Table 7.1. Subcritical, Single Parameter Limits for Plutonium Solutions and Metals
Plutonium Solutions and Metals
Mass of fissionable nuclide, kg
Diameter of cylinder of
Volume of solution, L
Concentration of fissionable
Cylinder, diameter, cm
Slab thickness, cm
Maximum density for which
Mass and dimension limits
are valid, g/cm3
(a) Oxides containing no more than 1.5% water by weight at full density.
(b) Oxides containing no more than 1.5% water by weight at no more than half density.
Double contingency requires that process designs incorporate sufficient factors of
safety to require at least two unlikely, independent, and concurrent changes in
process conditions before a nuclear criticality accident is possible.
Protection, or defense in depth, should be provided by either (a) the control of two
independent process parameters (which is the preferred approach, if practical) or (b) a
system of multiple controls on a single parameter. In all cases, no single credible
event or failure shall (DOE Order 420.1A, Section 4.3.3) result in the potential for a
criticality accident. The basis for selecting either approach is to be fully documented.
The two parameters that are controlled in the double contingency analysis process
must not be related by common mode failures. Judgment is required in determining
whether two events are related and, consequently, whether they represent two
contingencies or a single contingency. For example, exceeding a storage limit and
then flooding an area with water would constitute two independent events. However,
a fire followed by the flooding of a storage area with fire suppression water would
constitute a single event.
The double contingency principle is to be applied to all nuclear criticality safety
analyses for processes, systems and equipment, storage, and transportation of fissile
materials. Should contingencies be determined to be related, efforts are to be made to
separate the contingencies. If this is not practical, an exemption process is identified
in DOE Order 420.1A.