Plate-type research reactor cores have involute or rectangular coolant channels with channel gap size in the range 2 ≤ d ≤ 5 mm. Heat transfer under fully developed nucleate boiling (FDNB) and low-velocity (<0.15 m/s) upward flow conditions is important in accident situations where core cooling may be by natural convection. Using data from previous experimental work with 2 ≤ d ≤ 4 mm rectangular channels, it is shown that (a) wall superheat (ΔTsat) in thin channels under FDNB decreases with increasing probability of bubble contact, (b) ΔTsat is a function of the bubble departure diameter Db as well as d, and (c) ΔTsat can be significantly overestimated by the FDNB correlations that are conventionally used in plate-type research reactor analysis but that are based on higher pressure and larger d flow data and that predict ΔTsat as a function of local channel heat flux and pressure only (e.g., as in the Jens-Lottes and Thom correlations). A new FDNB correlation is proposed that represents the bubble contact mechanism through the dimensionless number (d — cDb)/d, where c is a fitting parameter that accounts for the statistical aspects of bubble formation and contact. The ΔTsat predictions of the new correlation agree with the experimental data to within 16% and approach those obtained from the Jens-Lottes correlation with decreasing Db/d.