Research reactor assembly

VVR_SM_Box_1.ipynb (14.9 KB)

Hi dear experts. I am trying to create a model of research reactor assembly as shown in figure. I also uploaded my code.

I am recieving an error below while corresponding cells that assign materials to these regions. :

I guess its becouse the way i assigned the regions.
Please, give advise how to resolve the coming error.
Thank you !