Modification of the Mimir-N2 Plant Dynamic Code based on JOYO MK-III Performance Test Results

Takamatsu, Misao ; Kuroha, Takaya*; Yoshida, Akihiro 

A study of passive safety test using JOYO has been carried out to demonstrate the inherent safety of sodium cooled fast reactors. In this study, emphasis was placed on the improvement of the accuracy of plant kinetics calculations. The Mimir-N2 analysis code, developed to analyze JOYO plant kinetics, was selected as the standard code for predicting plant behavior during transients. Mimir-N2 was Previously modified based on the data from plant characteristics and natural circulation tests during JOYO MK-II. Recently, the model of the core and the heat transport system of Mimir-N2 was upgraded to correspond to the modified heat transport system for MK-III. The MK-III performance test included manual reactor shutdown test and loss of power supply test etc. as transient tests. Although the sodium temperatures calculated by Mimir-N2 agreed well with the measurement results in the MK-III performance test, it was observed that the calculated sodium temperature descent rate at reactor vessel inlet and dump heat exchanger inlet etc. were slightly larger than measured. In order to further improve the accuracy of the calculation, the Mimir-N2 heat transport system models of the reactor vessel upper plenum, the hot leg of secondary heat transport system and the dump heat exchanger were modified based on the results of the MK-III performance test. As a result of the Mimir-N2 modification, the calculation results had improved agreement with the measurement results and it was confirmed that Mimir-N2 can accurately calculate plant behavior during transients such as reactor shutdown and loss of power supply.