125

HEAT &THERMODYNAMICS

110DULE B.2

STEAM GE[JERATOR
 125 - B.2

Heat & Thermodynamics

MODULE B.2

STEAM GENERATOR

Course Objectives

1. You will be able to explain how the temperature differ-

ence between the steam generator and the PHT system
changes during a "crash-cool" exercise.

2. You will be able to state how the PHT average tempera-

ture is affected by increasing the thermal resistance of
the steam generator tubes.

3. You will be able to explain why the programmed steam

generator level increases with power.

4. You will be able to explain one problem concerning high

boi ler level and three problems concerning low bei ler
level. You will be able to state the control action
which is designed to overcome these problems.

5. You will be able to state the three elements used for

boiler level control and explain why they cannot be used
at low loads.

6. You will be able to explain the response of the station

control system to a falling boiler pressure when control
is in the 'normal' mode and the control of the speeder
gear is in 'auto'.

7. You will be able to explain why the BPe program termin-

ates at 170 a c when in the 'cooldown' mode.

8. You will be able to explain how raising the pressure of

the steam generator improves the efficiency of' the
steam/water cycle.

9. You will be able to explain the limitation on raising

the steam generator pressure in the CANDU system.

November 1981 - 1 -
 125 - B.2

We have examined the basic thermodynamic principles and

must now apply these principles to the operation of the steam
generator and finally the reactor.

The steam generator removes the heat from the reactor un-
der normal conditions. The heat which is removed from the
fuel in the reactor channel by the heat transport D20 is re-
jected in the steam generator to the lower temperature light
water system.

The steam generator heat transfer takes place at the

tube bundles through which the primary heat transport fluid,
flows and around which the feedwater flows.

By varying the rate of heat removal in the steam gener-

ator we can control the rate at which the heat transport
temperature changes or we can ensure that it remains con-
stant, depending upon the power manoeuvring at the time.

In addition to acting as the major heat sink for the

reactor the steam generator produces high quality working
fluid that may be used to produce mechanical power in the
steam turbine.

The heat that is transferred from the PHT system to the

steam generator depends upon the temperature difference which
exists between the D20 and the lightwater in the steam gener-
ator.

As the temperature difference increases, more heat is

transferred. In a "crash-cool" exercise, this is exactly
what happens. By rejecting steam from the steam generator to
lower the pressure, the temperature falls as well and in-
creases the temperature difference between the steam generat-
or and the reactor. As a result, more heat is transferred
and the cool-down rate of the reactor is increased.

The heat which is transferred also depends upon the

thermal resistance of the tubes in the steam generator. If
these tubes become coated with oxide or other material, the
thermal resistance will increase which means that a higher
temperature will be needed in the PHT system in order to
transfer the same quantity of heat.

B.2.1

Explain how the temperature difference between the PHT

system and the steam generator changes during a "crash-cool"
exercise.

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 125 - B.2

B.2.2

Explain how an increase in thermal resistance, across

the steam generator tubes, affects the average PHT tempera-
ture.

* * * * *

Level Control

is important that the mass of light water in the

It
steam generator remains constant to provide an adequate heat
sink capacity for the reactor.

We have already seen that the liquid in the steam gener-

ator will expand as the temperature rises. This expansion
will cause an increase in the level of liquid in the steam
generator.

Do this exercise and compare your answer with the notes

at the end of the module.

B.2.3

Feedwater in the steam generator is heated until the

temperature rises from 170°C to 250°C. Determine the percen-
tage increase in volume that would occur due to this tempera-
ture rise.

* * * * *

In addition to this increase in level there is another

effect which will occur. As boiling takes place steam bub-
bles will form within the liquid and if the mass of water
stays constant this will cause the steam water mixture level
to rise. As the rate of ·steaming in the steam generator in-
creases the ratio of steam to liquid in the steam generator
will increase and cause an even higher level although the
mass of 'water' in the steam generator will not have changed.

