Numerical analysis on letdown system performance test for YGN 3

Integrated performance test of Chemical and Volume Control System (CVCS) was successfully performed in 1994. However, an extensive effort to correct hardware and software problems in the letdown line was required mainly due to the lack of adequate simulation code to predict the test accurately. Although the LTC computer code was used during the YGN 3 and 4 NSSS design process, the code can not satisfactorily predict the test due to its insufficient letdown line modeling. This study developed a numerical model to simulate the letdown test by modifying the current LTC code, and then verified the model by comparing with the test data. The comparison shows that the modified LTC computer code can predict the transient behavior of letdown system tests very well. Especially, the model was verified to be able to predict the 'Stiction' phenomena which caused instantaneous fluctuations in the letdown back pressure and flowrate. Therefore, it is concluded that the modified LTC computer code with the ability of calculating the 'Stiction' phenomena will be very useful for future plant design and test predictions