It will be apparent from our consideration of the magnox fuel cycle that it is essential to carry out the exchange of fuel on a continuous basis whilst the reactor is at power. Refuelling machinery has been designed to meet this requirement and whilst details vary from site to site the basic principles remain the same. A typical cycle of activities for a magnox station is as follows:

• New fuel will have been inspected and loaded to a transfer device. This usually takes the form of a carousel into which the fuel is loaded by hand.

• The refuelling machine is positioned over this trans­fer device and the fuel picked up element by ele­ment using the element grab and stored in suitable magazines within the machine.

• After pressurisation with carbon dioxide to reactor pressure the machine is attached to the reactor via an access point known as a standpipe. Often, a separate coupling device is required between the machine and standpipe.

• The interspace between the machine and the stand­pipe closure unit (shield plug) is pressurised with carbon dioxide to equalise the machine/reactor pressures. This procedure allows the closure device to unlock from the standpipe enabling the plug to Pe withdrawn tor >torage. In some cases, a grab ^uidinti device (charge chute) is loaded to the stand­pipe.

• Irradiated fuel is withdrawn from the selected chan­nel and stored within the machine. This is followed by loading of new fuel to the discharge channel.

• The discharee/charge of fuel is repeated as required until all selected channels have been dealt with,

• On completion of refuelling, the closure device is reloaded to the reactor and the standpipe/machine interspace depressurised allowing the shield plug to be relocked into the standpipe. The machine moves off the standpipe, is depressurised and the irra­diated fuel discharged from the machine to the storage ponds.

The more recent machines have been designed such that, in addition to carrying out the exchange of fuel, they are able to handle standpipe shield plugs, charge chutes and control rod shield plugs. These items are stored in the service turrets of the machine, A typical machine is shown in Fig 3.40.

The management of this cycle of activities is directed to achieving the following objectives:

• Ensuring that the selected channels are visited and discharged/charged as required.

• Ensuring that the irradiated fuel is safely discharged to the storage ponds.

• Ensuring that all operations are carried out with no effects on personnel or the environment.

The control of these objectives is embodied in ad­ministrative and operational procedures and in equip­ment design. In the first instance, documentation plays a major role in establishing the selection of the cor­rect type of fuel. (Reference to Section 8.4 on docu­mentation will indicate the extent to which this form oi control is employed.) Usually, during the discharge/ charge sequence, the operator has sight of the new tuel ма TV cameras and the distinguishing features ot polyzonal herringbone fuel and LTA/HTA type dі! Ierences can be identified.

The selection ol the correct channel is dependent to some degree on the design of the machine control circuits. However, each channel is provided with a bur>i cartridge detection (BCD) sample point and the behaviour of the channel signals can be used to con­orm the correct channel selection. A decrease in signal occurs as tuel is discharged or, alternatively, a signal change is induced by the insertion of a contaminated probe to the selected channel. Either method enables correct charge visitation to be confirmed. Usually the refuelling machines are provided with grab weight and height instrumentation. Observation of the erab weight-height behaviour is used to account for ele­ment movements.

All refuelling operations are conducted with close health physics monitoring of the activities for radio­logical and carbon dioxide hazards. Particular atten­tion is directed to the operation^ involving the cou­pling or uncoupling of the refuelling machine to the standpipe and during the discharge of the irradiated fuel to the ponds.

On-load refuelling has associated with it special pro­blems which influence the operation of the reactor as opposed to the handling of fuel and these must be taken into account during the refuelling process. In this context, recognition must be given to the fact that the fuelling machine when it is coupled to the reactor becomes an integral part of the pressure vessel. Also, the handling of new or irradiated fuel within the reactor is effecting a local change in the core which may result in variations of nearby temperature instrumentation.

In the first instance, the coupling of the refuelling equipment to the reactor via a standpipe is accom­plished by the equalising of a gas pressure above some form of closure device (Fig 3.41) with that of the reactor gas below that device. This process releases pins or ball catches to allow the closure device and shield plug to be withdrawn from the standpipe. Too great a pressure differential across the closure unit may result in seals being damaged or rolled, with the consequence that it may be impossible to remove the fuelling machine without a reactor shutdown and de- pressurisation. Although it is usual to carry a spare closure unit within the refuelling machine, tilting of the closure unit resulting from adverse pressure dif­ferentials or machine misalignment can result in the same situation due to the failure to withdraw the unit or re-seal it. In many cases, the reactor gas pressure is employed to maintain the react or/machine seal faces against the event of loss of operating gas pressure. With such designs, there may be operating conditions in which a lower than normal gas pressure is insuf­ficient to maintain this seal and a proper machine connection cannot be sustained, resulting in leakage of gas at pile cap level.

Fuel is handled by a grab constructed from steel components and on its insertion in a fuel channel re­presents the loading of some 13.6 kg of absorber into the core. The local temperature transients caused by the insertion and removal of the grab during re­fuelling may require special measures to be adopted to avoid excessive variations in the signals from safety line thermocouples.

The refuelling machine control circuits are designed so that any one feature is monitored by two inde­pendent means. Using the equalising of gas pressure

RETRACTABLE MAKE-UP SHIELDING

Fig. 3.40 Typical magnox fuelling machine

across the standpipe closure unit as an example, in one case the direct measurement of the pressure dif­ferential across it is to be zero and in the second instance the relative movement of the seal’s mechani­cal components is monitored by position switches which indicate that the seal is unlocked. When these two conditions are met the service hoist circuit is energised allowing the closure unit to be withdrawn.