**1. NAME AND TITLE**

TRIPLET: Two-Dimensional, Multigroup, Triangular Mesh, Planar Geometry, Explicit
Discrete Ordinates Code System.

TRIPLET is based, to a large extent, on the two-dimensional orthogonal mesh code CCC-222/TWOTRAN-II which is an improved version of CCC-195/TWOTRAN.

**2. CONTRIBUTOR**

Los Alamos National Laboratory, Los Alamos, New Mexico.

**3. CODING LANGUAGE AND COMPUTER**

FORTRAN IV; CDC 7600 (C00230C760000), CDC 6600 (C00230C660000), or IBM 360/370
(C00230I036000).

**4. NATURE OF PROBLEM SOLVED**

TRIPLET solves the two-dimensional multigroup transport equation in planar geometries using
a regular triangular mesh. Regular and adjoint, inhomogeneous and homogeneous (k_{eff} and
eigenvalue searches) problems subject to vacuum, reflective, or source boundary conditions are
solved. General anisotropic scattering is allowed and anisotropic distributed sources are permitted.

**5. METHOD OF SOLUTION**

The discrete ordinates approximation is used for the angular variables. A finite element method in which the angular flux is assumed to be given by a low-order polynomial in each triangle is used to solve the discrete ordinates equations. Angular fluxes are allowed to be discontinuous across triangle boundaries, and the order of the polynomial is input data to the code. Both inner (within-group) and outer iteration cycles are accelerated by either system or fine mesh rebalance.

Sources, fluxes, S_{n} constants, and cross sections may be input from standard interface files.
Creation of standard interface output files for S_{n} constants and scalar and angular fluxes is optional.
All binary data transfers are localized in subroutines called REED and RITE. Flexible edit options,
including dumps and restart capability, are provided.

**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 CDC machines, MAXLEN can be
about 40,000 words and peripheral storage is used for most group-dependent data. On IBM
machines, TRIPLET will execute in single precision (4 bytes per word) so that MAXLEN can be
several hundred thousand and most problems can be core contained.

**7. TYPICAL RUNNING TIME**

A six group, S_{2}, 1700 triangle, k_{eff} calculation of an EBR-II core requires about 4.4 minutes of
CDC-7600 time. Running times vary almost linearly with the total number of unknowns.

**8. COMPUTER HARDWARE REQUIREMENTS**

Six output (scratch) units, five interface units (use of interface units is optional), and two
system input/output units are required. A large bulk memory is necessary if core storage is
inadequate, as on the CDC machines. The GO step required 610 K bytes of storage on the IBM
360/91.

**9. COMPUTER SOFTWARE REQUIREMENTS**

A FORTRAN IV compiler is required. OVERLAY is necessary on the IBM 360.

**10. REFERENCE**

W. H. Reed, T. R. Hill, F. W. Brinkley, and K. D. Lathrop, "TRIPLET, A Two-Dimensional, Multigroup, Triangular Mesh, Planar Geometry, Explicit Transport Code," LA-5428-MS
(October 1973).

**11. CONTENTS OF CODE PACKAGE**

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

**12. DATE OF ABSTRACT**

July 1975.

**KEYWORDS: ** NEUTRON; GAMMA-RAY; DISCRETE ORDINATES; MULTIGROUP;
TRIANGULAR MESH; TWO-DIMENSIONS; FINITE ELEMENT METHOD