About when did a US/CCCP war become suicidal?

SNIP
Size and Weight. Nobody was capable of putting a 30-40 ton warhead of
that size at those heights. Well, that, and atmospheric attenuation -
all the prompt stuff, and the heat, gets absobed pretty quickly by the
Atmosphere, and there'd be no fallout. There would, if you chose the
right height, be pretty severe EMP effects, but you don't need a
whopping huge bomb for that.
SNIP:
No fall out? The 100 MT was achieved by wrapping a multi-ton U238
jacket about Ivan. The fast neutrons from Ivan fission the U238 and
now you have multi tons of fallout added to Ivan. This of course is
the fission-fusion-fission
bomb in mega-size. I make the fireball from 100MT about 67,000 feet in
diameter.
using known sizes and the W^1/3 relation. Walt BJ