Scope

An operating rule limit on coolant gas pressure is set, based on vessel integrity at specified temperatures:

• Excess coolant pressure is a trip parameter indi­cative of overfilling or severe boiler tube failure, the boiler operating at a higher pressure than the reactor.

• Rate of fall of pressure is a reactor trip parameter on magnox stations indicative of a depressurisation, for example, caused by a failure of the pressure vessel or circuit.

• Low coolant pressure is also a trip parameter.

• Rate of rise of coolant pressure is drawn to the attention of the operator as an alarm and he may decide to trip the reactor manually.

Typical trip settings are given in Tables 3.5 and 3.6. Magnox reactors

For reactors with steel pressure vessels, a coolant pressure limit based on vessel temperature will be set such that an adequate margin on vessel integrity is maintained if a shock loading was to occur. This limit is normally in the form of a graph of vessel pres­sure against minimum vessel temperatures, where safe conditions are to one side of a line on the graph.

Pressure Accurate measurement of the reactor gas circuit pressure is provided to enable the operator to comply with the station’s Operating Rules. In the case of Berkeley, a digital display of reactor gas pres­sure is prominently positioned on a panel in the CCR with individual gas circuit pressures indicated on the eight boiler/blower panels. The digital display unit also has high and low pressure alarm features to at­tract the operator’s attention to significant changes from the desired level. A recorder situated in the CCR annex also provides information about trends in pressure changes and is a reliable back-up to the CCR indications. An extremely accurate bourdon tube pressure gauge, which is directly connected to the pressure vessel and situated in the reactor blower/ boiler control room, is used by the plant attendant when changes in gas pressure are required for opera­tional purposes. This is a useful instrument as it is independent of all electrical supplies and therefore the operator can verify the reactor pressure at any time without difficulty. A high reactor gas pressure trip system is installed which is part of the safety

Table 3.5 Protection, alarm and trip settings /or a typical magnox reactor Margin alarm Trip alues Trip (unction setting Shutdown Stan-up Power Steady raising state Seutron Jinx Counter channel’ (і j Low counts — — 5 cps Vetoed Vetoed Vetoed (2) High counts — — 30 000 cps Vetoed V eioed Vetoed Low log power (1) Power ‘Intermediate 5-7 MW 10 MW power trip Vetoed abose 5 MW imminent’ — 20 s (21 Doubling item — Mam log power (I} Doubling time — — 20 s 20 s 20 s 20 s SDAs ‘SDA low trip 20 MW Not claimed as protection margin’ 25 MW margin 25 MW margin j 40-70 MW margin 1 Note: Maximum trip levels and margins as local operating rules Temperature (symmetrica!) Auto-reset FE 10s I5-8°C As local operating rules Absolute level dependent on 30-16°C dependent upon mean FE10 thermcouple errors Auto-reset margins ‘Low trip margin’ mean FE 10 2°C/minute upwards, 60°C/minute downwards above 27Q°C Re-setting rate thermocouple High margin trip errors Temperature (asymmetrical) Auto-reset CGO Absolute level As local operating rules Auto-reset margin ‘Low trip margin’ 7°C I5°C 15°C I5°C 15°C Resetting rate 2°C/minute upwards, 60°C/minute downwards above 270°C High margin trip 60° C 60°C 60°C 60°C COj coolant failure Rate of change of pressure 0.689 bar/min Blower failure (low current) Vetoed 56 A 56 A 56 A High gas pressure ‘Reactor C02 Reactor 1.7,4 bar pressure high’, also ‘reactor pressure warning’ (value set by operator) Reactor 2.7.9 bar Lo’-s of feed flow Feed range 32 bar Vetoed 29.6 bar 29.6 bar 29.6 bar pressure low

circuits and also a warning alarm set just below the

trip lev cl.

Level of reserves The Berkeley power station Operat­ing Rules require a minimum level of 61 t of CO: to be kept in reserve for reactor coolant duty. The station’s storage capacity of 213 t of CO: is far in excess of the minimum required, but for operational requirements, this higher level is maintained as far as is practically possible. There are no direct indications in the CCR of CO: stored, but stock control is administered by the CCR supervisor who orders direct from the supplier, A daily record of CO; stocks taken from direct-reading indicators on the storage tanks is kept by the CCR supervisor and CO; usage is con­trolled by the operator. This method of stock control

