Abstract
In experimental fast reactor "JOYO", natural circulation test with a trip of all main pumps in the 75MW full power operation was conducted on December 23, 1981. Transition of the main loop flow from forced flow to natural circulation was smooth, and high natural circulation capability was verified. The peak fuel subassembly outlet sodium temperature during this test was below that during 75MW normal operation. Previous to the test, natural circulation analysis code MIMIRN2 was developed. The predicted peak value of the fuel subassembly outlet sodium temperature was in a good agreement with the measured value.