Nordic Boiling Water Reactors employ filtered containment venting and ex-vessel debris coolability in the deep pool located under the reactor pressure vessel as cornerstones of their severe accident management strategy. This paper focuses on the uncertainty analysis of the source term in accident sequences that result in filtered containment venting to the environment using the MELCOR code. The impact of uncertain MELCOR modeling parameters and modeling options on the timing and magnitude of the source term released to the environment has been evaluated in accident sequences initiated by a large break LOCA and SBO. The performed simulations illustrate the effect of MELCOR modeling parameters and options on the code's predictions of severe accident progression, event timing, and the magnitude of the source term released to the environment in different accident scenarios. Furthermore, the results highlight the importance of various retention mechanisms that limit the release of fission products into the environment.