Estimation of critical mass of burned-up fuel

Critical masses of burned-up fuel of light water reactors (LWR) were calculated to show the effectiveness of taking burned-up rate into account for criticality safety control. Prior to the calculation, assay data of isotopic compositions of actinides in spent fuel of LWRs were collected, and the compositions were determined to give conservative calculated results from the criticality safety point of view. Concerning fission products, their physical and chemical properties were examined, about 10 isotopes which may be used for criticality safety calculation were selected. By using the isotopic compositions of actinides and selected fission products obtained in the above manner, critical masses were calculated and the results show that the critical mass increases with burn-up rate. (author)