BWR Steam Separator Modeling¶

Source

BWR Modeling Guidance — Separator/Dryer Modeling.

Overview¶

This section provides guidance on modeling BWR steam separators in TRACE using the SEPD component. The dryers are not explicitly modeled as a separate component — their effect on liquid-vapor separation is captured in the separator carryover/carryunder values, and the dryer fluid volume is included as part of the RPV VESSEL free volume. The guidance is applicable to:

GE BWR/3, BWR/4, BWR/5, and BWR/6

A typical BWR model includes four SEPD components representing separators in Rings 1 and 2 of the RPV (one per azimuthal sector). Separators are lumped together within each region as ideal separator models with realistic carryunder/carryover values that include the combined effect of separators and dryers.

Required Inputs¶

Nominal system pressure, feedwater flow rate, core flow rate
RPV nodalization (cell heights for axial levels)
Standpipe geometry (length, ID)
Separator geometry (lengths, ID/OD of all regions, swirl vane volume)
Separator carryover and carryunder
Pressure drop across separators
Loss coefficients for swirl vane and liquid return path
Total number of separators and fuel assemblies (total and Ring 1)

Component Setup — SEPD (5 main cells + 2 side cells, 9 edges)¶

Figure 1.2-1: Typical separator nodalization.

Figure 1.2-2: Typical 2-stage separator — internal volume division for cells 1–5.

Figure 1.2-3: Typical 3-stage separator — internal volume division for cells 1–5.

Initialization:

Main Tube Length = DZ_RPV,Level12 + DZ_RPV,Level13
Side Tube Length = height from swirl vane inlet bottom to bottom of last separation stage
NCELL1 = 5 (main tube), NCELL2 = 2 (side tube — liquid return)
Cosine Theta = −1.0 (downward side tube)

Geometry¶

Cell Layout¶

Cell	DX	Description
1, 2	½ × H_{average standpipe}	Standpipe
3	H_{swirl vane cone}	Swirl vane cone
4	H_{main stage} (2-stage) or H_{lower two stages} (3-stage)	Main separator body
5	H_{top stage}	Top stage
6, 7 (side)	½ × (DZ_RPV,Level13 − DX_cell5)	Liquid return path

Volume Formulas (per ONE separator)¶

Cell	Formula
1, 2	DX · π/4 · ID²_standpipe
3	π/12 · DX · (ID²_standpipe + ID²_{main sep} + ID_standpipe · ID_{main sep}) − VOL_{swirl vanes}
4	DX · π/4 · ID²_{main separator}
5	π/4 · DX · [ID²_{top stage} + (OD²_{liquid return} − ID²_{liquid return})]
6, 7	Annular liquid return path volume (see source for 2-stage vs. 3-stage formulas)

Edge Geometry¶

Edge	FA	HD
1–3 (standpipe)	π/4 · ID²_standpipe	ID_standpipe
4 (main separator)	π/4 · ID²_{main sep}	ID_{main sep}
5 (separator cap)	π/4 · ID²_{sep cap}	ID_{sep cap}
6 (steam outlet)	π/4 · ID²_{steam outlet}	ID_{steam outlet}
7, 8 (liquid return — main)	Annular: π/4·(OD²_outer − ID²_outer + OD²_inner − OD²_{main sep})	4·FA / (π·perimeter)
9 (liquid return — bottom)	Annular: π/4·(OD²_outer − ID²_outer + OD²_inner − OD²_standpipe)	4·FA / (π·perimeter)

CCFL: not applied at any edge (LCCFL = 0)

Orientation¶

Edges	GRAV
1–6 (main tube — upward)	+1.0
7–9 (side tube — downward)	−1.0

Initial Conditions¶

P = plant-specific or nominal system pressure
TL, TV = T_sat
ALPN_cells1-5 = plant-specific or 0.70 (main pipe)
ALPN_cells6-7 = plant-specific or 0.0 (liquid return)
PA = 0.0

Main Pipe Void Fraction

The void fraction in the main pipe cells should not be set to single-phase conditions — this will cause TRACE to switch the special SEPD modeling off and treat the component as a TEE. Use 0.70 if plant-specific values are unknown.

