The typical checks that are given in the following paragraphs are those that may be required for either дОК or magnox power stations. For a wide descrip­tion it is considered that the starting point commences with the reactor vessel depressurised, filled with air and all control rods except the safety group inserted. It is also assumed that all plant commissioning checks and tests have been completed. The following are typi­cal of the basic requirements the operator will need to be satisfied about before he proceeds to prepare the plant for start-up:

• Relevant maintenance and testing work completed on all available plant.

• Access to pressure vessel sealed and all joints made.

• Radiological de-zoning due to shutdown removed.

• All relevant safety documents (safety rules fourth edition and radiological) cleared and cancelled. All isolations removed and plant ready to set to work.

• Safety circuits — trip and alarm levels restored to required values for start-up and safety line trip setting certificate received from the instrument en­gineer.

• Temperature scanners alarm settings restored to normal for normal reactor operation.

Once satisfied on the above general activities, he will then proceed proper to the pre-start-up phase_and will consider the following:

• Boiler and feed system — minimum number of boilers to be totally available as specified in the operating instructions.

• Circulating water system — all systems in service with the required items of plant available.

• Main water treatment available and reserve feed — water tanks full with water at the correct purity for operation.

• Pressure vessel cooling system in service with water at correct purity.

• Sufficient gas circulators available as specified in the operating procedure and all commissioning sche­dules completed.

• All pressure closures (fuelling and control rod stand­pipes) seated in the correct position.

• Fuelling machine available for service.

• Emergency generator available as specified in the Operating Rules.

Having been satisfied that the above items are com­plete, carbon dioxide can be admitted to the vessel. During filling to the design density, checks will be made on the pressure circuit for leakage and any other abnormality.

The points of interest are leakage from the fol­low ing:

• Any pressure flange or dome at its joints.

• Pipework which has connection to the pressure vessel or boilers, e. g., oil pipework on circulator, major valve lands and standpipe assembly closures.

If these checks ave satisfactory then proceeding to the start-up is commenced. During this period, imme­diate:) prior to commencing the withdrawal of control rods, the following checks are made:

• That correct carbon dioxide purity is achieved (the maximum quantity of air is not exceeded — this is achieved by successive purging).

• That safety circuits, counter channels, flux channels and alarm systems are correctly set, active and in service.

• That temperature scanners are in service.

• That instrumentation is calibrated and in service.

• That burst cartridge detection equipment is in serv­ice and sufficient systems are available to satisfy the Operating Rules.

• That control rods are in the correct start-up po­sition and the sequence interlocks of control rod movement are checked and proven.

• That the reactor trip function operates and all electrical supplies are cut off from the control rod mechanisms.

When all the physical checks are completed on plant and equipment then it is necessary to satisfy technical considerations of limiting temperatures within the re­actor core and vessel. The temperatures which are sig­nificant and must comply with the limits as imposed by the Operating Rules are:

• Pressure vessel temperatures (concrete for concrete pressure vessels plus differential temperatures be­tween adjacent zones).

• Liner temperature (concrete pressure vessel).

• Fuel temperature (specifically for magnox reactors).

• Moisture in coolant gas (water or hydrogen in

CO:).

• Bulk reactor gas outlet temperature.

If all the above considerations satisfy the operator, the operating instructions and the Operating Rules then reactor start-up to criticality may proceed. Cri­ticality should be achieved close to the prediction, but if this does not occur {i. e., criticality is too soon or does not occur within ±200 milliNiles), then the se­quence must be stopped and investigated. Criticality prediction is discussed in Section 5.2 of this chapter.

Start-up checks during approach to power During the approach to criticality close observation of all operating parameters is essential. All control rods in the same groups should move in line and the dif­ference in height between rods in the same group should not vary by more than a few inches.