This increase of steam generator level is programmed in-

to the control system. The level setpoint in the steam gen-
erator increases linearly with steam flow until maximum steam
generator level is achieved at 100% steam flow.

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 125 - B.2

The effect of rapidly lowering the pressure of saturated

liquid may be seen on a temperature/enthalpy diagram.

Temperature,
°c
T - - - - A
_,#:_~ ...J
A
T. - - _./'-.....:,/1'.:...- /

Enthalpy, J/kg
Fig.2.1

The enthalpy remains constant and as the pressure rapid-

ly falls, the liquid has more heat than is needed for satura-
tion conditions and the excess heat produces vapour. What
happens to the level in the steam generator? It rises! You
can see this effect if a large steam reject valve or a con-
denser steam dump valve is open. The steam generator level
rises momen'tarily. If there had been a high level in the
steam generator then there would have been a danger of prim-
ing the steam lines with liquid from the steam generator.
This effect of increased volume due to a sudden decrease in
pressure or rise in temperature is called "swell".

The maximum swell effect in the steam generator would

occur when there is a large demand in steam flow, eg, an in-
crease in load from 50% to 100% power on a hot turbine. In
this case the swell would not cause a problem because the
programmed level would only be at the 50% power setpoint and
so priming is less probable.

In the event that an abnormally high level occurs in the

steam generator, a governor steam valve trip is initiated to
prevent liquid being carried into the turbine where massive
blade failure could occur.

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 125 - B.2

Look at the following questions and compare your answers

with those at the end of the module.

B.2.4

The mass of "water U is kept constant in the steam gener-

ator over a wide power range. As the steam flow increases
the programmed level in the steam generator also increases.
Explain why the progranuned level has to increase with steam
flow.
B.2.5

Explain why it is undesireable to have liquid enter the

steam turbine and state how the probability of this event
occurring is reduced.

* * * * *
The effect of swell is reversed when the pressure in the
steam generator is suddenly increased. This may occur with a
turbine trip when the steam flow is instantaneously reduced.
Any vapour bubbles which exist within the liquid are collaps-
ed and the liquid level falls. This causes the fluid in the
steam generator to "shrink". If the steam generator is oper-
ating at a low level when the turbine trip occurs, then the
resulting shrink may result in a very low steam generator
leve 1.

There are three potential problems with a very low

steam generator level. First, the level may fall below the
sensing point for level control, which is above the top of
the tube bundle. This means that the level control program
can no longer detect the level - it still operates at minimum
level signal.

Secondly, as the water inventory in the steam generator

falls the capacity as a heat sink for the reactor is also re-
duced and this is obviously an undesireable trend.

Thirdly, if the level in the steam generator falls any

further the tube bundle will be uncovered and dry out will
occur. The dissolved solids existing in the steam generator
will "bake out" on the tube surfaces and impede future heat
transfer.

The problem of low level is accommodated initially with

an alarm which may allow operator action and finally with a
reduction of reactor power, either by a setback or a trip de-
pending upon the operating rationale at the station concern-
ed. The effect of rapidly reducing reactor load ensures that
the reactor thermal power is more closely matched to the re-
duced heat sink capacity of the steam generators.

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 125 - B.2

Answer the following questions and compare your answer

with the notes at the end of the module.

B.2.6

Explain why the level in the steam generator initially

falls on sudden reduction of steam flow.

B.2.7

Explain three potential problems of low steam generator

level and how the effect of these problems is reduced in
practice.

* * * * *

There are three signals used for the level control pro-
gram,

(a) steam flow,

(b) feed flow.
{c} actual level.

The steam flow signal is used to produce a programmed

level setpoint for the steam generator which varies linearly
from 0% to 100% steam flow.

Control circuits compare steam and feed flows for mis-

matching, they also compare actual and programmed steam gen-
erator levels.

At low flows of steam and feedwater, measurement of flow

is unreliable. In addition to this problem any feedwater
regulating valve operation has a dramatic effect on the sys-
tem because the flowrates are so low. One minute there is
virtually no flow at all, then a regulating valve cracks open
and a great slug of water enters the system.

