Gadolinium is mixed with nuclear fuel for extended burnup, increased reactor cycle Lengths, and in-out core refueling schemes. The purpose of this work is to investigate the effect of Gadolinium on neutronic parameters. A comprehensive analysis of different fuel assemblies bearing gadolinia with different concentration and different number of gadolinia rods was carried out. The EPR fuel assemblies were chosen in this study. The fuel assemblies are of seven different types, each assembly has unique design characteristics in terms of enrichment, number of gadolinia rods and gadolinium concentration. The calculations were achieved using MCNP6 code and with ENDF/B-VII library. A fine mesh tally was superimposed on the geometry to illustrate flux distribution in the whole assembly and its change with burnup. The variation of reactivity and isotopic composition for each region of the assembly with burnup and its influence on flux distribution was studied. The results showed that the presence of gadolinium greatly affects the power distribution. It causes some skewness in axial power shape towards the top of the assembly, the assembly which has largest number of gadolinia rods and the lowest enrichment has the largest skewness of power distribution at BOC. At EOC the flux and axial power have a double hump shape due to the buildup of gadolinium isotopes and the large decrease in reactivity with burnup.
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