The commissioning of a nuclear power station extends from late in construction until full commercial op­eration is achieved. It is the process of setting the plant to work, providing evidence (as far as practicable) that the construction has been correctly carried out and that the design intent is met. It is also necessary to make certain that testing is sufficiently compre­hensive to demonstrate to the Licensing Authority that the plant is safe to operate and ensure that at all times during the commissioning process the appropriate safety constraints are observed. The administration of commissioning for a new power station in the CEGB is laid down in the ‘Plant Completion and Station Commissioning Procedure for New Power Stations’. However, for nuclear power stations there are also statutory requirements.

The site licence requires the licensee to set up a committee, known as the Station Commissioning Com­mittee (SCC) to administer the testing. There is also a requirement to divide the commissioning programme into stages which reflect the increasing nuclear safety implications in progressing through fuel loading, initial power raising and finally to full power operation. The licensee is not allowed to proceed from one stage of the commissioning programme to the next without the

explicit consent of the Health and Safety Executive <HSE).

The SCC is usually set up some two years before the expected fuel loading date. It is chaired by the station manager and has representatives from the con­tractor, GDCD design and project management, the health and safety department, the headquarters opera­tions department, the research department and the staff from the region in which the station is being built.

A second committee is also set up, the Plant Com­pletion Committee (PCC) responsible for ensuring that the preliminary testing of plant items is correctly carried out. The PCC is not specifically mentioned in the site licence but clearly is extremely important in laying the ground work for the activities of the SCC. It is chaired by the project manager who is respon­sible for the construction of the station and has a more site-orientated representation.

The SCC calls for Papers of Principle to be writ­ten for all the testing required and gives these papers its formal endorsement. These documents, which may also cover the PCC tests, are then furnished to the HSE. Detailed test schedules are drawn up which provide the step by step instructions to the commis­sioning team based on the principles already endorsed.

The detailed schedules for the PCC tests are en­dorsed by the PCC and those for the SCC tests by the SCC. As each test is finished, a Test Completion Certificate is drawn up and signed by representatives from the contractor and the operators. Where the test is more than a simple set of checks ticked off on a check sheet, a test report is written which not only describes the conduct of the test but lists any reservations or qualifications arising from the tests. If a reservation can only be. cleared by modifying the plant, such modifications must be processed using an agreed procedure. This procedure may involve the Nuclear Safety Committee and, if the modification is of sufficient safety significance, the approval of the HSE may be required before it is implemented. The SCC is then responsible for commissioning the modi­fied plant in the normal way. The test reports are endorsed by the appropriate committee who also agree on any action required concerning the reservations. The SCC test reports are furnished to the HSE and provide the necessary justification to them that it is safe to proceed to the next stage of commissioning. The HSE may wish to witness certain tests which they nominate in advance. It is the responsibility of the SCC to ensure that the Nil Site Inspector is informed when these tests are to take place with sufficient notice to allow him to be present. Failure to do this may result in the need for the test to be repeated with the inspector present.

The stages of the commissioning programme usually take the following form:

Stage 1: Core dryout cold unfuelled engineering test,

hot unfuelled engineering tests, control rod

drop tests and safety circuit checks. During this stage a very small amount of fuel may be loaded to check its mechanical behaviour under gas pressure and flow conditions.

Stage 2: Load full charge of fuel, physics tests at low

power, hot fuelled engineering tests. Fuel­ling machine test, control rod drop tests, check safety circuits.

Stage 3: Raise power to some intermediate value

usually less than 50% power, steam to main turbine-generator, reactor trip.

Stage 4: Raise power to maximum attainable, tests on

entire system under operational conditions.

The commissioning period usually terminates with a 72-hour run at maximum power and at that time, with the agreement of the HSE, the SCC is usually disbanded. If, however, there appears the need for major modifications to the plant during the course of commissioning, the work of the SCC may be ex­tended until this work is complete and the modified plant is commissioned.