In this low power/flow condition steam generator level

is essentially controlled by the level controller exclusive-
ly. Above -20% flowrate, when the large feedwater regulating
valves are in service the level control system can operate
with all three elements.

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 125 - B.2

Change of Steam Generator Programmed Level/Power

Change of Steam Generator 200

level vs. Power
193)
180 PNGSA

160

140
level,
om
0 120
BNGSA
(oIO)
-2 100
level, '
om
-40 80

-60 60

(-71)
,,)
-8 40
0 100 O· 100
Power, <70 Power, ~o
Fig.2.2

By comparison you can see that the programmed level at

Pickering NGS-A changes by 152 ems whilst the programmed
level at Bruce NGS-A only changes by 61 ems.

B.2.8
Why do you think this difference exists? Compare your
answer with the notes at the end of the module.

B.2.9
State the three elements which are used in a boiler
level control program. Explain how level control is effected
at low power levels.

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 125 - B.2

B.2.10

The high level alarm has been received on a boiler.

What actions can the operator take?

* * * * *

Boiler Pressure Control

Boiler pressure is used to control the mismatching which

may occur between the thermal power produced by the reactor
and the thermal power removed from the steam generator by the
steam flow.

As we have already discussed, in a saturated steam sys-

tem either temperature or pressure may be used to represent
the same heat quantities. In the Candu system we use pres-
sure because it is so sensitive to changes in the balance of
thermal power.

The main heat sink for the reactor is the stearn generat-
or. In turn, the steam generator has its own heat sinks,
some small, some large, some variable, some fixed.

Stearn Turbine

This is the normal consumer of stearn from the stearn gen-

erator. At Pickering NGS-A it is capable of using all the
reactor steam. At Bruce NGS-A the situation is complicated
by the supply of reactor stearn to the Heavy Water Plants.

At Bruce NGS-A the turbine cannot take all the reactor

steam and consumes 88% of the total reactor stearn if both the
reactor and turbine are at full load.

Changes in turbine or reactor power may be made by the

apc program to meet the designed pressure setpoint.

Steam Reject/Discharge Valves

These valves are capable of discharging any steam flow

necessary to restore system control. If the turbine is
available there is usually an offset before these valves
operate, to allow speeder gear operation to have an effect on
the steam flow via the GSV.

If the turbine is not available, the offset is removed

and these valves operate as soon as the pressure setpoint is
e>::ceeded. If the mismatching is large enough for the main
reject/discharge valves to operate, then a reactor setback

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 125 - B.2

is initiated until the large valves close and equilibrium is

restored.

Safety Valves

In the unlikely event that the turbine and/or the re j-

ect/discharge valve systems cannot control the pressure
excursion, then the stearn generator safety valves will allow
the excess steam to be vented to atmosphere.

Auxiliaries (D/A, Gland Steam, Steam Air Ejectors)

These loads are relatively fixed and although they may

account for up to 10% of the total steam flow, they do not
appear as controllable heat sinks from a steam generator
pressure viewpoint.

Boiler Blowdown

This is a variable heat sink and may affect the steam

generator. However, the flowrate is only 1-2% and as a re-
sult has an insignificant effect on boiler pressure.

B.2.11

List the three major heat sinks for the steam generator
and state when they are used.

* * * * *

Boiler Pressure Set Point "At-Power"

In all the BPe programs there is a pressure set-point at

various power levels.

At Bruce NGS-A the pressure set point is constant at 4.3

MPa(a).

At Pickering NGS-A the pressure setpoint falls from 5.09

MPa(a) at 0% power to 4 MPa(a) at 100% power.

The rationale for these two situations will be discussed

in further detail in Module B.l "Reactors". At this point
this is the set of conditions that have to be met by the
steam pressure control programs at each station.

