Gadolinia-bearing Assembly Design for Reduction of Critical Boron Concentration in APR 1400

The objective of this study is to develop a systematic approach to further reduce CBC at BOC and ultimately making MTC more negative, while maintaining cycle length and keeping pin peaking factor below the 1.55 safety limit. The methodology uses fuel assembly (FA) designs that contain both low and high Gd w/o rods. The APR 1400 fuel assembly and core design for Shin-Kori Unit 3 Cycle 1 (SK3C1), which is of PWR type, is used as reference case. The Gd-bearing assembly types in SK3C1, namely B1, B2, B3, C1, C2, and C3, uses Gd rods with 8 w/o gadolinia (Gd2O3) admixed in UO2, and 2 w/o U-235 enrichment. FA and core depletions are simulated using CASMO 3.0 and MASTER 3.0 design codes, respectively. The methodologies described in Section 2 are applied to assemblies B1, C1 and C3. Interpolation of these assemblies' quadratic model equations is performed using the 0.11 factor to obtain ΔkFA tables for B2, B3 and C2 assemblies. Once the design of all Gd-bearing assemblies are determined, core depletion is simulated. The summary of core depletion results is shown in Table II. It is clear that CBC at BOC is successfully reduced in the optimized core case, as also evident in Fig. 3. The amount of CBC reduction is less than the predicted 300 ppm. This is attributed to neutron leakage in core depletion simulation. The actual ΔkFA computed using results of the optimized core simulation is 0.078, which is well within the 1% error of the quadratic model solutions and close to the analytically calculated value. Thus, the assumption made in Eq. is validated