Optimization of the nuclear design of light gray control rods for PWR load-following operations

In order to perform PWR load follow operations without resorting to a boron concentration change, the light gray control concept (using the combination of one high worth control bank (the H-bank), several low worth control banks (the LG-banks) and the core average coolant temperature to control the reactivity and the core power distribution) has been introduced. Optimization of the nuclear design of the LG and H control banks was also performed in this work.;The performance index (or objective function) was set up to measure the performance of a system resulting from a given set of light gray control nuclear design parameters. The optimization of the light gray nuclear parameters was performed by minimizing the performance index. In order to efficiently minimize the performance index, several search methods were investigated. The perturbation method was found to be the most efficient for the single variable search (criticality search) and the conjugate gradient method was found to be the best for the multi-variable searches (LG-bank worths and Overlap search).;The optimum worth of the H-bank and four required LG-banks were found to be 1027.5 pcm and 120 pcm respectively. The optimum initial insertion depth of LG1 was determined to be 298 cm. for the load follow sequence used in the optimization process.;Three different types of load follow operations were simulated at both BOC and EOC and controlled using the foregoing concept control parameters. All six cases were successfully controlled without any boron concentration change and violating any of nuclear safety constraints.;For the simulation of the reactor core, two group, one dimensional time dependent diffusion equations were used with moderator temperature, Doppler and xenon-iodine feedback. A finite difference approximation was used to solve the resulting system equations and the difference step power shape approximation was used to integrate xenon-iodine dynamic equations.