**4. Framework of the proposed method**

The proposed framework has six steps shown in **Figure 2**. Step 1—based on the system requirement we model the stochastic Petri net.

In step 2—by executing the PN model, we generate possible tangible states. Based on the tangible states, we construct the reachability graph in step 3. In step 4—obtained Markov chain form reachability, the graph of SPN. In step 5, we estimate the reliability of the ISO system. In step 6, we validate the result with real-time operation data of NPP.
