The metrological support system for neutron measurements at nuclear re
actors has been developed on the concept of controlled neutron fields
applicable to organizations having a group of reactors. A metrological
classification is proposed for the fields and the relation between th
em and also for approaches to certification. This controlled-field sys
tem has been set up at the NIIAR reactors for use in nuclear materials
research. Data are given on neutron fields at five reactors.