The most critical and potentially hazardous phase is at the approach to criticality. This is because the deep penetration of rods produces criticality low down in the core. Since most of the instrument thermo­couples are housed in the top of the core, deviation of temperature at the bottom can be obscured from the operator. It is necessary for the operator to be extremely vigilant at this time. At criticality the power level will be flattened-off at about 100 kW thermal to allow the operator to make a true assessment of the conditions within the reactor before proceeding.

Methane is a colourless flammable gas and is the first member of the alkane series of hydrocarbons. It is soluble in alcohol and ether and slightly soluble in water. It is the major component of natural gas and is normally shipped as a compressed gas or occasion­ally in bulk as a cryogenic liquid. It is used in the production of ethanol, methyl chloride and methylene chloride. It is not considered a toxic gas but acts as an asphyxiant.

Oxygen is a colourless non-flammable gas essential for combustion, vigorously oxidising many elements directly at room temperature or aboe. it is slightly soluble in water and more soluble in organic solvents. It is used in many industries including iron blast fur­naces, glass manufacture, ТіСЬ production and in the direct oxidation of ethene to ethylene oxide. Oxygen is normally produced by the liquefaction-distillation of air and is supplied both as a gas and a liquid. Argon and oxygen are not completely separated by this process because of their relative boiling points. The argon under irradiation will transmute to the radioactive Ar-41 and hence oxygen for magnox and AGR use is produced elect го I у t icall у to minimise the argon concentration.

Carbon dioxide is used by many industries and’is supplied to the CEGB in liquid form, being delivered to each station in road tankers. It is stored in refri­gerated and insulated vessels at a pressure in excess of reactor operating pressure. The liquid is evaporated by passing the carbon dioxide through a steam jacketed heater and passed to the reactor through reducing akes.

The specification for the methane supplied to the

reactor is:

Maximum permitted concentration 20 ppm

Impurity Maximum permitted concentration

20 vpm 1 vpm 1 vpm (total) 10 vpm

Nil

I vpm 100 vpm Not specified Not specified Not specified

The methane used in an AGR can be supplied from two sources; either manufactured on site or delivered to site as a compressed gas or in liquid form. In both cases care has to be exercised in the transport of the methane to the main circuit due to its explo­sive properties when mixed with air. The flammability limits for methane are 5-15.4 Vo in air. This may invoke the safe routing of pipework, the use of double pipe tubing or its dilution with an inert gas (carbon dioxide) to below the lower explosive limit and the adequate ventilation of rooms through which the gas is transported.

The method of manufacture on site is by electrolysis of water using conventional electrolysis plant to form hydrogen and oxygen. The hydrogen is washed to remove alkali carryover and is mixed with either the stoichiometric or an excess quantity of carbon dioxide and passed over a two stage 0.5°!) ruthenium on an alumina support catalyst bed maintained at a tempera­ture of 350°C. Between the two stages is a cooler/ drier to remove moisture and to increase the effluent methane concentration from typically 49 Vo after the first stage to 60 ‘/о (dry volume) after the second stage (less than 5 Vo effluent hydrogen concentra­tion). The gas is then dried and compressed to above reactor pressure for transfer to the reactor gas cool­ant. The above processes follow* the following chemical reactions:

2H20 2H; + Cb and 4H2 + C02 CH4 + 2H2O.

The primary advantage of this process is that the requirements for methane and oxygen (excluding the small imbalance due to reactor leak rate and carbon monoxide production from graphite corrosion) is met from a readily available source, namely water, which is exactly balanced by the water removed in the drier system. The disadvantages of the process are the costs and difficulties associated with constructing and op — erating/maintaining a chemical plant compared to the much reduced initial costs of installing and maintain­ing bought-іп supplies of methane and oxygen.

Various attempts have been made to identify chemi­cals which will either inhibit the extent of corrosion beyond the required passivation layer, or selectively, inhibit the mechanism of corrosion product release by modifying the structure and adherence of the oxide film. A further possibility is the use of chelating re­agents to continuously and selectively remove active species by dissolution from deposited material and oxide films. Such soluble species could then be re­moved on an ion exchange bed possibly with regenera­tion of the chelating agent.

