As the importance of cyber security for nuclear power plants has recently increased, related organizations such as manufactures, operators, regulators, and research institutes have taken into consideration the many concerns for effective application of cyber security to critical digital assets at nuclear power plants. It is essential to analyze and research how to effectively apply nuclear cyber security requirements and what complementary factors are present in the overall process for a nuclear security technology system. The complementary points of nuclear cyber security technology could be analyzed through diverse cyber security methods such as cyber security assurance, assessments, and software development life cycle. It is necessary to apply nuclear cyber security technology effectively in accordance with the operation, assurance, and system development process of a nuclear facility. These analysis results could be applied to technical points for the cyber security of critical digital assets at nuclear power plants in the future and to technology that needs to be at the appropriate level of cyber security to ensure technical assurance. This paper is a new approach to compare and analyze various methods used in the cyber security field to discover complementary points for the application of cyber security to critical systems in nuclear power plants. Additionally, this paper also presents effective methods for developing and applying, evaluating, and regulating cyber security in nuclear digital critical systems using the complementary points of various cyber security methods identified in the analysis.