A nuclear reactor in operation is a source of a mixed neutron and gamma field, a shut-down nuclear reactor is a source of a gamma field. The purpose of the exercise is the measurement of the gamma field intensity in the vicinity of the fuel elements in the reactor core and in locations outside the reactor core, during reactor operation and in shutdown conditions. The fraction of the delayed gamma field magnitude vs. the total (composed of prompt and delayed gamma rays) during reactor operation is estimated by measurements following a rapid reactor shutdown (SCRAM).