**RSIC CODE PACKAGE CCC-222**

**1. NAME AND TITLE**

TWOTRAN II: Two-Dimensional Multigroup Discrete Ordinates Transport Code System in (x,y), (r,theta), and (r,z) Geometries.

TWOTRAN II is the second iteration of CCC-195/TWOTRAN. A generous number of comment cards were added and simplified programming was performed in order to make TWOTRAN more easily understood. CCC-129/TWOTRAN-SPHERE and CCC-195/TWOTRAN-(PN, FC, VW) are recommended for calculating spherical harmonics, first collision source, and variable weight.

**2. CONTRIBUTOR**

Los Alamos National Laboratory, Los Alamos, New Mexico.

**3. CODING LANGUAGE AND COMPUTER**

FORTRAN; CDC 7600 (A) and IBM 360/370 (B).

**4. NATURE OF PROBLEM SOLVED**

TWOTRAN II solves the two-dimensional multigroup transport equations in (x,y), (r,theta), and
(r,z) geometries. Both regular and adjoint, inhomogeneous and homogeneous (k_{eff} and eigenvalue
searches) problems subject to vacuum, reflective, periodic, white or input specified boundary flux
conditions are solved. General anisotropic scattering is allowed and anisotropic inhomogeneous
sources are permitted.

**5. METHOD OF SOLUTION**

The discrete ordinates approximation for the angular variable is used in finite difference form which is solved with the central (diamond) difference approximation. Negative fluxes are eliminated by a local set-to-zero and correct algorithm. Standard inner (within-group) and outer iterative cycles are accelerated by coarse-mesh rebalancing on a coarse mesh which may be independent of the material mesh.

Provision is made for creation of standard interface output files for S_{n} constants, angle-integrated
fluxes and angular fluxes. Standard interface input files for sources, fluxes, cross sections and S_{n}
constants may be read. All binary operations are localized in subroutines called REED and RITE.
Detailed edit options, including angular fluxes, dumps and restart capability are provided. Optional
use of an arbitrary rebalance mesh independent of the material mesh is allowed.

**6. RESTRICTIONS OR LIMITATIONS**

Variable dimensioning is used so that any combination of problem parameters leading to a container array less than MAXLEN can be accommodated. On the CDC machines, MAXLEN can be slightly greater than 40,000 words. Peripheral storage is used for most group-dependent data. On IBM machines, TWOTRAN-II will execute in the 4-byte mode so that MAXLEN can be several hundred thousand words and most problems can be core-contained.

**7. TYPICAL RUNNING TIME**

A six-group, S_{4}, 42 x 42 mesh point, k_{eff} calculation of an EBR-II model requires about 3.9 minutes
of CDC-7600 time. The sample problem took 10 minutes of CPU time on an IBM 360 computer.

**8. COMPUTER HARDWARE REQUIREMENTS**

TWOTRAN II is operable on the CDC 7600 (A) or IBM 360/370 (B) computers.

Five output units, five interface units (use of interface units is optional) and two system input/output units are required. A large bulk memory is desirable, but it can be replaced by disk, drum or tape storage.

**9. COMPUTER SOFTWARE REQUIREMENTS**

A FORTRAN IV compiler is required.

**10. REFERENCES**

a. Included in the documentation:

F. W. Brinkley, "TWOTRAN-II Memo," (April 19, 1974).

F. W. Brinkley, "TWOTRAN-II Memo," (March 18, 1974).

K. D. Lathrop and F. W. Brinkley, "TWOTRAN-II: An Interfaced, Exportable Version of the TWOTRAN Code for Two-Dimensional Transport," LA-4848-MS (July 1973).

b. Background information:

K. D. Lathrop and F. W. Brinkley, "Theory and Use of the General-Geometry TWOTRAN Program," LA-4432 (May 1970).

**11. CONTENTS OF CODE PACKAGE**

Included are the referenced document (10.a) and one (1.2MB) DOS diskette which contains the source code and sample problem input and output.

**12. DATE OF ABSTRACT**

September 1973; updated July 1975.

**KEYWORDS: ** DISCRETE ORDINATES; NEUTRON; GAMMA-RAY; TWO-DIMENSIONS;
MULTIGROUP