Improved diffusion coefficents for SPN axial solvers in the MPACT 2D/1D method applied to the AP1000® PWR start-up core models

Shane Stimpson, Fausto Franceschini, Benjamin Collins, Andrew Godfrey, Kang Seog Kim, Aaron Graham, Thomas Downar

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review

Abstract

As part of the Virtual Environment for Reactor Applications Core Simulator (VERA-CS), the 2D/1D capability in the MPACT code is being developed collaboratively by Oak Ridge National Laboratory and the University of Michigan. MPACT was used to model the AP1000® reactor start-up cores. One of the major shortcomings observed in initial results was the ability to accurately resolve the pin-wise fission rate distributions for cases with partial-length burnable poison pins. The primary source of the errors was determined to be the diffusion coefficients that are used in the axial transport solvers of the 2D/1D scheme. The work here demonstrates the deficiency of the previous method used to determine the diffusion coefficients by employing the out-scatter approximation to calculate the transport cross sections. New results are obtained by using the in-scatter and what is being termed the "Neutron Leakage Conservation" approximations to more accurately determine the transport cross sections being used to construct the diffusion coefficients. Additionally, the methods were applied to both 3D assembly and quarter core problems with comparisons of the Nodal Expansion Method and Simplified PN axial transport solvers with these improved diffusion coefficients. Significant improvements are observed, particularly in the single-assembly cases with partial-length Wet Annular Burnable Absorber pins. In the quarter-core cases, the improvements are less apparent because the power distribution is flatter, though the results demonstrate that it is still very worthwhile to incorporate these approaches for determining the transport cross sections.

Original languageEnglish
Title of host publicationMathematics and Computations, Supercomputing in Nuclear Applications and Monte Carlo International Conference, M and C+SNA+MC 2015
PublisherAmerican Nuclear Society
Pages1702-1716
Number of pages15
ISBN (Electronic)9781510808041
StatePublished - 2015
EventMathematics and Computations, Supercomputing in Nuclear Applications and Monte Carlo International Conference, M and C+SNA+MC 2015 - Nashville, United States
Duration: Apr 19 2015Apr 23 2015

Publication series

NameMathematics and Computations, Supercomputing in Nuclear Applications and Monte Carlo International Conference, M and C+SNA+MC 2015
Volume3

Conference

ConferenceMathematics and Computations, Supercomputing in Nuclear Applications and Monte Carlo International Conference, M and C+SNA+MC 2015
Country/TerritoryUnited States
CityNashville
Period04/19/1504/23/15

Keywords

  • 2D/1D
  • AP1000
  • Axial transport
  • Diffusion coefficient
  • MPACT

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