PWR Control Systems¶

Source

Rev 1 Modeling Guidance, §19 — Control Systems Guidance; Appendix A.19 — Control Systems Bases.

Overview¶

This section provides guidance on building selected sample control systems in TRACE using signal variables, control blocks, and trips. These are simplified control schemes representative of important aspects of facility behavior. When plant-specific control scheme descriptions are available, they should be used. The guidance is applicable to Westinghouse ⅔/4-loop, CE 2-loop, and B&W PWRs.

Control System Documentation

Analysts are strongly recommended to incorporate suitable comments in TRACE and/or logical views in SNAP which explain the function of each control component. Control systems are the most prone to facility-specific modifications.

Restart Calculations

TRACE has had difficulty properly overlaying controls into restart models. Include all control systems expected for future calculations in the base model. Deactivate unused systems by setting trip setpoints to values that will never be reached.

Control Systems Covered¶

System	Function	Scope
FW flow — normal	Temperature, flow rate, pressure control	SS
FW flow — coastdown	Post-trip FW pump rundown	Transient
TCV position	Valve position for steady-state steam flow	SS
TCV closure	Post-trip valve closure	Transient
AFW flow	Initiation and level control	Transient
ADV control (SG cooldown)	Controlled depressurization via ADVs	Transient
RWST switchover	HPSI/LPSI suction transfer from RWST to sump	Transient
Trip controller trees	Combining multiple RPS signals into one trip	Both
System availability	Single-failure and availability gates	Both
Pressurizer heater control	Heater on/off based on pressure setpoints	SS/Transient

Simple valve controls (IVTY=3 trip-and-table) are already covered in the component-specific guidance sections and are not repeated here.

Required Setpoints & Parameters¶

Steady-State Control¶

Nominal SG downcomer water level
Nominal FW flow rate (= nominal steam flow rate at SS)
Nominal FW temperature
Nominal SG steam dome pressure (W/CE) or SG outlet nozzle pressure (B&W) or turbine header pressure
Nominal steam flow rate

Transient Control¶

FW trip setpoints and pump coastdown curve
TCV closure setpoints and position-vs-time
AFW initiation setpoints; upper/lower SG level setpoints
SG cooldown initiation setpoints (RCS subcooling, time); target cooldown rate
RWST initial liquid mass and switchover mass
All RPS trip setpoints for trip controller trees

Feedwater Control System¶

The FW control system has four subsystems:

1. Flow Control¶

Controls the FW FILL component mass flow rate to maintain nominal SG downcomer water level. Uses a proportional-integral (PI) controller with:

Error signal = target level − actual level (from signal variable monitoring SG downcomer)
Controller output drives FW flow rate

2. Temperature Control¶

Controls the FW FILL liquid temperature to match the nominal FW temperature. Typically a simple constant or table lookup.

3. Pressure Control¶

Controls the FW FILL pressure if the FW injection line is modeled. Otherwise not needed.

4. Coastdown Control¶

Post-trip FW flow reduction — activates on FW trip signal, ramps flow down per pump coastdown curve. Uses trip-controlled table lookup with time-since-trip as independent variable.

TCV Control System¶

Position Control (Steady State)¶

Adjusts TCV flow area fraction to maintain target steam pressure or flow rate. Uses PI controller with:

Error signal = target pressure − actual steam header/dome pressure
Output controls TCV FAVLVE

Closure Control (Transient)¶

Trip-activated TCV closure — uses VTB1 table (flow area fraction vs. time since trip) controlled by turbine trip signal.

AFW Flow Controller¶

Activates on AFW initiation signal (typically low SG level). May include:

Simple on/off control: constant AFW flow when trip active
Level-controlled: throttle AFW based on SG level (reduce flow when level recovers above upper setpoint)

SG Cooldown Pressure Control¶

Uses ADVs to perform controlled depressurization:

Activates when RCS subcooling criteria are met
Controls SG pressure reduction rate by modulating ADV flow area
Target: reduce SG pressure at a specified rate (typically in °F/hr equivalent)

RWST Switchover Controller¶

Monitors RWST liquid inventory via control blocks:

Integrates HPSI + LPSI injection flow over time
When cumulative injected mass approaches initial RWST mass, switches ECCS suction from RWST to containment sump
Implemented via trip comparing integrated mass against switchover threshold

Trip Controller Trees¶

Multiple RPS signals that cause the same event (e.g., reactor scram) are combined using trip logic:

Individual trip components for each signal (e.g., high pressure, low flow, etc.)
Combined using logical OR via a controller trip that monitors all individual trips
Final trip output used as IRPWTR (scram) or IVTR (valve actuation)

See Power & Kinetics for the list of typical scram signals by vendor.

System Availability Gates¶

Model single-failure assumptions by deactivating specific systems:

Use a "gate" trip that is either always ON or always OFF
Connect the gate to the system's controlling trip
Toggle the gate to model equipment availability/unavailability scenarios

Pressurizer Heater Control¶

Models proportional and backup heater banks:

Proportional heaters: modulate power based on pressure error (target − actual)
Backup heaters: activate when pressure drops below a lower setpoint
Both controlled via signal variables monitoring pressurizer pressure
Output drives FLPOWER or equivalent heater power input

See Pressurizer for heater-specific inputs (PSET, DPMAX, ZHTR, POWIN).

Cross-References¶

Pressurizer — Heater control inputs, PORV/SRV trip setpoints
ECCS — FILL components controlled by these systems
Steam Circuit — TCV, TBV, ADV, MSSV valve trip connections
Power & Kinetics — Scram trip controller (IRPWTR)

This page faithfully extracts control system guidance from Rev 1 §19. Eight control systems are covered: FW flow (4 subsystems), TCV position/closure, AFW, SG cooldown via ADV, RWST switchover, trip controller trees, system availability gates, and pressurizer heater control. All systems use TRACE signal variables, control blocks, and trips. Plant-specific control schemes should be used when available.