The Japanese prototype fast breeder reactor Monju restarted its system startup test in May 2010 after a 14-year interruption. In the first stage of the test, reactor physics parameters have been measured at a zero power level.

The present paper describes the evaluation of the criticality data. The best-estimate value and its uncertainty are evaluated as accurately as possible, following the guidelines recommended by the International Criticality Safety Benchmark Evaluation Project.

The restart core contains 1.5 wt% of 241Am, which is three times more than the previous test. To extract the influence of the 241Am accumulation on calculation accuracy, criticality data obtained in the previous test are evaluated at the same level of detail.

The calculation accuracy is investigated with four major nuclear data libraries. It is confirmed that the accuracy is within 0.3% k/k, a 2 value of experimental uncertainty, with JENDL-3.3, JENDL-4.0, and ENDF/B-VII.0. The reactivity change due to the 241Pu decay can be simulated within an accuracy of 1% with JENDL-4.0 and JEFF-3.1.