Experimental investigation of partially ballooned fuel bundle under low injection flow rates

A postulated loss of coolant accident in a PWR with failure of emergency core cooling systems causes loss of heat sink for the fuel pins. Overheating of the fuel pins and difference in the internal and external pressures leads to clad ballooning over large portion of the fuel pin length. The re-flood behaviour of ballooned fuel pins has been studied extensively in the past for ballooned heater pin configurations, with up to 20 cm ballooned length of the total length, and with water injection rates typically equal to Emergency Core Cooling System (ECCS) injection rates. An experimental setup is thus developed to study the effect of large extent of ballooned region (up to 60 cm of the total length) on the re-flood behavior. The experimental setup employs 5 x 5 matrix of indirectly heated, pre-fabricated ballooned fuel pin simulator (FPS) surrounded with 20 heated non-ballooned and 12 dummy FPS. The objective of this experiment is to study the effect of water injection rate and initial temperature on the quenching behaviour of ballooned heated pins under bottom re-flood condition. The water injection rates are kept lower (0.11-0.45 g/s per unit length per FPS) than the typical SAMG injection flow rates for typical PWR (1.12 g/s per unit length per fuel pin) and the initial temperatures range from 260 degrees C to 650 degrees C. In all experiments FPS are observed to be quenched, and enhanced quenching is observed in the ballooned region towards the exit of the region owing to lesser carryover. Increase in water injection flow rate is found to result in a transition in the rewetting pattern from conduction controlled rewetting to fluid controlled rewetting.