# Can MCNP solve the geometric coincidence issue with a semi-cylinder and cuboid?

• chengmo
In summary, the conversation discusses how to solve a geometric coincidence problem in MCNP, where shapes need to intersect and be defined in a specific way. The conversation also addresses a specific issue with cell definitions and the use of positive and negative surfaces. The issue is eventually resolved by correcting an error in the x-axis, but there is still a problem with cell card 1. There is also mention of a discrepancy between MCNP and VisEd in terms of displaying the model.

#### chengmo

How do we solve the geometric coincidence problem? I need a semi-cylinder that fits into the cuboid but if I use the cuboid and the cylinder directly it's geometrically problematic

I'm not quite following the question. MCNP does Constructive Solid Geometry. Shapes are supposed to intersect and then you tell it which volumes are what part of the problem.

I set up four cubes but only two are displayed. In my setting, there should be cuboids in the middle and under the ring. Why？

#### Attachments

• A1++.txt
322 bytes · Views: 86
In a cell definition a positive surface typically means above or outside. A negative surface means below or inside. If you take a look at cell 3, you have -3 4, meaning below x=4 and at the same time above x=6. The space between x=4 and 6 is "3 -4". Now have a look at cell 4 and see if you can find something similar.

Thank you very much Alex A, I got the X-axis wrong. The sign of 1, 2,3, and 4 surfaces in the surface card is wrong

However, after I corrected the error of x axis, the modeling problem still occurred in cell card 1. Why is cell card 2 and cell card 1 written in the same way? cell1 caused the problem, but cell2 did not

#### Attachments

• QQ图片20230427163840.png
85.9 KB · Views: 67
• QQ图片20230427163848.png
135.9 KB · Views: 63
• A1++.txt
328 bytes · Views: 75
MCNP seems happy. When I try the interactive plotter I see the full D. VisEd does not seem happy from your picture. Maybe VisEd does not like the comment. I do not see an error.