**1. NAME AND TITLE**

VIM 4.0: Continuous Energy Neutron and Gamma-Ray Transport Code System.

**AUXILIARY CODE:**

SLICER: Plots a 2D slice of VIM input files (runs only on Linux).

**2. CONTRIBUTOR**

Argonne National Laboratory, Argonne, Illinois.

**3. CODING LANGUAGE AND COMPUTER**

VIM is written in Fortran 77 with a few subprograms in C. The geometry visualization program, Slicer, is in C++. Machines: Sun, IBM RS/6000, Linux PC (C00658/MNYWS/03).

**4. NATURE OF PROBLEM SOLVED**

VIM solves the steady-state neutron or photon transport problem in any detailed three-dimensional geometry using either continuous energy-dependent ENDF nuclear data or multigroup cross sections. Neutron transport is carried out in a criticality mode, or in a fixed source mode (optionally incorporating subcritical multiplication).

Photon transport is simulated in the fixed source mode. The geometry options are infinite medium, combinatorial geometry, and hexagonal or rectangular lattices of combinatorial geometry unit cells, and rectangular lattices of cells of assembled plates. Boundary conditions include vacuum, specular and white reflection, and periodic boundaries for reactor cell calculations.

The VIM 4.0 distribution includes data from ENDF/B-IV, ENDF/B-V, ENDF/B-VI and JEF2.2. Binary sequential data libraries for use with the code system on IBM or Sun workstations are included. ASCII data libraries and a convenient means to convert them to binary on a target machine are included for users on other systems.

In March 2004, a Linux version was added to the package. This release also includes additional libraries from JENDL 3.2 and a code that plots a 2D slice from VIM input files. The Unix version is unchanged from the previous release.

**5. METHOD OF SOLUTION**

VIM uses standard Monte Carlo methods for particle tracking with several optional variance-reduction techniques. These include splitting/Russian roulette, non-terminating absorption with nonanalog weight cutoff energy. The keff is determined by the optimum linear combinations of two of the three eigenvalue estimates - analog, collision, and track length. Resonance and smooth cross sections are specified pointwise with linear - linear interpolation, frequently with many thousands of energy points. Unresolved resonances are described by the probability table method, which allows the statistical nature of the evaluated resonance cross sections to be incorporated naturally into self-shielding. Neutron interactions are elastic, inelastic and thermal scattering, (n,2n), fission, and capture, which includes (n,), (n,p), (n,), etc. Photon interaction data for pair production, coherent and incoherent scattering, and photoelectric events are taken from MCPLIB. Trajectories and scattering are continuous in direction, and anisotropic elastic and discrete level inelastic neutron scattering are described with probability tables derived from ENDF/B data. VIM has an automatic restart capability to permit user-directed statistical convergence.

In eigenvalue calculations, the beginning source sites are from a random (flat) guess, or can be provided via ASCII input, or from a previous calculation. The starting neutrons for each subsequent generation are randomly selected from the potential fission sites in the previous generation.

Track-length or collision estimates of reaction rates are automatically tallied by energy group and edit region to facilitate comparison to other calculations. Groupwise edits include isotopic and macroscopic reaction rates and cross sections, group-to-group scattering cross sections, net currents, and scalar fluxes. Particle pseudo-collisions are used to estimate microscopic group-to-group (n,2n), inelastic, and PN elastic scattering. The serial correlation of eigenvalue estimates is computed to detect underestimated errors.

**6. RESTRICTIONS OR LIMITATIONS**

The maximum number of isotopes in one calculation is 40. The maximum number of splitting surfaces is 60. All other problem characteristics are accommodated by variable dimensioning.

The limit for the Linux version is now 100 isotopes. A maximum of 10000 active generations can be run.

**7. TYPICAL RUNNING TIME**

Varies widely, depending on geometric complexity, the number of isotopes, application of absorption weighting and splitting, overall scattering ratio, and desired statistics. A 6000-zone calculation of the LTR-IIa reactor keff to a 1 precision of 0.3% requires approximately 10 minutes on a Sun Sparc 20.

**8. COMPUTER HARDWARE REQUIREMENTS**

VIM runs on Sun, IBM RS/6000 workstations and PCs running Linux.

**9. COMPUTER SOFTWARE REQUIREMENTS**

In 2001, RSICC successfully tested VIM on a Sun Solaris 2.6 UltraSparc 60 using F77 5.0 and C/C++ 5.0 and on an IBM RS/6000 Model 270 running AIX 4.3.3, XL Fortran 7.1 and XL C 4.4. XSEDIT calls DISSPLA to plot cross section data, although this coding can be easily removed.

In 2004, a new Linux version was added. The Portland Group Fortran and the Gnu C and C++ compilers were used. The Gnu Fortran compiler also can be used. The DISSPLA coding was removed in this release. The author's Linux executables are included in the package. RSICC tested this release on Red Hat 7.3 with Portland Group Fortran 4.0-2 and GNU gcc 2.96.

**10. REFERENCE**

R. N. Blomquist, "VIM Monte Carlo Neutron/Photon Transport Code User's Guide Version 4.0" (December 27, 2000). Note that some of the links in this document to the web are not valid.

**11. CONTENTS OF CODE PACKAGE**

Included are the referenced document and one CD-rom with compressed Unix tar files which contain installation instructions, User's Guide, Fortran source, data libraries, test cases and Linux executables.

**12. DATE OF ABSTRACT**

April 1998, revised December 1998, June 2001, March 2004.

**KEYWORDS:** COMBINATORIAL GEOMETRY; REACTOR PHYSICS; CRITICALITY
CALCULATIONS; MONTE CARLO; NEUTRON; GAMMA-RAY; GAMMA-RAY HEATING; WORKSTATION