[en] Nowadays optimization goes with everything. So French engineering firms try to demonstrate that fuel transport casks and storage pools are able to receive assemblies with higher ²³⁵U initial enrichments. Fuel Burnup distribution contributes to demonstrate it. This instruction has to elaborate a way to take credit of burnup effects on criticality safety designs. The calculation codes used are CESAR 4.21-APOLLO 1-MORET III. The assembly studied (UO₂) is irradiated in a French Pressurized Water Reactor like EDF nuclear power reactor: PWR 1300 MWe, 17 x 17 array. Its initial enrichment in ²³⁵U equals 4.5%. The studies exposed in this report have evaluated the effects of: i) the 15 fission products considered in Burnup Credit (⁹⁵Mo, ⁹⁹Tc, ¹⁰¹Ru, ¹⁰³Rh, ¹⁰⁹Ag, ¹³³Cs, ¹⁴³Nd, ¹⁴⁵Nd, ¹⁴⁷Sm, ¹⁴⁹Sm, ¹⁵⁰Sm, ¹⁵¹Sm, ¹⁵²Sm, ¹⁵³Eu, ¹⁵⁵Gd), ii) the calculated abundances corrected or not by fixed factors, iii) the choice of one cross sections library used by CESAR 4.21, iu) the zone number elected in the axial burnup distribution zoning, u) the kind of cut applied on (regular/optimized). Two axial distribution profiles are studied: one with 44 GWd/t average burnup, the other with 20 GWd/t average burnup. The second one considers a shallow control rods insertion in the upper limit of the assembly. The results show a margin in reactivity about 0.045 with consideration of the 6 most absorbent fission products (¹⁰³Rh, ¹³³Cs, ¹⁴³Nd, ¹⁴⁹Sm, ¹⁵²Sm, ¹⁵⁵Gd), and about 0.06 for all Burnup Credit fission products whole. Those results have been calculated with an average burnup of 44 GWj/t. In a conservative approach, corrective factors must be apply on the abundance of some fission products. The cross sections library used by CESAR 4.21 (BBL 4) is sufficient and gives satisfactory results. The zoning of the assembly axial distribution burnup in 9 regular zones grants a satisfying calculation time/result precision compromise. (author)