A transient analysis has been performed for UO2 and MOX-fueled light water reactor cores based on Microscopic Reactor Physics, which treats the detailed distribution of temperature and effective cross section within a rod. Conventionally the volume-averaged temperature and the Rowlands’ effective temperature are used to calculate fuel rod-averaged cross sections, and applied to the transient analysis. The present method is considered as a reference and the result is compared with the conventional method for a mini fuel core containing eight fuel rods and a control rod. From numerical results, it is found that the Rowlands’ model underestimates the peak power and the volume averaged model produces rather good peak power results. After 1.0 sec, the Rowlands’ model yields the similar power as the reference, while the volume averaged model yields less power than the reference one.

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