VT Nuclear Education: Germany

Any amount of enriched U-235 or PU-239 over 80% will produce a "fizzle yield" if it is in a spherical form and given a proper Tamper Reflector.

Any fissile material of any size large or small, given a proper neutron reflector of 50% + efficiency or more, up to Infinity will explode.

Uranium gun based micro nukes are the easiest, cheapest and simplest to make due to the very slow internal neutron generation rate of 50 
neutrons per second in U-235.

This means you have to assemble a critical mass by placing it in a proper Neutron tamper / reflector in less than 1/200 of a second. Very 
easy to do with a low velocity gun type assembly. Therefore very small and cheap nuclear weapons can be made using this simple system.

However with PU-239 /240 the spontaneous neutron production rate is over 100,000 neutrons per second. This means an assembly time of less than 
1/100,000 of a second. That's why PU-239 requires a much faster implosion system for a full yield. But using a gun system with
PU-239 will still work only producing a much lower so called "fizzle yield".

At 1/1000 of a second for a gun design using PU-239 its yield would be 100 times smaller or about 200 tons. A Fizzle Yield is considered to be
less than 10% efficiency in explosive nuclear power or less than 2kt vs 20kt for a full weapons yield. This is still 90% less than the maximum 
100% yield of 200KT achievable with fission fusion boosting.

Little Boy used in WW2 had a fizzle yield of only 1.6 % for a 15KT total yield due to a neutron reflector that was too small. It used up to 3 critical masses of U-235
enriched to over 80% ? but the Tamper / Reflector was designed to only handle 1 critical mass. This was due to size and weight constraints. Therefore it was
a "fizzle dud" (low yield) that should have been equal to about 45 to 50 KT in size if it had a more massive Tamper. The 3 critical mass of over 300 lbs of
U-235 was to insure that it would work due to questions about the weapons enrichment level. (Documented in the Los Alamos Primer 1987).

Contrary to popular belief Heisenberg did calculate the proper critical mass of Little Boy at the end of the war while in Jail in the UK. His 
first calculation was too big by a factor of 100 however by his 3rd refinement of his critical mass formula 3 days later he got it to within 
10% of the actual amount of fissile material needed to make the weapon work. Heisenberg was working on a much smaller weapons design with a 
smaller yield (2KT) that was under 1,000 lbs in size so it could be fitted to existing German aircraft, V-1"s and V-2"s etc.
VS 20KT and 10,000 lbs. To the Manhattan Project this would be considered as a simple uranium fizzle weapon of 2 KT in size; way too small for them to 
even bother with; verse 20 KT for the US. To quote Neil Bohr "How big does it need to be?".

A B-52 only caries about 50,000 lbs of bombs or 50 tons. A 2kt weapon would be equal to over 40 fully loaded B-52 bomb loads; or over 150 WW2
era B-17's or B-24's bomb load. As you can see a fizzle yield of this size"2 kt" is still very big. Even a 100 ton yield at 1% efficiency is 
still equal to 2 full loaded B-52 bomb loads and the weapon would weigh less than 1 standard 1,000 lb bomb; not 100 bombs. The cost would be 
less than 250,000 dollars to make one. Cheaper than any tank, jet, ship or helicopter to make.
If you used recycled surplus nuclear weapons fuel the cost would be even less.