RSICC CODE PACKAGE PSR-431/ATHENA_2D
1. NAME AND TITLE
ATHENA_2D: Code System For Simulation Of Hypothetical Recriticality Accidents In a Thermal Neutron Spectrum.
2. CONTRIBUTORS
Fluor Daniel Northwest, Richland, Washington.
University of Washington, Seattle, Washington.
3. CODING LANGUAGE AND COMPUTER
Fortran 77; IBM PC and compatibles, DEC Alpha workstations, or other unix workstations (P00431MNYCP00).
4. NATURE OF PROBLEM SOLVED
ATHENA_2D was written to simulate a hypothetical water reflood of a highly-damaged light water reactor (such as the Three-Mile-Island Unit-2 after meltdown, with a packed debris bed near the center of the core), but with insufficiently-borated reflood water. A recriticality transient may result because of the potentially more reactive debris bed. ATHENA_2D solves the transient multigroup neutron diffusion equations in (r,z) geometry. Executing in parallel with the transient neutronics, is a single-phase computational fluid dynamics (CFD) model, driven by multichannel thermal hydraulics based on detailed pin models.
Numerous PV-Wave procedure files are included on the distribution media, useful for those who already have PV-Wave from Visual Numerics. These procedures are documented in the "README" files included on the distribution CD.
Some reactor lattice computer code such as WIMS-E, CCC-576/WIMSD4, or CCC-656/WIMSD5B is required for the creation of macroscopic cross section libraries, given pin-cell geometries. WIMS-E is a commercial product available from AEA Technologies, England, WIMS is not included on the ATHENA_2D distribution CD.
Several auxiliary routines are included in the package.
TFMAX: Utility that searches through ATHENA_2D binary output to find the maximum fuel temperature over space and time.
POST_VEL: Utility that searches through ATHENA_2D binary output to find maximum scalar and flow field values (over space) and outputs normalization factors as a function of time. These results are used to correctly scale animations.
CONVT: If executing ATHENA_2D on a PC under Windows, this utility converts one form of binary output (directly from ATHENA_2D) to another, which is readable by PV-Wave for Windows (PV-Wave is data animation and visualization software from Visual Numerics, Inc.)
CALC_MTX: Post-processing utility for calculating the model coefficients for the calculation matrix.
5. METHOD OF SOLUTION
Both the neutronics and CFD equations are solved by successive-over relaxation (SOR) iteration. Chebychev extrapolation is available for the neutronics equations, although this option has not been thoroughly tested. The CFD equations use the semi-implicit method for pressure-linked equations (SIMPLE) iteration to achieve self-consistent solutions for mass, momentum, and energy. Asymptotic acceleration of the pressure-correction equation (part of the SIMPLE iteration) is available. The one-dimensional thermal hydraulic fuel pin models employ fast tri-diagonal matrix inversion for a single time step.
6. RESTRICTIONS OR LIMITATIONS
ATHENA_2D assumes moderator boiling occurs before fuel remelt, as there is no mechanism to handle fuel remelt and relocation. However, even the most severe transients simulated during the course of the PhD work showed that this is reasonable. In a thermal neutron spectrum, time constants are longer than those in a fast spectrum. Given typical fuel piece dimensions, moderator boiling always occurred before the peak fuel temperature reached the melting point.
Another limitation is the single (liquid) phase CFD model. Boiling is treated, but only insofar as to calculate local void fractions that feed back to the neutronics equations through local cross section interpolation based on reduced moderator density. The reactivity effects of voids being explicitly transported away from their point of origin is not treated. However, these effects are believed to be small as the introduction of voids tends to be a primary shutdown mechanism for these severe transients. Improved two-phase modeling would only affect the details of the shutdown phase of the transient, not the total energy release.
Other Limitations:
Reactivity feedback effects arising from any potential fluidized bed motion of fuel particles in the debris bed is not treated. No fuel motion is modeled.
Blackbody (radiative) heat transfer is not modeled.
Radiolytic gas bubble formation and its effect on reactivity is not modeled.
There is no treatment of gamma or neutron thermalization heating in the moderator.
