Non-replaceable core components control the service life of the nuclear power plants. The structural integrity and functional capabilities of these components need to be ensured during all operating states of the plant. To demonstrate the capabilities of the core components for Long Term Operation (LTO) of Pressurized Heavy Water Reactor (PHWR) and Boiling Water Reactor (BWR), Time Limited Ageing Analyses (TLAAs) are performed considering time dependent potential degradation mechanisms. Major degradation mechanisms considered for the assessment are neutron embrittlement, environment assisted fatigue, thermal embrittlement, irradiation assisted stress corrosion cracking and corrosion. Assessment is primarily based on two parameters viz., a time dependent parameter used to define time scale for the analysis and the other parameter is analysis parameter which provides the criteria for acceptability of the component for the extended period of operation. The core components of a typical BWR and PHWR have been assessed for the applicable degradation mechanisms to demonstrate structural integrity and functional capabilities for their entire service life, including LTO, while retaining the same safety margins as established during the design stage. For BWR, the components considered for the assessments are reactor pressure vessel and its internals, bottom head control rod drive assembly, reactor pressure vessel nozzles and recirculation piping. For PHWR, the components considered are Calandria-End shield assembly and its supports, inaccessible Calandria nozzles and connected piping. Comprehensive assessment of these components is carried out based on either the threshold values specified in the screening criteria and/or detailed assessment of the flaws observed during in-service inspection or postulating a flaw at critical location of the component. Screening based evaluation has been carried out based on the results of experimental studies conducted under simulated reactor condition or experience-based database, whereas, assessment of the observed or postulated flaws is carried out based on ASME Section XI requirements to demonstrate the structural integrity of the component. Based on the assessment, structural integrity and functional capabilities of the core components are demonstrated for the extended period of operation or LTO.