PWR General Modeling Guidance¶
Source
Rev 1 Modeling Guidance, §1 — General Modeling Guidance; Appendix A.23 — Generically Applicable Basis for Thermal Hydraulic Components.
Overview¶
This section provides general guidance on modeling approaches applicable to all PWR components in TRACE. The guidance is applicable to Westinghouse ⅔/4-loop, CE 2-loop, and B&W PWRs. Topics covered:
- Main model options
- NAMELIST variables
- Material specifications
- Component orientation
- Friction factor application
- Pipe wall modeling
- Solute tracking (boron)
- Valve flow capacity tests
- Stand-alone system initialization
Main Model Options¶
| Variable | Recommended Value | Notes |
|---|---|---|
| INOPT | 1 (NAMELIST after title cards) | — |
| ID2O | 0 (H₂O) | Default fluid |
| DSTEP/TIMET | Auto (0/0.0 for standalone; −1/−1.0 for restart) | — |
| IPAK | 1 (water packing on) | — |
| EPSO | 1.0 × 10⁻⁴ | Pressure convergence |
| EPSS | 1.0 × 10⁻⁴ | Steady-state convergence |
| OITMAX | 10 | Pressure iterations |
| SITMAX | 10 | Steady-state iterations |
| ISOLUT | 0 (off) or 1 (on) | Enable for boron-sensitive transients |
Component Orientation¶
Component orientation is specified at each edge via three parameters:
- Orientation — vertical, horizontal, or angled
- Angle (θ) — angle of the pipe segment in radians (+π/2 = vertical up, −π/2 = vertical down, 0 = horizontal)
- GRAV — sin(θ), the gravitational projection factor
For curved pipes (elbows, U-bends), the approach is to approximate the curve with straight segments at intermediate angles. The modeled elevation change across each segment should match the physical plant.
Friction Factor Application (NFF)¶
The friction factor correlation option (NFF) controls how TRACE computes wall friction at each edge:
- NFF = 1 — TRACE computes friction internally using the Churchill correlation (recommended for most edges)
- NFF = 0 — Friction not computed; analyst provides all losses via KFAC/KFACR (required at edges with significant flow area changes)
Nodalization Criterion for NFF = 0
When NFF = 0 at an edge, the cells on both sides of that edge must satisfy a nodalization criterion: the cell length DX must be at least 0.625 times the hydraulic diameter (DX ≥ 0.625 · HD). This ensures that the unmodeled wall friction is insignificant compared to the form losses. This criterion may be relaxed for very short components (plenums, pump nozzles).
Additive Loss Coefficients (KFAC/KFACR)¶
Form losses at edges with flow area or direction changes:
| Geometry Change | KFAC (Forward) | KFACR (Reverse) | Reference |
|---|---|---|---|
| Sudden contraction | 0.5 · (1 − β²) | (1 − β²)² | Crane pg. 2-11 |
| Sudden expansion | (1 − β²)² | 0.5 · (1 − β²) | Crane pg. 2-11 |
| 90° standard elbow | 30 fT | 30 fT | Crane pg. A-29 |
| 45° standard elbow | 16 fT | 16 fT | Crane pg. A-29 |
| Close-pattern return bend | 50 fT | 50 fT | Crane pg. A-29 |
| Standard tee (through branch) | 60 fT | 60 fT | Crane pg. A-29 |
where β = Dsmall/Dlarge and fT = Darcy friction factor for smooth commercial steel pipe (Table 1.5-1).
Wall Surface Roughness (EPSW)¶
| Material | EPSW |
|---|---|
| Commercial steel pipe | 4.572 × 10⁻⁵ m (1.5 × 10⁻⁴ ft) |
| Drawn tubing (SG tubes) | 1.524 × 10⁻⁶ m (5.0 × 10⁻⁶ ft) |
| Plant-specific | Use measured value if available |
Pipe Wall Modeling¶
Two approaches for modeling metal mass heat transfer:
- Internal pipe wall — TRACE spawns a simple pipe wall heat structure. Suitable for simple cylindrical components with one-sided heat transfer.
- HTSTR components — Manually created heat structures. Required for structures coupled to multiple hydraulic components, complex geometries, or fuel rods.
Most SG and RPV structures require HTSTR components (see Heat Structures).
Solute Tracking (Boron)¶
When ISOLUT = 1, boron concentration (CONC) is tracked in all hydraulic cells:
- Primary system: Nominal steady-state boron concentration (ppm)
- SG secondary side: 0.0 ppm
- ECCS injection: Analysis-specific RWST boron concentration
- BWR systems: 0.0 (no boron)
Valve Flow Capacity Tests¶
After modeling PORV, SRV, MSSV, or other valves, verify that the modeled valve achieves the correct flow rate at the rated pressure. This is done by:
- Setting the valve fully open (FAVLVE = 1.0)
- Setting upstream pressure to rated pressure
- Running a short transient
- Comparing calculated flow rate to rated capacity
Adjust KOPEN or AVLVE if needed.
10:1 Volume Ratio Guidance¶
The ratio of the volume of a cell to the volume of any adjacent cell should not exceed 10:1. This prevents numerical instabilities from large volume mismatches. If exceeded:
- Subdivide the larger cell
- Adjust DX values to redistribute volume
- Exception: 1-D to 3-D cell connections (CRGT to RPV, etc.)
Cross-References¶
- Component Numbering — Standardized numbering scheme
- Initialization — SS and transient setup procedures
- Heat Structures — HTSTR vs. pipe wall guidance
- CCFL Sensitivity — CCFL model recommendations from NRC assessment
This page faithfully extracts general modeling guidance from Rev 1 §1. Key topics include NFF=0 nodalization criterion (DX ≥ 0.625·HD), standard KFAC formulas for area changes and elbows, wall roughness values, and the 10:1 volume ratio guidance.