Thermal-hydraulic analysis of a spectral shift reactor core

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Virginia Polytechnic Institute and State University


A computer program has been developed which performs a thermal-hydraulic analysis of a mechanical spectral shift reactor fuel assembly. In this reactor type, the fuel to water volume ratio is changed to shift the neutron energy spectrum and control reactivity. The ability to handle different fuel to water ratios is the unique feature of this program.

The required input parameters are the coolant inlet pressure and temperature, the mass flow rates, and the power density. The cladding temperature and the coolant pressure and temperature are calculated throughout the fuel assembly.

Three test cases were performed using a conceptual reactor core design. The fuel to water ratio of this design can be varied from 1.25 to 0.75 by removing blank rods from the core. Each fuel assembly is enclosed in a hex can. The fuel assembly flowrates can be independently selected.

The test case results indicate that proper flowrate selection can produce equal fuel assembly enthalpy gains. The maximum allowable pressure drop determines the maximum coolant flowrate and the maximum power density variation between fuel assemblies. The coolant exit properties are relatively unaffected by changing the fuel to water ratio. This indicates that little heat is transferred into the moderator regions that are formed when void rods are removed.

The most useful program modification would be the addition of a routine that defines the coolant channel parameters for any fuel assembly.