Experimental evaluation of high-temperature fuel-coolant thermal interaction behavior using unirradiated fuel powder

Mihara, Takeshi ; Urano, Kenta; Udagawa, Yutaka  ; Kakiuchi, Kazuo 

Mechanical energy generated during fuel failure under reactivity-initiated accident (RIA) conditions, such as pressure pulse and water hammer, strongly depends on the fragmentation state and temperature of the fuel. When failure caused by pellet/cladding mechanical interaction (PCMI) occurs rapidly at low temperature, fission gas release drives pellet fragments to move at high velocity in water, leading to extremely efficient heat transfer between the fuel and coolant. This results in rapid vapor generation and the production of impulsive mechanical energy. These observations indicate that both the particle surface area and the highly efficient heat transfer associated with high-velocity fragment motion are key influencing factors. In the 264-2 and 264-24 experiments, test conditions were designed to simulate fuel-coolant interaction under conditions where the driving force for pellet fragment motion, which is characteristic of RIA events, is absent. In the 264-24 test, the specific surface area of the pellet particles (surface area per unit mass) was designed to exceed that of previously tested high-burnup fuels. In addition, the fuel enthalpy (thermal energy per unit mass) was set based on prior observations to conditions where significant mechanical energy generation is expected. As a result, both the pressure pulse and water hammer energies were significantly lower than those observed in high-burnup fuel failure cases, where the driving force for pellet fragment motion is considered to be present. This clearly demonstrates the critical importance of the driving force for pellet fragment motion in the generation of mechanical energy.