Development of simulation-based testing environment for safety-critical software in digitalized nuclear power plant

An issue on incorporating the software reliability within the NPP PRA model has been emerged in the licensing processes of digitalized NPPs. Since software failure induces CCFs of the processor modules, the reliability of the software used in NPP safety-critical I&C systems must be quantified and verified with proper test cases and environment. In order to prove the software to be error-free or have very low failure probability, an exhaustive testing of software is required. In this study, a software testing method based on the MCS-based exhaustive test case generation scheme combined with the simulation-based test-bed is proposed. The software test-bed was developed by emulating the microprocessor architecture of PLC used in NPP safety-critical applications and capturing its behavior at each machine instruction. For the test case generation, the software logic model was developed from the formal definition of FBD/LD and the sets of MCSs which represent the necessary and sufficient conditions for the software variables' states to produce safety software output were generated. The MCSs were then converted into the test sets which are used as inputs to test-bed to verify that the test cases produce correct output after software execution. The effectiveness of the proposed method is demonstrated with the safety-critical trip logic software of IDiPS-RPS, a fully digitalized reactor protection system. The method provides a systematic way to conduct software exhaustive testing while effectively reducing the software testing effort by emulating PLC behavior in machine-level compared to existing software testing methods