Theoretical safety calculations were done with proved codes utilized by the staff of the RERTR program in the HEU to LEU core conversions. The studies were designed to evaluate the reactivity coefficients and kinetics parameters of the reactor involved in the evolution of peak power transients by reactivity insertion accidents. It was done to show the trend of these reactivity coefficients as a function of the core size and fuel depletion for RP10 cores. It was useful to get a better understanding of the progression of the reactivity insertion transients monitoring the critical thermal hydraulic parameters to avoid core damage. To confirm the accuracy of these studies the results were compared with experimental data of the SPERT I reactors. The microscopic cross section calculations were condensed to 15 broad groups using the WIMSD4M[1] code for all the isotopes from eight different regi...