BWR Control Rod Guide Tube Modeling

Source

BWR Modeling Guidance — Control Rod Guide Tube Modeling.

Overview

This section provides guidance on modeling BWR control rod guide tubes (CRGTs) and the associated CRD cooling flow in TRACE. CRGTs are modeled as PIPE components with connected FILL components for CRD cooling flow. The guidance is applicable to:

• GE BWR/3, BWR/4, BWR/5, and BWR/6

Two CRGT PIPE components (with accompanying FILL components) should be included in the model, representing the CRGTs associated with Rings 1 and 2 of the RPV VESSEL component.

Required Inputs

• Axial heights of lower 7 levels in RPV VESSEL component
• Flow area of edge connecting axial levels 8 & 9 in RPV (Rings 1, 2)
• CRGT inner diameter (ID)
• Total number of CRGTs
• Number of fuel assemblies in Rings 1 & 2 of RPV
• Control blade wing geometry (thickness, width)
• Leakage flow area at bottom of CRGT
• Fuel support piece flow area (unorificed center channel)
• Nominal system pressure and core inlet subcooling
• Nominal total CRD cooling flow rate and liquid temperature
• CRD cooling flow coastdown time

Figure 1.1-1: Geometric parameters for approximating flow area through typical fuel support piece.

Component Setup — PIPE (6 cells, 7 edges)

Figure 1.2-1: Typical CRGT nodalization.

Initialization:

• Total Length = Σ DZ_i,RPV for levels 2 through 7
• NCELLS = 6
• NPIPES:
  • Ring 1: (N_{fuel assemblies Ring1} / 2·N_{total fuel assemblies}) × N_{total CRGTs}, rounded
  • Ring 2: 0.5 × N_{total CRGTs} − NPIPES_Ring1
• Orientation = Vertical

Geometry

Cell Layout

• DX_{cell i} = DZ_i+1,RPV (cell heights match RPV axial levels)

Volume — Account for Control Blade Displacement

Figure 1.3-1: Diagram of typical control blades (from above).

Square-tipped blades:

\[ VOL_{cells} = DX_i \cdot \left[\frac{\pi}{4} \cdot ID_{CRGT}^2 - 2 \cdot w_{blade} \cdot t_{blade} + t_{blade}^2\right] \]

Rounded-tipped blades:

\[ VOL_{cells} = DX_i \cdot \left[\frac{\pi}{4} \cdot ID_{CRGT}^2 - 2 \cdot w_{blade} \cdot t_{blade} - (2\pi - 3) \cdot t_{blade}^2\right] \]

where w = blade width, t = blade thickness.

10:1 Volume Ratio

The 10:1 check is generally not applied for connections between 1-D and 3-D cells. Check cells 2–5 for compliance. If adjustments are needed, subdivide existing cells rather than modifying them (to simplify heat structure setup later).

Edge Geometry

Edge	FA	HD
1 (bottom leakage)	Leakage flow area at bottom of CRGT	2 · FAedge1 / (π · IDCRGT)
2–6	Same as VOL/DX (blade displacement formula)	4 · FA / (π · IDCRGT + 4 · wblade)
7 (fuel support piece)	FAfuel support center channel − blade displacement	4 · FA / [(π−8)·R + 4·H + 4·wblade]

Fuel Support Piece FA

If the actual flow area is not available, it can be approximated from measurements as: FA = 2·W·H − W² + (4−π)·R² where W, H, R are as shown in Figure 1.1-1.

CCFL at Top (Edge 7)

CCFL may occur at the top of the CRGTs due to countercurrent flow:

Parameter	Value
CBETA	1.0 (Kutateladze correlation)
CCFLM	1.0
CCFLC	2.0493902

• LCCFL_edges1-6 = 0
• LCCFL_edge7 = CCFL model "N"

Orientation

All edges vertical upward: GRAV = +1.0

Initial Conditions

• P = plant-specific or nominal system pressure
• TL = T_sat − ΔT_subcooling; TV = T_sat
• ALPN = 0.0; PA = 0.0
• VL_edge1 = 0.0 (assume all CRD flow exits through top)
• VL_edges2-7 = ṁ_{CRD cooling} / (N_{total CRGTs} · ρ · FA_edge)
• VV = 0.0

Friction

Edge	NFF	KFAC	KFACR	ICFLG
1	0	0.5·(1−β₁²) + (1−β₂²)²	0.5·(1−β₁²) + (1−β₂²)²	0
2–6	1	0.0	0.0	0
7	0	0.5·(1−β₁²) + (1−β₂²)²	0.5·(1−β₁²) + (1−β₂²)²	0

• Edge 1: β₁ ≈ 0, β₂² = FA_edge1/FA_edge2
• Edge 7: β₁² = FA_edge7/FA_edge6, β₂² = FA_edge7/FA_{core axial edge}

KFAC Tuning

Edge 1 and 7 KFAC/KFACR values may need adjustment to obtain correct flow distribution due to a bias in TRACE's momentum calculation for this type of geometry.

General Inputs

• ICONC = 0 (or per guidance); CONC = 0.0
• IPOW = 0; EPSW = plant-specific or 4.572 × 10⁻⁵ m
• NSIDES = 1 (for CRD cooling flow FILL connection); THETA = 0°; IENTRN = 0
• IACC = 0; Use Pipewall = per heat structure guidance

CRD Cooling Flow — FILL Component

Setup

Parameter	Value
IFTY	8 (mass flow with trip)
IOFF	2 (state properties held constant)
DXIN	DXCRGT,cell1
VOLIN	VOLCRGT,cell1
ALPIN	0.0
TLIN	CRD cooling flow temperature (after CRD seals)
TVIN	Tsat at input pressure
PIN	CRD cooling water pressure, or nominal system pressure
PAIN	0.0
CONCIN	0.0
TWTOLD	0.0
RFMX	10²⁰ (no restriction on rate change)
Couple to CONTAN	False
VMSCL	1.0

Initial Mass Flow

\[ FLOWIN = \frac{NPIPES_i}{N_{total,CRGTs}} \cdot \dot{m}_{CRD\ cooling} \]

Set FLWOFF = FLOWIN (prevents confusion when controlling trip returns to OFF).

Fill Table (VMTBM)

The independent variable form should be set to negative (time since trip). Typical scenarios:

• CRD cooling termination: VMTBM = [(0.0, FLOWIN); (rampdown time, 0.0)]
• Operator action: Table of flow values as a function of time after trip

Trip Setup

For a simple trip to terminate CRD cooling at transient initiation:

Parameter	Value
Setpoint Range Type	1 [On(For) || OFF]
Initial SET	0 (On)
SETP1	−1.0
SETP2	0.0
ITST	1 (Signal Variable)
IDSG	Signal variable for problem time
Reverse Trip	False

Cross-References

• Reactor Vessel — RPV VESSEL component that CRGTs connect to
• Steam Separators — Upper RPV flow paths

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This page faithfully extracts CRGT and CRD cooling flow modeling guidance from the BWR Modeling Guidance. CRGTs are PIPE components with 6 cells matching RPV axial levels 2–7, with CCFL at the top edge (Kutateladze, CCFLC=2.049) and FILL-driven CRD cooling flow entering via side junction.