From: dingbat@cix.compulink.co.uk (Codesmiths)
Newsgroups: sci.military
Subject: Re: accidental nuclear detonation
Date: 7 Apr 92 03:39:00 GMT
Organization: AT&T


From Codesmiths <dingbat@cix.compulink.co.uk>


> From briand@anasaz.UCAR.EDU (Brian Douglass)
> 
> >>From <TWZ101@psuvm.psu.edu>
> >>     If a strike aircraft carrying a nuclear weapon was shot down by ground
> >> fire (SAM or AAA), is there any chance of the bomb detonating?  What if the
> >> fire hit the bomb itself?
> >
> 
> [deleted stuff about bombs not going off]
> 
> In theory this is all true, however, there was one notable exception.  A
> few years ago, it was discovered at the Nevada Test Site that a particular
> type of nuclear weapon (Army tactical I recall) could in theory go off if
> it fell off the back of a truck and landed a certain way.


Modern weapons use implosion assembly; either spherical or radial. It
is likely that some linear assembly designs still exist, either as
small diameter earth-penetrating weapons, or in the pages of the
Anarchist's Cookbook. To trigger a corect explosion of an implosion
design, it is necessary to fire all detonators at their correct
times, with a high degree of accuracy. Any timing errors will reduce
the degree of compression & hence the yield.

"One point safety" is an important concept in practical stockpile
weapons. In any serious accident, it is quite likely that the
conventional explosives surrounding the device core will detonate. A
weapon is said to be "one point safe" if a nuclear explosion will
only occur after simultaneous triggering of the detonators, and it is
safe from any accidental detonation of the explosive, starting from
one point.


The use of electronic neutron generators, rather than the old
polonium/beryllium "golf balls" is an additional safety measure. A
frangible initiator would always go off when compressed by an
explosion, but an electronic generator requires the fuzing system to
be operating normally.

Tritium injection is not reliable as a safety measure. Although
design yield relies on the correctly timed injection of a tritium
booster, the device can still fizzle even without any boosting. A
tiny fizzle would still be orders of magnitude than any weapon
accident to date.

I've never heard of any device using a steel plate as a safety
device, and the physics make it seem very unlikely. One of the early
designs did use aluminium chains as a neutron absorber, which were
removed manually during the in-flight arming procedure, although this
was not regarded as a particularly reliable safeguard.


In the early days of free fall bombs, it was common practice for the
bomb to be loaded into the aircraft with the "pit" (containing the
fissile materials) carried separately. Only in-flight, would the
weaponeer insert the pit into the bomb casing & high explosive
assembly. Naturally this caused many problems, so much of the early
effort in weapons engineering was directed towards a "wooden bomb",
which could be sealed into its casing, left on a shelf for tens of
years, then immediately deployed without any need for maintenance of
manual assembly. One advantage of a separate pit was that in the
event of an aircraft accident, the bomb & its explosives could be
jettisoned relatively safely (as indeed happened many times). The pit
was stored in a strengthened transport case, designed to survive a
crash without releasing the fissile material.


One of the most famous nuclear weapons accidents was that over
Palomares, Spain in 1966. A B52 carrying 4 Mk28RI weapons collided
with a KC135 tanker. One weapon landed intact, one went into the sea
(recovered intact from 2,850 feet) and 2 exploded. No nuclear
explosion took place, although the explosive dispersal of plutonium
involved an extensive clean-up operation, involving the removal of
hundreds of tons of contaminated soil.

A number of deployed devices have later been shown to not be one
point safe. These include the Mk28 bomb, W40 warhead (Bomarc,
Lacrosse), Mk43 bomb & W47 warhead (Polaris). As can be seen by the
Palomares accident, even if a weapon is known to be one point unsafe
in some situations, this is not a guarantee that it will fission in
all accidents.


          Andy Dingley    dingbat@cix.compulink.co.uk