CFAST Validation (NUREG 1934 Appendix B)

This case extends the Appendix A MCR validation to a scenario with fire growth and spread: the U.S. NRC NUREG-1934 (EPRI 1023259), Appendix B — Cabinet Fire in a Switchgear Room. Where Appendix A validates deck replication of a fully prescribed fire, Appendix B additionally validates FRI3D's fire-spread modeling: the NRC model prescribes the secondary cable-tray fire as a hand-built HRR table derived offline with the FLASH-CAT method (NUREG/CR-7010), while FRI3D computes that secondary fire online — tray ignition from the CFAST-predicted thermal exposure, and tray HRR from the FLASH-CAT methodology — from nothing but the plant model geometry and the cable library. The published NRC CFAST model is run unmodified as the baseline, the FRI3D-generated deck for the same room is run with the same CFAST 7.7.3 build, and the two are compared over the full 3600 s simulation.

Reference case

A 464 kW electrical cabinet fire (PE/PVC fuel, C₂H₃.₅Cl₀.₅, heat of combustion 20.9 MJ/kg, radiative fraction 0.49, CO yield 0.147, soot yield 0.136) in a 26.5 × 18.5 × 6.1 m switchgear room, igniting a stack of three horizontal cable trays above the cabinet. Ventilation is three supply and three return fans (0.472 m³/s each, pressure cutoffs 200–300 Pa) with the room otherwise sealed except for a door undercut crack (1.09 m × 0.013 m); the room pressurizes to ≈ +250 Pa. Simulation duration 3600 s.

Input deck comparison

Every input group of the FRI3D-generated deck was compared field-by-field against the NRC model: ambient conditions, compartment geometry, leakage, mechanical ventilation, fire chemistry and yields, and the ignition-source HRR curve. All fields match, with three understood differences:

Secondary fire representation. NRC prescribes one combined tray fire as a fixed table at a fixed 3.8 m elevation. FRI3D computes each tray's ignition time and HRR contribution, and emits the burning-elevation history directly in the fire table (the fire rises from the cabinet at 2.4 m to the burning trays at 3.8–4.9 m as they take over). The FRI3D tray fuel loading uses the generic FLASH-CAT library defaults, not values tuned to this scenario.
Leakage representation. NRC models the door crack as an explicit wall vent; FRI3D represents the same open area (0.0142 m²) through CFAST's leakage-area ratio. Both reach the same fan-cutoff-limited pressure plateau; the low-pressure onset transient differs by ≈ 3 minutes.
Target representation. FRI3D targets are thin gas-exposure probes because cable failure (THIEF) is evaluated outside CFAST; NRC embeds THIEF plates in-deck. Gas-layer quantities are unaffected.

Results

Matched-time comparison over the 3600 s run (FRI3D-generated deck vs NRC baseline):

t (s)	HRR (kW) FRI3D / NRC	Upper layer temp (°C) FRI3D / NRC	Layer height (m) FRI3D / NRC	Upper layer O₂ (%) FRI3D / NRC
300	88.8 / 81.2	23.0 / 23.1	5.4 / 5.4	20.4 / 20.4
600	616.8 / 440.9	38.1 / 33.3	4.8 / 4.6	19.8 / 20.1
900	1228.1 / 1054.8	68.9 / 67.5	4.3 / 3.9	18.3 / 19.1
1200	1672.8 / 1598.8	99.5 / 100.9	4.1 / 3.5	16.2 / 17.5
1800	873.2 / 755.3	83.2 / 76.9	4.1 / 3.1	15.8 / 16.8
3600	713.2 / 678.0	71.1 / 68.4	3.6 / 3.0	17.6 / 17.5

Peak / extreme values over the full run:

Quantity	FRI3D	NRC	Difference
Peak upper layer temperature	104.1 °C	103.6 °C	+0.5 %
Peak HRR (computed vs prescribed)	1673 kW	1599 kW	+4.6 %
Peak compartment pressure	243 Pa	249 Pa	−2.3 %
Minimum upper layer O₂	15.3 %	16.5 %	−1.2 pt
Minimum layer height	3.6 m	3.0 m	+0.6 m
Tray target surface temperature peaks	812 / 985 / 1004 °C	808 / 1000 / 999 °C	≤ 1.5 %

FRI3D generated CFAST deck vs NRC CFAST baseline — switchgear cabinet fire with FLASH-CAT tray spread

The heat release rate is the strongest statement in this case: FRI3D's peak of 1673 kW and post-burnout tail of 713 kW are computed by the FLASH-CAT engine from generic library cable data, against NRC's offline-derived 1599 kW peak and 678 kW tail — the +4.6 % difference is the methodology reproducing the reference, not a copied input. The mid-transient HRR differences (e.g. 617 vs 441 kW at 600 s) reflect tray ignition timing: FRI3D ignites trays from the CFAST-predicted exposure, while NRC prescribed a fixed 480 s ignition.

The layer height difference (3.6 vs 3.0 m late-time) is a direct consequence of the fire-elevation representation: the zone-model interface settles where plume entrainment from the lower layer balances extraction, and the entrainment path is measured from the fire base. NRC holds the tray fire at a constant 3.8 m, while FRI3D raises the fire elevation with the burning front (to ≈ 4.7 m once only the upper trays burn), shortening the entrainment path and stalling the interface ~0.6 m higher. The difference is bounded by the 1 m height of the tray stack itself — the ambiguity inherent in assigning any single elevation to a multi-tray fire — and does not affect the layer's temperature, species, or pressure, which match to within a few percent.

Conclusion

The FRI3D-generated CFAST deck reproduces the NRC NUREG-1934 Appendix B switchgear baseline within ±0.5 % on peak upper layer temperature, ±2.3 % on peak pressure, and +4.6 % on peak HRR — with the secondary cable-tray fire computed by FRI3D's FLASH-CAT implementation from generic library data rather than prescribed. Differences in mid-transient timing and late layer height are traceable to the documented representation differences (computed vs prescribed ignition, dynamic vs fixed fire elevation, leakage-ratio vs explicit-crack modeling) and are bounded by the values above.

Reference

NUREG-1934 / EPRI 1023259, Nuclear Power Plant Fire Modeling Analysis Guidelines, Appendix B: Cabinet Fire in Switchgear Room.
NUREG/CR-7010 Vol. 1, Cable Heat Release, Ignition, and Spread in Tray Installations During Fire (CHRISTIFIRE) — FLASH-CAT method.
NUREG-1824 / EPRI 1011999, Verification and Validation of Selected Fire Models for Nuclear Power Plant Applications (CFAST volume).