It is applicable to operations with unirradiated mixed uranium-plutonium oxide (MOX) outside nuclear reactors.
A classical validation approach for these systems is difficult because of the paucity of critical experiments for MOX fuel.
Various reference systems, in terms of isotopic compositions, thicknesses of water reflection, and densities of oxide are evaluated by different combinations of calculation codes and nuclear data libraries (i.e. different calculation schemes).
The critical values defined in ISO 11311:2011 are the lowest of those calculated by each of these calculation schemes and accepted as credible.
The values in ISO 11311:2011 are reference values and not absolute critical values.