Effect of a Transport Lag on Stability of a Reactor with a Nonlinear Controller and Multiple Lags

Many nuclear reactor systems have transport lag elements in their control loops. In such systems, the examination of stability using lag time as a continuously variable parameter will be found useful to the preliminary design of control systems or to evaluate the effectiveness of control systems when a lag time changes from a designed value in normal or abnormal condition. In this paper, the stability of a reactor system with multiple transport lags and a nonlinear control element is analyzed by using the parameter plane method in which variables are transformed into a logarithmic function. Taking lag time as the continuously variable parameter, stability analyses have been conducted for both simplified and realistic reactor models. From the former, many interesting phenomena are observed. For example, under some conditions, the system repeats the periodic sequence of stability and instability with the increment of lag time, while the frequency of a limit cycle changes discontinuously at specific lag time. In the latter model, similar phenomena are observed.