Safety guide for in-service inspection of nuclear power plants
1 Introduction
The Regulations on operation safety of nuclear power plants (HAF0300, hereinafter referred to as “the Regulations”) has put forward the principle requirements for the in-service inspection of nuclear power plants, and this guide is an explanation and supplement to the relevant provisions in the Regulations.
This safety guide is a guideline prepared for instructions on the scope, acceptance criteria, evaluation of inspection results, equipment, techniques and methods used in inspection, organization and management for the in-service inspection of nuclear power plants. Methods and schemes different from those given in this guide may be adopted in practice, provided that it is proved to national nuclear safety authority that those adopted in practice are at the same safety levels as those given in this guide and they will not pose more risks to the public and personnel at the nuclear power plant site.
1.1 General
1.1.1 During the service life of a nuclear power plant, components may be subject to multiple effects, and it is difficult to predict what effects a single component or combined ones may have on the service life of nuclear power plant with the accuracy required for nuclear safety. The most important effects are stress temperature, irradiation, hydrogen adsorption, corrosion, vibration and wear, all of which depend on time and history of power plant operation. These effects will cause changes in material properties, such as the formation and/or development of aging, embrittlement, fatigue and sunken. Defects refer to imperfections, discontinuities, irregularities or damages in component materials, such as cracks, laminates, weld inclusions or pores.
1.1.2 Therefore, it is necessary to inspect the systems and components of nuclear power plant for possible damage, so as to judge whether they are acceptable for the continued safe operation of nuclear power plants or whether it is necessary to take remedial measures. The inspection focuses on the key components and systems of reactor’s main coolant system, since they are important for safety and a failure may cause serious consequences. It shall be noted that the components given in this safety guide belong to a complete item of nuclear power plant, such as piping, a pump, a valve, and a vessel. In such terms, piping is a component consisting of pipelines, pipes, pipe fittings, pipe fittings, supports and hangers, etc.
This guide defines such inspection during the service life of nuclear power plants as "in-service inspection", and recommends relevant main methods, frequency and management measures. Where appropriate, in-service inspection also includes leakage test of system.
1.1.3 For in-service inspection, it is necessary to take appropriate measures in the design of nuclear power plant to make inspected components accessible and to keep the radiation exposure to inspectors as low as reasonably achievable (ALARA).
The scope and rigorousness of in-service inspection must be related to the safety importance of the inspected and tested systems and components. This issue may be determined with the safety guide HAF0201 Safety functions and component grading for boiling water reactors, pressurized water reactors and pressure tubular reactors.
It is necessary to select relevant acceptance criteria for inspection and test and corrective actions, such as repairing or replacing unaccepted components after the inspection. The safety levels specified in the design of nuclear power plants may be used for prioritizing safety importance of systems or components, and for dividing the level of in-service inspection.
In-service inspection program is an integral of measures taken by the nuclear power plant operating units to ensure the safe operation of nuclear power plants.
1.2 Scope
1.2.1 This guide outlines the requirements for preparing in-service inspection program for the key systems and components referred to in 1.1.2.
1.2.2 In addition to recommending the scope of in-service inspection of these systems and components, this guide also provides general guidance on the methods, techniques and minimum frequency of inspection and test. This guide proposes sampling method as a method to inspect the same defects of the same components, explains the evaluation method of the obtained results, and specifies the acceptance criteria. Where necessary, it requires repeated or supplementary inspection and test. Where inspection and test results are unacceptable, it recommends necessary corrective actions.
1.2.3 The in-service inspection program includes the inspections and tests to be carried out during the service life of nuclear power plants. It shall be noted that, in order to implement the program successfully, inspections, tests and evaluations must be completed prior to the operation of nuclear power plants to provide benchmark data with which the results of inspections and tests in the in-service inspection program may be compared, so as to evaluate the possible change of defects and the acceptability of components.
