View full screen; the lens he's using costs more than a car.
https://www.youtube.com/embed/REP4S0uqEOc
https://www.youtube.com/embed/REP4S0uqEOc
The demon core was a 6.2-kilogram (14 lb), 89-millimetre-diameter (3.5 in) subcritical mass of plutonium
Available here:
http://www.areaware.com/products/golden-section-finder
http://www.indiana.edu/~jkkteach/Q550/Pennings2003.pdf
Timothy J. Pennings, The College Mathematics Journal, vol. 34, no. 3, May 2003, pp. 178-182.