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 125 - B.2

BPC Pressure Setpoint ys. Unit Power

5.2

5.0
4.8

Pre$wre 4.6
MPo(a) ,
4.4I--,B",N~G,,5~A,- ::::::,_::::- _
4.2

4.0

3.8+------------------r
a 100
Unit Power, '70
Fig. 2.3

We are examining the pressure of the steam generator.

Suppose we want to raise the pressure in the steam generator,
how could we do this? The reactor is rejecting heat to the
steam generator and the steam generator is rejecting heat via
the steam system.

If we wish to raise the pressure in the steam generator

we have to produce an imbalance which results in more heat
being supplied to the steam generator from the reactor than
is being removed from the stearn generator via the steam.
There are two ways that we could to this:

(a) Raise reactor power.

(b) Decrease steam flow from the boiler.

In practice the method used would depend upon the mode

of control.

On the other hand, if we wanted to lower the steam gen-

erator pressure, there are two actions that could be taken:

(a) Reduce reactor power.

(b) Increase steam flow from the steam generator.

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 125 - B.2

Reactor Leading Mode

In this mode, the reactor power is kept constant and the

steam f low from the steam generator is varied to meet the
programmed BPC setpoint pressure for the reactor power. This
mode is used at Pickering NGS-A as the 'normal' operating
mode and is used at Bruce NGS-A for low power operation and
for abnormal conditions.

Reactor Lagging Mode

In this mode, the generator load is kept constant and

the reactor is controlled to maintain the boiler pressure
setpoint. This is the 'normal' mode used at Bruce NGS-A.

Boiler Pressure Control - Reactor Leading

In this mode, the reactor power is changed to the new

value and the BPC program makes sure that the rest of the
system follows.

Suppose we want to raise unit power. Initially we can

change demanded reactor power and produce more heat. There
will now be more heat rejected to the steam generator than is
being removed by the steam. As a result the pressure will
rise in the steam generator. The BPC program sees the rise
in pressure and opens the governor steam valves to allow more
steam to flow out of the steam generator into the steam tur-
bine, thereby reducing the steam generator pressure back to
the programmed setpoint for that reactor power.

The turbine provides the primary heat sink for the steam
generator. In the event that the turbine could not reduce
the steam generator pressure, then the secondary heat sink
would be used, ie, Steam Reject Valves (SRV's).

I f the speeder gear is not under BPC control and the

mismatch causes the steam pressure to rise above the pressure
setpoint the small SRV's will open. If this does not reduce
the steam pressure then two events will follow:

(a) the large SRV's will open to reduce the steam generator
pressure.

(b) the reactor power will be reduced until the large SRV's
shut, thereby quickly reducing the mismatch in power.

If the unit power is to be reduced, a reduced demanded

reactor power is input. The steam pressure starts to fall as
now more heat is being removed from the steam generator than
is being supplied by the reactor.

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 125 - B.2

The BPC program monitors the falling steam generator

pressure and reduces the steam flow into the steam turbine
via the GSV' s to restore the setpoint pressure.

B.2.12

Describe how a rising boiler pressure signal would be

handled with a "reactor leading" mode, at power I when the
speeder gear is not controlled by the BPC program.

* * * * *
Boiler Pressure Control - Reactor Lagging

In this mode the generator power is kept constant and

the reactor power setpoint is adjusted to maintain the pres-
sure setpoint.

Suppose we wanted to raise unit power. Initially an in-

crease in demanded power would result in an opening of the
GSV's which would result in a lowering of the steam generator
pressure because more heat is being removed with the steam
than is being supplied by the reactor. The BPC program re-
sponds to the falling boiler pressure by raising the reactor
power setpoint until the boiler pressure returns to the pro-
grarruned value.

As already mentioned, this mode applied only at Bruce

NGS-A. In extreme cases where the reactor manoeuvring cannot
control the pressure, the BPC program reverts to reactor
leading. In the high pressure situation atmospheric steam
discharge valves relieve the excess pressure. If the boiler
pressure error is too large because of low pressure, a slow
speeder runback is initiated until boiler pressure is restor-
ed.

