[en] MARCH 2 calculations for the TMLB' accident (a transient with loss of all ac power) at Surry provide a test bed for addressing problems with computer models in an analysis situation typical of those expected in applications of the MELCOR code system for severe accident analysis. This paper describes a sensitivity analysis for the TMLB' accident sequence at Surry with and without loss of coolant from the primary system through degraded pump seals. Screening calculations were performed with a modified version of the MARCH 2 severe accident analysis code, which was coupled with the CORCON MOD 2 code for core-concrete interactions. Uncertainty and sensitivity analysis techniques compiled as part of the NRC-sponsored MELCOR program were used