New-generation Nuclear Power Plants (NPPs) may be exposed to unknown hazards due to novel design-specific features. These hazards may lead to unexpected accident scenarios. In this paper, we develop a framework to integrate Importance Sampling (IS) in Monte Carlo Dynamic Event Tree (MCDET) analysis to efficiently explore potential accident scenarios in NPPs. The framework, called IS-MCDET, accounts for the specific features of new-design safety protection systems. The use of the proposed framework for scenario exploration is shown for a case study concerning the analysis of loss-of-coolant accident scenarios in a small modular dual fluid reactor equipped with a new-design auxiliary cooling system.