To investigate the coolability of a uniformly heated tube by free convection of atmospheric air, heat transfer experiments were conducted using vertical open an-nuli with adiabatic outer walls. To examine the effect of the annulus ratio on the coolability of the heated tube, the experiments employed four annuli (diameter ratios of 1.155, 1.33, 1.63, and 12.0). The operating parameters included heat fluxes up to 1.38 W/cm2 with a corresponding surface temperature of 856K. The results, extrapolated to 1200 K, were used to provide a qualitative estimate of the coolability of multirod bundles, as a function of the equilibrium surface temperature and the pitch-to-diameter (P/D) ratio. Although the decay heat removal rate for P/D values <1.5 increased rapidly with P/D ratio, for larger P/D values the decay heat removal rate was insensitive to either the P/D value or the rod arrangement in the bundle. These results suggest that in TRIGA-type reactors at a typical P/D ratio of 1.12, the maximum decay heat removal level is ∼1 kW/m. This maximum corresponds to an initial decay power following sustained operation at ∼12.5 kW/m.
