On the anomaly in sizes of minimum critical mass cores

• Analysis of core sizes of reflected minimum critical mass (MCM) systems.
• MCM core size as a function of reflector thickness shows a minimum.
• A diagram of partial neutron currents at the core–reflector interface is presented.
• The new concept of intergroup albedo is introduced.
• Size anomaly is explained on the basis of fast flux and intergroup albedo behavior.

The anomaly in sizes in minimum critical mass cores, as detected by Mike Williams, is analyzed by using intragroup and intergroup neutron reflection coefficients. In this way the three components in the partial neutron currents from the reflector to the core can be calculated. It is shown that the anomaly is the result of an interplay of these partial neutron currents.