Once-Through Steam Generator (OTSG) Modeling¶

Source

Rev 1 Modeling Guidance, §7 — SG Primary Side (OTSGs); §14 — SG Secondary Side (OTSGs); Appendix A.7 — SG Primary Side Bases (OTSGs); Appendix A.14 — SG Secondary Side Bases (OTSGs).

Overview¶

This section provides guidance on modeling the primary and secondary sides of once-through steam generators (OTSGs) in TRACE. The guidance is applicable to:

Babcock & Wilcox (B&W) PWRs

This guidance is not applicable to U-tube steam generators used in Westinghouse or Combustion Engineering plants — see the UTSG Modeling page — or to helical coil SGs proposed for some SMRs.

Key differences from UTSG modeling:

Straight tubes (no U-bend) — primary flow enters at top, exits at bottom
No steam separator — OTSGs produce superheated steam directly
Upper/lower tubesheet — tubes span the full SG height
Upper and lower downcomer — separated by a divider, with aspiration flow path
Finer nodalization at top — needed to resolve steam superheating above dryout point
Three SJC flow paths — liquid (lower DC → boiler), aspiration (boiler → lower DC), and steam (boiler → upper DC)

Required Inputs¶

Primary Side¶

Reactor vessel outlet and inlet fluid temperatures
Primary loop nominal system pressure
SG inlet and outlet plena geometry (ID, height, total volume, wall thickness)
SG plenum material properties (thermal conductivity, heat capacity, wall roughness)
SG tube geometry (ID, total length)
SG upper and lower tubesheet geometry (thicknesses)
SG tube material properties (roughness)
Number of SG tubes
% Steam generator tube plugging (SGTP)
Loss coefficients for fittings, nozzles, etc.

Secondary Side¶

Nominal SG steam dome pressure
Nominal feedwater and steam outlet flow
Nominal aspirator steam flow
Main steam line diameter
SG secondary side geometry (boiler region ID, shell OD, shroud dimensions)
SG tube geometry (OD, pitch)
SG tube external wall roughness
Loss coefficients for tube support plates
Flow path geometry for: liquid flow (lower DC → boiler), steam flow (boiler → upper DC), aspiration flow (boiler → lower DC)

Primary Side¶

The OTSG primary side is modeled as three PIPE components: inlet plenum, SG tubes, and outlet plenum.

SG Inlet Plenum — PIPE (1 cell, 2 edges)¶

Figure 7.2-1: Typical OTSG inlet plenum nodalization.

Component setup:

Total Length = SG inlet plenum height
NCELLS = 1, NPIPES = 1
Orientation = Vertical
Component Number = (Loop Number) × 100 + 1005

Geometry¶

Parameter	Value
DX_cell1	SG inlet plenum height
VOL_cell1	Half spheroid = 2π/3 · (ID_plenum/2)² · DX₁
FA_edge1	π/4 · ID²_{inlet nozzle}
FA_edge2	N_tubes · π/4 · ID²_{single tube} (adjusted for SGTP)
HD_edge1	ID_{inlet nozzle}
HD_edge2	ID_{one SG tube}

Plenum Geometry

The OTSG inlet plenum is modeled as half a spheroid, unlike the UTSG which uses a half spherical cap. If the inner diameter is not explicitly known, it may be assumed equal to the SG shell ID.

Friction¶

Edge	NFF	KFAC	KFACR
1	0	16 f_T (standard elbow, 45°)	16 f_T
2	0	0.5 · (1 − β²) contraction	(1 − β²)² expansion

β = ID_{one tube} / HD_{inlet plenum} (where HD_plenum = (4 · FA_plenum / π)^0.5)
ICFLG = 0 at all edges; CCFL not applied at plenum edges

Initial Conditions & General Inputs¶

Same approach as UTSG inlet plenum:

P = nominal primary pressure; TL, TV = RV outlet temperature
ALPN = 0.0; PA = 0.0
EPSW = plant-specific or 4.572 × 10⁻⁵ m (commercial steel)
IPOW = 0, IACC = 0, NSIDES = 0
Use Pipewall = False

OTSG Tubes — PIPE (11 cells, 12 edges)¶

Figure 7.2-2: Typical OTSG tube nodalization (11 cells, 12 edges).

Component setup:

Total Length = tube length + upper tubesheet thickness + lower tubesheet thickness
NCELLS = 11
NPIPES = total tubes (or adjusted for SGTP: [1 − SGTP%/100] × total)
HD = ID_{one SG tube}
Component Number = (Loop Number) × 100 + 1010

Cell Layout¶

Cells	DX	Description
1	Upper tubesheet thickness	Upper tubesheet
2–3	Tube length (upper section) / 2	Upper tube section (finer — see basis)
4–10	Tube length (remaining) / 7	Main tube section
11	Lower tubesheet thickness	Lower tubesheet

Basis — Appendix A.7

"The nodalization was developed based on modeling experience with LOCA analyses to adequately capture the key thermal hydraulic phenomena as documented in the applicable PIRTs without unnecessary increases in computational time, with the exception of non-uniform wetting of the SG tubes."

