1. NAME AND TITLE

THT: Three-Dimensional Neutron Coarse Mesh Code System to Evaluate Average Bundle Fluxes and Power in Light Water Reactors.

2. CONTRIBUTOR

AgipNucleare, Milano, Italy, through the OECD Nuclear Energy Agency Data Bank, Gif-sur-Yvette, France.

3. CODING LANGUAGE AND COMPUTER

Fortran VS; IBM 360/370.

4. NATURE OF PROBLEM SOLVED

THT is a 3-dimensional neutron coarse mesh program to evaluate average bundle fluxes and power in LWRs. The method employed in THT is aimed at being more precise than current industrial design methods without being too slow. The horizontal meshes in THT correspond exactly to the horizontal sections of the bundle. The vertical mesh size has no equivalent limitations but should usually be chosen near to the horizontal meshes. The approximations involved in the method require that the mesh size be much larger than the thermal and epithermal mean free paths in the bundles. This condition is always met when dealing with standard LWR bundles.

5. METHOD OF SOLUTION

The basic equations in THT are similar to those of some widely used industrial design methods. They are derived from diffusion theory and use 3-group neutron constants, but only the fast fluxes are explicitly calculated. The THT theory includes, however, two features, both of which result in a refinement of the system coefficient evaluations with respect to the other methods: (1) a new spectral model to evaluate the thermal and epithermal average flux inside the assemblies and (2) discontinuity factors, as defined by Henry, are used in the equations to connect the fast, epithermal, and thermal fluxes across the node boundaries. THT is particularly suited for comparison with reference data and allows the user to choose different values for some model parameters and different convergence strategies. For the same reason, however, there exists no interface with other programs.

6. RESTRICTIONS OR LIMITATIONS

The length of the container array is independent of input. At present, this array has a length of 100,000 words which allows THT to treat problems as large as 1600 nodes cases. On the PRIME 750 computer, the dimension of the container array can be modified, if necessary, by changing the corresponding DIMENSION and COMMON statements throughout the program. On other large computers, it may be sufficient to modify only the DIMENSION statement in the main program.

7. TYPICAL RUNNING TIME

Execution times depend upon the number of meshes and the required precision. The test cases included in the package were executed by NEA-DB on an IBM 3081 K. THT required 1.8 seconds for test case 1 and 0.4 seconds for test case 2.

8. COMPUTER HARDWARE REQUIREMENTS

A problem with 768 nodes can be run on a PRIME 750 computer in 200 K bytes of main storage. With a container array of 100,000 words, main storage requirements for running the test cases are 524 K bytes on an IBM 3081 K.

9. COMPUTER SOFTWARE REQUIREMENTS

THT is written in standard Fortran IV. It was developed on a PRIME 750 computer but, to run it on an IBM 370/168, only the time routines had to be replaced. It should run on any computer supporting Fortran IV.

10. REFERENCES

E. Biscaretti and O. Chiovato, "THT - An LWR 3-Dimensional Neutron Coarse Mesh Method Including Discontinuity Factors," Informal Report (July 1984).

E. Biscaretti and O. Chiovato, "THT - How to Use" (September 1984).

E. Biscaretti and O. Chiovato, "ADICOM - A New Coarse Mesh Diffusion Method for LWRs," (In Italian) Paper presented at the "Meeting on Reactor Physics" at the University of Bologna (March 1983).

11. CONTENTS OF CODE PACKAGE

Included are the referenced documents and one (1.2MB) DOS diskette which contains the source code and sample problem input.

12. DATE OF ABSTRACT

June 1986.