[en] As appears by recent accident of Fukusima, low safety classification of equipment (emergency diesel generator, spent fuel storage pool) have a major impact as the accident conditions. It's necessary to give a supplementary revision about definition of safety functions and safety classification in Korea. And conducting research on the safety classification is necessary to nuclear safety and public acceptance. The need to classify equipment in a nuclear power plant according to its importance to safety has been recognized since the early days of reactor design and operation. The existing methods for safety classification of structures, systems and components (SSCs) have evolved in this light of lessons learnt during the design and operation of existing plants, mainly with light water reactors. Although the concept of a safety function as being what must be accomplished for safety has been understood for many years, and examples based on experience have been provided, the process by which safety functions can be derived from the general safety objectives has not been described. Therefore, it was mainly from experience and analysis of specific designs that classification systems identified those SSCs that were deemed to be of the highest importance in maintaining safe operation, such as the continuing integrity of the primary pressure boundary, and classified them at the highest level. The purpose of safety classification in a nuclear power plant is to identify and categorize the safety functions and to identify and classify the related SSC items on the basis of their safety significance. This will ensure that the appropriate engineering design rules are determined for each safety class, so that SSCs are designed, manufactured, constructed, installed, commissioned, quality assured, maintained, tested and inspected to standards appropriate to their safety significance. ASME (American Society of Mechanical Engineers) requires designers to undertake a number of steps to perform safety classification and to justify the assignment of SSCs to safety classes