Most unfolding news stories about uranium-fueled bombs seem to inadvertently focus on what are in fact intermediate steps on the technical road to obtaining a weapon. Words like “centrifuge” and “uranium hexafluoride” have entered the common vocabulary, recognized (if not entirely understood) by the average man or woman on the street. As with all things bearing on national defense, knowledge is power, for everyone. Events on the other side of the world, in secret laboratories or clandestine arms-dealer meets, can affect our daily lives right here at home. So it’s good that Americans and other peace-loving nations’ civilians are climbing the learning curve about nukes, even as their governments experience the pitfalls of trying to negotiate with pariah regimes. It’s regrettable that such multi-layered learning effort is needed, but this is one context where ignorance most certainly isn’t bliss.

Yet a knowledge gap exists, in the spread of beneficial understanding into popular culture, regarding two aspects of bad guys getting their hands on working nukes. The intermediate technical stages referred to above, covered well in the media, amount to industrial processes for which access is all too widespread already, complex and expensive though these processes might be. That is to say, they amount to specific means for going from raw material to a finished product. They’re phases in the middle of a journey that requires a beginning and an ending to pose real danger. It’s the beginning and the ending that appear to be somewhat neglected in the willy-nilly, bootstrap education going on. Reducing these knowledge gaps would seem worthwhile, both providing a better perspective on the overall problem of rogue nukes, and suggesting further points at which proliferation attempts could be detected and restrained.

The two issues deserving further exposure are:

1. How hard is it to get unprocessed uranium?

2. How much processed U-235 is needed for a powerful bomb?

To cut to the chase, the answer to question 1 is: Not as hard as you might think. The answer to question 2 is: Less, and maybe a lot less, than you might suspect.

Obtaining raw uranium: Uranium ore, as a mineral, occurs naturally as a type of uranium oxide, known as yellow cake because of its color and consistency. Early in the Atomic Age, yellow cake was believed to be rare. Rich deposits, suitable for mining, were highly valued -- so valued that in the 1950s investment frauds based on fictitious uranium mines were common. Then it was realized by geologists that yellow cake, in varying degrees of concentration, could be found throughout much of the crust of Planet Earth. (A sign of this is that uranium is the precursor of a familiar homeowner’s headache today: radon gas. Radon, a radioactive element itself, is one product in the chain of elements produced as uranium atoms decay. Radon is a hazard because, being a gas, it percolates upward through the crust to accumulate in poorly-ventilated basements.) Unrefined uranium as a commercially viable resource, whatever its intended use, exists in large reserves in countries ranging from the U.S. and Canada to South Africa, Australia, China, Mongolia, many parts of the former Soviet Union, and elsewhere. The total of these worldwide reserves has been estimated to run to tens of millions of tons. Since the uranium isotope U-235 useable for fueling reactors and bombs averages about 0.7% of all uranium, the reserve of potential bomb fuel runs to a few hundred thousand tons. That’s a lot of bomb-grade uranium. And that’s just on dry land.

Another source of uranium is common seawater, where a dissolved form of yellow cake constitutes about three parts per billion. This doesn’t sound like much, until you combine several tidbits of info, which you might classify as uranium trivia factoids -- except there’s nothing trivial about U-235-fueled atom bombs. Factoid one is that the total volume of seawater in all the world’s oceans, measured in liters (roughly a quart), is the number 14 followed by twenty zeroes. That’s a very big number. Factoid two is that the total of pure U-235 dissolved in all this seawater -- the 0.7% of the three parts per billion -- weighs about thirty million tons. That’s one spicy meatball.

How hard is it to extract ocean-borne uranium? Japan, one country always interested in energy independence, in the 1970s ran an experimental plant to filter out uranium from the sea. The all-up cost of obtaining uranium ore this way, as measured in the price of a kilowatt-hour of electricity, was competitive with the cost of obtaining that power from barrels of oil.

