Optimization for steam generator level control system setpoints after power up-rate of Ulchin units 1 and 2

Setpoints of reactor control system are evaluated in order to confirm the plant operability by best-estimate analysis methodology after 6% power up-rate and replacement of steam generators in Ulchin Units 1 and 2. First of all, because of feedwater increase after power up-rate and replacement steam generator, one issue is to analyze the steam generator level control system performance to confirm the operability of SG level and decide speed and differential pressure of feedwater pump, turbine runback target value and runback rate in case of one feedwater pump loss. The another issue is to connect smoothly to house load operation in case of the large load rejection event without reactor trip. In this event, we experience reactor trips such as departure nucleate boiling ratio (DNBR) and steam generator level high/low reactor trip. To prevent the DNBR reactor trip, we should extend steam dump capacity or sharpen the steam dump control system setpoints. However this prescription may make it more difficult the steam generator level control. Therefore, the determination of the optimal steam dump control system setpoints are made by analyzing both of the DNBR and steam generator level transients. An optimal set of steam dump control system setpoints is proposed after 6% power up-rate in Ulchin Units 1 and 2. Finally, the steam generator level control system performance is checked to remain stable by performing the step load change analysis and one feedwater pump loss analysis with turbine runback. Also, the steam generator control system is affected by shrink and swelling due to open and close of the steam dump valve and inflow of cold feedwater. We analyzed the effect of the phenomenon due to operation parameter change after power up-rate. (authors)