Both of these approaches are under development and will have to be proved effective and acceptable to the existing complex primary circuit chemistry be­fore they can be recommended for use by reactor operators.

Ail the magnox stations except Wvlfa use cooling t‘■akh about 6 m deep for spent fuel storage in Tip’- tor a minimum ot some three months before Tspateh, this allows the post-irradiation heat and radiation to decay to an acceptable level. Water is a cheap and elfective medium for cooling and shield­ing whilst allowing the handling operations to be observed.

A typical sequence of events is shown diagramma — tically in Fig 2.29, which shows the irradiated fuel handling and storage arrangements at Oldbury. There are essentially three basic routes depicted on the dia­gram; the incoming flask and skip route (stages /-6), the outgoing irradiated fuel route (stages 7-17) and the splitter flask route (stages 18-26).

A brief description of the facilities required and the 26 stages shown on Fig 2.29 is given as follows:

Incoming flask and skip

1 An empty skip arrives inside a road transport flask.

2 The flask is transferred to the storage bay by the flask crane.

3 When required, the flask is transferred to the washdown bay where it is washed, the lid bolts removed and the lid jacked open.

4 The prepared flask is transferred into the dispatch bay of the cooling pond: the lid is removed and returned to the decontamination bay, and the empty skip is removed from the flask and trans­ferred to the pond storage bay by the skip crane.

5 Flask lid seals are inspected, if defective, the lid is decontaminated and transferred to the leak testing bay where seals are renewed.

6 When required, the empty skip is transferred to a pond handling bay.

Outgoing irradiated fuel route

7 Fuel elements and bottles discharged from the reactor arrive in the unloading tray in the cooling pond. From here they are transferred to a storage skip by manipulators.

8 When full, the skip is moved to the pond storage bay for a cooling period.

9 After cooling (for at least 90 days), the skip is returned to the pond handling bay w’here elements are removed to check that the correct cooling period has elapsed.

10 Polyzonal fuel elements are desplittered and placed in a second skip. Herringbone fuel elements are not desplittered and are placed in a separate skip,

11 When a skip is ready to leave the pond, it is transferred to the caesium sampling position in the storage bay.

Fig. 2.29

Irradiated fuel handling and storage arrangements at Oldbury

A Road Transport Flask containing an empty Fuel Element Skip arrives at Oldbury on a Low Loader and is transferred to the Storage Say by the Flask Crane.

When the Flask is to be processed it is transferred to the Washdown Bay for a Pre-Ponding Wash. The Lid Bolts are removed and the Flask Lid is jacked open.

After cleaning and preparation the Flask is transferred to the Despatch Bay in the Cooling Pond by the Flask Crane.

Once the Flask is in the Cooling Pond the Lid is removed and transferred to the Washdown Bay and the empty Skip inside the Flask is removed and transferred to a Storage Bay by the Skip Crane.

The Lid Seals are visually inspected ready for replacement on the Flask. If the Seals are seen to be defective the Lid is decontaminated and transferred to the Leak Testing Bay where the Seals are renewed.

When the empty Skip is required it is transferred to a Handling Bay.

Fuel Elements and Bottles discharged from the Reactor pass through the Unloading Well equipment and arrive in the Unloading Tray in the Cooling Pond. From here they are removed by manipulators and placed in a Storage Skip.

When the Skip is full it is moved to the Storage Bay by the Skip Crane and left to cool.

After the necessary cooling period has elapsed (at least 90 days) the Skip is brought back into the Handling Bay.

Polyzonal Fuel Elements are removed from the Skip, monitored to check that the correct Cooling Period has elapsed, Desplittered and placed in a second Skip. Herringbone Fuel Elements are monitored but not Desplittered and are put in a separate Skip.

Once a Skip is ready to leave the Pond it is transferred to the Caesium Sampling Position in the R1 Storage Bay.

