THT, 3-D Coarse Mesh LWR Bundle Fluxes and Power with Discontinuity Factors

1 - Description of program or function: 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 in fact the mesh size to 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. 2 - 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 which both result in a refinement of the system coefficients evaluations with respect to the other methods: (1) A new spectral model to evaluate the thermal and epithermal average flux inside the assemblies, based on local solutions of the thermal and epithermal flux equations. (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 was mainly written to study the new coarse mesh method developed at AGIP Nucleare. The program is therefore 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. 3 - Restrictions on the complexity of the problem: 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