Reactor Control System Purpose: Through a combination of automatic and manual (operator) actions, maintain the NSSS’s characteristics on their targeted (programmed) values during “normal” operation. Ex: Nuclear Power = Steam Power = Electrical Power/efficiency Steam P S/G (PWR) or Reactor Vessel (BWR) level PRZ (PWR) level PRZ P (PWR) Primary Coolant Temp (PWR) [related to steam P] Core Void Content (BWR) PWR

Reactor

SG Turbine

Condenser

T

T

T

P

T

P

P

WT

T

TCV

hot

cold

avg

rx

sg sat

sg

cond

T

imp

=

stm

At steady-state /RX sg TQ Q W

[ ]RX RX RX avgQ UA T T 2

hot coldavg

T TT

[ ]sg sg avg satQ UA T T

Reactivity: 0 CR B MOD RX FP 0 reactor critical and at

steady state

0( )MOD MOD avg avgT T 0MOD

0( )RX RX RX RXT T 0RX

“NATURAL” RESPONSE TO POWER MANEUVERS

Power maneuvers are typically initiated by actions on the steam side. What is the “Natural” system response (no automatic control operations) to perturbations initiated on the secondary (steam) side?

Reactor

SG Turbine

Condenser

T

T

T

P

T

P

P

WT

T

TCV

hot

cold

avg

rx

sg sat

sg

cond

T

imp

=

stm

Suppose we desire T stW m Open TCV

sg satP T

If sgUA constant (UTUBE) avgT

( )avg MODT RxPwr

RxPwr 0RX RXT

The reactor will settle out at a new higher power level such that RxPwrnew = [ ] [ ] ( / )new new new

RX RX avg sg avg sat TUA T T UA T T W with 0newsg sgP P

Continued increases in TW Continued decreases in sgP

Reactor

SG Turbine

Condenser

T

T

T

P

T

P

P

WT

T

TCV

hot

cold

avg

rx

sg sat

sg

cond

T

imp

=

stm

Suppose we desire T stW m Close TCV

sg satP T avgT

( )avg MODT RxPwr

RxPwr 0RX RXT

The reactor will settle out at a new lower power level such that RxPwrnew = [ ] [ ] ( / )new new new

RX RX avg sg avg sat TUA T T UA T T W with 0newsg sgP P

Continued decreases in TW Continued increases in sgP

Pressurized Water Reactors are natural load followers. However, this mode of operation results in relatively large swings in Psg, and Psg moves in directions opposite of that desired.

Suppose we wish to keep Psg approximately constant over the entire power range.

Reactor

SG Turbine

Condenser

T

T

T

P

T

P

P

WT

T

TCV

hot

cold

avg

rx

sg sat

sg

cond

T

imp

=

stm

T stW m Open TCV however, require sg satP T and impP

[ ]

sgQ

sg avg satst

g fd

UA T Tm

h h

For h and sat st avgT m T

( )avg MODT

In addition [ ]sg RX RX RX avg RXQ Q UA T T T

( )RX RXT

To maintain Psg constant at higher powers implies negative reactivity which must be compensated for. Solution Withdraw control rods.

Reactor

SG Turbine

Condenser

T

T

T

P

T

P

P

WT

T

TCV

hot

cold

avg

rx

sg sat

sg

cond

T

imp

=

stm

Similarly if we desire T stW m Close TCV impP

[ ]

sgQ

sg avg satst

g fd

UA T Tm

h h

For h and sat st avgT m T

( )avg MODT

In addition [ ]sg RX RX RX avg RXQ Q UA T T T

( )RX RXT

To maintain Psg constant at lower powers implies positive reactivity which must be compensated for. Solution Insert control rods.

CONTROLLING Tavg IMPLIES CONTROLLING Psg

Pro

gram

med

Tav

g

Electrical Power0 % 100 %

The Tavg program is chosen based on PRZ size and steam P considerations.

where ( )sg AVG sat sat Sat SGSGQ UA T T T T P

Given avgT and /RX sg TQ Q W T across S/G Fixed satT of steam fixed

SGP fixed

In OTSGs, SGQ while holding both Tavg and Tsat constant can be accomplished by

changing the steam generator level (UA). However, TRX still as before requiring control rod motion.