When the Skip is ready to leave the Pond it is checked under the Caesium Sampling Hood and then placed inside an empty Flask in the Despatch Bay.

Once the Skip has been placed in the Flask by the Skip Crane the Flask Crane collects the Flask Lid trom the Washdown Bay. lowers it onto the Flask and then transfers the Flask to the Washdown Bay. The Flask is Decontaminated and the Lid is bolted down. Health Physics check that decontamination is satisfactory then the Flask is transferred to the Leak Testing Bay. The Flask is Leak Tested and Nitrogen purged, if required by the Transport Approval, it is Fluoride Dosed then transferred to the Storage Bay. When a Low Loader is available the Flask is checked tor contamination and then transferred to the Low Loader.
14
15
16
Once clearance has been obtained, the Low Loader leaves Site for either the British Rail Railhead, Berkeley Power Station or Berkeley Nuclear Labs.
17
When a Splitter Flask is full of Magnox debris it is transferred to the Despatch Bay by the Skip Crane and deposited in a Cradle. The Flask Crane now transfers the Splitter Flask and Cradle to the Washdown Bay for Decontamination. After decontamination the Splitter Flask and Cradle are transferred to the Splitter Flask Transporter on the Magnox Debris Corridor. The splitter Flask Transporter transfers the Sputter Flask and Cradle along the Corridor to the Active Waste Oump Loading Bay. At the Active Waste Dump the Splitter Flask is removed from the Transporter and Cradle by the Dump Crane and lowered onto a Magnox Debris Vault Door. The Splitter Flask contents are emptied into the Vault and the Flask is then transferred back onto the Transporter by the Dump Crane. The Splitter Flask Transporter transfers the Splitter Flask and Cradle along the Magnox Debris Corridor to the Flask Crane. The Splitter Flask and Cradle are transferred to the Despatch Bay by the Flask Crane. When required, the Splitter Flask is removed from the Cradle by the Skip Crane and is transferred to a Handling Bay adjacent to a Desplittering Machine.
19 20
21
22
23
24
25
storage arrangements at Oldbury

12 Caesium sampling complete, the skip is placed inside an empty flask in the pond dispatch bay.

Note: At Sizewell A and Hinkley Point A, the skip

is removed from the pond up a shielded ramp before being placed in a flask, but the arrange­ment at Oldbury is more representative.

13 The flask crane collects a flask lid from the decon­tamination bay, lowers it onto the flask and returns the flask and its lid to the decontamination bay (see Fig 2.30).

14 The flask is decontaminated and the lid is bolted down. Health physics check for satisfactory de­contamination and the flask is transferred to the leak testing bay.

15 The flask is leak tested, nitrogen purged and, if necessary, fluoride dosed.

16 The flask is transferred to the storage bay to await road transport and a final contamination check.

17 Transfer to road transport for delivery to re­processing plant.

Splitter flask route

18 Splitter flask full of magnox debris.

19 Full splitter flask deposited on a cradle in the dispatch bay.

20 Flask and cradle transferred to the washdown bay for decontamination.

21 Decontamination flask and cradle transferred to the splitter flask transporter for delivery to the active waste dump loading bay.

22 Splitter flask removed from transporter and cradle and lowered onto a magnox debris vault door by the dump crane.

23 Magnox debris emptied into the vault and the empty flask returned to its cradle and transporter.

24 Splitter flask and cradle transported to flask crane.

25 Splitter flask and cradle transferred to the pond dispatch bay.

26 When required, splitter flask is removed from cradle and transferred to the pond handling bay, next to the desplittering machine.

Whilst ponds provide convenient shielding, careful

and rigorous water treatment and pond management

is necessary if magnox fuel can corrosion and re­

lease of contamination are to be acceptable. The decay heat also has to be removed. Ponds there­fore are provided with water circulating systems em­bodying filters, coolers and water treatment equipment. Contamination is removed by the filter and water treat­ment media and hence there is supplementary shield­ing provision for the exchange and storage of these arisings. Early experience with storage ponds was un­satisfactory, particularly when storage periods were excessive. Can-corrosion occurred and contamination spread from the water surface and from equipment as it was moved in and out of the water. Considerable ingenuity by station and other staff was necessary to cope with the difficulties.

