Materials used for PWR internals include ferritic steel, wrought austenitic stainless steel, cast stainless steel (CASS) and Ni alloys. The internals main­tain the soundness of the geometrical core. The core consists of the upper core structure, core baffle/former/barrel, thermal shield and lower core sup­port structure. The factors which influence degradation of these parts are: thermal plant transient, flow-induced vibration, radiation, high temperature, mechanical and thermal stress, and corrosive coolant. The main degradation mechanisms are: fatigue; radiation and thermal embrittlement; void swell­ing; and irradiation assisted stress corrosion cracking (IASCC). IASCC is a type of SCC indicated by a large quantity of neutrons in a material. The main objective of degradation management in the case of reactor internals is to ascertain if the internals support the core and can protect the CRDM (Morgan and Livingston, 1995).

In-service inspection and surveillance and changing of the materials are some of the measures used to manage degradation of the internals.

It is difficult to inspect the inside of a nuclear reactor, but it is possible to obtain information on physical damage, leakage and mechanical and structural states through visual inspection of the accessible zone. When an in-service inspection is being conducted, all flange closure studbolts and heads are removed. At this time, damaged equipment can also be removed. Equipment moved to the pool or which remains in the pressure vessel can be inspected using a remote control camera. It is difficult to conduct ultra­sonic testing on this equipment or to interpret the results, but eddy-current testing is effective in measuring reduced thickness of pipes. Inspecting the inaccessible zone using the monitoring systems is complicated. Therefore, more effective remote control inspection equipment is needed (Morgan and Livingston, 1995).

The general regulation of in-service inspection of reactor vessel internals in ASME Section XI requires a visual inspection every ten years. Recently the requirement has become a visual inspection (VT-1, VT-3) supplemented with ultrasonic inspection of the baffle former bolts. The baffle former bolts comprise the weakest part of the internals. Supplementary ultrasonic exam­ination is carried out in accordance with ASME Section XI subsection IWB, examination category B-N-3 in the United States and some other countries. Development of the ultrasonic examination equipment used for inspecting these bolts should take into consideration the existence of locking bar style bolts and the accessibility problems.