The consent to operate a reactor is given for two year periods. Within a maximum of two years since the licensee was given consent to start it up, it must be shut down for inspection. The object of the in­spection is to give sufficient confidence and assurance that the reactor can run a further two years in a safe and untroubled way.

The inspection looks at all fixtures and fittings that are both inside and outside the reactor pressure vessel. The following is a list showing the type of compo­nents inspected, although the list may vary slightly from location to location: [31]

ness and corrosion, and any signs of failure, es­pecially welds.

• Use of NDT techniques to look at the soundness of welds, especially those associated with pressure vessel penetrations.

• Inspection of any moving part or link within the pressure vessel for freedom of movement and cor­rosion.

• Take samples of graphite core for scientific analysis to look at its condition and deterioration,

• Check fuel channels and control rod channels for straightness to identify any distortion of the core,

• Check the soundness of pressure vessel thermal insulation in the case of concrete pressure vessels.

• Check the insertion rates of entry of control rods under free-fall in a hot reactor core.

• Check the soundness of the pressure vessel cooling system to ensure there are no internal leaks to the concrete pressure vessels.

• Inspect the soundness of gas circulator or boiler gas side isolating valves and ensure they function correctly.

• Check boiler structures and inspect for any boiler leaks that may have been occurring.

• Carry out a debris check in the bottom of boilers and be satisfied that debris is as expected.

This list represents the main reasons for the biennial overhaul, but others may be included at the request of the Nuclear Installations Inspectorate.

Following the inspection, a meeting with the Nil will be held to satisfy them it everything is in order and a further two year period of operation can occur without any major failure or problem. At this meet­ing the results of the findings of both the biennial inspection and the maintenance schedule results will be tabled. There will also be discussions on future work and the expectations of the conclusions from any test carried out. Providing the Inspectorate is satisfied, the chief inspector will issue a certificate to the licensee to give permission for the reactor to be started up. During the process of the biennial overhaul, the inspectors will have been watching the results of all activities and satisfying themselves as the work proceeds.

Within 14 days of the start-up certificate being given, the licensee must have presented a report on the result of the inspection together with all technical data of inspections and work carried out. If any test has indicated that a problem exists remedial work must be identified or it must be shown that the defect has no material effect on safety of the component.