Numerical computation of a reactor vessel melting under external cooling conditions

Numerical computations are performed for melting and natural convection in the liquefied region of a reactor vessel under external cooling to find more thermal margin for in-vessel retention. Existing typical experiment and calculations for gallium melting are used for the validation. The transient flow field in the liquefied region and the melt front movement analyzed are compared with those from finite-element and finite-volume methods. Reasonable agreements are achieved with respect to melt progression and flow configuration in the liquefied zone. A three-dimensional geometrical model for an azimuthally 3° angular section of the APR1400 pressurized water reactor vessel is prepared based on this verification, and a conservative heat flux profile from the corium inside with a concentrated heat flux from the metallic layer of 2.1 MW/m², which is greater than maximum critical heat flux, is applied to the vessel model assuming constant exterior temperatures of 400 and 1000 K. The results show that even though the vessel inside heat flux is much greater than the critical heat flux, this does not intensively melt a vessel due to combined effects of latent heat absorption during the melting and the remaining heat spreading through the entire vessel.