Any country with a coastline and the necessary technology can presumably do what Japan did thirty years ago. Seawater extraction as a means toward getting raw ore for refining U-235 is therefore a dual-use technology. Refinement processes might be disguised in any coastal desalination plant, whose outward purpose is producing large amounts of drinking water. Such plants are increasingly common, especially in the Third World. Should the International Atomic Energy Agency start inspecting desalination plants for embedded, covert uranium extraction? Maybe we do need to put a cork in this vast U-235 pipeline.

How much U-235 will make a big bomb? The type of U-235-based atom bomb that was dropped on Hiroshima is known as the gun bomb design. In essence, super-critical mass is achieve within the device by uniting one sub-critical chunk of U-235 with another, by shooting it down the barrel of an artillery tube, where the propellant used is similar to conventional artillery-shell propellant. The Hiroshima weapon’s yield was about 12 kilotons. Some sources put the total amount of U-235 refined into bomb grade (say, 90% or better purity) that’s needed in this design as something like forty pounds. Less than that, and the gun bomb won’t accomplish super-criticality: no big blast and no mushroom cloud. So, it would seem that forty pounds is a barrier, a limiting factor, in this approach to mass destruction. Less than forty pounds, the bad guys don’t got an A-bomb.

However, less widely known is that uranium can be used in the different type of bomb design that was tested in New Mexico and dropped on Nagasaki. This other approach, the implosion design, must be used if the fuel is plutonium. (Because of its differing, more temperamental nuclear properties, plutonium in a gun bomb would predetonate, or fizzle.) An implosion bomb is a much more subtle and delicate thing, harder to construct from scratch than a gun bomb. However, an implosion bomb can indeed be fueled by weapons grade uranium instead of plutonium. Sources estimate that an implosion design U-235 bomb would need only twenty pounds of fuel, instead of forty. This is because the implosion approach assembles a super-critical mass more efficiently. The barrier to WMD implementation has just been cut in half. Not good news.

It gets worse. There are variants of the implosion design. The simplest one uses specially shaped high-explosive lenses to collapse a hollow sphere of plutonium -- or uranium. The hollow sphere represents latent atomic blast power, waiting to be unleashed. The weight of fuel is adequate to achieve super-criticality. Its spatial arrangement, however, is all wrong -- not enough fuel packed down to small enough volume. But when that hollow sphere is squashed at extremely rapid speed into a smaller but solid sphere, the new configuration has the density needed to achieve a “fast prompt critical disassembly” -- the technobabble euphemism for a nuclear detonation.

More sophisticated implosion designs do more than simply collapse a hollow sphere into a solid one. They achieve such a strong and perfectly shaped implosive force that the solid sphere is actually compressed. Compression into a volume as small as one-fourth that of unstressed solid metal has been reported. In other words, the density of the bomb fuel is as much as quadrupled. Because of the physics involved, this higher density requires less total weight of fuel for a large atomic explosion to occur. The degree of reduction in the amount of fuel required by such a compression-implosion device is classified. I make a semi-educated guess that if the bomb quadruples the density, the weight of fuel needed might be cut in half. If so, ten pounds of weapons grade U-235 would achieve a yield of several kilotons, maybe even a dozen -- Hiroshima-scale devastation with only one quarter the fuel. Or, viewed differently, four times as numerous an arsenal for the same amount of fuel. Frightening thoughts.

Conclusion: A centrifuge cascade alone does not an atom bomb make. You need the raw uranium, and you need the means to assemble a super-critical mass of bomb grade fuel. But yellow cake in quantities of interest to rogue states and terrorists is everywhere, so restriction of the supply is much more difficult that it looks. And advanced implosion bomb designs, plus the technology to make them, might become more widely available soon, if they aren’t already. When these designs greatly shrink the weight of bomb grade U-235 needed to produce a Hiroshima-sized nuclear blast, Armageddon may be closer than we realize.

JoeBuff.Com / Joe Buff Inc. Joe Buff, President Dutchess County, New York E-Mail readermail@JoeBuff.Com