There is no treatment of high-temperature, fuel-clad-water chemical interaction which is a potential hydrogen gas source term.
There is no turbulence energy dissipation in the CFD model.
The current library of heat transfer correlations is limited and could be improved.
7. TYPICAL RUNNING TIME
The "Case E05" sample problem required about 12 hours of run time on a 150 MHZ DEC3300, 64-bit Alpha workstation running OpenVMS 7.1. This same case executed in just under 4 hours on a 450 MHZ, Pentium II Xeon Intergraph PC running Windows NT 4.0.
8. COMPUTER HARDWARE REQUIREMENTS
ATHENA_2D runs on IBM PC (Pentium-class) running Windows 95 or later, on DEC Alpha or other unix workstations. At RSICC it was tested on DEC Alpha 3000 running Digital Unix Version 4.0D with Fortran 77 Version 5.1-8 and on a Sun UltraSparc 60 running Solaris 2.6 with f774.2.
9. COMPUTER SOFTWARE REQUIREMENTS
There are no special software requirements for ATHENA_2D except as noted below. If there is a need to modify and recompile the program, any 32-bit Fortran compiler should work. The IBM PC executable included on the distribution CD was created using the Microsoft Fortran PowerStation 4.0 compiler. (The older 16-bit Microsoft Fortran 5.1 compiler failed to work because of the size of data arrays). If color animations are desired of results, procedure files are included on the distribution CD for the PV-Wave software from Visual Numerics, Inc. However, this is not required to merely run ATHENA_2D.
In order to create new ATHENA_2D input, a case-specific (enrichment, fuel composition, etc.) macroscopic cross section library must be created using some lattice transport code such as the WIMS code. The format for this library is standard ASCII text and is defined in documentation found on the distribution CD.
10. REFERENCES
a)Included in documentation:
K. N. Schwinkendorf, Notes on package contents (September 1999).
K. N. Schwinkendorf, Notes on cross section formats (September 22, 1999).
K. N. Schwinkendorf, Excerpt from PhD dissertation, "Appendix D, ATHENA_2D Input Manual," (1996). The full dissertation is included on the distribution CD in both WordPerfect and Adobe Acrobat formats.
b) background information:
K. N. Schwinkendorf, "Recriticality Energetics of a Hypothetical Water Reflood Accident in a Damaged Light Water Reactor," Nuclear Science & Engineering," Vol 132, Number 1 (May 1999). (Full paper)
K. N. Schwinkendorf, "Recriticality Energetics of a Hypothetical Water Reflood Accident in a Damaged Light Water Reactor," ANS Annual Meeting, Orlando, Fl, vol 76, pp. 277-278 (June 1-5, 1997). (Summary)
K. N. Schwinkendorf, "Nuclear Energetics Analysis of Light Water Reactor Recriticality Under Severe Core Accident Conditions," Ph.D. dissertation, UMI number: 9638022, University of Washington (June, 1996).
K. N. Schwinkendorf, "ATHENA_2D: A Computer Code for Simulation of Hypothetical Recriticality Accidents in a Thermal Neutron Spectrum," International Conference on Mathematics and Computations, Reactor Physics, and Environmental Analyses, Portland, Oregon, vol 2, p. 1154 (April 30-May 4, 1995). (computer code abstract)
K. N. Schwinkendorf, "ATHENA_2D: A Two-Dimensional Computer Code for Coupled Neutronic Computational Fluid Dynamics Simulation of Thermal-Spectrum Recriticality Accidents," Simulators International XII, Proceedings of the 1995 Simulation MultiConference, vol 27, number 3, p. 153 (April 9-13, 1995).
11. CONTENTS OF CODE PACKAGE
Included are the documents referenced in (10.a) and one CD-ROM with the Fortran source, PC executables, documentation, and test cases written in both compressed DOS and Unix formats.
12. DATE OF ABSTRACT
September 1999.
KEYWORDS: DIFFUSION THEORY; FLUID DYNAMICS; MULTIGROUP; REACTOR SAFETY; THERMAL HYDRAULICS