B.2.13

Describe how a falling boiler pressure signal would be

handled with a "reactor lagging" mode at power.

* * * * *
Warm Up Mode

In this mode the Heat Transport system temperature may

be raised by requesting a constant rate of change of boiler
setpoint pressure.

The excess steam is vented to atmosphere via t_he steam

reject valves at Pickering NGS-A or the atmospheric steam
discharge valves at Bruce NGS-A.

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 125 - B.2

By increasing the pressure in the steam generator the

temperature is also increased. A common example is an auto-
mobile radiator. (Why increase the radiator pressure? If
overheating was a problem raising the pressure may prevent
boiling and would increase the heat removal rate from the
radiator due to the higher coolant temperature resulting .from
the higher pressure.)

Cooldown Mode

In the cooldown mode heat has to be removed from the re-

actor until the reactor can be cooled with shutdown cooling.

If the turbine is available the turbine load can be re-

duced using the BPC program so that the electrical output re-
duces with the reduced steam flow available from the steam
generator.

It should be noted that as the steam pressure falls, the

quality of steam in the turbine is deteriorating and this in-
creasing wetness in the turbine may be a very good reason for
not allowing the BPe program to use the turbine all the way
down. In this case switching the speeder control to "Manual"
would bring the steam reject values into operation.

If the turbine is not available, as in a turbine trip,

then steam is rejected either to atmosphere via steam reject
valves at Pickering NGB or to the main condenser via condens-
er steam discharge valves at Bruce NGS-A. This process con-
tinues until the temperature of the PHT falls to around 170°C
at which point the SRV's are full open and no longer capable
of reducing the PHT temperature. It is at this point that
the shutdown cooling takes over.

B.2.14

Explain why the BPC program terminates at 170°C when in

the 'coo1down' mode.

Cycle Efficiency

As stated in Module B.3.1, we can get best use (ie, most

efficiency) from steam when the temperature difference be-
tween the steam in the steam generator and the steam in the
condenser is at maximum.

If we raise the steam pressure in the steam generator,

how does this affect the steam temperature?

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 125 - B.2

Since the water in the steam generator is at saturation

conditions. if the pressure of the water is raised the water
will boil at a higher temperature. Thus, the temperature of
the steam produced will increase - this increases the temper-
ature difference between the steam in the steam generator and
that in the condenser. The efficiency of the cycle will in-
crease as well.

There is a limitation on the pressure of the steam gen-

erator. As the pressure and temperature of the water/steam
system are increased, the temperature difference across the
steam generator tubes is decreased and less heat is transfer-
red from the primary heat transport fluid. The temperature
of the primary heat transport fluid in the tubes will in-
crease. This will cause the temperature of the primary heat
transport fluid in the reactor to increase. Less heat will
be transferred through the fuel sheath and the fuel and fuel
sheath temperatures will rise.

The limiting temperature of the primary heat transport

fluid in the reactor is 290 to 300 D C. At this limit, the
temperature in the fuel reaches a maximum of 2300 D C and the
fuel sheath temperature is approximately 350 to 400 DC. If
the heat transport fluid temperature rises above 300 D C, (with
no boiling), the maximum fuel temperature approaches the
melting point (about 280a D c).

If melting of fuel occurs, fission product gases normal-

ly held at the fuel grain boundaries are released, building
up high pressures inside the fuel sheath. The fuel sheath
temperature is increasing rapidly (and its mechanical
strength is decreasing) as the heat transport fluid and fuel
temperatures increase. The high pressures on the inside of
the fuel sheath will contribute to failure of the sheath
which will lik.ely occur in the range of 800 to 1100 DC. When
sheath failure occurs there will be release of fission
products into the primary heat transport system.

Answer the following questions and compare your answers

with those at the end of the Module.

B.2.15

How does raising the pressure of the steam generator im-

prove the efficiency of the steam/water cycle?

