Dryout heat flux for core debris bed. III Mixture of intact fuel and particulated clad.:Mixture of Intact Fuel and Particulated Clad

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Mixture of cylindrical steel pellets and Al2O3 balls, which simulated the intact fuel pellets and fragmented claddings, respectively, was inductively heated in a 50 mm I.D. pyrex glass cylinder filled with water, to investigate the coolability of TMI-2 type degraded core debris bed. The size of steel pellets was 11 mm dia. × 11 mm for BWR, 8 mm dia. × 12 mm for PWR and 5.5 mm dia. × 9 mm for FBR and Al2O3 balls were about 2 mm in diameter. The height of the debris bed was 25 cm or lower. The dryout heat flux does not level off up to a bed height of 25 cm or over for the TMI-2 type bed while 8 cm or so in the bed of only steel balls. The dependence of dryout heat flux on the system pressure agrees with the Lipinski's 0-D model by adopting a proper equivalent diameter. When a simple number-weighted average is used as the equivalent diameter, the prediction gives a fairly good agreement with the experiment for FBR type bed but underestimations for the PWR and BWR type beds. It should be noted that the small balls of less fraction, not the large pellets, substantially govern the dryout. When the coolant flow is allowed from the bottom, however, the dryout heat flux is enhanced up to the level for the complete vaporization of coolant, and small amount of mass flux or circulation head can greatly improve the coolability.
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