On-Line Computers in Optimal Reactor Shutdown

The buildup of xenon-135 following the shutdown of high-flux thermal reactors with low excess reactivity may render these reactors subcritical for many hours. It has long been known that preshutdown power variations can modify the xenon-135, iodine-135 transients to correct this problem. Recently the techniques of Pontryagin and others have been used to optimize power programs which control the xenon buildup. Point reactor models have always been used. This paper reviews the solution associated with a minimum time optimality criterion. The effect of spatial variations in power, xenon, and iodine are shown to affect greatly the optimal power trajectories. Numerical comparisons between point and one-dimensional models are presented for a bare sphere reactor. The role of a digital computer in analyzing and controlling the shutdown procedure is discussed.