The original double differential scattering kernel developed by Rothenstein and Dagan for heavy isotopes, differs from the current NJOY solution procedure. The main difference is in the exclusion of the strong scattering resonances in the latter. The scattering kernel of many materials of interest like U238 is strongly dependent on the cross section profile, mainly in the vicinity of resonances, which leads to changes in the absorption and in the Doppler-effect. Moreover, former investigations and the current one indicate that the MCNP treatment in the epithermal region is questionable as the code applies unjustified approximations. New mathematical formalism by Rothenstein enabled the implementation of the modified kernel within the THERMR module of NJOY. The problematic of the existing kernel in NJOY is thus demonstrated and preliminary results emphasize the importance of using the new formula for the scattering kernel for nuclides with strongly energy dependent scattering cross-sections. The essential steps to improve the MCNP sampling methods are also discussed.