T xBt. E 3 6

Typical AGR (ripping schedule

Trip parameters	Trip setting nominal	Redundancy	C omments
Main guar dime
1	Pulse count rate htah ties el onl>)	500 1ЛЧ-	2 out of 4
2	Plus period (log DC l	24 ч iJoahiing time 2n,]	2 out of 4
3	Excess flux (linear channelsi rate and lex el stop		200 MW min up 1000 MW nun down Q margin lO^o (40 MW min}	2 out of 4
4	Excess flux (log DC channels — from linear output} rate and level stop		Upper 1" MW	2 out of 4
5	Low reactor pressure 1	28.5 bar a	2 out of 4
6	Low reactor pressure 2	28.5 bar a	2 out of 4
7	High reactor pressure 1	44.8 bar a	2 out of 4
8	High reactor pressure 1	44.8 bar a	2 out of 4
9	High CGO temperature rate and level stop	10°C min up 200°C/min down Q margin 40°C Upper 7|0°C	2 out of 4 in each case of two separate sets
10	High circulator outlet gas temperature rate and level stop	7°C/min up 7°C/min down Q margin 20°C Upper 320°C	2 out of 4
11	Circulator undervoltage		Less than 3.3 kV for greater than 200 m/s	Two 2 out of 4
12	!GV position	14°	1 out of 2 per quadrant into 2 out of 4
13	Any one quadrant extra high circulator outlet gas temperature	370°C	2 out of 3 per circulator
14	High boiler half-unit outlet gas temperature (via quadrant trip initiated)	320°C	2 out of 3 per half­unit into 1 out of 6 per quadrant into 2 out of 4
15	Circulator underspeed (via quadrant trip initiated)	2520 r/min	2 out of 3 per circulator into 1 out of 2 per quadrant into 2 out of 4
16	High CACS demineralised water temperature (via quadrant trip initiated)	40°C	2 out of 3 per quadrant into 2 out of 4
17	Two quadrant trips initiated	—	2 out of 2
18	Safety room high temperature	40°C	2 out of 4
19	Impact vibration	10 dВ above background	2 out of 4
2U	fuelling maG. me grab load	U LI 1880 kg U — L2 2)80 kg O-‘L 2775 kg ROC 80-85 kg/s	2 oul of 3
21	Pile cap air lemperaiure high	130°C	2 out of 4	Distributed sets. Channel trip if one ther­mocouple rises 30° above ambient of 50° noting two series thermocouples to one input. Local gas temperature at trip 80°C nominal.

T fil t 3.6 icontd)
Typical AGR tripping schedule

Гир parameters	Trip ‘Citing nominal	Redundancy	Comments
Dnc’e — juardhne j H і e h COO temperature rattr. ми! e ■ e! ‘top	Identical to Item 9 — main guardline	2 out of 4
3 hvcC’S lT. iv Circaґ Jt. mr. eNt rate and level v;op	Identical to item 3 — mam guardline	2 out o; 4
3 Quadrant trip initiated	—	2 out of 4
Auxiliary guardline I Pulse couni rate high	>U0 kW	2 out of 4
Quadrant protection I Half-unit outlet gas temperature high, loss	320°C 2‘0°C	2 out of 3 per half­unit into 1 out of 6 per quadrant	Safety U"Ociaied
2 Low superheater transition joint metal temperatures	See Note l	2 out of 3 per half­unit into l out of 6 per quadrant
3 Circulator underspeed.’overspeed	2520 r/min 3220 r^min	2 out of 3 per circulator into 1 out of 2 per quadrant	Safety a-oociaied
4 Differential oil pressure across circulator bearings very low	0.75 bar	2 out of 3 per circulator into 1 out of 2 per quadrant
5 Oil level from circulator compartment very high	See Note 2	2 out of 3 per circulator into I out of 2 per quadrant
6 Circulator lub oil tank level very low	See Note 2	2 out of 3 per circulator into 1 out of 2 per quadrant
7 Circulator high differential pressure between reactor and motor compartment	4 bar	2 out of 3 per circulator into 1 out of 2 per quadrant
8 CACS demineralised water temperature high	40° C	2 out of 3 per quadrant	Safety associated
9 Circulator outlet gas temperature high SSD initiation	435°C	2 out of 3 per circulator into l out of 2 per quadrant
Bulk group 1 insertion	69^0
Delay timer	4.35 s

S’oies

1 Trip >eiting is a function of steam pressure

2 Trip is based on integral level switches with fixed setting

3 {,) = quiescent

ihen enables the operator to have a reasonable assess­ment of CO: reserves at any time and is able to en­sure that minimum levels are maintained.

Shutdown action — coolant pressure There are auto­matic reactor trips on high pressure and high rate- ol-ehange of pressure.

To cater for maximum credible depressurisation faults on steel pressure vessel stations, emergency shut­down devices (ESD) are installed. These shutdown devices are individually tripped on high rate of change of pressure, independently of the guard lines. The rapid shutdown is to guard against coincident core distortion caused by the depressurisation fault.

