – say 26 of those ingots naked. But without the packaging, the neutron flux would fry the backpack’s wearer, and 26 cans wouldn’t fit in that backpack.

So add “nuclear explosive” to the list of misleading terminology concocted by reporters who seem not to have read Wikipedia’s articles on nuclear weapons. Perhaps Birch and Smith could say exactly what they do mean: “uncanned large metal ingots of HEU” is what they’re implying; it didn’t take long for others to conclude that in the Twitter discussion. They’re not keeping anything secret that way. Or, of course, “nuclear explosive” could simply be one more reportorial synonym for HEU in any form.

South Africa’s HEU is a concern. It should be stored under high security, or in a chemical form or isotopically diluted to where it cannot be used in a bomb. Unfortunately, all we learn from these articles is that Douglas Birch and R. Jeffrey Smith believe that that is not the case.

 

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