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 125 - B.2

B.2.16

Explain the limitation (in the CANDU system) on raising

the steam generator pressure.

* * * * *

We have covered the major points concerning the stearn

generator. You should turn to the objectives and read them
carefully. If you feel that you can satisfy these require-
ments, ask the course/shift manager for the Criterion Test.

* * * * *

When you have completed the test, ask for the self
Evaluation Sheet and compare your answers.

When you are ready, ask the course/shift manager to re-

view your work. If you identify areas that need further
practice, return to the relevant section and then try the
test again when you feel you are ready.

When you are both satisfied with your work, have the
manager sign off the progress summary sheet and proceed to
the final Module, 8.1 II Reactor".

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 125 - B.2

Answers

MODULE B.2

STEAM GENERATOR

In a "crash-cool" exercise, the steam is rejected from

the steam generator fast enough that the pressure will
fall. In this situation, the temperature in the steam gener-
ator falls with the pressure. The result of the falling
temperature is to increase the temperature difference between
the PHT system and the steam generator which increases the
rate of heat removal from the reactor and reduces the time
for reducing reactor temperature.

B.2.2

The effect of increased thermal resistance means that a

higher temperature difference is required to transfer the
same amount of heat. This is exactly the same as in the
electrical analogy where the voltage applied to a higher
resistance has to be increased to transfer the same amount of
power through the circuit.

The higher temperature difference can only be produced

by an increase in the PHT average temperature. So an in-
crease in the thermal resistance of the steam generator
tubes, due to corrosion products and other material contamin-
ation, will result in an increase of the average PHT tempera-
ture.

B.2.3

Using the steam table, we can compare the specific vol-

ume of liquid vf at 170°C and 250°C using table I,

Vf at 170°C = 1.1144 £/kg

vf at 250°C = 1. 2513 £/kg.
Change in volume 1. 2513 - 1.1144

= 0.1369 £Ikg.
This percentage increase in volume = (0.1369/1.1144) x 100

- ...12.3%.
,---

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 125 - B.2

Obviously there is some increase in level solely due to this

expansion effect.

B.2.4

Suppose the steam generator is at operating temperature

but producing no steam. At this condition the boiler would
be full of liquid containing no vapour bubbles. The level of
the liquid would be that corresponding to the programmed
level at 0% power.

If the heat input to the steam generator is increased

boiling will now occur and vapour bubbles will be produced
within the liquid. This will have the effect of "floating"
the surface of the liquid to a higher level.

As the rate at which heat is being supplied to the boil-

er increases, to the maximum, so the generation of vapour
bubbles reaches a maximum. At this full power steaming rate
the steam generator level reaches its highest value.

Steam is leaving the boiler and the fluid is being re-

placed by feedwater entering the boiler to maintain a level,
programmed to the rate of steaming, to keep the mass of water
in the vessel sensibly constant.

At full load approximately 10% of the weight of fluid in

the boiler is due to vapour bubbles. These vapour bubbles
produce an increase in the total fluid volume of approximate-
ly 5 times. when steaming at full power.

B.2.5

Liquid has a high density in relation to vapour. It is

also relatively incompressible. This means that when a
change of direction is needed with liquid flows at high
veloci ties and large flowrates very large forces can result.
Water hanuner is an illustration of this effect. The liquid
will tend to move in a straight line. Can you imagine a slug
of water passing through the high pressure turbine in a
straight line? Slugs of water in a steam turbine produce the
same type of problem as birds flying into aviation g.as tur-
bines.

The blading at tempts to change the direction of the

liquid flow into the turbine and it is even money at best as
to whether the blade is strong enough to withstand the impact
or the water breaks the blading and wholesale blade shedding
results.

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 125 - B.2

Needless to say the presence of water is to be avoided

and this event is anticipated by a high level alarm on the
steam generator which may allow some operator action before a
high level trip operates the governor steam valves on the
turbine to exclude the liquid.

