In the Finnish nuclear industry, model checking, a formal verification technique, is used as an additional means of safety assurance for instrumentation and control (IC) system design. Since the code of vendor-specific basic function blocks used in IC is commonly closed, these blocks need to be modeled manually based on available specification. This modeling introduces an additional source of human factor into the verification process. To increase the reliability of the library of basic blocks used in nuclear IC verification, we apply formal synthesis techniques, which can construct finite-state models of reactive systems from behavior examples and temporal properties. Since these techniques have computational limitations and synthesized models are hard to understand even by an analyst, we do not use them in the final verification process. Instead, in an iterative process, behavioral differences between a synthesized model and a manual model implementation are identified and used to create a list of features of manual implementations which either violate the specification or show that the specification is ambiguous.