How to overlay coolant channels on top of universe

Hi there,

I followed the CANDU Bundle tutorial to implement a circular lattice. I actually used it twice, once to build the unit cell and again to place those unit cells around the core. In the tutorial where they fill the water_cells with heavy water is where I am now placing moderator, and I would like to place coolant channels in that moderator between unit cells.

Currently I’m trying to add the coolant channel cells to the same universe as the unit cells, and also adding their regions to the equivalent of water_cells[i].region. This method hasn’t been working, so I was thinking of creating another universe for the coolant channels and overlapping that on top of the moderator. Any tips would be much appreciated, please let me know if I can clarify anything. Thank you!

I was not adding the regions to the correct cells. Some of the coolant channels belonged to two reasons so I just brute forced it by adding every coolant channel to every cylinder cell.