Analyses of possible explanations for the neutron flux fluctuations in German PWR

During the last 15 years the neutron flux fluctuation levels in some of the German PWR changed significantly. During a period of about ten years, the fluctuation levels increased, followed by about five years with decreasing levels after taking actions like changing the design of the fuel elements. The increase in the neutron flux fluctuations resulted in an increased number of triggering the reactor limitation system and in one case in a SCRAM. Several models based on single physical effects are used to simulate the neutron flux. Each of these simple models can reproduce some of the characteristics of the observed neutron flux fluctuations.