The Rationale of Leakage Parameters adopted in Leakage Feedback Method

A conventional two-step core calculation procedure employing B1 critical leakage correction has been adopted as the standard procedure for reactor design analyses for a long time because of its indispensable merits due to far lower cost than Direct Whole Core (DWC) calculations and acceptable accuracy. The method proved its validity and its performance for various LWR cores but a proper rationale of the functionalization formula is not clearly verified yet. In this study, the fitting formula of GCs for LFM is introduced with the explanation of how each leakage parameter impacts on GCs. From the evaluation of the fitting formulas for APR1400, the statement is verified by proving that GCs can be functionalized with respect to the fast and the intermediate leakages. SI shift is a representative of spectrum inside the whole-fast group. Since the tendencies of SIS are different for the edge and the corner PAs, the consideration of SIS for PAs results in the accuracy enhancement of fitted GCs. Among four different fitting formulas for leakage dependent GCs, using three variables, fast and thermal LFs and SIS, showed the best accuracy for both IA and PA. The estimated GC error of LFM is less than 30 pcm in RMS. With a help of advance in a leakage correction method, the two-step method employing LFM achieved less than 36 pcm in reactivity and 0.7 % in assemblywise power error in RMS for the 2D APR1400 core compared to the DWC reference. Assessment of LFM on various core problems with thermal feedback and rod insertion will be presented in the upcoming paper.