**1. NAME AND TITLE**

OMEGA: Monte Carlo Criticality Code System.

**AUXILIARY ROUTINES**

THLIB: BCD-to-binary Conversion of Thermal Library.

SUBLIB: Conversion of Epithermal Subgroup Library.

**2. CONTRIBUTOR**

Akademie der Wissenschaften der DDR, Zentralinstitut fur Kernforschung Rossendorf, German
Democratic Republic, through the OECD NEA Data Bank, Gif-sur-Yvette, France.

**3. CODING LANGUAGE AND COMPUTER**

ALGOL; BESM-6.

**4. NATURE OF PROBLEM SOLVED**

OMEGA is a Monte Carlo code for the solution of a time-independent neutron transport equation for a general three-dimensional geometry. The sub-critical problem (with an outer source) as well as the critical (K-effective) problem can be solved for a special three-dimensional geometry. The geometry consists of a rather general arrangement of three basic shapes (prisms with circular, rectangular, or hexagonal cross section with a finite height and different material layers along the axis). The cross sections are calculated using built-in data libraries.

OMEGA evaluates the multiplication factor, the flux distribution and reaction rates. It mainly
calculates the criticality of complicated arrangements of fissionable materials. Calculated results are:
the K-effective eigenvalue, the flux distribution, reaction rates, and spatially and energetically
condensed cross sections for later use as in diffusion calculations.

**5. METHOD OF SOLUTION**

The Monte Carlo method is used with neutrons started from an initial source distribution. A batch of neutrons is followed from collision to collision until the histories are terminated by capture, fission, or leakage. If the eigenvalue problem is to be solved, the starting positions of the next generation of neutrons are determined by the fission points of the preceding generation, and the first few generations are omitted.

OMEGA uses the 26 group ABBN library in the subgroup representation and a THERMOS library
for the cross sections in the epithermal and thermal region, respectively.

**6. RESTRICTIONS OR LIMITATIONS**

All the arrays are dynamically declared. Therefore, large demands for memory in one part can
be compensated by a small demand in another part of the program, and fixed limitations of parameters
cannot be given.

**7. TYPICAL RUNNING TIME**

The running time is strongly dependent upon the complexity of the problem. The average time for a real problem is 1 hour.

The results are calculated during the Monte Carlo calculation and not finally from the recorded
collisions. Therefore, the running time depends on the amount of the desired results.

**8. COMPUTER HARDWARE REQUIREMENTS**

OMEGA was developed for the BESM-6 computer. It uses 28 K of fast central memory, 1 disc
(or drum), and 1 library tape. The program is executed under the BAMOS operating system.

**9. COMPUTER SOFTWARE REQUIREMENTS**

An ALGOL compiler is required.

**10. REFERENCES**

E. Sartori, "The Monte Carlo Criticality Code OMEGA," memo (January 1983).

E. Seifert, "The Monte Carlo Criticality Code OMEGA," ZfK-364 (October 1978).

**11. CONTENTS OF CODE PACKAGE**

Included are the referenced document and one (1.2MB) DOS diskette which contains the ALGOL
text and sample problem input, plus the THERMOS cross section library and the ABBN subgroup
library.

**12. DATE OF ABSTRACT**

January 1983, updated July 1991.

**KEYWORDS: ** COMPLEX GEOMETRY; MONTE CARLO; CRITICALITY CALCULATIONS;
NEUTRON; REACTOR PHYSICS