Tvpical values Berkeley Hinktey Роті.-1 Oldbury

Normal operating pressure, bar ’.’9 12."5 25.17

High pressure trip, bar 7.93 13.38 26.34

High oP’6i, bar/min 0.68948 2.07 4.%

{The above are illustrative only, the pressures may be measured at different parts of the gas circuits.)

AGRs

Reactor gas pressure Apart from maintaining a cool­ant pressure to permit adequate fuel cooling, there are also important safety implications:

• Reactor pressure failing at higher than the usual rate to be expected from normal vessel leakage implies some breach in the primary circuit pressure bound­ary. The assessment is facilitated by an on-line cal­culation of mass of gas in the vessel, based on measurements of reactor pressure and temperature.

• High reactor pressure implies that either there is a boiler leak or the CCb admission valve is not closed when it should be. This prompts operator action to isolate the leaking boiler or close the CO2 valve,

• It is necessary to ensure that the reactor is not overfilled prior to the start of power operation.

Pipework is installed so that reactor gas pressure can be measured at four points under the vessel top slab, one in each quadrant above the boiler annulus. The pipework is 17.2 mm o. d. and 3.2 mm thick stainless steel. The pipework for each quadrant is routed through the quadrant instrument penetration at the + 2.3 m level and thence to the quadrant outer re­actor gas instrumentation racks.

For quadrants A and D there is a transducer, giving a 4-20 mA signal for a range 0-60 bar with an accuracy of 0.5%. For quadrant В there is a low pressure range transducer giving a 4-20 mA signal for a range 0-6 bar with an accuracy of 0.5%, The pipe­work from quadrant C is not used, being isolated by a valve in the primary isolation valve rack at the +3.9 m level and capped in the quadrant outer gas instru­ment rack.

The signals from quadrants A, В and D are used

as follows:

Indication-alarm Quadrant signal used

CCR unit desk high range indication A

CCR unit desk ІО’л range indication В

CCR diree(-wire facia low pressure alarm (via A, D

alarm amplifier set at 32 bar) (grouped)

CCR direct-wire facia high pressure alarm A. D

(via alarm amplifier set at 42.5 bar) (grouped)

CR post-trip mimic analogue indication D

CCR post-tnp mimic low pressure digital signal A, D

LIC panel indicator В

НІС post-trip mimic analogue indication A

Valves are provided to enable the transducers and alarm amplifiers to be calibrated and tested while the reactor is at power.

All signals are also fed to the data processing system.

The pipework, transducers, alarm amplifiers and ca­bling are aseismic. Gas pressure reactor trip signals are not derived from this equipment.

Gas baffle dome pressure differential This is impor­tant for safety in three respects:

• High gas baffle dome pressure differential indicates the approach towards the limit of design pressure dif­ferentia! plus the margin allowed in the design code.

• Reduced limits on gas baffle dome pressure differ­ential must be observed during refuelling to ensure that partially withdrawn fuel sleeves are not sub­jected to excessive tensile stress due to internal pres — surisation or buffeting due to excess gas cross-flow.

• Low gas baffle dome pressure differential may in­dicate that the gas flow over the fuel elements is insufficient for adequate cooling and, in particular, the flow may not be sufficient to give proper CGO thermocouple response and hence may render re­actor protection from CGO temperature signals in­effective.

In the case of Heysham 2, the pressure differential across the gas baffle dome is measured at four points, one in each quadrant of the reactor.

The pipework is 17.2 mm o. d. and 3.2 mm thick stainless steel, and is used solely for differential pres­sure measurement; no gas sampling flows are drawn down the pipework since this would affect the differ­ential pressure measurement. The pipework from each measuring point is routed through a different pene­tration for each channel, i. e., the relevant quadrant instrument penetration at the +2.3 m level.

There are four transducers measuring the pressure differential, one in each quadrant outer reactor gas instrumentation rack, approximately 6.5 m above the instrument penetration and about 3.5 m from the ves­sel wall. The accuracy is 0.5% full-scale with a range of 0-4 bar and they are designed for operation at up to 60 bar static pressure.

Each channel provides a 4-20 mA signal. The sig­nals from quadrants A and C are sent to the data processing system. Those from quadrants В and D are fed to a 2-way switch on the unit desk in the CCR so that either may be displayed on the conventional indicator on the desk.

Additionally, alarm amplifiers on quadrant В and D channels generate alarms for high gas baffle dome differential pressure {at least 2.70 bar). These are grouped to give a single direct-wired facia alarm in the CCR and are individually repeated to the data processing system. Valves are provided to enable the transducers and alarm amplifiers to be calibrated and tested while the reactor is at power.

The data processing system derives alarms from the analogue signals from quadrants A and C in the event of abnormally high and low values.

The pipework, transducers, alarm amplifiers and ca­bling are aseismic.