NEA-0430 SNAP.

1. NAME OR DESIGNATION OF PROGRAM:  SNAP-3D.

3. NATURE OF PHYSICAL PROBLEM SOLVED

The group neutron diffusion equations in one, two, or three dimensions are solved. The available geometries are (x,y,z), (r,theta,z), (hex,z), and (tri,z).
External and internal boundaries of the usual logarithmic form are allowed. Optional reflective or periodic symmetry conditions are available.
Scatter matrices may be full, restricted to down scatter or have a band form. The emergence spectrum may be a function of position. Adjoint fluxes may be computed.
     There are five classes of problems:
(a) Eigenvalue search for K-effective.
(b) Eigenvalue search for critical buckling.
(c) Eigenvalue search for critical time constant.
(d) Fixed source problems in which the sources are functions of zone.
(e) Fixed source problems in which the sources are provided for     every mesh point and group.

4. 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.

5. RESTRICTIONS ON THE COMPLEXITY OF THE PROBLEM

(a) No more than 200 groups.
(b) There is a limit of 300 different types of material but this can be lifted by altering a single statement.
(c) The overall maximum problem size is determined by the capacity  of one array. On ICL computers, this can be changed from run to  run by means of suitable JCL. On IBM computers 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.

6. TYPICAL RUNNING TIME:  This is problem dependent.

7. UNUSUAL FEATURES OF THE PROGRAM

Arbitrary scatter, including up- scatter, is allowed between groups. Every problem can be solved in the normal or adjoint mode. Very general, group-dependent boundary conditions may be specified in fixed source problems and might, for example, represent the neutron current entering a shield from a reactor core.

8. RELATED AND AUXILIARY PROGRAMS:   SNAP. WIMS-E modular computational scheme.

10. REFERENCES

- C.W.J. McCallien:
  'The Solution of Reactor Diffusion Problems'.
   The Computer Journal, Vol. 13, no. 4, pp. 369-377 (1970).
- C.W.J. McCallien:
  'Multi-Dimensional Neutron Diffusion Calculations and Coarse-
   Mesh Rebalancing'.
   Proceedings of Institute of Physics Conf, Glasgow (Aug.1975)
   To be published by Transcripta books, Washington.

- C.W.J. McCallien:
  SNAP-3D, A Three-Dimensional Neutron Diffusion Code.
  TRG Report 2677(R) (October 1975).
- C.W.J. McCallien:
  SNAP, A Two-Dimensional Neutron Diffusion Code.
  TRG Report 1990(R) (1970).
- C.W.J. McCallien:
  SNAP - A Three-Dimensional Neutron Diffusion Code
  AEA-FR-0008(R) (February 1993).
- C.W.J. McCallien:
  SNAP - A Three-Dimensional Neutron Diffusion Code
  AEA-RS-1214 (February 1993).

11. MACHINE REQUIREMENTS:  At least 220k bytes of fast store. 11 disk files.

13. OPERATING SYSTEM OR MONITOR UNDER WHICH PROGRAM IS EXECUTED:  ICL 4/72 DOS 7J.

14. ANY OTHER PROGRAMMING OR OPERATING INFORMATION OR RESTRICTIONS

The code will accept macroscopic cross sections on cards, on an x-tape, on a WIMS-E file, or on a COSMOS file.
There is an edit for calculating the mean neutron life time from the normal and adjoint fluxes. There is also an edit that prints neutron currents in selected directions over specified sub-regions. Linear combinations of the fluxes, such as powers and reaction rates, can be printed.
SNAP-3D was originally conceived and designed as a code for solving neutronic problems within a reactor core. It has subsequently been used successfully for shield problems with high attenuation and for gamma diffusion problems.

