The overall purpose of commissioning is to demonstrate that the design requirements of the SSCs are met and to bring them to operating mode. Testing should establish that the NPP can operate in all the modes for which it has been designed. Commissioning is further addressed in Chapter 22.

Commissioning can be divided into four stages (IAEA, 2003d): (1) pre­operational tests; (2) fuel loading and subcritical tests; (3) initial criticality and low power tests; and (4) power tests. These stages are further subdi­vided into subtasks that are required by the licensee or regulator, or that depend on the technology being commissioned.

The pre-operational stage requires that construction activities associated with the system should be completed and documented, including all ele­ments of the quality assurance programme. The construction company nor­mally also carry out various pre-commissioning activities, such as flushing, cleaning and hydrostatically testing each system and piece of equipment individually. Also, the licensee must ensure that all equipment is ready for operation. This involves:

• Inspection of the SSCs to ensure proper construction, manufacturing and installation, such as welding, quality of workmanship, loose parts, and cleanliness

• Checking of electrical and protective devices

• Calibration of instruments

• Verification of operability of instrument loops and required response times

• Adjustment and settings of process controllers and limit switches.

Once the above has been completed, the pre-operational stage can be subdivided into two activities: cold performance tests and hot performance tests. These tests in most cases will be carried out sequentially; however, some cold tests, such as containment pressurization and leakage rate tests, might not be done until the end of the testing period, before fuel loading. Cold performance tests include the start-up of the fluid systems and support systems. The tests yield data that verify the operational functions of com­ponents and the compatibility between systems. If pressure tests on the primary and secondary systems were not previously done by the construc­tion group, the operator will also perform these tests at this sub-stage. Hot performance tests verify that systems conform to design requirements. These tests, where practicable, should simulate anticipated operational occurrences at typical plant operating conditions.

The tests should verify the effectiveness of the various heat transport phenomena as well as checking for vibration, clearances, effectiveness of insulation, thermal expansion, and the effects of high temperature on elec­trical and mechanical equipment performance. Hot performance testing should be carried out at least to the point where steady-state operating conditions are reached. Completion of the initial rotation test of the tur­bine-generators would typically mark the end of the hot commissioning phase. Operating staff should also use this opportunity to verify the operat­ing procedures, such as hot to cold shutdown, before fuel loading begins.