When the geometry is written as cells, a cell combination algorithm may be invoked to reduce the total number of cells and thus increase the efficiency of the MCNP calculation. Two cells are combined if they share a common boundary, are bounded by the same surfaces in the directions perpendicular to the common boundary, and contain the same material. The cell combination algorithm first searches for candidate cells with common boundaries in the X direction, then in the Y direction, and then optionally Z.
This figure shows a converted image where cells have been combined. The black lines show the cell boundaries. The color of the central regions that originally were black (see this figure ) has been changed to green to show the outlines in those areas. Without combining, the cell boundaries would be the same as the grid lines in this figure .
Combination Passes. The algorithm is repeated in a sequence of passes until no more cells have been combined or another termination condition is met. These criteria are set on the MCNP Options dialog where one may set a limit on the number of passes and/or minimum number of c ombined cells M-the algorithm terminates if M or fewer cells are deleted in the current pass. The number of cells deleted in each pass and calculation time for the pass (if Time Operations is checked on the Options dialog) are written to the transcript window. The time may increase as the number of neighboring cells checked increases with each pass.
The combination process can be halted by pressing the <Esc> key.
Fast Combination. An alternative fast combination algorithm searches for combination candidates in only one direction in a single pass. The net reduction in cells will not be as much as with the full algorithm but can still be substantial. The alternative algorithm is used if Fast Combine is checked on the MCNP Options dialog. This figure shows an example of using the fast combine algorithm.
Combination Between Scan Planes. The combination algorithm may attempt to combine cells in the direction perpendicular to the scan planes according to the Combine in Z checkbox on the MCNP Options dialog (combining in both directions in the scan plane is always done). Our experience so far shows that combining in the Z results in only a few more cells being combined at the expense of a much larger computation time. Combine in Z is ignored if Fast Combine is checked.
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