Experimental investigation on effect of flow blockages on quenching behaviour under low injection flow rates

Quenching studies of ballooned fuel pins have indicated enhancement of coolability for flow blockages ranging upto 90% with blockage extension of 6% (20 cm blockage length) under typical Emergency Core Cooling System (ECCS) injection rates. Similar enhancement is also observed for flow blockage of 45% with higher blockage extension of 60% and lower injection rates. An experimental setup is developed to assess the coolability under high flow blockage (upto 80% of the flow area) and longer ballooned length extensions (up to 60% or 600 mm). The setup employs 5 X 5 matrix of indirectly heated, pre-fabricated ballooned fuel pin simulator (FPS) surrounded with 20 heated and ballooned FPS which are further surrounded with 12 dummy FPS. The objective of this experiment is to study the effect of water injection rate on the quenching behaviour of large scale ballooned heated pins simulating early phase of severe accident. Bottom re-flood condition is considered for the study. The water injection rates (0.11–0.45 g/s per unit length per FPS) are kept lower than the typical PWR specific SAMG injection flow rates to assess minimum flow rate requirement. The FPS is observed to be coolable only when the injection rates are higher than a certain value. Higher quenching rate is observed in the region towards the entry of the ballooned length as compared to the region towards the exit of the ballooned length. Conduction controlled rewetting is found to be dominant for the entire range of injection rates considered for the experiments. Flow rates (0.11–0.45 g/s per unit length per FPS) are found to successfully quench the bundles. However, the FPS temperatures exceed the oxidation run-away threshold temperature for 10–25 g/s injection flow rates (0.11–0.275 g/s per unit length per FPS).