Fundamental Data Relating to the Criticality of Simple Systems

The criticality guide recently produced in the French Commissariat à l'Energie Atomique by the Critical Mass Sub-Committee gives the basic data for undertaking criticality studies on isolated systems and systems in interaction. It contains information and standards taken from foreign as well as French documents, and adds to and clarifies this information. The paper summarizes the original studies underlying parts of this guide to the extent that they throw new light on the fundamental data relating to the criticality of isolated fissile units. Reflected critical values (water reflector of infinite thickness) are proposed as a function of the concentration of fissile materials. These values, mainly obtained by the DSN method, supplement the known results for medium- and low-enriched uranium and weakly moderated low-density systems. Differences from the published minimum critical values for low enriched uranium are interpreted. The standards proposed apply to neutronically homogeneous media, whose dilution curves (density of fissile product versus moderation ratio HA) correspond to the following reference systems: metal-water mixture; salt (which when dry has a density of fissile material in the neighbourhood of 3.2 g/cm3. The choice of the salt dilution curve is discussed. Finally the various types of heterogeneity are defined and rules are proposed for determining whether a unit can be regarded as neutronically homogeneous. (author)