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CCFL Sensitivity Summary

Source

Rev 1 Modeling Guidance, Appendix A.26 — Countercurrent Flow Limit Sensitivity Summary (Krotiuk).

Overview

This appendix documents the results of NRC sensitivity studies (W.J. Krotiuk) to support CCFL modeling recommendations for PWR LOCA analyses. The study compared TRACE predictions with varied CCFL inputs against test data from:

  • Bethsy small cold leg break Test 9.1.B
  • ROSA-IV small cold leg break Test SB-CL-14
  • Semiscale small cold leg break Test S-NH-2
  • LOFT small cold leg break Test L3-7
  • ROSA-2 intermediate cold leg break Test 2

Key Findings

Executive Summary

  • Small breaks: CCFL inputs may be included without substantially affecting PCT predictions — all PCTs were close to test data regardless of whether CCFL was specified
  • Intermediate breaks: CCFL inputs are required to predict PCT measurements, particularly at the upper core plate and end box
  • Recommendation: Include CCFL models in all small, intermediate, and large break PWR LOCA analyses

Small Break Assessment

For small breaks, PCT predictions are close to measured data whether CCFL is included or not. The recommended models from publication review are:

Location Model CBETA CCFLM CCFLC
Upper core plate (D < 2.86 cm) Bankoff −1.0 1.07 0.00433
Upper core plate (D > 2.86 cm) Wallis 0.0 1.22 0.88
SG tubes Wallis 0.0 1.0 0.8
Hot legs at SG inlet Wallis 0.0 0.74 0.38
Cold legs at pump suction Kutateladze 1.0 1.0 1.79
Guide tubes Not needed — — —
Downcomer bypass Not needed — — —

Intermediate Break Assessment (ROSA-2 Test 2)

For intermediate breaks, CCFL inputs are essential. The recommended models from the publication review generally work well, with exceptions at the UCP and end box:

Location Model CBETA CCFLM CCFLC Notes
Upper core plate Kutateladze 1.0 1.0 1.0 Publication review suggested Wallis/Bankoff, but Kutateladze better matches data
End box Kutateladze 1.0 1.0 1.0 Same as UCP
Guide tubes Wallis 0.0 1.0 0.8 Additional location from TRACE assessment
Downcomer bypass Not needed — — — Minimal effect on PCT; generally not included
SG tubes Wallis 0.0 1.0 0.8 Consistent with publication recommendation
Hot legs at SG inlet Wallis 0.0 0.74 0.38 Consistent with publication recommendation
Cold legs at pump suction Kutateladze 1.0 1.0 1.79 Consistent with publication recommendation

CCFL Model Types

CBETA Value Model Description
0.0 Wallis Dimensionless superficial velocity correlation
1.0 Kutateladze Kutateladze number correlation
−1.0 Bankoff Bankoff flooding correlation

Application in This Guidance

The CCFL models recommended in the component-specific guidance pages are derived from this sensitivity study:

Component Page Location Model Applied
Primary Loop Hot leg at SG inlet (edge 5) Wallis (0.0/0.74/0.38)
Primary Loop Cold leg at pump suction Kutateladze (1.0/1.0/1.79)
Steam Generator (UTSG) SG tubesheet inlet (edge 2) Wallis (0.0/1.0/0.8)
Reactor Vessel Upper core plate Per break size assessment

Test Facilities

Test Break Area (m²) Break Diameter (mm) Initial Pressurizer P (MPa)
Bethsy 9.1.B 2.09 × 10⁻⁵ 5.16 15.51
ROSA-IV SB-CL-14 7.99 × 10⁻⁴ 31.9 15.50
Semiscale S-NH-2 4.71 × 10⁻⁶ 2.45 15.58
LOFT L3-7 1.26 × 10⁻⁵ ~1.265 14.90
ROSA-2 Test 2 1.32 × 10⁻³ 41.0 15.53

Cross-References

This page faithfully extracts the CCFL sensitivity study from Rev 1 Appendix A.26 (Krotiuk). The key finding is that CCFL models should be included in all PWR LOCA analyses — they have minimal impact on small-break PCT but are essential for intermediate breaks, particularly at the upper core plate where the Kutateladze model (1.0/1.0/1.0) outperforms the publication-recommended Wallis/Bankoff models.