Requirements to a Norwegian national automatic gamma monitoring system

An assessment of the overall requirements to a Norwegian gamma-monitoring network is undertaken with special emphasis on the geographical distribution of automatic gamma monitoring stations, type of detectors in such stations and the sensitivity of the system in terms of ambient dose equivalent rate increments above the natural background levels.

The study is based upon simplified deterministic calculations of the radiological consequences of generic nuclear accident scenarios. The density of gamma monitoring stations has been estimated from an analysis of the dispersion of radioactive materials over large distances using historical weather data; the minimum density is estimated from the requirement that a radioactive plume may not slip unnoticed in between stations of the monitoring network.

The sensitivity of the gamma monitoring system is obtained from the condition that events that may require protective intervention measures should be detected by the system. Action levels for possible introduction of sheltering and precautionary foodstuff restrictions are derived in terms of ambient dose equivalent rate.

For emergency situations where particulates contribute with only a small fraction of the total ambient dose equivalent rate from the plume, it is concluded that measurements of dose rate are sufficient to determine the need for sheltering; simple dose rate measurements however, are inadequate to determine the need for foodstuff restrictions and spectral measurements are required.