It is important to determine the radiation dose received by key operational equipment in the containment areas of CANDU reactors. A dosimetry system was developed to evaluate the radiation environment under operational conditions. The key component in the system is a passive MOS (semiconductor) dosimeter which is hermetically sealed and is not affected by the high humidity in the containment area. Doses were measured in a number of Ontario Hydro operating reactor sites over periods of 4 000 to 11 000 hours. As a result of this dose mapping study, several remedial actions have taken place and it is recommended that similar reactors undergo this type of study.