NUCLEAR THREAT
The drawback to this manual system is the time required to insert the nuclear capsule. On some U.S. warheads, it took between eight to 20 minutes. In addition, this method of assembly suffers from many operational setbacks, ranging from one-point safety of the assembled warhead to the length of time to mate the combat-ready reentry vehicle to the liquid-fueled missile (the DPRK does not have a solid-fuel intercontinental ballistic missile (“ICBM”) or submarine-launched ballistic missile (“SLBM”) capability yet). Several early U.S. nuclear weapons by 1951 used automatic in-flight insertion (“AIFI”), where an electro-mechanical screwjack mechanism moved the capsule into the weapon’s high-explosive sphere on command just before combat drop, or final missile arming and fusing. A weapon using this mechanism could sit ground alert with the capsule stored in a nearby secure building or van, ready to be placed into the warhead by trained specialists when directed (this method was done with the Matador ground launched missile overseas in the ‘50’s). From a nuclear-safety standpoint, the capsule of nuclear material was kept separate from the weapon's high explosive and stored in a carrying case (nicknamed a “birdcage”). This was not possible for the Trinity device, nor the Fat Man Mark 3 bomb assembled during the 1946 Operation Crossroads Able test. A major drawback of the fully assembled solid- pit design is the lack of one-point safety. A declassified Strategic Air Command historical study postulated that in case of a one-point detonation of the weapon’s high explosive (“HE”), there was a 15% chance of a 40-ton
nuclear contribution. If the capsule were outside the central cavity, but adjacent to the HE sphere as in the AIFI weapons, at most the contribution would be equal to the HE content of the warhead – unintentional detonation of a warhead with a 500-pound HE content could theoretically result in a 500-pound nuclear contribution. 6 Thus, early DPRK warheads may not be safe from an operational standpoint, posing a risk if they are assembled and mated to rockets and missiles during an international crisis. The prospect of an accidental low-yield nuclear detonation in North Korea is real. As for statements that the North detonated a “thermonuclear warhead,” the possibility exists that the DPRK used thermonuclear fuels in a device primary. A variety of thermonuclear fuels and methods could be used, ranging from a Soviet style “sloika” (layer cake) single-stage, thermonuclear- implosion system, to gas boosting of the primary. It should be noted that all modern sealed-pit U.S. nuclear warheads are “inherently one-point safe.” 7 A nuclear weapon is considered one-point safe if the probability of achieving a nuclear yield greater than four pounds of TNT equivalent in an accidental high-explosive detonation is no higher than one in a million, per the Nuclear Matters Handbook from the U.S. Department of Defense. Perhaps the most effective means, and safest from an operational standpoint, was the development of the hollow-boosted primary, where a thin shell of nuclear material is used. 8
40 | August 2017
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