Final summary report of fuel-dynamics tests H2 and E4

Results of two failure experiments using LMFBR-type fuel during simulated unprotected transient overpower accidents are reported and analyzed. In both experiments, a single fresh fuel pin in a Mark-IIA loop was subjected to a temperature-limited, step-reactivity irradiation in the TREAT reactor. Total energy was 490 MJ in Test H2 and 690 MJ in Test E4. Except for their timing, the sequence of events in the failure scenario was the same for both tests. Local coolant boiling began 25-50 msec before failure. Significant upward fuel flow in the center of the pin started as early as 100 msec before cladding failure. Cladding failure was due to melting after contact with molten fuel and occurred at the top of the fuel column. Formation of an outlet flow-channel blockage began about 10 msec after failure and was complete by 50 msec. Inlet blockage began later and was less extensive. No significant amount of fuel sweepout was observed. Fuel remains separated into a small group of 50-1000-μm fragments and a macroscopic group of chunks and clinkers. The final distribution of fuel remains may have resulted from a delayed fuel/steel interaction in the inlet region