Skip Navigation LinksIn Service Inspection (ISI)

 

1. What is ISI?

On an AGR, In Service Inspection (ISI) refers to the periodic inspections carried out within the reactor pressure vessel to ensure that no changes have occurred which could threaten safety or economic operation. In most cases this is carried out using remote handling equipment because of the difficulties of access and the constraints of heat and radiation, although man access is employed in some cases.

The remote handling equipment can vary from simple hoist mechanisms or more complex manipulators and the inspections are usually carried out using television cameras and the results are recorded onto laserdisc or video. In same cases, photographic cameras are also used.

Because the results from ISI are used to support the reactor safety case, particular base is taken in recording, interpreting and logging of the results. During inspections, there is a close liaison with independent assessors and the Nuclear Installations Inspectorate (NII).

2. Why is ISI carried out?

ISIS is carried out to check that there has been no degradation of the reactor internals which might affect the safety case or continued operation of the reactor, or to give early warning of such a problem. The items subject to inspection are set out in a maintenance schedule, and for each inspection, a set of detailed inspection proposals is agreed with NII.

Some potential degradation mechanisms will be readily visible, such as fretting, wear, debris, carbon deposition, cracking, oxidation etc. In other cases, the degradation mechanism itself cannot be detected by the techniques used for ISI and it is necessary to check for the resulting damage or distortion to the reactor structure – such mechanisms include creep, fatigue, corrosion and radiation embrittlement.

3. When is ISI carried out?

Inspections are carried out with the reactor shut down. Conventional inspection equipment could not operate under the operating conditions of temperature, gas flow and pressure. In most cases, inspections are carried out to a planned schedule during statutory outages every two or three years. Typically these last about 20 days. Inspections are usually carried out with the reactor depressurised and purged to air at temperatures between 50 and 80°C. In future in order to shorten outage times it would be beneficial to carry out the work at higher temperatures, with the reactor still containing its carbon dioxide coolant.

4. How is ISI carried out?

ISI is carried out using a variety of tools including camera systems, remote manipulators and man access.  These as described in the context of the following chapter.

Note that ISI refers to the inspections of the reactor core and of the pressure vessel.  To differentiate between the two, headings with “Reactor Core Inspections” embedded in the title refer to equipment and practices used to inspect the reactor core itself (i.e. fuel channels, bricks, etc), whereas other items described as “Pressure Vessel Inspections” refers to equipment and practices used for inspecting the remaining components of the pressure vessel (i.e. boilers, core restraints, etc). Some equipment is applicable to both areas – such as the camera units.