Plant ageing if uncontrolled may increase the probability of failures, possibly leading to accidents. Examples of safety implications of unchecked ageing are hydrogen pickup during normal operating conditions (NOC), the formation of hydride blisters on the pressure tube

leading to pressure tube rupture, PT sag leading to PT/CT contact, flow assisted corrosion (FAC) in piping and outlet feeders, leading to leaks and breaks.

There are failures that are not covered in reliability studies. These are failures that typically go undetected and therefore unpredicted by system testing such as in the case of the MOV torque switch settings. As friction increases settings are increased and may be increased to the point where the motor seizes or other system failures occur. Another similar instance is instrument loop reliability and the effects of the instrument error on performance and safety. Signal drifts, as the instrument ages, may lead to out of specification signals and failures, for example neutronic instrument dynamics and response.

Safety system settings can also be affected by ageing due to changes in process conditions (trip margins). Similarly, during an accident, safety system settings can be affected by the ageing state of the equipment and by changes due to the progression of the accident itself such as the case of the pressure tube (PT) blisters, stress corrosion cracking (SCC) of boiler tube and subsequent leaks, cable deterioration due to radiation exposure, material properties (affecting their ability to withstand failures), PT diametral creep creating flow redistribution in the core, bundle bypass, etc. whereby the critical channel power (CCP) effect is initially positive and then goes strongly negative.