B. 2.6

One of the easiest ways of analyzing this effect is to

return to the temperature/enthalpy diagram and plot the
initial condition and raise the pressure keeping the enthalpy
constant.

Temperature,
°c
T, B
,,'-- P,
-'-'- J

T, - __ /-_~
P,
'-'- '-/
A

Enthalpy. J/kg
Fig. 2.4

Initially the steam generator has fluid as liquid/vapour

mixture at pressure PI as shown at point A. When the pres-
sure suddenly increased to P2 the mixture is now below the
saturation temperature corresponding to the higher pressure
and the vapour bubbles condense as the latent heat of vapour-
ization is used to raise the liquid to the new saturation
temperature.

The condensation process causes the vapour to di'sappear

and the volume shrinks resulting in a reduced steam generator
level.

B.2.7

There are basically three problems that arise from a

very low steam generator level.

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 125 - B.2

First, as the water inventory in the steam generator is

reduced there is less capacity as a heat sink for the reac-
tor. This means that from a control point of view we are
moving in a direction where we have more reactor thermal pow-
er than we can handle. Not a desireable situation!

Secondly, if the level falls below the low level tapping

on the steam generator, the level control program will not
recognize this event and actual level measurement will be
lost.

Thirdly I if the steam generator level falls below the

top of the tube bundle, dry out will occur and dissolved
solids existing in the steam generator will "bake out" onto
the external tube surfaces and impede future heat transfer.

The probabilities of the above events occurring are re-

duced by a low level alarm which may allow some operator
action. If this is not successful, a significant reduction
in reactor power occ~rs to restore the match of thermal power
of the reactor to the reduced heat sink capacity of the steam
generator. The reduction of reactor power may be a setback
or trip depending upon operating rationale at the specific
station.

B.2.8

The whole concept of changing the programmed level with

steaming rates revolves around maintaining adequate heat sink
for the reactor.

If you don't think about it, it would appear that the

Bruce NGS-A reactor which is 60% larger than Pickering NGS-A
doesn't require as large a heat sink. This obviously is not
the case. There is a large design difference in the steam
generators at Bruce NGS-A not the least of which is the com-
mon steam drum which is partly full of liquid and therefore
presents a much larger capacity than at Pickering NGS-A.
This is the primary reason for the smaller change in steam
generator level with power, there is more capacity available
for the same level change.

B.2.9

The three elements are:

(a) Steam flow,

(b) Feedwater flow,
(c) Actual level.

The steam flow is used to produce the programmed level.

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 125 - B.2

The comparator circuits look at:

Steam/Feedwater flow
Actual/Programmed level.

At low power levels measurement of steam flow and feed-

water flow is not very accurate and control of the feedwater
flow via the feedwater regulating valves is insensitive. At
this point the steam generator level is more easily handled
by the level controller alone without the other two elements.

When the steam flow is in excess of 20% and the large

feedwater regulating valve is being used, the three elements
may be used to monitor steam generator level.

B.2.10

Every station is going to have different systems and

constraints. As a result we can only examine the concepts
and then see how the concepts are applied in the operating
manuals.

The question does not state whether the boiler is asso-

ciated with a bank of boilers, furthermore it does not state
whether all the boilers have the same high level.

We must make some assumptions. We 11 assume that the

I

boiler is in a bank of boilers and is the only boiler with a

high level.

At low loads it is common for different boilers to have

different steaming rates due to physical positions within the
system. It is important to identify the boiler which has the
highest steaming rate and ensure that the feedwater trim/iso-
lating valves are left in the full open position.

The high level in the boiler should be reduced by

slightly opening the trim valves on the remaining boilers.
The objective is to have all the boiler levels at sensibly
the same value.

If after adjusting trim valves the levels overall remain

high, then this situation may be corrected by reducing the
setpoint of the feedwater control valve controller.

This situation is most likely either at low loads where

small changes in actual flowrates are going to have a very
significant effect, or when reactor power distribution to the
boilers is changed by changes in reactor zonal power produc-
tion.

