Tim Worstall

That Korean Test

Hhhmm:

There was still “no definitive proof” that a nuclear device had been
detonated at an underground facility in northeast North Korea,
according to Western intelligence sources. “For a first nuclear test,
one would have expected radioactivity to have been released into the
atmosphere, unless the device was small enough for the North Koreans to
have contained all the radioactivity underground,” said one source.

If
no trace of radiation is discovered by the Constant Phoenix, it would
indicate that the size of the detonation was more in line with the
estimate given by the South Koreans — less than one kiloton of TNT.

A kiloton? I’m not sure you can make a bomb that small can you? Or at least, not one that has worked properly? So, was it really TNT as some have suggested? Or an attempt at a nuclear bomb which then fizzled?

The thing is, if you actually have the right metals, (highly enriched uranium or the right type of plutonium) a nuclear bomb is amazingly simple to make. If you made two great big plates, like the very largest pieces used by weightlifters, then stuck one on top of a ladder, another on the ground, then pulled away the ladder so that A falls on B, you would have a bomb.

It would be a very mild one, not an enormous amount would happen but you would get a bit of fission. All of the complexity in bomb making is in two things: reducing the amount of metal that you use and ensuring that more of it undergoes fission. To make an efficient bomb is actually quite difficult, you want shaped charges, superfast switches and all sorts of other technological gadgetry.

Anyway, here’s my unsupported guess about the N Korean bomb test. It was a failure, a fizzle, and what it shows is that while they may have fissile material, they haven’t yet mastered 60 year old technology.

In

3 responses

  1. triticale Avatar
    triticale

    The nuke plans I have call for a three story drop between two 35 pound hemispheres. I’m not sure if the relationships are linear, or how critical the shapes are, but I reckon that would have to be a pretty tall ladder. This is a crude implementation of the “gun-type” bomb, obsolete after Nagasaki. The usual approach uses those shaped charges and fast triggers to compress a single lump. Khan supposedly sold Pakistan’s info on how to do this to Iran and NK.

    Reply
  2. David Gillies Avatar
    David Gillies

    Gun-type bomba are amazingly simple to make. They suffer from drawbacks that make it very unlikely any aspiring nuclear power would attempt to build one.
    Firstly, they’re inefficient. For a given yield you require much more fissile material than a plutonium bomb. This makes them much less attractive as missile warheads. Secondly, enriching uranium is much harder than breeding plutonium in a reactor (isotopic separation is hard; elemental separation is easy).
    But you can’t make Pu into a gun type bomb, at least not a practical one. Reassembling the sub-critical pieces requires higher speeds than can be achieved with explosives, and the inevitable presence of Pu240 as a contaminant in the Pu239 results in premature detonation and a fizzle. Pu bombs must be implosion weapons, and as Tim says, this is a non-trivial engineering task. You can’t simply wrap the pit (core) in a sphere of explosive and set it off, as the converging shock fronts from the detonation points will cause an effect called the Rayleigh-Taylor instability and effectively squirt the fissile material out of the ‘gaps’, The shock fronts must be focused by using explosive materials with different detonation velocities, machined into very precise shapes (and using materials with very high dimensional stability). The detonators are high-power devices, exploding bridgewire or slapper types, and they must be fired with timing precision in the sub-microsecond range. The calculations required to model the detonation fronts (hydrocodes) are very complex and are considered national security assets. Plutonium is also a notoriously difficult susbstance to work with from a metallurgical perspective. Then you have the problem of machining the pusher, the tamper/reflector and the initiator (the last of which these days quite often involves tritium-enriched scandium).
    Miniaturisation is a further challenge in itself. There are competing desiderata. Obviously, one challenge is to make the smallest and lightest warhead for a given yield. Another is to increase efficiency so that more fissile material is involved in the reaction (which sets the upper bound of yield). The latter indicates higher compression of the pit, which means more powerful explosives and heavier tamping, which militates againt the former. There are other techniques to increase efficiency, notably boosted fission, in which a charge of tritium is injected into the hollow pit just before detonation, to provide a large supply of available neutrons. But this is a refinement.
    Could the NORKs have been trying a sub-scale test? It’s not out of the bounds of possibility. If they had a pit made largely of a non-radioactive metal, alloyed with a small amount of Pu, they could have engineered a deliberate fizzle to see if their implosion design was viable without using a warhead’s worth of fuel. An unintentional fizzle still remains the most likely explanation, in which case if I were a North Korean nuclear scientist I’d be very worried right about now.

    Reply
  3. Chris White Avatar
    Chris White

    Probably commenting too late for anyone to read this, but…
    The Trinity test was ~20kT, Fat Man was 32kT, and the smallest first-test to date was India’s 12kT.
    A one-kiloton explosion for a first nuclear test? Not likely, unless they’ve got hold of some seriously hi-tech kit.

    Reply

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