Dear Tomfey,
Many thanks for your message. The problem you are addresing is rather general, do you already have a FLUKA input file describing it?
In the meantime, you could start by having a look at Volumetric source of complex geometry, there you can see how to create a volumetric source of a given shape (download both flair file and source file, create an executable including your source file, and use the executable file when running your simulations).
Additionally, in the estimation of the dose delivered to the people surrounding your facility, I imagine you not only have to consider direct radiation exposure but also inhalation and injestion of radionuclides. In this view, you would probably need estimates on the type radionuclides (and corresponding activity) reaching your regions of interest, depending on, for instance, environmental aspects.
I hope this helps,
Cheers,
Marco