In addition to maintenance and surveillance checks on various equipment and instrumentation items, it is necessary to undertake special periodic in­service inspection (ISI) of certain structures and components that are criti­cal for safety and continued operation of the NPP and are not easily replaceable. ISI results form an important input for the ageing assessment of systems, structures and components for long-term operation of the reactor. Examples of ISI are the assessment of the extent of radiation — induced embrittlement of the reactor pressure vessel through periodic examination of test coupons installed for the purpose, integrated leak rate testing of the reactor containment, periodic assessment of loss of prestress­ing in the reactor containment building, checking of steam generator tubes for thinning and existence of flaws, periodic examination of welds in the primary coolant pressure boundary, and checking of critical piping of the primary and secondary coolant system and steam system for loss of thick­ness or any fatigue-induced degradation. ISI is done by highly trained personnel using special techniques such as ultrasonic testing, eddy current testing and radiography. Some of the inspections, such as those of test coupons for reactor pressure vessel health assessment, may have to be carried out in hot cells of post-irradiation examination laboratories. Thus it is necessary to develop technical competence in the operating organization and in the technical support organization such that ISI can be done follow­ing specified standards and the inspection results can be properly inter­preted to arrive at important decisions concerning long-term operation of the NPP