Where ψ is the neutron fluence rate , ซิกมา is the reaction cross sections, and RRa is the reaction rate . The original continuous functions of energy was simplified by the group approximation. The neutron energy spectrum was obtained from the used activation monitors by deconvolution in the Sand-ll code. The group cross section was gained in the NJOY99 code with the ENDBVII.0 nuclear data library. Then, the following equation was used: