Evaluation of Sodium Radioactivity in the Primary System of the Prototype Fast Reactor Monju

In this study, the sodium radioactivity of ²⁴Na and ²²Na in the primary system measured in the prototype fast breeder reactor Monju was evaluated, and the reliability of the measurements and calculations was examined. The calculated-to-experiment values and their uncertainties for ²⁴Na and ²²Na radioactivities were 0.97 to 1.07 and 8.1% to 11.0% and 1.03 to 1.16 and 23.3% to 24.1%, respectively, using the JENDL-4.0 nuclear data library. The ²²Na radioactivity calculated with ENDF/B-VIII.0 was larger by 40% than those calculated with JENDL-4.0 and JEFF-3.3 due to the ²³Na(n,2n) cross-section discrepancy. The importance of the ²²Na neutron capture effect is also confirmed herein for the accurate evaluation of the ²²Na radioactivity. The experimental data were judged to be useful for validating the calculation method for improving the reliability of future designs of sodium-cooled fast reactors.