Reactor Core Stability Analysis for a Boiling Water Reactor by a Transient Model

Abstract

Applying an autoregressive (AR) technique to a boiling water reactor stability test yields two kinds of reactor stability performance indexes. One is characterized by the neutron flux to reactor pressure open-loop transfer function. The other is characterized by the closed-loop transfer function. Studies were performed on these reactor core stability indexes, using a one-dimensional transient model. To simulate these two kinds of stability characteristics in the time domain, the input/output relation for the system considered is important. In both cases, the output variable is the neutron flux. For the input state variable, in the case of the open-loop stability index, reactor pressure was chosen and adopted as a boundary variable to enable neglecting the feedback due to change in the reactor dome pressure. In the case of the closed-loop stability index, the vessel steam flow to the main steamline was adopted to separate the reactor response from the main steamline.

Employing this procedure, both stability indexes were estimated by the one-dimensional reactor transient model. Comparing these indexes to those evaluated by the AR fitting and Fourier transform of small perturbation test data, it was concluded that the one-dimensional transient model predicts well the open- and closed-loop stability performance. Further, it was shown that the open-loop index conventionally used is a somewhat conservative one.