Thursday, October 12, 2006

A Deadly Kind of Fizzle

It is probably unwise to taunt a man who commands a million-man army. It is also probably unseemly to make light of the man’s ballistic and nuclear weapons, even if they don’t work.

Right now people around the world are running around in circles, pulling their hair and wringing their hands over North Korea’s underground atomic bomb test, when it ought to be the subject of late-night television comedians.

In the old days opposing armies faced each other in an open field, war paint on their faces, feathers in their helmets, rattling their shields and making rude comments about their opponents’ manhood.

Today global leaders rattle their ballistic missiles and nuclear weapons rather than spears and battle axes, but it is still a macho world. And in macho terms, Kim Jong-il is a man who literally can’t get it up.

In July North Korea tried to launch a multi-stage ballistic missile that fizzled out over the Sea of Japan minutes after it was launched. This was the second test firing in eight years and it was even less successful than the one in 1998, which at least flew over Japan, landing somewhere near the Aleutian islands.

Two months later the North conducted a purported nuclear weapons test, which may not have been an obvious fizzle, but its extremely low yield of less than one kiloton of conventional explosives (about 550 tons to be precise) strongly suggests it was a dud.

Sub-kiloton nuclear tests are not unheard of. Pakistan set off five bombs in 1998, three of which were supposedly of sub-kiloton yield, although that has never been confirmed.

It is confirmed that the British in a series of above ground tests in South Australia in the mid-1950s set off a half-dozens explosions ranging from 27 kt down to less than one kiloton (930 tons).

The US has conducted numerous sub-kiloton tests in its quest to miniaturize atomic bombs to put in artillery shells and other delivery systems. But no nuclear weapons state - from America’s “Trinity” test in July 1945 (20kt) to Pakistan (40kt) - has ever made its world debut with such a low yield.

Initial detonations have all been in the 10kt-60kt range. So the North’s explosion was lower than the lowest by a factor of nearly 20. “No one has ever duded their first test of a simple fission device,” says “North Korea’s nuclear scientists are the worst ever.”

There are several reasons why a nuclear test might fizzle. Indeed, “fizzle yield” is a recognized nuclear term, indicating the complexities of igniting a fission bomb, especially one that is made from plutonium.

A fizzle can happen when the bomb literally blows itself apart too fast for the nuclear chain reaction to take place properly and generate the large amount of energy needed for a full-scale explosion. Should that happen a bomb meant to be in the Nagasaki range (20kt) may yield only about a kiloton.

There are two ways this can happen. One is contamination with Plutonium-240. The key ingredient for a plutonium bomb does not exist in nature; it is manufactured in a nuclear reactor when atoms of Uranium-238 capture loose neutrons and are converted to Plutonium-239, the stuff of bombs.

But these plutonium atoms can, in turn, capture neutrons, becoming Pu-240 and even Pu-241. They are not fissionable. With a bomb contaminated with Pu-240, the probability of a fizzle is very large. The US maintains a standard that none of its bomb-grade plutonium will have more than 6% Pu-240.

Presumably North Korea’s physicists, not to mention their Russian and Chinese advisors, understand this problem. But whether they applied the skill needed to suppress PU-240 build-up is another question.

The North could avoid the problem of contamination entirely by using uranium as the basic ingredient of its bomb. Much less is known of the North’s purported uranium enrichment program, but it very unlikely that it progressed to the point of producing sufficient weapon’s grade material.

The other problem concerns detonation. Plutonium bombs work on the “implosion” principle. A sub-critical core of plutonium about the size of soft ball is surrounded by conventional explosives. The pressure from the explosion squeezes the plutonium into a critical mass, setting off the nuclear explosion.

But the shaped charges must be so precisely engineered that they go off simultaneously. If even one charge explodes prematurely, even by a nanosecond, it may blow the bomb apart, cutting short the chain reaction and reducing the yield.

Of course, even a sub-kiloton bomb can cause a lot of damage. A one kiloton bomb will have the radius of destruction of about a third of the Hiroshima bomb, not exactly a city-flattener, but perhaps something that could work as a terrorist weapon.

North Korea should understand the destructive power of even sub-kiloton explosions from experience. On April 22, 2004 in Ryongchon near the Chinese border an explosion of presumably ammonium nitrate killed or injured about 3,000 people (roughly the same number as killed on September 11) and flattened acres of houses.

Some speculate that the North may have skipped the debut stage and moved directly to testing a deliberately weaponized bomb. Such a low-yield bomb would be more suitable for a export to terrorist. If that were the cased, the test may not have been aimed at world opinion in general but was, in fact, a demo for potential buyers.
So one is left really with two possibilities: Was it meant to be a normal, moderate-to-high yield test that fizzled out, or it was a deliberate low yield test, a subtle advertisement for the atomic marketplace?

If, as seems likely, the test was designed for show or for its purported deterrent effect on the US, then it was almost certainly a failure. A yield so small as to leave the world wondering what really happened should not count as a major propaganda victory.

A deliberately low-yield test is oddly more nefarious but also so difficult to pull off that it would seem to be beyond the competence of North Korea in view of its shortage of top-tier physicists and engineers.

Of course, Pyongyang may set off more nuclear tests (using up their limited stocks of plutonium). Someday they might build a multi-stage ballistic missile that really works; someday they may conduct a successful nuclear test; someday they may marry the two. Yes, and Kim Jong-il may live to be 100.


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