By surrounding the fissionable material with a suitable neutron "reflector", the loss of neutrons can reduced and the critical mass can be reduced. read more
Thanks for asking. I do not want to go into details of it. But, two things I can tell you here: 1. Critical mass is a term most commonly used in context of nuclear weapons. read more
Plutonium-239 can also absorb neutrons and fission along with the uranium-235 in a reactor. Of all the common nuclear fuels, Pu-239 has the smallest critical mass. A spherical untamped critical mass is about 11 kg (24.2 lbs), 10.2 cm (4") in diameter. read more
Simple calculation of the critical mass for highly enriched uranium and plutonium-239 Christopher F. Chybaa) Department of Astrophysical Sciences and Woodrow Wilson School of Public and International Affairs, read more