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PWR Initialization

Source

Rev 1 Modeling Guidance, §20 — Steady State Initialization; §21 — Transient Initialization; Appendix A.20 — SS Initialization Bases; Appendix A.21 — Transient Initialization Bases.

Overview

This section provides guidance on initializing PWR models in TRACE, covering both steady-state setup and transient restart preparation. The guidance is applicable to Westinghouse ⅔/4-loop, CE 2-loop, and B&W PWRs.

Part 1: Steady-State Initialization

Target Steady-State Values

The base model should represent nominal Hot Full Power (HFP) conditions. Transient-specific analyses may require different initial conditions (e.g., 102% power for Appendix K LOCA).

Typical targets:

Category Parameters
Core power Total power, radial/axial distribution
Water inventories RPV mass, total primary mass, SG mass
Primary thermal-hydraulics Thot, Tcold, pressurizer P and level, loop flow rate, core flow, bypass flows
Secondary thermal-hydraulics SG dome P, SG outlet T, steam flow, SG level, recirculation ratio, FW flow/temperature
Pressure drops RCP head, cold leg ΔP, vessel nozzle ΔP, downcomer ΔP, core ΔP, hot leg ΔP, SG primary ΔP, SG secondary ΔP, MSL ΔP
External systems Containment P, injection water T/P, AFW T

Pressure Boundary Condition

A temporary BREAK component is added to the pressurizer during steady-state initialization to hold the system at the target pressure. This is connected via an isolation VALVE that is closed at the start of the transient.

Pressurizer Boundary Isolation Valve (VALVE):

  • Zero-cell VALVE connected between pressurizer and BREAK
  • IVTY = 3; initially open (FAVLVE = 1.0)
  • Closed at transient start via trip (VTB1 ramps to 0.0)

Steady-State Pressure BREAK:

  • IBTY = 0 (constant pressure)
  • PIN = target pressurizer pressure
  • ISAT = 3; DXIN = 10⁻⁶ m; VOLIN = 10⁶ m³
  • ALPN = 1.0 (all gas — matches pressurizer steam space)

Signal Variables & Control Blocks

Set up monitoring of key parameters before running the CSS calculation:

  • Liquid level signal variables — for pressurizer, SG downcomer
  • Pressure drop signal variables — across major components (core, SG, pump, etc.)
  • Mass flow signal variables — for loop flow, core flow, bypass fractions

Constrained Steady-State (CSS) Controllers

CSS controllers automatically adjust selected parameters to achieve target values:

  • Pressurizer level → adjust letdown flow
  • SG level → adjust FW flow (or use FW control system)
  • SG pressure → adjust TCV position
  • Core bypass flow fractions → adjust bypass K-loss coefficients

Core/Bypass Loss Coefficient Tuning

Iterative process to match target flow distribution:

  1. Set up mass flow signal variables and sum control blocks
  2. Calculate flow fraction control blocks
  3. Build PI controllers to adjust K-loss coefficients
  4. Run CSS calculation
  5. Check convergence; iterate if needed

Running the CSS Calculation

  • STDYST = 2 (CSS Calculation)
  • Run for sufficient time to reach equilibrium (typically 200–1000 s)
  • Monitor key parameters for convergence

Review Steady-State Results

Four levels of verification:

Check Description
Equilibrium All parameters stable (derivatives ≈ 0); no oscillations
Tuned variables CSS-controlled parameters match targets (level, pressure, flow)
Iterative tuning K-loss coefficients converged; bypass flow fractions correct
Target validation All target values from §20.1 matched within acceptable tolerance

Problem Diagnosis/Debugging

Common issues:

  • Code failures at initialization → check for inconsistent ICs, zero flow areas, extreme temperatures
  • Oscillations → check for control system instability, reduce CSS controller gains
  • Incorrect pressure drops → adjust KFAC values iteratively
  • SG level issues → check separator parameters, FW control system

Finalize Base Model

  1. Update model with steady-state values from the converged calculation
  2. Remove CSS controllers (or deactivate)
  3. Close pressurizer boundary valve
  4. Run null transient to verify stability (TRANSI=1, no perturbations, ~100 s)
  5. Confirm all parameters remain stable

Part 2: Transient Initialization

NRC Staff Experience

Most NRC analytical experience is focused on LOCA analyses. Guidance for other transients is based on more limited sample sizes and will evolve with future experience.

General Transient Procedure

  1. Determine if a new steady state is needed (changed power level, different axial shape, break renodalization)
  2. If yes: re-run as GSS (STDYST=1) with the modified model — iterative K-loss tuning is generally not needed since the base model is already validated
  3. Create restart input deck
  4. Set main problem variables:
Parameter Value
DSTEP −1 (Auto)
TIMET −1.0 (Auto)
TRANSI 1 (Transient)
DTSTRT −1.0 (brief null transient first) or 10⁻⁴ (immediate start, typical for LOCA)

Restart Consistency

Ensure all initial conditions for components in the restart deck match the end-of-steady-state values. This is especially critical for break components — mismatched P/T/void across edges causes artificial transients or code failures. Import ICs from the steady-state restart file before saving.

Transient-Specific Checklists

LOCA

  • Break location and type (DEG/split) — see Break Modeling principles
  • Break pipe renodalization (L/D criteria)
  • BREAK component IBTY (0 or 7)
  • ECCS actuation trips and setpoints
  • Containment backpressure assumptions
  • Reactor scram timing
  • RCP trip timing (coastdown model)
  • Initial power level (102% for Appendix K)
  • DTMAX: 10⁻⁴ s initially, may relax after blowdown

Loss of Feedwater

  • FW trip timing and coastdown
  • AFW actuation criteria and flow rate
  • Feed-and-bleed initiation criteria (if applicable)
  • PORV/SRV setpoints
  • SG dryout timing considerations
  • RCP trip criteria

Feedwater Line Break

  • Break location (inside vs. outside containment)
  • Affected and unaffected SG identification
  • MSIV closure timing
  • AFW to affected vs. unaffected SG
  • Containment conditions

Turbine Trip

  • TSV/TCV closure characteristics
  • MSSV/ADV actuation
  • Reactor scram (if credited)
  • Secondary side pressure transient
  • DTMAX: may need smaller values during rapid pressure changes

Steam Generator Tube Rupture

  • Tube rupture location and flow area
  • Affected SG identification
  • Operator actions (isolation, cooldown)
  • MSSV/ADV behavior on affected SG
  • Primary-to-secondary leakage rate

Cross-References

This page faithfully extracts steady-state initialization (§20) and transient initialization (§21) guidance from Rev 1. The SS process uses a temporary pressure BC BREAK with CSS controllers to achieve target conditions, followed by validation against plant data. Transient initialization provides checklists for LOCA, LOFW, FWLB, turbine trip, and SGTR scenarios.