PWR Break Modeling¶
Source
Rev 1 Modeling Guidance, §22 — Break Modeling; Appendix A.22 — Break Bases.
Overview¶
This section provides guidance on modeling PWR pipe breaks in TRACE, covering two break types:
- Double Ended Guillotine (DEG) — LBLOCA (complete pipe severance)
- Split Break — LBLOCA, IBLOCA, and SBLOCA (partial pipe opening)
The guidance is applicable to Westinghouse ⅔/4-loop, CE 2-loop, and B&W PWRs.
Required Inputs¶
- Break type, location, flow area, orientation (top/side/bottom for split breaks)
- Break loss coefficients
- Initial and transient containment conditions (temperature, pressure)
- Broken pipe fluid temperature, pressure, wall roughness
- Pipe material and dimensions
Break Pipe Nodalization Criteria¶
For both break types, control volumes within 2.0 meters of the break must be renodalized:
Break-Adjacent Cell Sizing
- Cell adjacent to break: 0.25 ≤ L/D ≤ 3.0 (preferred: 1.0 < L/D < 2.0)
- Volumes within 2.0 m: double each successive cell until approaching unmodified model cell volume
- 10:1 adjacent cell volume ratio must be maintained
Basis — Appendix A.22
"The node resolution near the break needs to be sufficiently fine to result in reasonably accurate pressure/void fraction values at the break edge. The criteria was based on studies performed by NRC staff to determine nodalization strategies that would lead to 'converged' break flow results."
DEG Break Modeling¶
Steady State¶
Split the broken pipe into two PIPE components joined at the break location. Renodalize per criteria above.
Transient¶
Attach two BREAK components to the pipe ends in the restart file:
Pipe component updates:
| Parameter | Vessel-side edge | Loop-side edge |
|---|---|---|
| NFF | 0 | 0 |
| KFAC | (1−β²)² ≈ 1.0 (expansion) | 0.5·(1−β²) ≈ 0.5 (contraction) |
| KFACR | 0.5·(1−β²) ≈ 0.5 (contraction) | (1−β²)² ≈ 1.0 (expansion) |
| ICFLG | 1 (choked flow) | 1 |
DEG BREAK Component Parameters¶
| Parameter | IBTY = 0 | IBTY = 1 | IBTY = 7 |
|---|---|---|---|
| Description | Constant P (maximize PCT) | P vs. time table | CONTAN coupled |
| ISAT | 3 (Tsat) | 3 | 3 |
| IOFF | 0 | 0 | 0 |
| DXIN | 10⁻⁶ m | 10⁻⁶ m | 10⁻⁶ m |
| VOLIN | 10⁶ m³ | 10⁶ m³ | 10⁶ m³ |
| ALPN | 1.0 | 1.0 | 1.0 |
| TIN | Containment T | Containment T | Containment T |
| PIN | Min containment P (or 14.5 psia) | Initial containment P | Initial containment P |
| PAIN | Containment P | Containment P | Pcont − Psat(TIN) |
| ADJPRESS | False | False | False |
| RBMX | 10²⁰ Pa/s | 10²⁰ Pa/s | 10²⁰ Pa/s |
| PSCL | 1.0 | 1.0 | 1.0 |
| PTB | Not used | Containment P vs. time | Not used |
| IBSV | 0 | Problem time signal | 0 |
| IBTR | 0 | Break trip ID | 0 |
Split Break Modeling¶
Steady State¶
The broken pipe remains as a single PIPE component, renodalized per criteria above.
Transient¶
A BREAK component connects via a SJC (single junction component) attached by side junction to the pipe:
Pipe component updates:
- NSIDES = 1 (add side junction at break cell)
- THETA = 90°
- IENTRN = 1 (tee offtake model ON)
SJC setup:
| Parameter | Value |
|---|---|
| FA | Desired break flow area |
| HD | √(4·FA/π) — circular break assumed |
| NFF | 0 |
| KFAC | 60 fT + (1−β²)² ≈ 60 fT + 1.0 (tee branch + expansion) |
| KFACR | 60 fT + 0.5·(1−β²) ≈ 60 fT + 0.5 (tee branch + contraction) |
| ICFLG | 1 |
| VL, VV | 0.0 |
Basis — Appendix A.22
"For split breaks, a SJC component is used to model situations where the flow path through the pipe wall would result in violating the 10:1 criteria. The split breaks are assumed to be circular in computing the hydraulic diameter for convenience. Modeling experience indicates that hydraulic diameter in this specific situation is not a major determining factor in the break flow rate."
SJC orientation (depends on break position):
| Break Position | Angle | GRAV |
|---|---|---|
| Top of pipe | +π/2 | +1.0 |
| Bottom of pipe | −π/2 | −1.0 |
| Side of pipe | 0.0 | 0.0 |
The split break BREAK component uses the same parameter table as DEG breaks above.
Cross-References¶
- Containment — BREAK boundary conditions (IBTY options)
- Primary Loop — Hot leg and cold leg break locations
- Initialization — LOCA transient setup checklist
- BWR Break Modeling — Similar BWR break guidance (includes injection line SEG breaks)
This page faithfully extracts PWR break modeling guidance from Rev 1 §22. DEG breaks use two BREAK components on a split pipe; split breaks use a SJC + BREAK via side junction with tee offtake model. Break-adjacent cells require L/D 0.25–3.0 with 2× volume grading within 2 m. Containment backpressure options: constant (IBTY=0, maximize PCT), time-dependent (IBTY=1), or CONTAN-coupled (IBTY=7).