It was these difficulties that led to the adoption of dry spent fuel storage at Wylfa (Fig 2.31). Each dry store cell consists of a bundle of vertical thimble tubes with a carbon dioxide atmosphere in w’hich the spent fuel is placed by the charge machine. The outside of the tubes is cooled by natural convection air. After the decay period the elements are discharged using a separate hoist to a shielded vault where they are de- lugged, placed in skips and dispatched in the trans­port flask. Subsequently, further dry storage has been provided at Wylfa in low head air-cooled vaults.

2.3 Reactor charge/discharge In principle, refuelling consists of coupling the pres­surised FM to the reactor, removal of the spent fuel elements one at a time from the selected channel and subsequent charging of the new elements.

5.5.1 Types of measurement

The measurements fall into two main categories:

• Condition monitoring to monitor the chemical con­stituents that are critical to corrosion of and de­position on metal and graphite components.

• Indication of gross effects such as the ingress of water into the reactor caused by boiler leaks.

5.5.2 Condition monitoring

Lists of magnox reactor gas analysis measurements indicating duty, typical ranges and type of measure­ment are shown in Table 2.I2.

Helium is added to the coolant gas and its mea­surement is used to indicate coolant leak rate. The measurement of nitrogen indicates air ingress that may occur during refuelling operations, whilst the measurement of hydrogen may indicate oil in-leakage (hrough the gas-circulator gas/oil seal. Hydrogen, carbon monoxide and methane are effective condition
monitoring measurements with respect to graphite and/or steel corrosion, these factors being more cri­tical for the AGRs because of the increased tempera­tures and coolant pressures.

Typically for condition monitoring purposes, a magnox station may use a katharometer and/or flame ionisation gas chromatograph with a local integrator/ controller arranged to sample each reactor in turn and provide a data printout every 30 minutes. Cali­bration gas mixtures are used to provide standards which have been checked against research laboratory standard mixtures.

The equipment used to place the viewing device at the various inspection positions inside the reactor may be divided into hoist units and units that move the viewing device out sideways once access has been ob­tained from the inspection penetration.

Hoist units form part of the inspection equipment of all AGRs and are used where gravity, together with a combination of fixed and temporary guides, can be utilised to insert the viewing device to the required position. In these cases the viewing device is lowered on a metal flexible hose which carries the cooling gas and the weight of the viewing device and any attach­ments; it also houses the electrical cabling required for services, control and telemetry of the viewing device. A typical hoist unit is shown in Fig 2.103, working in conjunction with an articulated TRIUMPH camera.

In order to inspect the relatively large area above the gas baffle dome and below the vessel roof, there is a requirement to traverse the camera horizontally once it has been lowered through the 7 m long small diameter (105 to 260 mm) access penetration. The first manipulator designed and developed for inspecting this area was for Hinkley Point B. It is capable of lower­ing the camera (housed within a 10 ш mast) through a 260 mm diameter peripheral fuel channel into the reactor, extending a telescopic boom horizontally up to 5 m and subsequently lowering the camera on its service hose to a further depth of 13 m, Additionally the whole machine is capable of slewing through 360°, can elevate or lower the telescopic boom and is fitted with a knuckle joint at the end of the boom to pivot the camera up through the horizontal. Figure 2.104 shows the general arrangement of the manipulator.

