During various flow transients in a sodium-cooled reactor, localized boiling can occur. If this boiling does not result in dryout, significant reactor core damage is not likely. A full-length electrically heated 19-pin bundle was used to determine the extent to which dynamic boiling can be sustained before dryout occurs. Over 30 boiling runs were made with runs at three flow-power conditions culminating in dryout. Continuous boiling for time periods exceeding 20 sec was observed. Preliminary data analysis suggested that thermal inertia of the duct walls, which were backed with thermal insulation, was higher than designed and was contributing to boiling incoherence. Posttest examination confirmed that the insulation annulus had become permeated with sodium, resulting in a significantly increased thermal inertia. Detailed comparisons of experimental results with the results of several different analytical techniques indicate that incoherent boiling caused by bundle thermal inertia was responsible for the long time periods between boiling inception and dryout. This suggests that thermal inertia designed into the reactor core could prevent or delay core damage during various flow-power mismatch transients
