Skip to content

Fine Mesh Nodalization Sensitivity Summary

Source

Rev 1 Modeling Guidance, Appendix A.25 — Fine Mesh Nodalization Sensitivity Summary (Bajorek).

Overview

This appendix documents NRC sensitivity studies (S.M. Bajorek) on the effect of fine mesh (FM) renodalization on fuel rod heat transfer during reflood transients. The study examined the parameters that control FM node size:

  • NFAX — Number of permanent heat transfer nodes per axial hydraulic cell
  • DZNHT — Minimum size of a fine mesh node
  • NZMAX — Upper limit on total heat transfer nodes (permanent + FM)

Test Configuration

  • PWR "hot assembly" with constant forced reflood (1.5 in/s = 0.0381 m/s)
  • Single VESSEL with 264-rod Westinghouse 17×17 assembly heat structure
  • Nuclear rod model (burst + metal-water reaction enabled)
  • Assembly outlet pressure held at 40 psia (0.76 MPa)
  • Power adjusted for PCT > 1900°F (1310 K) with quench after 350 s

Results

Base Case (NFAX=3, DZNHT=1 mm)

  • PCT ≈ 1335 K
  • Bundle quench at 372 s
  • Burst at 52.94 s
  • ECR ≈ 8.25%

NFAX Sensitivity

NFAX DZNHT PCT (K)
3 1 mm 1334.86
5 1 mm 1319.77

Increasing NFAX from 3 to 5 reduced PCT by ~15 K — a small effect.

DZNHT Sensitivity (NFAX=5)

DZNHT (mm) PCT (K)
0.1 1319.77
0.5 1319.77
1 1319.77
2 1318.01
4 1319.93
16 1319.76
32 1337.65
64 1379.18

Results converge when DZNHT ≤ 16 mm. Results with DZNHT ≤ 1 mm are identical.

Fixed Node Size (No FM Renodalization)

When FM is turned off (NZMAX = N×NFAX + 2), results converge at NFAX = 11 (~20 mm node size). Above this value, results change very little.

Key Conclusions

Summary

  1. TRACE becomes insensitive to heat transfer node size when nodes are smaller than approximately 20 mm, regardless of whether FM renodalization is used
  2. The recommended base configuration is NFAX = 3, DZNHT = 1 mm with FM renodalization on — this provides effectively converged results
  3. Increasing NFAX to 5 provides a modest PCT reduction (~15 K) at moderate computational cost
  4. Very fine fixed meshes (NFAX = 331) cost ~45× more CPU time with only ~5 K PCT improvement over FM renodalization
  5. Caution: These results are for reflood transients; blowdown may have different sensitivity

Computational Cost

Configuration CPU Time (s) PCT (K)
NFAX=3, DZNHT=1 mm (base) 429 1335
NFAX=5, DZNHT=1 mm 495 1320
NFAX=331, no FM 19,292 1315

Application

The recommended fine mesh settings used throughout this guidance:

  • NFAX = 3 (default, adequate for most analyses)
  • DZNHT = 0.001 m (1 mm minimum FM node size)
  • FM renodalization = On

These settings are referenced in the Heat Structures and Power & Kinetics guidance.

Cross-References

This page faithfully extracts the fine mesh sensitivity study from Rev 1 Appendix A.25 (Bajorek). The bottom line: NFAX=3 with DZNHT=1 mm and FM on provides effectively converged PCT results for reflood transients at modest computational cost.