This paper presents the architecture of the Safety Programmable Logic Controller (SPLC) for advanced nuclear safety systems and describes the evaluation and analyses of reliability for the SPLC using the Markov model. The SPLC is designed to have structural flexibility for users to select module redundancy according to the requirements of specific applications. To be used for the nuclear safety system, the SPLC is configured for multiple modular redundancy composed of dual modular redundancy and triple modular redundancy. Markov models were developed for three types of existing safety-grade Programmable Logic Controller (PLC) architectures and the SPLC, and the reliabilities of the architectures were then evaluated and analyzed using the models. The results show that the reliability of SPLC is up to 1.6 times better than those of the three PLC architectures, and the mean time to failure (MTTF) of the SPLC is up to 22 000 h better than those of the three. From the reliability analyses, the failure rate of each module in the SPLC should be <2 × 10-4 /h, and the MTTF average increase rate depending on the fault coverage factor (FCF) increment, i.e., MTFF/FCF, is 4 months/0.1.