Critical heat flux for downward-facing pool boiling on CANDU calandria vessel

Under a severe accident condition in CANDU reactors, damage to the reactor is expected such that a significant amount of core material can melt and relocate downward to the bottom of the vessel. It is essential that the vessel remains intact and mechanically strong to accommodate the core melt. However, a thermal creep failure is postulated to occur if sustained critical heat flux is instigated in the surrounding shield tank water. A mechanistic model is developed to predict the critical heat flux variations along the downward facing outer surface of calandria vessel. The hydrodynamic model considers a liquid macrolayer beneath an elongated vapor slug on the heated surface. Local dryout is postulated to occur whenever the fresh liquid supply to the macrolayer is not sufficient to compensate for the liquid depletion within the macrolayer. A boundary layer analysis is performed, treating the two phase motion as an external buoyancy driven flow, to determine the liquid supply rate and the local critical heat flux. Model shows good agreement with the available experimental data. The model has been modified to take into account the effect of subcooling and has been validated against the empirical correction factors.