Principles for Control Rod Withdrawal Strategy During the Startup of Boiling Water Reactors

Control rod withdrawal strategy during boiling water reactor startup is explored and verified by examples simulated with a three-dimensional nuclear-thermal-hydraulic coupled computer code with xenon dynamics included. The study leads to the following two general principles. 1. Use of the shallow control rods should be minimized in the intermediate pattern insofar as the power peak near the bottom is within the limits for the xenon transient control. 2. Every control rod should be withdrawn at least once to the same depth or to more than that of the rated pattern, and use of two different control rod patterns is necessary for the effective stretching out of the preconditioned envelope.