The true criticality of that basic political rationale is being demonstrated in the 1990s. Colin S. Gray
But the inspector general of the Department of Homeland Security, in a report released Tuesday, found that the list was not child's play: all these "unusual or out-of-place" sites "whose criticality is not readily apparent" are inexplicably included in the official federal antiterrorism database. Eric Lipton
specifically, physics: the state in which a mass of fissionable material sustains or is able to sustain a nuclear chain reaction
Criticality can be achieved by increasing mass (adding more plutonium) or … by compressing a barely subcritical mass to increase its density. Daniel J. Lenihan
How big is a critical mass? That depends on the proportion of fissile material and on its shape and configuration. Surrounding the mass with a neutron-reflecting substance further reduces the size needed for criticality, by returning neutrons that would otherwise escape. Richard Wolfson