The main objective of this study is the knowledge of the impact of the atomic density on the uncertainty of the effective multiplication factor (Keff) due to the uncertainties in elastic and inelastic scattering, capture and fission cross sections. Six thermal cases of the benchmark (HEU-SOL-THERM-001) have been studied by using a recently updated nuclear data evaluation JENDL4 to calculate the sensitivity vectors for ¹H, ¹⁶O, ²³⁵U and ²³⁸U isotopes. These sensitivity profiles are calculated by using the adjoint-weighted perturbation method based on the Kpert card; and we validated them with the KENO code results, with the differential operator technique of the MCNP5 code and with two nuclear data evaluations (ENDF/B-VI.8 and ENDF/B-VII.0). Kpert card is used by the Monte Carlo code MCNP6. Thus, the Keff uncertainties induced by nuclear data uncertainties have been calculated by combining the sensitivity vectors with the covariance matrices that are generated by the ERRORJ module of the recently updated of the nuclear data processing system NJOY99. This study shows that: it must the cross sections and covariance matrices adjustment of the isotopes that have the great atomic densities and it must the cross sections and covariance matrices adjustment of the fissile isotopes even if they have small atomic density in nuclear reactors.