MPACT Overview

MPACT is a transient one-step code that primarily relies on the 2D/1D method for whole core transport and has been applied to PWRs (dozens of operational cycles), and BWRs. With this approach, 3D geometries are separated into 2D-radial and pin-homogenized, 1D-axial problems.  For the 2D solvers, the method of characteristics is used and for the 1D solvers, P3 transport with a fourth-order Legendre-based spatial flux expansion is used.  These 2D and 1D solvers are coupled through axial and radial transverse leakages and accelerated with a 3D-coarse mesh finite difference solver. MPACT uses inline cross section generation based on the subgroup method and uses ORIGEN for depletion.

Contact Ben Collins at or for more information

Applications and Integrations

Nuclear reactor physics steady-state eigenvalue (core follow) and transient simulation (load follow and NRC Chapter 15 licensing events) with cross section feedback and material isotopic depletion

Full integration with the thermal-hydraulics code CTF, fuel temperature codes CTFFuel and BISON, crud chemistry code MAMBA, and ex-core transport code Shift.

Planned capabilities for the future include:

  • BWR Support
  • Extended transient analysis for series of operational transients


  1. MPACT Team, “MPACT Theory Manual,” Technical Report, CASL-U-2019-1874-001, Oak Ridge National Laboratory, Nov. (2019). 
  2. MPACT Team, “MPACT User Manual,” Technical Report, CASL-U-1873-001, Oak Ridge National Laboratory, Nov. (2019). 
  3. B. Collins, S. Stimpson, B. Kelley, M. Young, B. Kochunas, A. Graham, E. Larsen, T. Downar, A. Godfrey, “Stability and accuracy of 3D neutron transport simulations using the 2D/1D method in MPACT,” Journal of Computational Physics, 326 (2016), 612-628.
  4. B. Kochunas, D. Jabaay, T. Downar, B. Collins, S. Stimpson, A. Godfrey, K. Kim, J. Gehin, S. Palmtag, F. Franceschini, “Validation and Application of the 3D Neutron Transport MPACT within CASL VERA-CS, NURETH-16, Intl. Top. Mtg. on Nuclear Reactor Hydraulics, Chicago, Aug 30-Sep 4, 2015.
  5. B. Kochunas, B. Collins, S. Stimpson, R. Salko, D. Jabaay, A. Graham, Y. Liu, K. Kim, W. Wieselquist, A. Godfrey, K. Clarno, S. Palmtag, T. Downar, J. Gehin,  “VERA Core Simulator Methodology for Pressurized Water Reactor Cycle Depletion,” Nuclear Science and Engineering, 185 (2017), 612-628.

MPACT Images


3D Rod-by-Rod Power Distribution

2-D Slice of the MPACT Model (left) and MPACT Flux Distribution (right) of the NuScale SMR with Detailed Reflector Model