Fig, 2.101 Arrangement of boiler closure viewing equipment

Aboe-dome manipulator at Hlnkley Point В

At Dungeness B, the only available access penetra­tion for viewing the above-dome region has a diameter of 170 mm. This restraint has led to the concept of the ‘links manipulator* where a ‘break-back* chain is used to feed the camera into the reactor and the chain itself forms the boom of the manipulator. This principle has been extended to cover inspection re­quirements for Hartlepool, Heysham 7, Heysham 2 and Torness. Fig 2.105 shows the Heysham 2 inter­stitial manipulator which is an example of the ‘links’ principle. For Hartlepool and Heysham 7, a manipu­lator has been built which has a boom extension of up to 7 m; it has five movements of freedom — slew, elevation, extension plus ‘wrist and knuckle’ at the boom tip.

The various motions of these manipulators are pushbutton-controlled from a central console and there is also a microprocessor-based ‘teach and repeat* control system to allow semi-automatic control. All manipulators are filled with hand drive facilities for emergency retrieval in case of power failure.

A variant of the links idea is used to deploy ca­meras from penetrations in the bottom of the pressure vessel. For these routes, the links are stored on a drum and they are driven up the standpipes to get across either to the boiler annulus (Heysham 2) or into the undercore region (Hartlepool/Heysham 7). For the former, the viewing angle of the camera is extended by using the combined ‘wrist and knuckle* action of a drive head fitted to the end link.

In addition to the above, boom manipulators have been developed to view the under-boiler region (Hink — ley Point B), outlet duct viewing (Hartlepool/Heysham 7) and the sub-diagrid region (Heysham 2) using the access available when a gas circulator is removed. For the first two, the boom consists of a series of tubes that are screwed together and extended from the ac­cess position. For the latter, a telescopic rectangular section boom can be extended from the access position and the camera viewing angle extended by using the combined ‘wrist and knuckle* action of a drive head fitted to the end of the boom.

This system is provided to protect the boilers against the ingress of chloride from the CW system or caus­tic solutions from the condensate polishing plants. A high level of protection is required and thus, as far as practicable, two diverse sets of equipment are provided, each using ‘2 out of 4’ logic.

9.11 Heating and ventilating system

There are over 50 separate heating and ventilating systems on Heysham 2, of which about 40 are related to safety and/or contaminated zone systems. Consi­derable effort has been put into ensuring that the standard of C and I design and equipment on these systems is of a comparable level to that employed on the main reactor/turbine-generator plant.

A number of systems are controlled by program­mable logic controllers (PLC) also known as Program­mable Controllers (PC), which implement both sequen­tial and modulating control. These PLCs are linked to a microprocessor, programmed in Cutlass, which provides centralised logging and display.

T BLE M 5 want’d) Typical ACR mppins schedule

I Гпр -.cuing is a function of steam pressure — trip is based on integral level switches with fixed setting

‘ Q = quiescent

There is now an increasing awareness of the potential for common mode failures arising from shortcomings or defects both in plant design and in subsequent manufacture, installation and operation. There is no agreed mathematical method of calculating a value for the limiting effects of common failure on system reliability. There is, however, agreement that systems with identical redundant channels should not be as­sumed to be capable of achieving reliabilities better than about 10”4 to 10”s per demand. This limitation is applied even though random failure rate data might indicate a lower failure probability.

The design safety guidelines issued by the CEGB specify a protection system reliability of better than 10~4 failures per demand for faults which are pos­tulated to occur at frequencies of 10”3 per year or greater. To design a system to meet these stringent requirements has required the specification of two diverse protection systems each with four-way redun­dancy. For the PWR it was decided to base one system on multiple distributed microprocessors with digital processing, and the other on the well proven Laddie guard lines with analogue trip amplifiers as used on existing magnox and AGR stations.

The microprocessor system does employ core logic for the guard line voting, but the two systems are sufficiently different in design and hardware to avoid common mode effects.

The two systems are referred to respectively as the primary protection system (PPS) and the secondary

protection system (SPS).

The safety case for the station is made on the basis that the PPS and SPS each independently provide protection for all frequent faults (frequency of 10~3 per year or greater). The PPS provides protection for all faults (frequent and infrequent) that are within the station design basis, and two independent para­meters are generally available to detect each fault.