PWR: Nuclear Power

Electrical Power Turbine Impulse Pressure Reactor Power ( relP )

Core Av. Coolant Temp. AvgT

Turbine Impulse Pressure (psia)

0 200 400 600 800 1000

Rel

ativ

e P

ower

(%

)

0

20

40

60

80

100

120

Reactor PowerElectrical Power

Automatic Control System: Operator sets Electrical Power (Load or Demand Setpoint) Ref Pimp

TCV moves which automatically sets high pressure turbine impulse pressure

Impulse Pressure Automatically sets Programmed AVGT Automatically

moves control rods in or out to make AVGT Programmed AVGT

Speed of rod motion AVG AVGT Programmed T

ISSUE: Time lags in system can result in undershoot/overshoot Example: Step load increase

SOLUTION: Add an “anticipatory” signal based on the difference between turbine output and reactor thermal power

rel rel

1

Electrical P Core P

E =( ) ( )

load RX

load ref RX ref

W Q

W Q

2 avg avgE = Programmed T T

1 1 2 2E=G E +G E

Rod Speed E

What if control rods result in undesirable positions? Ex: Totally withdrawn Cannot withdraw anymore

Too deep inserted Violating control rods insertion limits not enough rod

worth to trip plant to required subcritical condition Power shape violates power peaking limits, e.g. F or QF .

How are rods repositioned? Change soluble boron concentration in the primary loop!

Rods too inserted Borate Rods too withdrawn Dilute

How much should PPM be changed?

0 1 0 1rodsPPM PPM Position Position

Special situation of rapid load reduction

Electrical relP (with time) rapidly AVG AVG

rel rel

T Programmed T

Core P Electrical P

rapidly

Control rods insert Core relP

Problem: Control rods don’t insert fast enough Core relP Electrical relP

AVGT continues AVGT Programmed AVGT .

What happens to steam P?

Electrical relP rapidly

AVG

Turbine control valves partially close

T

Steam P

Automatic Control System Actions

Rapid decrease in Electrical AVG

rel

T

P

Steam P

Control rods inserted

Core relP

Core relP but Core relP ElectricalAVG

rel

T

P continues

Steam P

AVG AVG Set Point

Set Point

T Programmed T ΔT

Steam P > Steam P

If

Open Steam Dump (Turbine Bypass)

Valves Blow Steam (to condenser or atmosphere) Steam P . Steam Temp SG AVGΔT T …Steam dump valves stay open until setpoint

values not exceeded.

Feedwater Pump Speed Control

F/W Pump

Feed Control ValveSteam Generator

Turbine Control Valve

High Pressure TurbineRCS Flow

PressureP

P P

f/w

steamImpulse

Typical Pump Curve

P

G

RPM

p

RPM 1

RPM 2

Pressure Drop Across the Feed Control Valve

2

( )2

FWFCV dis SG FCV FCV

GP P P K H

0

2( )

FCV

FCV FCVFCV

KK H

H

0

22 2 FCVFW FCV FW FCV FCV

FCV

HG P G P H

K

If the pressure drop across the feed control valve is constant, then feed flow increases linearly with valve position (linear control valve). This simplifies the control problem. Pump speed controller adjusts pump RPMs such that

dis SG FCVP P P Constant

For SGP Constant

FCV GFW Pdis FCVP

RPMs Pdis FCVP Constant

Turbine Control

P Psteam impulse

Turbine Control Valve

Pimpulse Controller

Set Elec. Prel Set Prog. impulse impulse impulseP P Prog P 0 Close

down on turbine control valve impulseP

How is Prog impulseP determined?

Require RPMT/G = Prog. RPMT/G (1800 @ all Elec. Prel AC frequency) With fixed RPMT/G and Elec. Prel Pimpulse uniquely specified RPM Controller T/G T/GRPM Prog RPM 0 Close down on control valve

Pimpulse RPMT/G Error signal for T/G controller controls both impulseP and T/GRPM utilizing Turbine

Control Valves

S/G Level Control (UTUBE) Programmed level to assure S/G tubes covered to assure adequate heat transfer and separators not flooded.