"All of the tubes are lumped into a single PIPE component with the NPIPES input used to set the number of parallel identical components. Non-uniform wetting phenomena in OTSGs, where certain tubes experience different conditions than others, have been shown to be applicable to OTSG-specific thermal hydraulic phenomena. Scaled versions of the OTSGs used in the MIST facility have a relatively long spray region in which liquid from the EFW header is not distributed to all SG tubes. Therefore, in an OTSG, a large number of tubes will not be directly sprayed."

Edge Geometry¶

FA_{all edges} = π/4 · ID²_{one tube}
HD_{all edges} = ID_{one tube}
VOL_cells = FA × DX (cylindrical tubes)
All edges vertical: GRAV = −1.0 (primary flow descends from inlet plenum at top to outlet plenum at bottom)

CCFL¶

CCFL is not applied at any OTSG tube edges in the standard nodalization. (Unlike UTSGs, there is no U-bend or tubesheet flooding concern in the standard configuration.)

Friction¶

Edge	NFF	KFAC	KFACR
1	0	0.5 · (1 − β²) contraction	(1 − β²)² expansion
2–11	1	0.0	0.0
12	0	(1 − β²)² expansion	0.5 · (1 − β²) contraction

Edge 1: β = ID_tube / HD_{inlet plenum}
Edge 12: β = ID_tube / HD_{outlet plenum}
ICFLG = 0 at all edges

General Inputs¶

EPSW = plant-specific or 1.524 × 10⁻⁶ m (drawn piping)
NSIDES = 0, IACC = 0, IPOW = 0
Use Pipewall = False (HTSTR components attached manually)

SG Outlet Plenum — PIPE (1 cell, 2 edges)¶

Figure 7.2-3: Typical OTSG outlet plenum nodalization (two intermediate legs).

Component setup:

Total Length = SG outlet plenum height
NCELLS = 1, NPIPES = 1
Component Number = (Loop Number) × 100 + 1015

B&W plants have two cold legs per loop. Both intermediate legs are connected via side junctions:

NSIDES = 2
Side junction angle = plant-specific or 135° (consistent with UTSG outlet plenum guidance)
FA_edge2 = 0.0 (flow exits via side junctions)
HD_edge2 = 1.0 × 10¹⁰ (TRACE requires HD > 0)

Basis — Appendix A.7

"B&W plants have two cold legs per loop. Therefore, both intermediate legs are connected via side junctions with an appropriate angle (consistent with the orientation of the SG outlet nozzles). This will ensure that both legs are treated equally by the code and analyst."

Primary Side Summary¶

Figure 7.6-1: Typical OTSG primary side nodalization summary.

Secondary Side¶

The OTSG secondary side is modeled as two PIPE components (boiler region and downcomer) and three SJC components (lower liquid flow, aspiration flow, and upper steam flow).

SG Boiler Region — PIPE (15 cells, 16 edges)¶

Component Number = (Loop Number) × 100 + 1410

Cell Layout¶

The boiler region is nodalized consistently with the primary side tube component. The upper portion is finely nodalized to resolve steam superheating above the dryout point.

Cells	DX	Description
1	H_{lower DC-to-boiler flow path}	Bottom (flow path height)
2	DX_{cell10,OTSG tube} − H_{flow path}	Coupled to tube cell 10
3–8	DX_{cells9-4,OTSG tube}	Coupled to tube cells (1:1)
9–11	⅓ · DX_{cell3,OTSG tube}	Fine mesh — superheating region
12–14	⅓ · (DX_{cell3,OTSG tube} − H_{boiler-to-upper DC})	Fine mesh continuation
15	H_{boiler-to-upper DC flow path}	Top (upper flow path height)

Basis — Appendix A.14

"Modeling experience and studies indicated the need for finer nodalization in the upper part of the boiler region to obtain finer resolution of the change in temperature above the point at which dryout occurs, when the vapor becomes superheated by the primary side fluid. The fluid temperature does not change as quickly on the primary side due to the much higher heat capacity of liquid, but the vapor temperature changes quickly and is a very important parameter in indicating proper OTSG performance."

Volume — Account for Tube Displacement¶

\[ \text{VOL}_{cells} = \left(\frac{\pi}{4} \cdot ID_{boiler}^2 - N_{tubes} \cdot \frac{\pi}{4} \cdot OD_{tube}^2\right) \cdot DX \]

Note: Unlike UTSGs, the factor of 2 on N_tubes is not needed because OTSG tubes are straight (each tube displaces volume only once per axial level).

