6.1.1 The requirement for monitoring

When a reactor unit is running at a steady appreciable power, absolute vigilance is still required although ac­tions are not occurring at the same rate as in the start-up or shutdown phase. Nevertheless, frequent monitoring of conditions and systems is necessary. The frequency of monitoring is preset in the operating in­structions and it is incumbent on the operator to carry out specific items of monitoring at stated times or frequency.

Two risks are present:

• The risk of commercial loss from either lost gen­eration or damage to plant and equipment.

* The risk of a nuclear accident from a rupture of the pressure containment accompanied by the failure of the nuclear fuel.

Although commercial loss is a serious event for CEGB financial considerations, it is not a danger to the public in that they can suffer injury to health, unless accompanied by a nuclear incident. Therefore those items of plant and equipment in which failure cannot result in a nuclear accident are monitored and protected in the same way as any commercial plant. Special considerations to protect the public are not necessary, thus a high proportion of plant and equipment on a nuclear power station is standard equipment similar to that in a conventional station.

The second risk of nuclear incident is the one that gives concern and is treated either to eliminate it or reduce it to negligible proportions (of the order of 1 in 106). Nuclear incidents that can affect the public in any serious way can only result from damage to the fuel whereby fission products are released into the gas stream and gas is lost from the reactor by way of a breach of the pressure circuit. With this effect in mind it is necessary to see how conditions can be monitored to give the operator advance warning or information for him to correct the situation.

The following list gives those items that need special consideration to ensure safe operation:

• Temperature of fuel, core and pressure vessel.

• Condition of fuel, to detect mechanical failure of the fuel cladding material.

• Loss of coolant flow due to circulator failure re­sulting in circulator run down.

• Loss of coolant due to failure in the pressure circuit.

• Run away nuclear power could result in damage to fuel by overheating.

These aspects are considered in the following para­graphs which address the systems that give informa­tion to the operator to enable him to manage his plant. It must be stressed that the probability of fail­ure leading to a nuclear incident Is very small, but it is essential that the operator is able to intercept any conditions which are not normal.