Releases of radionuclides and the production of aerosols during the molten core/concrete interaction (MCCI) phase of degraded core accidents in light water reactors are termed “ex-vessel releases.” The VANESA and METOXA codes were respectively developed by the U.S. Nuclear Regulatory Commission and the Industrial Degraded Core Rulemaking (IDCOR) program to quantify ex-vessel releases. Comparison of calculations by VANESA and METOXA (under identical initial and boundary conditions) show that except for niobium and strontium species, the predicted ex-vessel radionuclide release rates are within an order of magnitude of each other. In an actual application of these two codes to the source term quantification of severe accidents, the initial and boundary conditions for the calculations could be significantly different, as demonstrated in an analysis of an anticipated transient without scram accident sequence in a boiling water reactor. For the same amount of debris, the MCCI thermal-hydraulic results provided for METOXA from a DECOMP calculation tend to drive more radioactive material from the debris pool than those provided for VANESA from a CORCON/MOD2 calculation. The MAAP code, however, predicts that less mass is involved in the MCCI.