Reactor gamma noise studies

Abstract

A review is given of reactor gamma noise studies, with the main emphasis laid on experience gained from subcritical reactors at Otaniemi, Finland, along with a survey of the development of stochastic models, numerical studies and gamma noise measurements. Within the framework of a space-independent two-energy-group model that has been developed, all of the simplified correlation functions associated with the detector signals i j and i 1 assume the form 〈 i j ( t). i 1( t + τ)〉 = A + B 1 exp (− ω 1 τ) + B 2 exp (− ω 2 τ); the subindices 1 and 2 here refer to the fast and thermal groups of neutrons respectively. The detection model includes the detection of gamma rays, thermal neutrons and fast neutrons, absorption, recoil and fission type detectors, gamma-compensated detectors, and response fluctuations. Explicit expressions are given in regard to the influence of a large number of reactor and detector parameters. The gamma radiation, which is emitted with long delay, can be regarded as spurious, and in the main degrades measurement quality. With respect to detector effects, the model further provides some insight into the analysis of power reactor noise. It has been experimentally confirmed that information on the thermal and fast groups of neutrons in a thermal reactor is derivable from noise measurements by means of photon detection, or alternatively the joint use of thermal neutron and photon detection. Ex-core measurements, and measurements on fast reactors, have also been effected. The use of gamma detection in reactor noise measurements may be regarded as a complement to and a substitute for thermal neutron and fast neutron detection. Other distinct and useful features of gamma detection include the possibility of acquiring energy-sensitive detectors with excellent time resolution, the possibility of making use of energy selection by the agency of electronic amplitude discrimination, the relatively small disturbance in the system that arises from a gamma detector, particularly in ex-core measurements, and the great relaxation length of high energy photon radiation as compared with thermal neutrons.