A Review of Criticality Safety Models Used in Evaluating Arrays of Fissile Materials

The methods commonly used in this country to evaluate the criticality safety of fissile material arrays include density analog , surface density, equilateral hyperbola, albedo, and solid-angle techniques. These can be divided into array unit interaction and semiempirical methods. The albedo and solid-angle techniques fall into the former class; the rest fall into the latter class. A study reveals that interaction methods are useful in treating arrays of arbitrary mesh patterns (e.g., triangular or hexagonal) having only a few units, while the semiempirical techniques are more applicable to arrays with a large number of units. The density analog and surface density approaches are easy to apply but typically require more auxiliary calculations, while other methods are more difficult to use, but more broadly applicable. None of the methods satisfactorily handle nonuniformly spaced arrays, arrays with arbitrary amounts of internal moderator, or “clumped” arrays, i.e., arrays of arrays. Most of the methods have some provision for treating mixed arrays, but these provisions often do not apply to arrays of arbitrarily arranged fast (e.g., metal) and slow (e.g., solution) units.