MCNP: Is it possible to make a "hole" in an fmesh tally?

[ssapp]

Hello,

I am running a MCNP calculation where there is a source in the center of a building. I have put a fmesh tally parallel to the ground so that I can see how the particles disperse throughout the building and outside the building. However, I only really care about the results from the fmesh tally voxels outside the building's walls. Is there any way I can make a "hole" in my fmesh tally so the particles aren't tracked by the fmesh inside the building?

Thank you.

 

[DEvens]

If you are doing a rectangular mesh then you can limit it to whatever rectangular region you are interested in. Then you can cover various regions with multiple tallies. From your description I would guess that to be the way to go.

If you are doing a cylindrical tally then you can't skip tallying from r=0 to some radius. The best you can do is one radial division for the part you want to ignore. It will have the same angular division at every radius, so if you have N angular divisions you will need to ignore all of them for the radius you want to ignore.

Tallying only slows the code a small amount. So having a small number of tallied voxels in places you don't care about is not a huge deal.

 

[raining Pete]

Hi there,

I'm not an expert in MCNP, but I did some research and it seems like there is a way to exclude certain regions from the fmesh tally. You can use the "exclude" command in your input file to specify which regions you want to exclude from the tally. So in your case, you would want to exclude the building's walls from the tally.

Here's an example of how the exclude command would look like in the input file:

FMESH 1 EXCLUDE 1 2 3 4

In the above example, the "1 2 3 4" represents the regions that will be excluded from the tally. You would need to replace those numbers with the regions that correspond to your building's walls.

Hope this helps! Good luck with your calculation.