11.1.6 System design

The primary and secondary protection system have measurements of almost identical parameters from which to generate reactor trip and ESF action. The instrumentation used for the measurements in each system is chosen to be as diverse as practicable within the limitations of the transducers available.

An example listing of the parameters that are mea­sured for trip protection purposes for the two systems is given in Table 2.17.

The primary protection is implemented with mi­croprocessors and is therefore able to provide for an extensive range of parameters and functions with com­plex logic. The use of microprocessors permits protec­tion to be derived using algorithms to assess limiting core conditions dependent upon control rod positions and operating power levels. This protection would not have been possible using analogue based equipment without excessive complexity.

Axial macroscopic

At the start of a magnox reactor’s life, the uniform fuel loading leads to a flux (and hence rating) shape similar to the theoretical cosine shape of a uniform bare reactor, but with some distortion caused by the partially inserted regulating rods. A typical magnox axial flux (and rating) shape is shown on Fig 3,5. As burn-up proceeds the competing processes of U-235

burn-up and Pu-239 build-up result in a slight in­crease in peaking of the rating shape followed by a degree of flattening. The changes in ratio of peak — to-mean axial rating are however only a few percent, considerably less than the effects caused by rod bank movements during normal reactor operation.

In magnox reactors the radial flux shape is flattened by the use of neutron absorbers. A typical average ra­dial macroscopic rating shape is shown on Fig 3.6 (a). This shape, dearly showing the large flat central re­gion, is maintained throughout core life. Superimposed on this shape is a local channel-to-channel variation due to variation in isotopic content of the fuel. The power generated in a newly refuelled channel rises relative to its surroundings as plutonium builds up until the loss of U-235 begins to outweigh the in­crease in Pu-239. This behaviour is demonstrated on Fig 3.7, which shows the ratio of individual channel power to surrounding channel power, known as the ‘age factor’, for a typical magnox reactor. Of the lO^o or so increase in relative power, the dominant effect is the increase in fission cross-section with a

small {less than 2%) contribution due to the higher heat generation per fission of plutonium. The changes occurring in the channel flux level relative to its surroundings is small (less than 1%). This constancy of flux level arises because the channels are close together relative to neutron migration lengths and hence the neutron flux in the channel derives main­ly from neutrons produced in surrounding channels which, because of the on-load refuelling in magnox reactors, are (on average) under constant conditions when the fuel cycle has reached an equilibrium state.

If an abnormal situation is detected with the reactor at power, requiring rapid shutdown, all the control rods are dropped into the core to shut down the reactor, the safety rods are then withdrawn (when it is deemed safe to do so). Safety rods provide a re­serve of negative reactivity which can be dropped into the reactor core if an abnormal situation is detected while the reactor is shut down.

In most magnox reactors this function is provided by a group of black rods, 10-20 in number, specially designated for this duty, and these rods are normally held fully withdrawn at all times. At Dungeness A, safety rods were originally provided, but these have now been incorporated into the bulk groups and the satetv function, while shut down is provided by the regulating rods and trim rods. At Wylfa, the last of the magnox series, no safety rods are provided be­cause the complement of other rods is deemed to be adequate to ensure safety at all times.

In AGRs the safety rod function is provided by some of the grey rods while the reactor is shut down. Prior to start-up the safety group are lowered to the same height as the remaining grey rods.

Safety rods are not formally claimed to provide protection in any faults, and the reactivity margin at shutdown does not depend on safety rods.

Secondary shutdown rods

In magnox reactors with steel pressure vessels some bulk rods, typically 25-40 in number and designated secondary shutdown (SSD) rods, are fitted with a me­chanical latch which releases the rod from its sup­porting wire or chain in the event of a rapid gas depressurisation, to ensure a rapid shutdown of the reactor. In all other respects the rods operate as bulk rods, and in normal operation an SSD rod is in­distinguishable from other bulk rods.