PWR Heat Structures Modeling¶
Source
Rev 1 Modeling Guidance, §17 — Heat Structures Modeling; Appendix A.17 — Heat Structures Bases.
Overview¶
This section provides guidance on modeling PWR heat structures (HTSTR components) in TRACE. These are used for metal masses not already modeled with the internal pipe wall model, including:
- Structures thermally coupled to multiple components — SG U-tubes, SG/RV dividers (shroud, barrel)
- VESSEL component structures — no internal pipe wall model available
- Complex geometry structures — separators, plenums, internal masses
- Fuel rods and heated rods
The guidance is applicable to Westinghouse ⅔/4-loop, CE 2-loop, and B&W PWRs.
Required Inputs¶
- Fuel rod geometry (pellet ID, gap thickness, cladding ID/OD, active height, gap conductance/pressure)
- Core design information (assembly count, pitch, burnup grouping, peaking factors)
- SG tube geometry, tubesheet geometry, barrel/separator geometry
- RPV baffle, barrel, outer shell geometry
- RCP internal surface area and metal volume
- Control rod guide tube geometry
- Material properties for all structures
Minimum Required Heat Structures¶
| Structure | Count per Model | Geometry | Material |
|---|---|---|---|
| Fuel rods | 6 × Naz-sectors | Cylindrical (HSCYL=1) | Fuel/gap/clad composite (MATRD=1) |
| Core thimble tubes | 2 × Naz-sectors | Cylindrical | Zircaloy (MATRD=2) |
| RPV baffle | Naz-sectors | Flat slab (HSCYL=0) | Carbon steel (MATRD=9) |
| RPV barrel | Naz-sectors | Cylindrical | Carbon steel (MATRD=9) |
| RPV outer shell | Naz-sectors | Cylindrical | Carbon steel (MATRD=9) |
| SG tubes | 1 per SG | Cylindrical | Inconel 600 (MATRD=12) |
| SG barrel (shroud) | 1 per SG | Cylindrical | Carbon steel (MATRD=9) |
| CRGTs (upper plenum) | 2 × Naz-sectors | Flat slab | Carbon steel (MATRD=9) |
| SG plena, tubesheets, separators, RCP, RPV internals | As needed | Varies | Varies |
Key Initialization Parameters¶
Geometry Type¶
- Cylindrical (HSCYL=1): SG tubes, SG barrel, RPV barrel/shell, fuel rods, thimble tubes, SG tubesheets, SG primary separators
- Flat Slab (HSCYL=0): RPV baffle, CRGTs, SG secondary separators, RCPs, SG plena, RPV internal structures
Radial Nodes¶
| Structure | Nodes | Rationale |
|---|---|---|
| Fuel rods | 8 | 5 in fuel pellet + gap + 2 in cladding |
| SG tubes | 4 | Adequate for thin-walled Inconel tube |
| RPV barrel, baffle, SG barrel, separator, thimble tube, CRGT | Wall thickness (in) / 0.1 + 1, rounded | ~0.1 inch per node |
| All other structures | log₂(1 + thickness/0.1) + 1, rounded | Logarithmic spacing for thick walls |
Initial Temperature¶
| Structure | RFTN |
|---|---|
| SG tubes, barrel, separators, tubesheets | SG secondary-side Tsat |
| RPV barrel, baffle, shell, thimble tubes, CRGTs, internals | RV average moderator temperature |
| Fuel rods | Average core coolant temperature |
| Other | Temperature of surrounding fluid |
Axial Cells¶
| Structure | NZHTSTR | Notes |
|---|---|---|
| SG U-tubes | 22 | Spans hot + cold legs + U-bend |
| OTSG tubes | 15 | Straight tube span |
| SG barrel (UTSG) | 12 | Downcomer/boiler coupling |
| SG barrel (OTSG) | 13 | |
| RPV outer shell | NZ (total RPV axial levels) | |
| RPV baffle | NZ − 10 | Core region only |
| RPV barrel | NZ − 5 | Core + upper plenum |
| Fuel rods | NZ − 12 | Active fuel height |
| Thimble tubes | NZ − 11 | |
| SG plena, RCP, tubesheets | 1 | Single axial node |
| Primary separators | 2 | |
| Secondary separators | 1 |
Thickness¶
- Fuel rods: ODclad / 2
- SG tubesheet: (ID²SG shell − Ntubes · ID²tube) / (4 · Ntubes · IDtube)
- All others: Wall thickness, or approximate as Metal Volume / Total Wetted Surface Area
Width (Flat Slab Only)¶
- RPV baffle: (# peripheral assembly faces × Pitchassembly) / Naz-sectors
- CRGTs: 4 × (Outer Width − 2 · Thickness)
- Others: Total Wetted Surface Area / Length
Fuel Rod Heat Structures¶
Fuel rods are the most complex heat structures with multiple material zones:
- Zone 1: Fuel pellet (UO₂) — nodes 1–5
- Zone 2: Gap (gas) — node 6
- Zone 3: Cladding (Zircaloy) — nodes 7–8
Key considerations:
- 6 fuel rod groups per azimuthal sector: fresh/once-burned/twice-burned × Ring 1/Ring 2
- Supplemental (hot) rod modeled within each HTSTR for PCT tracking
- Hot rod peaking factor applied to supplemental rod
- Power fractions assigned via POWER component connections (§18)
Connection Guidance¶
Each HTSTR must be connected to the appropriate hydraulic components on both sides:
- SG tubes: Inner surface → SG primary PIPE; outer surface → SG secondary PIPE (boiler region)
- SG barrel: Inner surface → SG boiler region; outer surface → SG downcomer
- RPV barrel: Inner surface → RPV interior ring; outer surface → RPV downcomer ring
- RPV baffle: Inner surface → Ring 1; outer surface → Ring 2
- RPV outer shell: Inner surface → RPV downcomer; outer surface → adiabatic or ambient
- Fuel rods: Inner surface → adiabatic (center); outer surface → RPV fuel channel cell
Cross-References¶
- Power & Kinetics — POWER component that drives fuel rod heat generation
- Steam Generator (UTSG) — SG tube and barrel heat structure connections
- Reactor Vessel — RPV structure heat structure connections
This page faithfully extracts heat structure modeling guidance from Rev 1 §17. At minimum, a PWR model requires fuel rods (6 groups × N sectors), thimble tubes, RPV baffle/barrel/shell, SG tubes, SG barrel, and CRGTs. Fuel rods use 8 radial nodes (5 fuel + gap + 2 clad); SG tubes use 4 nodes; thick structures use ~0.1 inch per node.