Level %

Relative Power0 100 %

1. S/G level correct.

If Steam Mass Flow Rate = F/W Mass Flow Rate S/G level constant. If S/G level correct, adjust F/W Flow Rate such that it equals Steam Flow Rate. Adjustment done by valve and/or variable speed F/W Pump

2. S/G level not correct.

Adjust F/W Flow Rate to bring steam generator level to programmed value.

Issue: Load follow maneuver

Electrical relP Turbine Control Valves Open

Steam flow

Steam PsatT

Water in S/G Flashes S/G Level

“Swell” effect (short lived phenomena)

Electrical relP … S/G level … Called “Shrink” effect

Automatic Control System Actions

Electrical relP S/G swell F/W Flow

Steam Flow Desire F/W Flow

Conflict!

How resolved?

a) Delay (lag compensation) S/G level signal to automatic control system b) Control F/W flow by Steam/F/W Flow mismatch

Electrical relP S/G Swell

Steam Flow

F/W Flow

S/G level {due to initial swell effect passing} then Prog. Level F/W Flow until S/G level restored and F/W flow = S/G flow.

Integration of Control Directly Controlled: AVG S/G impulse T/GT , Level , P , RPM

Not Directly Controlled: rel rel Steam Steam F/WElec. P , Core P , m , P , m

Question: How do we know these not directly controlled parameters will have desired values? Assume Steady-State

Set rel impulse T/GDemanded Elec. P P /RPM Elec. Prel Core Prel RXQ

Pimpulse TAVG

RX SG SG AVG sat steamQ =Q =UA [T -T (P )] Psteam

sg steamsteam steam F/WQ = h P -h m

steamm SG

g steam F/W

Q=

h (P )-h

Elec. Prel LevelS/G

LevelS/G steam F/W m = m

Conclusion: Specifying: Elec. relP

Directly Controlling: AVG S/G impulse T/GT , Level , P , RPM

Assures

Not Directly Controlled: rel sg steam f/w SteamCore P ,Q , m , m , P

Have Desired Values!

Note constraints: Steam impulseP >P

F/WP has to be such to meet this constraint.

Response to Load Maneuver relElec. P Prog impulseP Turbine Control Valve Pimpulse

Turbine Control Valve steamP Steam S/G sgT T Q

AVG rel

AVG

T Core P

Prog. T Control Rods Insert

relCore P

When WR sg AVG Steam SteamCore P < Q T T P

steam lvl

steam

P SG then

TCV m

F/W Control Valves f/w m

Note: T/GRPM control is also required to assure relElec. P demanded satisfied

Indirectly control steamm to correct value.

Prz level

Electrical relP Programmed AVGT Primary loop Av. Water Density Primary Loop

Water Volume Unique Prz Level (Programmed Prz Level)

Prz level maintained by CVCS Prz level Prog. Prz level CVCS Charging Rate CVCS Letdown Rate.

Automatic Control System Tavg Programmed Prz. Level CVCS Rate dependent upon Prz. Level

Programmed Prz. Level When Tavg and Prz Level = Prog. Prz level CVCS Charging Rate = CVCS Letdown Rate.

Prz Pressure

Objective: Hold Primary System P constant set pointP

Assume SS operation P = set pointP

Prz Heaters on sufficient to offset heat losses from Prz and spray circulation (some to keep Prz chemistry same as rest of RCS & Minimize Thermal Shock)

Electrical relP ( ) AVGT ( ) initially Prz level ( ) Prz P ( )

Automatic Control System

(Prz P – PSet Point)

0 Prz Heaters Output

0 Prz Heaters Output

If (Prz P – PSet Point) Set Point(1)ΔP Prz Sprays Prz Steam Condenses Prz P

If (Prz P – PSet Point) Set Point (2) Set Point (1)ΔP >ΔP Prz Pilot Operated Relief Valve (PORV)

Opens Prz Steam blows down to Prz Relieve Tank Prz Pr If (Prz P – PSet Point) < Set Point(3)ΔP Backup Heaters On

Longer Term Effect:

rel AVG AVG AVGElec. P Prog. T T via T control ...