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Fundamental principles for designing the ideal field montoring network have been studied from the viewpoint of monitor detecting capability.The detecting capability is specified by a radius of orbicular "detecting domain" such that the radjoactive cloud flowing into the domain gives an exposure-rate above the alarm level to the detector; for example, 100 m for a cloud-gamma detector at 2 km downwind, 400 m for a cloud-concentration monitor at lkm downwind, assuming a radioactive cloud that inflicts a dose of 100rems to the thyroid of a child staying for lOhr at the point of maximum concentration.The ideal distribution of detectors are obtained by adjoining their detecting domains so as to surround the reactors, keeping a outlying distance from the reactors, i. e., 1.8km, to retain complete capability for detecting a radioactive cloud, and to eliminate the direct gamma radiation assuming the 106Ci of fission products confined in the reactor in an accident. In the neighborhood of reactors, film-dosimeters or TLD are advisable instead of the y-monitors.The concept of detecting domain is also applicable to the evaluation of the probability of detecting a radioactive cloud by a deficient monitoring network, composed of insufficient number of field monitors.

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The exposure rates of 7-rays from 41Ar clouds issuing from the stack of a nuclear reactor were calculated on the basis of Pasquill's cloud diffusion formula, using exact 7-ray attenuation and build-up formulas. The results have agreed well with the experimental values of a "Ar release test performed in 1961 a t the Tokai Research Establishment, Japan Atomic Energy Research Institute.The cumulative dose of 7-rays received at a given point from shifting 4lAr cloud for a long period was also calculated by considerations based on the probability concept. The validity of the method has been confirmed by comparison between the value estimated with statistics of meteorological data and the cumulative value of observed 7-exposure. By this method, the maximum allowable release rate was obtained for the Tokai area.