Experimental investigation of sodium boiling in partially blocked fuel subassemblies

Abstract

Out-of-pile experiments were conducted to examine local sodium boiling in two wire-wrapped 37-pin bundles simulating LMFBR fuel subassemblies. The central 24 subchannels were blocked in the first bundle, and a 1 2 edge part of bundle cross-sectional area was blocked in the other. The boiling modes observed in the wake were irregular nucleate, oscillatory and stationary. Three different types of boiling transition were identified, which were characterized by the temperature gradient around the two-phase voiding region. Dryout occurred not in the irregular nucleate boiling mode, but in both the stationary and oscillatory boiling modes. The coolability margin is considered to be 20 to 30% in terms of power-to-flow ratio from the incipient boiling to the occurrence of dryout.