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BWR Steam & Feedwater System Modeling

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BWR Modeling Guidance — Steam & Feedwater System Modeling.

Overview

This section provides guidance on modeling the BWR steam network and feedwater system in TRACE, covering all components from the RPV steam outlet to the turbine/condenser boundary conditions, plus the feedwater line back to the RPV. The guidance is applicable to:

  • GE BWR/3, BWR/4, BWR/5, and BWR/6

Required Inputs

  • MSL geometry (ID, length or volume), material properties, wall thicknesses
  • MSL flow restrictor geometry (Venturi throat ID, inlet/outlet IDs, length)
  • Equalizing header geometry
  • MSIV seat IDs, locations, loss coefficients, closing times, response delays
  • SRV line geometry, rated flow capacities/pressures, seat IDs, loss coefficients, opening/closing times/setpoints, ADS response parameters
  • Turbine inlet/bypass line geometry
  • TCV/TSV geometry and opening/closing characteristics
  • FW line geometry, FW sparger geometry (ID, length, radius of curvature), FW injection flow area
  • Condenser and wetwell nominal conditions (pressure, temperature)
  • System nominal pressure, total rated steam/FW flow

Component Summary

The steam network typically consists of:

Component Type Typical NCELLS Notes
MSL/MSIV (×3) VALVE 1 + L/(8·ID), rounded up Two individual + one lumped pair; DX/HD ≤ 8 for pressure propagation
SRV banks (×9) VALVE 0 or 2 Three per MSL; zero-cell (config a) or 2-cell (config b for ADS)
SRV/ADS BREAKs BREAK — Connected to wetwell CONTAN
Turbine inlet/TCV VALVE L/(8·ID) + 1 Cell 1 = equalizing header; TSV/TCV lumped
Turbine bypass (TBV) VALVE 0 Side-junction connected, zero-cell
Turbine BREAK BREAK — Turbine backpressure BC
Condenser BREAK(s) BREAK — Condenser backpressure BC
Feedwater line PIPE 3–5 typical From FW BC to RPV

Figure 1.2-1: Example steam network nodalization diagram.

MSL Nodalization

The DX/HD ratio of ~8 is based on recent NRC experience with turbine trip transients where pressure propagation through the steam lines is important. For LOCA analyses where secondary side effects are less important, coarser nodalization (DX/HD ≤ 30) may be used to save computational time. For MSL break scenarios, break nodalization guidance applies.

Main Steam Line / MSIV — VALVE


Example MSL/MSIV nodalization.

Key features:

  • 4 physical MSLs → 3 VALVE components (2 individual + 1 lumped pair)
  • Average MSL length/volume used to facilitate lumping
  • Cell 1 models MSL flow restrictor (Venturi — frustum geometry)
  • MSIV modeled as valve at appropriate edge (IVTY=3)
  • MSL connects to equalizing header at outlet
  • SRV/ADS components attach via side junctions

MSL/MSIV valve inputs: Same IVTY=3/INTLOSSOFF=1/KOPEN pattern as recirculation valves. MSIV-specific: FAVLVE=1.0 (initially open), VTB1 closing table uses MSIV closing time with response delay.

Safety Relief Valves / ADS — VALVE


Typical SRV/ADS valve nodalizations: (a) zero-cell SRV, (b) 2-cell ADS.

Configuration:

  • 9 SRV components per MSL arrangement (3 per MSL component)
  • 3 pairs use config (a) — zero-cell side-junction VALVE
  • Last set uses config (b) — 2-cell VALVE for ADS function
  • Each SRV/ADS VALVE connects to a wetwell BREAK (IBTY=7 CONTAN)

SRV valve inputs: IVTY=3, trip uses 4-setpoint hysteresis (same pattern as PWR PORV/SRV):

  • Setpoint Range Type: 3 [ON(For) || OFF || On(Rev)]
  • SETP1 = closing pressure, SETP4 = opening pressure
  • SETP2 = average(SETP1,SETP4) − 0.1%, SETP3 = average(SETP1,SETP4)
  • SRV banks have different setpoints reflecting the plant stagger

ADS trip: Separate trip logic with specific ADS initiation criteria (low water level + timer).

SRV/ADS BREAK Components

Connected to wetwell CONTAN. Key parameters:

  • IBTY = 7 (CONTAN coupled), COMPID = wetwell component ID
  • Compartment Region = Liquid Space (negative COMPID — SRVs discharge below suppression pool surface)
  • ISAT = 0, DXIN/VOLIN match upstream cell, BDCOND = plant-specific

Turbine Inlet / TCV — VALVE


Typical turbine inlet line and TCV configuration.
  • Cell 1 = equalizing header (all MSLs feed in)
  • TSV and TCV lumped at TCV location
  • NCELLS = L/(8·ID) + 1, DX/HD ≤ 8
  • Valve at last edge; IVTY=3

Turbine BREAK

  • IBTY = 0 (constant pressure) or IBTY = 1 (pressure table)
  • PIN = condenser pressure (for constant) or pressure schedule
  • ISAT = 3; ALPN = 1.0; DXIN = 10⁻⁶ m; VOLIN = 10⁶ m³

Turbine Bypass Valve — VALVE (0 cells)


Typical turbine bypass valve configuration.
  • Zero-cell VALVE, side-junction connected to turbine inlet line or MSL
  • May be single bypass line or two (one per MSL), depending on plant
  • IVTY=3; trip and table define TBV opening characteristics

Condenser BREAK

Same pattern as turbine BREAK (IBTY=0 at condenser pressure).

Feedwater Line — PIPE


Typical BWR FW line diagram.
  • Models the FW line from the FW regulation boundary condition to the RPV
  • Last cell = FW sparger ring (if modeled): DX = (θ/360°) · π · Rsparger
  • FW FILL connected upstream; injects at appropriate downcomer elevation
  • Typically 3–5 cells; orientation varies with plant-specific routing

Key notes:

  • FW line is included primarily for FW line break transients and FW mass inventory
  • For non-FW-break transients, the FW line has minimal impact on results
  • FW sparger ring volume included as last cell; injection ports as last edge FA

General Notes — All Steam/FW Components

Parameter Value
ICONC 0 (no boron in BWR)
IPOW 0
EPSW Plant-specific or 4.572 × 10⁻⁵ m
INTLOSSOFF 1 (Off) for all VALVE components
Use Pipewall Per heat structure guidance (manually attached HTSTR)

Steady-State Checklist

Steady-State Verification

  • Total steam flow matches rated value
  • Steam dome pressure matches nominal
  • MSL pressure drop is reasonable
  • SRVs are closed during steady state
  • TCV is at appropriate position for rated power
  • TBVs are closed during steady state
  • FW flow matches rated value
  • FW temperature is correct

Cross-References

This page faithfully extracts steam and feedwater system modeling guidance from the BWR Modeling Guidance. The steam network includes 3 MSL/MSIV VALVEs (DX/HD≤8 for pressure propagation), 9 SRV VALVEs (zero-cell + 2-cell ADS variants with 4-setpoint hysteresis trips), turbine inlet/TCV, TBV bypass, and associated BREAK BCs. The FW line is a simple PIPE primarily important for FW line break scenarios.