15. NAME AND ESTABLISHMENT OF AUTHOR

    C.W.J. McCallien
    Computer Section
    United Kingdom Atomic Energy Authority
    Risley, Warrington
    England.

test-case data   mag tapeALLCASES.DAT;16 (4) test problems input    DATTP
test-case output mag tapeALLCASES.DEC6;1 test pb long out. DEC UNIX OUTTP
test-case output mag tapeALLCASES.DEC7;1 test pb short out. DEC UNIXOUTTP
test-case output mag tapeALLCASES.VAX6;7 test pb long out. VAX VMS  OUTTP
test-case output mag tapeALLCASES.VAX7;7 test pb short out. VAX VMS OUTTP
test-case data   mag tapeALLTESTS.DAT;30 (65) test problems input   DATTP
test-case output mag tapeALLTESTS.DEC6;2 test pb long out. DEC UNIX OUTTP
test-case output mag tapeALLTESTS.DEC7;2 test pb short out. DEC UNIXOUTTP
test-case output mag tapeALLTESTS.VAX6;9 test pb long out. VAX VMS  OUTTP
test-case output mag tapeALLTESTS.VAX7;9 test pb short out. VAX VMS OUTTP
test-case data   mag tapeBIGCASES.DAT;3 (11) test problems input    DATTP
test-case output mag tapeBIGCASES.DEC6;1 test pb long out. DEC UNIX OUTTP
test-case output mag tapeBIGCASES.DEC7;1 test pb short out. DEC UNIXOUTTP
test-case output mag tapeBIGCASES.VAX6;8 test pb long out. VAX VMS  OUTTP
test-case output mag tapeBIGCASES.VAX7;8 test pb short out. VAX VMS OUTTP
test-case data   mag tapeCDFR.DAT;1 single case fast reactor test pbDATTP
test-case output mag tapeCDFR.DEC6;1 test pb long output DEC UNIX   OUTTP
test-case output mag tapeCDFR.DEC7;1 test pb short output DEC UNIX  OUTTP
test-case output mag tapeCDFR.VAX6;1 test pb long output VAX VMS    OUTTP
test-case output mag tapeCDFR.VAX7;1 test pb short output VAX VMS   OUTTP
test-case data   mag tapeEXECSEVERAL.;1 Script to run SNAP on UNIX  DATTP
test-case data   mag tapeEXECSNAP-UNIX.;3 Execut. procedure for UNIXDATTP
miscellaneous    mag tapeREAD-ME.LIS;5 Information file             MISTP
test-case data   mag tapeSMALLCASES.DAT;1 artificial cases test pb  DATTP
test-case output mag tapeSMALLCASES.DEC6;1 test pb long out.DEC UNIXOUTTP
test-case output mag tapeSMALLCASES.DEC7;1 test pb short out.DECUNIXOUTTP
test-case output mag tapeSMALLCASES.VAX6;1 test pb long out. VAX VMSOUTTP
test-case output mag tapeSMALLCASES.VAX7;1 test pb short out.VAX VMSOUTTP
source program   mag tapeSNAP-FOR-4I5H.;2 Portable SNAP-4I5H source SRCTP
source program   mag tapeSNAP-FOR-DEC.;1 SNAP source for DEC UNIX   SRCTP
source program   mag tapeSNAP-FOR-VAX.;1 SNAP source for VAX VMS    SRCTP
test-case data   mag tapeSNAP-VAX.COM;7 JCL for VAX                 DATTP
test-case data   mag tapeSNAP-VAX.RUN;35 Proc.to run SNAP on VMS    DATTP
test-case data   mag tapeSNAP-VAX.SUB;10 Batch submiss.command (VMS)DATTP
test-case data   mag tapeSNAP-VAX.SUB;9 Batch submission command-VMSDATTP
report                   AEA-FR-0008(R) (February 1993)             REPPT
report                   AEA-RS-1214 (February 1993)                REPPT

File name	File description	Records
NEA0430_01.001	ASSEMBLER SUBROUTINES	87
NEA0430_01.002	SOURCE PROGRAM (F4)	8391
NEA0430_01.003	RISLEY SUBROUTINES (ASSEMBLER)	140
NEA0430_01.004	RISLEY SUBROUTINES (F4)	689
NEA0430_01.005	OVERLAY CARDS	42
NEA0430_01.006	JCL	23
NEA0430_01.007	SAMPLE PROBLEM DATA	124
NEA0430_01.008	SAMPLE PROBLEM PRINTED OUTPUT	1688