**1. NAME AND TITLE**

HEXAB-3D: Three-Dimensional Few-Group Coarse Mesh Diffusion Code for Neutron Physics Calculation of Reactor Core in Hexagonal Geometry.

**2. CONTRIBUTOR**

Institute for Nuclear Research and Energy, Bulgarian Academy of Sciences, Sofia, Bulgaria.

**3. CODING LANGUAGE AND COMPUTER**

FORTRAN IV(G) and Assembler; IBM 370 and IBM 3031.

**4. NATURE OF PROBLEM SOLVED**

HEXAB-3D solves the three-dimensional few-group diffusion model for the calculation of the basic neutron physical characteristics of power reactors in hexagonal geometry.

**5. METHOD OF SOLUTION**

The nine-point mesh-centered finite difference approximation of the neutron diffusion equation is used. The standard inner-outer iteration strategy is employed. Inner iterations are solved using two different incomplete factorization techniques: AGA two-sweep iterative method and modified AGA. Successive overrelaxation is applied. The power method, combined with two- or three-term Chebyshev polynomial acceleration, is applied for outer iterations.

**6. RESTRICTIONS OR LIMITATIONS**

The limits are a maximum of 10 energy groups, 30 horizontal layers and 100 material compositions.

**7. TYPICAL RUNNING TIME**

The sample problem took 4 minutes, 8 seconds on the IBM 3031 under OS/VS1.

**8. COMPUTER HARDWARE REQUIREMENTS**

HEXAB-3D runs on the IBM 370/145 and IBM 3031 computers.

**9. COMPUTER SOFTWARE REQUIREMENTS**

The code is written in FORTRAN IV and Assembler. The FORTRAN IV (G) compiler is required together with the IBM Assembler running under OS/VS1.

**10. REFERENCES**

**a. Included in the documentation:**

K. Ivanov, M. Manalova, T. Apotolov, ``IBM User's Guide to HEXAB-3D Code,'' Informal Notes Pr.11-22.07.1987 (March 1991).

K. Ivanov, T. @Apostolov, M. Manalova, ``HEXA-3D: A Three-dimensional Few-Group
Coarse-Mesh Diffusion Code for Reactor Calculations,'' *Proc. of International Topical
Meeting on Advances in Mathematics, Computations and Reactor Physics, Pittsburgh,
Pennsylvania (April 28 -- May 2, 1991.) *

**b. Background information:**

K. Ivanov, M. Manalova, T. Apostolov, ``Application of Two-Sweep Iterative Method
AGA DSOR in the three-dimensional Hexagonal Programme HEXAB-3D for Group
Diffusion Calculations of Fast Breeder Reactors,'' *Nuclear Energy* 28, Sofia, pp. 13-18
(1990).

**11. CONTENTS OF CODE PACKAGE**

Included are the referenced documents and one DS/HD 5.25-in. diskette (1.2 MB), containing the source code, sample input and output.

**12. DATE OF ABSTRACT**

October 1991.

**KEYWORDS: ** COMPLEX GEOMETRY; DIFFUSION THEORY; MULTIGROUP;
NEUTRON; REACTOR PHYSICS