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In the construction of a complicated geometry model, it is possible that some space is undefined or some volume belongs to more than one zone. These geometry errors will cause MCNP to lose particles and should be avoided. Ray tracing of models with errors present, especially models described by surfaces, can give some very strange images. A fuzzy boundary between cells which changes appearance on changes of view or scaling is a good sign that an error may be lurking in the geometry description.
Geometry errors may be introduced by the incorrect definition of a body, simple user error, or many other causes. Because of the limited precision of a computer, it may not be possible to place two bodies with planar surfaces with those planes touching, i.e., in a configuration in which they would share a plane if described in surface geometry; some or all of the boundary may be flagged as an error.
Geometry checking in Sabrina is controlled by Test. The checking mode is in effect when the Test item in the Options menu is checked. The TEST ON command also enables checking. Error checking may take place during the generation of a ray-traced or line-drawing image. Use of the Query, which interactively fires a single ray, will also detect errors.
When testing in MCNP surface geometry, cell descriptions containing greater than 999 elements (surfaces and operators) are not allowed. Cells described by a single infinite surface (such as a cylinder) may cause fatal errors. When the plane of a 2D plot lies exactly between 2 cells that touch only along a line, false errors may appear. For example, with 2 cylinders of radius 1 parallel to the Z axis that are centered at (x, y) = (-1, 0) and (1, 0), a slice at x = 0 will show a false error. A slice at x = 0.0000001 will not.
Testing works with repeated structures in 3D plots, but
we cannot be certain that we have encountered and fixed
every possible situation that might produce a false error.
Only cells in the same universe are checked to see if they
overlap. In some cases, the error report may list only
two overlapping cells where more than two occupy the same
space. Because of the additional time required to checking
complicated structures, the user should first test separately
the components that will be repeated. When using a lattice,
a test should be first performed on a lattice containing only
a few elements rather than the full lattice.
Testing does not work well with repeated structures in 2D
(SLICE) plots.
If errors are detected, messages are written to the file errfile.txt, a text file that can be examined while still running Sabrina by selecting Cat errfile.txt under the Files menu or by using the CAT or Read (each line of errfile.txt is treated as a comment by Sabrina). The messages in the error file tell where space was found undefined, for instance
! EXITING CELL 2 ON SURFACE 7 NO CELL WAS FOUND ON THE OTHERSIDE
or what cells were overdefining space.
! EXITING CELL 0 ON SURFACE 7 THE FOLLOWING CELLS WERE FOUND:
! 1 2
When error checking in a model with overdefined space, a track may either uncover the multiple definition, or become lost. If it gets lost, messages about undefined space may be present. Messages about undefined space accompanying messages of overdefined space should be ignored at first. Often, the undefined space messages will go away after an overdefined space error is successfully corrected.