Building a critical-mass cube requires more fissionable material than building a critical-mass sphere.
Work Step by Step
A sphere has the least surface area for any given volume. This minimal surface increases the chance that the neutron will be captured and initiate another fission event before it leaks through the surface and out of the sample. Therefore building a critical-mass sphere requires less fissionable material than building a critical-mass cube. This is discussed on page 640.