Edge Geometry¶

Edge	FA	HD
1	0.0 (SJC provides flow path)	0.0
2–15	π/4 · ID²_boiler − N_tubes · π/4 · OD²_tube	4 · FA_{unit cell} / (π · OD_tube)
16	0.0 (SJC provides flow path)	0.0

Basis — Appendix A.14

"The hydraulic diameter is calculated based on a unit cell of the SG tube lattice. The region between the SG tubes and the SG shroud is not significant enough to affect the hydraulic calculation, so the unit-cell hydraulic diameter is simply used for convenience."

Friction¶

NFF_edges1,16 = 0; NFF_edges2-15 = 1
KFAC_edges2-15 = k_{tube support plates} (if known)
ICFLG = 0 at all edges
No CCFL applied

Basis — Appendix A.14

"Some consideration was given to the possibility that a CCFL model was necessary due to AFW injection in the upper part of the boiler region. Currently available information does not seem to suggest that one is necessary, so the current guidance was developed without one."

Initial Conditions¶

P = nominal SG pressure
TL = saturation temperature at SG pressure (subcooled cells) or feedwater temperature
TV = saturation temperature (lower cells) or superheated temperature (upper cells where dryout has occurred)
ALPN = plant-specific or estimated based on boiling characteristics
VL, VV = estimated from nominal flows, or 0.0

General Inputs¶

NSIDES = 3 (for three SJC connections); THETA = 90° for all
IENTRN = 0 (offtake model off for all)
IACC = 0, IPOW = 0
Use Pipewall = False

SG Downcomer — PIPE (11 cells, 12 edges)¶

Component Number = (Loop Number) × 100 + 1400

The downcomer is a single PIPE component representing both the upper and lower downcomer regions. The flow area at the divider elevation is set to zero, effectively separating the upper and lower regions while keeping them in one component for numbering convenience.

Basis — Appendix A.14

"The downcomer in OTSGs is a simple annular cylinder. It is subdivided into an upper and lower downcomer. Many models use separate PIPE components, but a single component was assumed in the modeling guidance for convenience in component numbering standardization. The two approaches are fundamentally identical. The flow area at the elevation where the downcomer divider is located is set to zero."

Nodalization Constraints¶

The downcomer nodalization is driven by:

Heights of the upper and lower flow paths between downcomer and boiler region
Location of the divider between upper and lower downcomer
Consistency with boiler region for heat structure coupling via the shroud

The upper and lower cells are sized to preserve flow path heights. The remaining cells are uniformly distributed within each downcomer region.

Volume and Flow Area¶

VOL = π/4 · (OD²_downcomer − ID²_downcomer) · DX — annular geometry
FA at divider edge = 0.0 (no flow across divider)
FA at other edges = π/4 · (OD²_downcomer − ID²_downcomer)

Connections¶

Feedwater injects at the cell at the appropriate elevation
Main steam line connects at the cell at the appropriate elevation
Auxiliary feedwater connects at the appropriate elevation
The analyst should renodalize if feedwater and main steam line would connect to the same cell

Three SJC Flow Paths¶

Three SJC components model the flow paths between downcomer and boiler region:

SJC	Flow Path	Component Number
Lower Liquid	Lower downcomer → boiler region bottom	(Loop) × 100 + 1405
Aspiration	Boiler region → lower downcomer (steam heating of incoming liquid)	(Loop) × 100 + 1415
Upper Steam	Boiler region top → upper downcomer	(Loop) × 100 + 1420

All SJC junctions: angle = 90°, IENTRN = 0 (offtake model off).

Basis — Appendix A.14

"Three flow paths exist between the downcomer and the boiler region of OTSGs: the lower liquid flow from the lower downcomer to the boiler region, the upper superheated flow from the boiler region to the upper downcomer, and an aspirator flow path at the upper part of the lower downcomer. The aspirator allows fluid from the boiler region to flow into the lower downcomer, where it condenses and heats the incoming liquid, while the other two flow paths act as inlet/outlet flow paths for the boiler region."

"No offtake models are activated for the side junctions. The only flow path for which the offtake model might be significant is the aspirator flow path, and no well-defined water level is expected across the span of the boiler region."

Steady-State Checklist¶

Steady-State Verification

Cross-References¶

UTSG Modeling — U-tube SG guidance for Westinghouse/CE plants
Primary Loop — Hot leg (with candy-cane bend for B&W), intermediate leg, RCP
Pressurizer — Pressure control
Heat Structures — SG tube and shroud HTSTR guidance (§17)

This page faithfully extracts OTSG modeling guidance from Rev 1 §7 (primary side) and §14 (secondary side), with associated bases from Appendix A.7 and A.14. B&W plants may have raised or lowered loop configurations; this guidance assumes a lowered loop (most common). For raised-loop configurations, the basic nodalization is expected to be applicable with the straight-pipe segment relocated from the intermediate leg to the hot leg.