**RSICC CODE PACKAGE CCC-434**

**1. NAME AND TITLE**

SNAP-3D: A Three-Dimensional Neutron Diffusion Code.

**2. CONTRIBUTOR**

** **UKAEA Technology, Risley, United Kingdom, through the OECD Nuclear Energy Agency
Data Bank, Issy-les-Moulineaux, France.

**3. CODING LANGUAGE AND COMPUTER**

** **Fortran 77; PC and workstation (C00434MNYCP01).

**4. NATURE OF THE PROBLEM SOLVED**

SNAP-3D is based on SNAP2 and is a one- two- or three-dimensional multigroup diffusion code system. It is primarily intended for neutron diffusion calculations, but it can also carry out gamma-ray calculations if the diffusion approximation is accurate enough. It is suitable for fast and thermal reactor core calculations and for shield calculations.

SNAP-3D can solve the multi-group neutron diffusion equations using finite difference methods in (x,y,z), (r,theta,z), (TRI,z), (HEX,z) or (spherical) coordinates. The one-dimensional slab and cylindrical geometries and the two-dimensional (x,y), (r,z), (r,theta), (HEX) and (TRI) are all treated as simple special cases of three-dimensional geometries. Numerous reflective and periodic symmetry options are available and may be used to reduce the number of mesh points necessary to represent the system. Extrapolation lengths can be specified at internal and external boundaries.

The problem classes are: 1) eigenvalue search for critical k-effective, 2) eigenvalue search for critical buckling, 3) eigenvalue search for critical time-constant, 4) fixed source problems in which the sources are functions of regions, 5) fixed source problems in which the sources are provided, on disc, for every mesh point and group.

**5. METHOD OF SOLUTION**

The finite difference equations are solved by a combination of source iteration and successive over-relaxation. If desired, a coarse mesh rebalancing algorithm may be used at intervals to accelerate the source iterations.

**6. RESTRICTIONS OR LIMITATIONS**

No more than 69 energy groups are permitted. There is a limit of 300 different types of material, but this can be lifted by altering a single statement.

The overall maximum problem size is determined by the capacity of one array. The main program must be recompiled with two cards amended in order to adjust the size of the array. The amount of I/O necessary during the iterations depends on the size of this array and can often be reduced by enlarging the array.

**7. TYPICAL RUNNING TIME** The running time for SNAP-3D is problem dependent. The sample problems ran on

the RSICC Sun in 15 minutes. The sample problems ran on the Micron PC in <5 minutes.

**8. COMPUTER HARDWARE REQUIREMENTS**

SNAP-3D has run on DEC Vax 4000, DEC ALPHA Station 200-4/166, Pentium 166MHz, HP 9000/7xx, IBM RISC/6000, and Sun SPARCStation.

**9. COMPUTER SOFTWARE REQUIREMENTS**

** ** The code is expected to run on any operating system on which a Fortran 77 compiler is
available. Fortran compilers are required on all Unix machines. An executable created with
Microsoft Powerstation 4.0 is included for PC users.

** **

**10. REFERENCES**

McCallien, C. W. J., "SNAP - A Three-Dimensional Neutron Diffusion Code,"

AEA-RS-1214 (February 1993).

McCallien, C. W. J., "SNAP-3D - A Three-Dimensional Neutron Diffusion Code,"

Group Computer Services, Risley, TRG Report 2677(R) (October 1975).

**11. CONTENTS OF CODE PACKAGE**

The distribution CD contains the referenced documents and a self-extracting compressed file and a compressed tar file with the source, PC executable and test cases.

**12. DATE OF ABSTRACT**

December 2002.

**KEYWORDS**: COMPLEX GEOMETRY; ADJOINT; GAMMA-RAY; NEUTRON;
DIFFUSION THEORY; MULTIGROUP; ONE-DIMENSION; TWO-DIMENSIONS