The principal factors that must be taken into account in assessing criticality safety are as follows: [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42]

For given fuel composition (factors 1 and 2 specified), the simplest but most restrictive condition to ensure subcriticality is one of items 3, 4, 5, or 6 (limitation of mass, dimensions, volume, or concentration of fissile material). These so-called single-parameter limits for fissile nuclides are spelled out in American National Standard ANSI N16.1-1975 [А4]. They were abstracted in Table 4.11 of Chap. 4 and are amplified somewhat in Sec. 8.2, following. These single-parameter limits give the largest mass, size, volume, or concentration that will be safely subcritical no matter what other criticality-limiting conditions may be present.

Use of a single-parameter limit often leads to an inconveniently small size of batch or equipment. To permit safe operation on a larger scale, combinations of two parameters that together are safely subcritical are sometimes specified, provided that the simultaneous presence of both parameters can be assured. For example, if the maximum concentration of plutonium in aqueous solution can be limited to 20 g/liter, the maximum safe diameter of a cylinder may be increased from the single-parameter limit of 15.7 cm (Table 4.11 or 10.25) to 25 cm (Fig. 10.35).

By restricting the concentration of moderators (item 7) or the presence of reflectors (item 8), the dimensions or concentrations of safely subcritical systems may be increased further. The presence of neutron-absorbing poisons such as boron, cadmium, or gadolinium (item 9) also sometimes permits such increase. On the other hand, heterogeneity (item 10, such as lumping of fuel containing 238U or interaction between two systems containing fissile material (item 11, such as adjacent pipes carrying fissile material) reduce the dimensions, mass, or concentration of safely subcritical systems.