The spatial behavior of neutron flux during power reactor transients as effected by the xenon-iodine chain

Abstract

The current trends in the design of commercial nuclear power reactors are toward higher thermal neutron flux levels, more uniform power distributions, and increased core dimensions, all of which tend to make the reactor more economically efficient. Each of these conditions, however, is also more favorable to the sustaining of Xenon-induced spatial power oscillations, which can cause power peaking that will eventually result in overpower conditions in some fuel elements leading to fuel element failure. Since it appears that all previous investigations of the phenomenon have assumed that a system is operating at a constant power level, a study was undertaken of the characteristics of spatial neutron flux variations, induced by the xenon-iodine chain, occurring during power reactor transient behavior. Using the physical model of a uniform ring reactor, and the mathematical techniques of perturbation theory and model analysis, azimuthal oscillatory variations in power were examined as a function of the design and operating parameters of a nuclear power plant system. The types of transient behavior studied included power startups and power decreases of a step and ramp function form. The nature of the resulting xenon-induced spatial power oscillations was discovered to be quite unique when compared to their counterparts occurring during steady-state operation. Their behavior was affected by the time during a power level change when they were initially induced and the rate of the power level change..