Void reactivity (Cmn) coefficients as indicators of boiling water reactor stability

Paper in proceeding, 2016

This paper presents the results of some investigations performed in the area of the stability of Boiling Water Reactors (BWRs). These systems are known to become unstable under certain conditions, and thus the development of different methods for the quantification of their stability properties is of prime importance. Preliminary studies with Reduced Order Models showed that the stability behavior of the system can be described in terms of so-called Cmn-coefficients. These coefficients represent the effect of a change in the void fraction on the eigenmodes of a nuclear core. It turned out that when these coefficients are positive, the system is clearly unstable because of the corresponding positive void feedback. On the other hand, in the case of negative Cmn-coefficients, it has been found, both using ROMs and dynamic core simulators like SIMULATE-3K, that the system becomes less stable for the Cmn coefficients becoming more negative. A thorough examination of the dependence of Decay Ratio on the Cmn coefficients using ROMs also demonstrated that there is a strong correlation between the DR and the Cmn coefficients what regards the stability properties of a BWR system. As a result, the estimation of the Cmn coefficients opens up the possibility of using such coefficients as a qualitative measure of core stability in a predictive manner. This could be used for instance as means to compare the relative stability of several core loadings without the need of running lengthy time-dependent three-dimensional core calculations, and could be of great help to nuclear engineers when designing nuclear cores. Therefore the main purpose of the paper is to investigate the possibility of using Cmn-coefficients as an alternative stability indicator for Boiling Water Reactors, as well as to determine their advantages and disadvantages as compared to the traditional Decay Ratio.