The need to carry out maintenance on in-core items resulted from the oxidation problems in 1968. At that

time, fractured bolts were discovered within the core area of the Bradueil reactors. The bolts were identi­fied as being associated with the steel sample baskets fixed to the roof of the reactor. Equipment was pro­duced and used to torque-off remaining bolts and to remove the brackets from the reactor. It was also found necessary to reinforce the top restraint beams of the core. This was effected by the placement of new beams onto the existing beams. The beam end> were coupled by a tongue and tee-slot coupling and the whole ring structure tensioned into place. An awareness ot the oxidation problems prompted theo­retical assessments of in-core components at all the CEGB reactors. The assessments identify areas re­quiring attention, and subsequent monitoring by in­spection is used to ensure their continued satisfactory condition. Where areas are identified as being prone to failure, programmes of repair or reinforcements are initiated. The programmes have individually led to the development of an extensive range of purpose — built equipment. Most engineering bench operations have been carried out within reactors and include drill­ing, grinding, cutting and welding. There has been only a limited exchange of equipment between stations because each reactor is of unique dimensional design, necessitating the development of purpose-built equip­ment. Repair packages in themselves frequently require the provision and development of built-in viewing/ illumination devices.

It is not possible to detail all the repairs that have been carried out on the CEGB reactors, so comment is confined to a number of examples. Further details may be obtained by reference to the proceedings and symposia of the institutional bodies, in particular the Institutions of Mechanical and Nuclear Engineers and the British Nuclear Energy Society.

A repair at Bradwell necessitated the removal of- . a component from the lower end of the standpipe. The component (latch assembly) is larger than the standpipe internal diameter and this required it to be reduced in size by plasma arc cutting to facilitate its removal. Retaining bolt nuts were torqued off, the latch ring lowered to a cutting table and then reduced in size by plasma arc cutting. The debris was held within the device which was then withdrawn from the reactor for disposal of the pieces. Thirty operational — equences were required for each latch and a com­puter prompt program was used as an operational guide.

An early ‘routine’ repair operation at Oldbury re­quired the removal of the jubilee clips used to clamp thermocouple runs and their replacement by suitably designed clips. The runs were located at eight sites tn each reactor and extended over some 3 m of the mield wall. A manipulator (Fig 3.59) was designed with >een functions of movement. A variety of tools be attached to the arm of the unit so that up to 1- different operations can be carried out. There being ■wwera] hundred Jip$ in each reactor necessitated the

work being spread over a Ю-year period. Work was carried out on a 24-hour shift cycle for about 10 weeks at each shut down.

Using the same manipulator another ‘routine’ task was that of fixing a flanged assembly to the boiler shield wall (Fig 3.60). In this case a repair package was designed to drill holes through the flanges and into the wall, insert and butt weld a retaining stud and to finally nut-torque the retaining plate into position. The manipulators are powered by air motors and capable of a 36,3 kg pay load at 2 m radius.

Retaining plates in the reactor roof were predicted to fail by weld cracking (via oxide jacking). A more powerful hydraulic manipulator was build for this re­pair — 90.7 kg at 2.75 m radius (Fig 3.61). The unit handled a drilling package and a bolt loading package and this was used to drill holes and to insert and torque-up the bolts. Again, a large number of bolts were involved and located around the periphery of the reactor roof thus requiring the work to be extended over a number of years.

Inevitably, welding repairs will be required within the core space of reactors, and work on a remotely controlled welding process has developed to the stage where extensive repairs have already been completed. The system known as ‘Warrior’ (Welding And Repair Robot In Oldbury Reactor) consists of a heavy duty manipulator known as the ‘serving manipulator’. This handles a six movement of freedom ‘work performing manipulator’. The latter carries a MIG welding head incorporating a laser ranging system (Fig 3.62).