Usually power assessments are carried out and re­corded every eight hours. This enables the operator to look at reactor power historically and at the same time gives an indication of the relative accuracy of his reactor desk instrumentation. In addition it gives an accurate indication of normal reactor power so that a true estimate of xenon can be made if shutdown occurs.

6.2 Shutdown of a reactor system

A reactor shutdown may occur under these principal situations:

* A reactor may trip due either to fault conditions or to conditions arising that require the reactor to be tripped.

* Controlled shutdown where there is no urgency to complete the sequence and prior consideration can be given to each step.

* Emergency shutdown where it is essential to close the reactor down but does not warrant the manual trip button being operated to give immediate shut­down.

Where possible the controlled shutdown is preferred for two reasons, firstly, it is not good practice to thermally cycle the cores and structures of reactors since this accelerates wear and tear, and secondly, a controlled shutdown gives the operator more time to prepare and think out his course of action thus avert­ing any mistakes which may have an adverse effect on plant and machinery.