WPNCS Expert Group on Minimum Critical Values

Basic minimum critical values are important physical constants needed for assessing safety margins in criticality and are used for licensing. The expert group was formed to compile minimum critical values* of U235/U238-, Pu-, MOX-, and U233-systems. Homogeneous systems with uniform distribution of the fissile material will be covered. Discrepancies in the data were identified and an explanation of discrepancies were sought.


Under the guidance of the Working Party on Nuclear Criticality Safety, the expert group:

  • collected data from different countries, including a short description of the methods used to achieve the data;
  • identified discrepancies and propose explanations;
  • addressed effects of engineering data, of density formulae, reflector materials;
  • provided technical input to the international community, e.g. ISO;
  • supplied a general reference for criticality safety analyses, that use/include minimum critical values.


The expert group's final report can be downloaded from the NEA website. With the completion of the report, this activity has, for the moment been discontinued and the expert group disbanded.

Related links

Reference Values for Nuclear Criticality Safety
Homogeneous and Uniform UO2, "UNH", PuO2 and "PuNH", Moderated and Reflected by H2O

NEA Working Party on Nuclear Criticality Safety

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Last updated: 9 July 2010