Particle Bed Heat Removal with Subcooled Sodium: D-4 Results and Analysis

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Abstract
The results of a fission-heated sodium-U02 particle bed heat removal experiment (D-4) are presented and the effects of cooling the overlying sodium below saturation are discussed. Single-phase convection began at a Rayleigh number an order of magnitude smaller than for water. Bed disturbances were observed to occur at the onset of boiling, but only after a previous boiling cycle. The disturbances are believed to be due to the flashing of superheated liquid sodium after noncondensable gases had been removed during a previous boiling cycle. The start of bed dryout was observed with two different overlying sodium temperatures (300 and 600°C). The dryout power was 0.77 kW/kg with 300°C overlying sodium (and 29 kPa pressure) and 3.58 kW/kg with 600°C sodium (and 43 kPa). It is believed that cold overlying sodium reduces the large heat-removal capability of shallow beds by causing vapor condensation within the bed and suppressing channel formation. Steady-state temperatures above the boiling temperature were observed at the bed bottom for several power levels above the incipient dryout power, indicating stable dry zones.