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 125 - B.2

If there is a danger of boiler high level tripping the

turbine then the boiler blowdown valves may be opened to try
and prevent this happening.

B.2.11

The three main heat sinks for the steam generator are:

(a) Steam Turbine

(b) Steam Rejection System
(e) Boiler Stearn Safety Valves.

Steam Turbine

This is the normal heat sink and is used as a heat sink

when the turbine steam flow is used to control the boiler
steam pressure.

Steam Rejection System

This is used as the second heat sink and may reject

steam to atmosphere or the condenser depending upon the sta-
tion in question. This system is used if the turbine is not
available to remove the excess steam. In this case the off-
set is removed and the SRV's operate as scon as the pressure
setpoint is exceeded.

Boiler Safety Valves

In the unlikely event that neither the turbine nor the

SRV's can restore the over pressure the boiler safety valves
will lift to protect the steam generator from overpressure.

B.2.12

The "reactor-leading" mode is the 'normal' mode for

Pickering NGS-A which means that the reactor power will stay
constant whilst the steam flow is adjusted to maintain the
pressure setpoint.

If the turbine speeder gear is not controlled by the BPC

program then no change in steam flow to the turbine can occur
and steam flow from the steam generators will be achieved by
opening of the reject steam valves_

The offset which normally applies to the steam reject

valves, when the turbine is available to the BPC program, is
removed. As soon as the boiler pressure exceeds the setpoint
pressure the steam reject valves will start to open.

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 125 - 8.2

If the over pressure is such that the large reject valves are
needed, then a reactor setback will be initiated to reduce
the time taken to restore control.

The reactor setback would stop when the large steam re-
ject valves closed. If this did not happen the reactor would
reduce power to 2% FLP.

8.2.13

In the 'reactor lagging' mode of operation the variable

power is associated with the reactor. If the steam pressure
started to fall below the setpoint pressure the demanded
reactor power would be increased to restore the steam gener-
ator pressure.

In the event that the steam pressure continued to fall

the unit control would change and initiate a slow speeder
gear runback until the steam pressure was restored.

8.2.14

The BPC program relies upon being able to change the

steam flow from the boiler to change the boiler pressure.

As the steam pressure in the boiler drops the volume of

steam increases. For example at 250°C, I kg of dry steam has
a volume of 50 liters. As the temperature and pressure fall,
this volume increases. At 130°C the volume has now increased
to 668 liters per kg which is an increase of more than 13
times.

The effect of this increasing steam volume causes the

SRV's to open until they reach a point where they are fully
open and can no longer reduce the pressure in the steam
generator.

This happens at around 170°C. As a result, this is the

termination point of coo1down using BPC. Further cooling of
the PHT system will take place using the shutdown cooling
circuits.

8.2.15

As the pressure in the steam generator is raised, the

water boils at a higher temperature, ie, the steam tempera-

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 125 - B.2

ture increases. This has the effect of increasing the temp-

erature difference between the steam in the steam generator
and that in the condenser. This increased temperature dif-
ference means there is more work available in the turbine,
which increases the cycle efficiency.

B.2.16

The fuel limits the operating pressure and temperature

of the steam generator. The fuel is a ceramic which has very
poor heat transfer characteristics. With centre fuel temper-
ature about 2300°C, the sheath temperature is only 350 to
400°C. Allowing for heat transfer from the fuel to the 020
and heat transfer from the D20 to the light water in the
steam generator, the operating temperature in the steam gen-
erator is around 250°C.

If the centre fuel temperature reaches the melting point

(at about 2BOO°C), release of fission product gases from the
fuel may contribute to sheathing failure and escape of fis-
sion products into the primary heat transport system.

Thus the limiting fuel temperature is 2300°c (allowing a

safety margin), which means the maximum pressure available in
the steam generator is the saturation pressure corresponding
to 250°C, ie, about 4 MPa(a).

J. Irwin-Childs

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