Introduction  Essential service water system is responsible for providing cooling water to the nuclear island and bringing heat to the ultimate heat sink,implementing safety classification is helpful to achieve the optimal safety function under the premise of economy.

  Method  SSG-30 was the method to classify items according to the importance of their safety functions and the severity of their failure consequences.The security functions of the items were decomposed according to the system process,different functions of the same system could be classified and graded differently, different devices of the same function could be classified differently, and different components of the same equipment could be further graded,functional redundancy could be used to satisfy the single failure criterion.

  Result  Grading results can be used to review system functions,for example, the safety level components must meet the seismic requirements and be equipped with emergency power supply before they can be put into operation.The safety level of items can not be reduced according to the mature "engineering experience" in the design of existing PWR.

  Conclusion  The method of item classification by functional decomposition can provide references for other PWR project design.