ISO 11311:2011 specifies common reference critical values (of which the effective neutron multiplication factor, k_eff, is equal to 1) for homogeneous water-moderated plutonium-uranium oxide mixtures based on an inter-code comparison of calculated critical values.

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.