The principal difference between a conventional and vented assembly in a liquid-metal fast breeder reactor is that a vented assembly has holes cut in the duct wall at locations above the active core region and below the load pads above the core. The purpose of the vent holes is to allow coolant to leave the assembly and partially pressurize the interstitial spaces between the assemblies before exiting to the upper plenum through the load pads. Two primary benefits are anticipated from a core composed of vented assemblies

  1. extension of assembly life by pressurizing the core interstitial region (spaces between the assemblies) to reduce duct stress and radial growth
  2. reduction of the reactor coolant pressure drop by providing an additional bypass flow path to the upper plenum for the coolant after it cools the active core.
To allow accurate prediction of the thermohydraulic performance of a core containing vented assemblies, a thorough understanding of the fundamental flow characteristics of coolant flowing through the vents and between the assemblies is needed. A comprehensive analysis and hydraulic test verification were conducted at the General Electric Company to characterize the key flow coefficients that affect the design and evaluation of the vented core concept.