Operating margin recovery to deal with RCS hot leg temperature fluctuations

Although the existence of temperature streaming phenomenon was expected and steady state temperature streaming has been considered in the design of Temelin I and C, the impact of its related effect, temperature fluctuations, was unfortunately underestimated with regard the operational margin. At Temelin VVER-1000, incompletely mixed turbulent flow streams of different temperatures flowing out of the core causes rapid peak-to-peak fluctuations in hot leg temperature with fast response temperature detectors. These fluctuations then propagate into the indicated delta temperature core power fluctuations. Past operation showed, that the further action shall be taken to allow operate with the aggressive loading pattern without high risk of inadvertent reactor trip. The real plant data were evaluated for two first cycles of the Unit 1. Several solutions were outlined and evaluated and as the most appropriate the reduction via hot leg temperature signals averaging is recommended. (author)