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1. NAME AND TITLE**

GNOMER: Multigroup 3-Dimensional Neutron Diffusion Nodal Code System with Thermohydraulic Feedbacks.

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2. CONTRIBUTOR**

Institute "Jozef Stefan" University of Ljubljana, Ljubljana, Slovenia, through the NEADB, Issy-les-Molineaux, France.

**3. CODING LANGUAGE AND COMPUTER**

Fortran 77 for IBM PC's and compatibles, VAX, and other computers (C00625/MNYCP/01).

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4. NATURE OF PROBLEM SOLVED**

GNOMER is a program which solves the multigroup neutron diffusion equation on coarse mesh in 1D, 2D, and 3D Cartesian geometry. The program is designed to calculate the global core power distributions (with thermohydraulic feedbacks) as well as power distributions and homogenized cross sections over a fuel assembly.

**5. METHOD OF SOLUTION**

Using a variant of the Green's function method for the one-dimensional equations and applying the Nodal approach for extension to multidimensional cases, a very efficient algorithm was obtained and incorporated into the main GNOMER code. A simple thermohydraulic module to calculate temperature distribution is included. The Reactivity Coefficient method is applied for small corrections to cross sections when core conditions differ from nominal (at which the cross sections have been tabulated) due to thermohydraulic feedbacks. -Cyclic quadrant symmetry and octant symmetry options are available.

**6. RESTRICTIONS OR LIMITATIONS**

There must be enough available system memory to run all the included sample problems. Sample problem 4 will not run using the included MS Fortran created executable because of limitations involving allocated array sizes (the Microway created executable must be used to run sample problem 4 on a PC). On the VAX using sample problem 4, format rounding errors of one unit may occur in the thousandths decimal place.

**7. TYPICAL RUNNING TIME**

Approximate run times for GNOMER using the five included sample cases were as follows:

Tests VAX Lahey Fortran Microsoft Fortran

T1 02.93 sec 00.66 sec 03.29 sec

T2 33.48 sec 44.05 sec 3 min 18.94 sec

T3 50.09 sec 1 min 06.62 sec 5 min 18.57 sec

T4 3 min 36.22 sec 3 min 44.53 sec ------------

T5 18.10 sec 38.39 sec 2 min 01.55 sec

**8. COMPUTER HARDWARE REQUIREMENTS**

An IBM PC or compatible with 640K of RAM or a DEC VAX.

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9. COMPUTER SOFTWARE REQUIREMENTS**

There are no special software requirements for GNOMER on a PC unless there is a need to compile the source code. MS Fortran v5.1 (or higher) or NDP Fortran-486 v4.4.0 are the recommended compilers for the PC and were used to create the two executables included in the package. NDP Fortran-486 or another compiler with an extended DOS capability are required for creating an executable that is capable of running sample problem 4 or other more complex problems. NDP compiler was used to create executable included in package. VAX Fortran is required for compiling the source code on a DEC VAX under VMS.

**10. REFERENCES**

a) included in documentation:

A. Trkov, "Multigroup 3-Dimensional Neutron Diffusion Nodal Code with Thermohydraulic Feedbacks," IJS-DP-6688, Revision 2 (July 1994).

b) background information:

A. Trkov, M. Najzer, L. Skerget, "Variant of Green's Function Nodal Method for Neutron Diffusion," Nucl. Sci. Technol., 27(8) 766-777 (August 1990).

**11. CONTENTS OF CODE PACKAGE**

The referenced document and one 3.5-inch (1.44 MB) diskette written in DOS format are included. The code package contains sample input and output, the executable, and the GNOMER source files.

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12. DATE OF ABSTRACT**

November 1994, April 1995.

**KEYWORDS: **DIFFUSION THEORY; MULTIGROUP; COMPLEX GEOMETRY;
MICROCOMPUTER