Action levels for automatic gamma-measurements based on probabilistic radionuclide transport calculations

In the event of a nuclear or radiological emergency resulting in an atmospheric release of radioactive materials, stationary gamma-measurements, for example obtained from distributed, automatic monitoring stations, may provide a first assessment of exposures resulting from airborne and deposited activity. Decisions on the introduction of countermeasures for the protection of the public can be based on such off-site gamma measurements. A methodology is presented for calculation of gamma-radiation action levels for the introduction of specific countermeasures, based on probabilistic modelling of the dispersion of radionuclides and the radiation exposure. The methodology is applied to a nuclear accident situation with long-range atmospheric dispersion of radionuclides, and action levels of dose rate measured by a network of monitoring stations are estimated for sheltering and foodstuff restrictions. It is concluded that the methodology is applicable to all emergency countermeasures following a nuclear accident but measurable quantities other than ambient dose equivalent rate are needed for decisions on the introduction of foodstuff countermeasures.