ASSESSMENT OF THE BEST ESTIMATE THERMAL DESIGN METHOD USING THALES SUBCHANNEL CODE

摘要

In order to maintain the safety of the reactor core, the minimum DNBR in the PWR. core have to remain higher than the DNBR limit during Condition I and II events. Therefore, it is important to adequately determine the DNBR limit for the reactor safety and economic feasibility. To realistically evaluate the relationship among the uncertainties and reduce the conservatism resulting from the unknown phenomena, KEPCO NF introduces the Monte Carlo method for the modification and improvement of the existing thermal design method. For the best estimate thermal design method, various researches are conducted as follows. First, the random sampling methods are investigated to generate the Gaussian random numbers. Second, the normality test methods are reviewed to verify the normality of the generated random numbers. Third, the subchannel code, THALES, is used for the subchannel analysis. THALES is a COBRA family code developed by KEPCO NF. Through the subchannel analysis, the DNBR_V distribution can be obtained. Forth, the DNBR_V distribution is statistically combined with the uncertainties of the other parameters, e.g. CHF correlation etc. Then, the total DNBR distribution is generated and the DNBR limit can be determined to avoid DNB at a 95% probability at a 95% confidence level. Finally, through the example calculation, it is verified that this method produces reasonable results. If more studies are done, the best estimate thermal design method is useful to determine the DNBR limit.