Coupled Nuclear-Thermalhydraulic Calculations for Fort St. Vrain Reactor
NURETH-14 - 2011 September 25-30

Presented at:
2011 September 25-30
Toronto, Canada
Session Title:
G4-2 - Gas Cooled Fast Reactors and Very High Temperature Reactors; System Calculations

Benjamin Betzler (University of Michigan)
Eva Sunny (University of Michigan)
John Lee (University of Michigan)
William Martin (University of Michigan)


The 842-MWt Fort St. Vrain (FSV) helium-cooled, graphite-moderated reactor was officially decommissioned in 1989 after 14 years of operation. The active core has 247 fuel columns divided into 37 fuel regions, with six axial layers of 79 cm tall hexagonal fuel blocks. Graphite blocks outside, above, and below the active core serve as reflectors. Four types of blended TRISO particles produce the 13 compositions distributed in the fuel regions. An MNCP5 model for FSV uses an average kernel size that preserves fuel loading and a 58% packing fraction for each composition, either smearing TRISO particles within the fuel rod or explicitly modeling each particle in a regular lattice. A coupled MCNP5-RELAP5 setup accounts for thermal feedback effects. For eight axial and six radial regions of the FSV model, MCNP5 determines the power fractions required by RELAP5 to calculate axial temperature distributions. The MCNP5 model receives these temperatures and the process repeats until convergence to a solution. Similar work on the Very High Temperature Reactor uses Ratio and PIKMT methods for both homogeneous and heterogeneous models. Both methods calculate energy deposition from neutrons, fission products, beta particles, and prompt and capture gammas, but the Ratio method uses pre-calculated fractions to account for delayed gamma contributions to energy deposited. The PIKMT method is computationally costlier, more accurate, and converges faster than the Ratio method. The PIKMT method is applied to FSV.

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