The heat flux that leads to onset of nucleate boiling qONB is an important quantity for plate-type research reactors since it is frequently used as a thermal design constraint and also indicates the transition point from single- to two-phase heat removal in transient analyses. Recent experimental work has shown that qONB can be sensitive to both channel gap size d and flow velocity v under laminar, upward flow conditions that are encountered in such reactors under naturalconvection core cooling. New experimental data are presented to test the validity of the correlation proposed from the results of the previous work in extended d and local pressure p ranges. The correlation predicts the new experimental data for mixed or pure buoyancy-driven upward flows in 2.0 ≤ d ≤ 5.0 mm channels with 0.03 ≤ v ≤ 0.16 m/s and 1.05 × 105 ≤ p ≤ 1.70 × 105 Pa within 25%. The new d range covers almost all the existing and planned plate-type research reactors. The p range extends the applicability of the correlation to the analysis of a number of accident scenarios in open-pool reactors with power levels up to 5 to 10 MW, such as partial loss of pool water or coolant pump trip. The pressure range is also relevant to the analysis of similar accidents in higher power pressurized systems if the accident is accompanied by system depressurization. In the implementation of the correlation for such analyses, it is important to note that the correlation implicitly assumes that the wall superheat is nonnegative.
