Functional Block Diagrams (FBD) are commonly used as a graphical representation for probabilistic risk assessment in a wide range of complex engineering applications. An FBD models the stochastic behavior and cascading dependencies of system components or subsystems. Within FBD-based safety analysis, Event Trees (ET) dependability modeling techniques are typically used to associate all possible risk events to each subsystem. In this paper, we conduct the formal modeling and probabilistic risk assessment of a nuclear power plant in the HOL4 theorem prover. Using an FBD modeling in HOL4 of the nuclear Boiling Water Reactor (BWR), we formally determine all possible classes of accident events that can occur in the BWR. We compare our formal analysis in HOL4 with those obtained analytically and by simulation using Matlab and the specialized Isograph tool. Experimental results showed the superiority of our approach in terms of scalability, expressiveness, accuracy and CPU time.