Liquid velocity estimates:

\[ VL_{edge1} = \frac{\dot{m}_{core}}{[(\rho_l - \rho_g)(1 - ALPN_{RPV,Level11}) + \rho_g] \cdot FA_{edge1}} \]

\[ VL_{edges2-5} = \frac{\dot{m}_{core}}{[(\rho_l - \rho_g)(1 - ALPN) + \rho_g] \cdot FA} \]

VL_edge6 ≈ 0.0 (all vapor exits top)
VL_edges7-9 = (ṁ_core − ṁ_FW) / (ρ_liquid · FA)
VV_edges1-5 ≈ VL_edges1-5
VV_edge6 = ṁ_FW / (ρ_steam · FA_edge6)
VV_edges7-9 ≈ 0.0

Friction¶

Edge	NFF	KFAC	KFACR
1	0	0.5 (infinite contraction)	1.0 (infinite expansion)
2–3	1	0.0	0.0
4	0	Facility value or 0.5 (swirl vane — tune to ΔP)	Same
5	0	0.5·(1−β²) for 2-stage only	(1−β²) for 2-stage only
6	0	1.0 (infinite expansion)	0.5 (infinite contraction)
7	0	Facility value, or 9.5 (from Idelchik 6-27)	Facility value, or 9.7 (from Idelchik 6-28)
8	1	0.0	0.0
9	0	1.0 (infinite expansion)	0.5 (infinite contraction)

Edge 5 (2-stage only): β = ID_{2^nd stage} / ID_{1^st stage}
ICFLG = 0 at all edges

General Inputs — Separator Parameters¶

Parameter	Value
ISTAGE	0 (ideal separator with constant carryover/carryunder)
JCELL	5 (liquid return side tube connects at cell 5)
IENTRN	0 (offtake model off)
NDRYR	0 (dryers included in carryover, modeled in RPV)
XCO (carryover)	Plant-specific, or 1.0 × 10⁻³ (net through separator + dryer)
XCU (carryunder)	Plant-specific, or 2.5 × 10⁻³ (typical GE separator)
ALPSMN	0.05 (minimum barrel void — avoids oscillation)
ALPSMX	0.9 (maximum barrel void)
IPOW	0; EPSW = plant-specific or 4.572 × 10⁻⁵ m
Use Pipewall	False (HTSTR components required for separator walls)

NSEPS — Number of Separators per Component¶

Each SEPD component represents separators in one ring/sector:

\[ N_{sep,Ring1} = \frac{N_{fuel\ assemblies,Ring1}}{N_{total\ fuel\ assemblies}} \cdot N_{total\ separators} \quad \text{(rounded)} \]

\[ NSEPS_{Ring\ i} = \frac{N_{sep,Ring\ i}}{2} \quad \text{(per azimuthal sector)} \]

Carryover/Carryunder Sensitivity

Certain transients can be sensitive to carryover and carryunder fractions. Plant-specific data should be used when available. Using a value of 0.0 is not recommended because it leads to non-physical dynamic level response behavior.

Steady-State Checklist¶

Steady-State Verification

Separator ΔP matches plant data (adjust KFAC_edge4 and KFAC_edge7)
Steam quality at separator outlet is physically reasonable
Liquid return flow matches expected recirculation
RPV downcomer level is within expected range
Main pipe void fractions are two-phase (SEPD mode active, not TEE mode)

Cross-References¶

Reactor Vessel — RPV VESSEL component connections (standpipe inlet, steam outlet, liquid return)
Jet Pump — Recirculation flow driving the separator inlet
Recirculation Lines — Loop flow paths

This page faithfully extracts separator/dryer modeling guidance from the BWR Modeling Guidance. Four SEPD components model the separators across two rings and two azimuthal sectors, with dryer effects lumped into separator carryover/carryunder values. The guidance covers both 2-stage and 3-stage separator designs.