| The critical heat flux for natural convection boiling on the outer surface of a simulated reactor vessel surrounded by a thermal insulation structure was studied experimentally to obtain basic information for the assessment of external cooling of reactor vessels under severe accident conditions. Two different insulation geometries were investigated, one similar to the insulation structure for AP600 and the other for KNGR. For both insulation geometries, the local critical heat fluxes were found to be much higher than the corresponding values without thermal insulation. The spatial variation of the local critical heat flux, which exhibited a minimum near the minimum gap of the flow channel formed between the insulation structure and the reactor vessel, was found to depend strongly on the geometry of the insulation structure in the lower part of the vessel. On the other hand, there was very little difference in the local critical heat flux between the two geometries in the upper part of the vessel. |
