Abstract: The critical safety of nuclear fuel elements from a research reactor and two NPPs under normal and accidental conditions was analyzed through the calculation of k_eff by MCNP.Through the calculation,an upper threshold of the enrichment degree which could be stored in the facility was obtained,the effects of distance and water density on k_eff were analyzed;and several fuel pellets storage schemes were compared.The calculation results indicate that,under normal or accidental conditions the critical safety is acceptable;the enrichment threshold is 7.44%;k_eff approximated linear increases and quadratic decreases as the enrichment degree increases,and approximately linear decreases as the distance increases;fuel pellets with different enrichment degrees should be districted in a certain area according to their enrichment degrees,and fuel pellets with comparative high enrichment degrees should not be stored in the center of the storage area.