Proposal for a new method of measuring the decay heat of fission products through noise analysis of gamma rays

High power operation of nuclear reactors depletes the nuclear fuel and accumulates fission products (F.P.'s) in the fuel. Almost all the F.P.'s are unstable and emit γ and/or β in the course of their transmutation. Their total energy varies depending on the fuel depletion and on the time elapsed after the production of the F.P.'s. When the power reactors are shut down, the decay heat plays the role of heat source, the emmision rate of which do not have, however, a deterministic but has rather a stochastic nature. The fluctuating heat source gives rise to fluctuations in the temperature of the core material. When the dirty core is restarted and operated at relatively low power level, the fluctuations in neutrons, and in the high-energy photons emitted upon nuclear fission, as well as the temperature noise are caused primarily by the fluctuations in the decay heat. There is a possibility of obtaining some information on the magnitude of the decay heat through noise analysis of the high-energy photons or of the temperature. The present paper examines theoretically the stochastic nature of the decay heat and its relation to the fluctuations in temperature, neutrons and prompt gammas. It should be of interest to perform experiments with use made of the present new method of deter- mining the decay heat and/or the in-core fuel depletion. (author)