MCNP - z-coordinates of cylindrical mesh >0?

[19matthew89]

TL;DR Summary

z-coordinates of cylindrical mesh tally must be greater than zero?

Hi everyone,

I am using MCNP6.2 and trying to set up a cylindrical coordinate in a reactor channel. The origin as the midplane of the channel.

In my attempt of setting up a cylindrical FMESH with the origin on the z-axis at the bottom of the channel (so z<0) I got this fatal error message: "z-coordinates of cylindrical mesh tally must be greater than zero"!!!
Is it really like that? Do the cylindrical meshes really only work the z>0 halfspace?
Is there a workaround (like a transformation) that could be used? As it says that z-coordinates have to be positive it doesn't even work putting the origin at the top of the channel.

Thanks a lot in advance.

 

[19matthew89]

EDIT: Problem solved

I am writing here as I solved the issue and it might be useful for someone else.

In the cylindrical mesh, the value of jmesh have to be positive! The description in the manual is misleading as it says:

"Locations of the coarse mesh points in the y direction for rectangular geometry or in the z direction for cylindrical geometry"

so you might expect it's the position of the coarse meh point expressed with respect to the original system of reference but actually no! It refers to the cylindrical system of reference of the mesh itself and since the origin has to be considered at the bottom of the mesh, the z can consequently only be positive.

I hope it helps in case.

 

[Grelbr42]

If I am not mistaken, you can get the effect you want by choosing the axis to be in the direction you need. So if you point your axis in the -ve z direction, then choose the intervals in the +ve sense of that vector, you get what you need.