I was under the impression that metal can absorb radiation and hold it. I also understood that the density of your protection from radiation was important. Back in the '50s, they advised having a lot of books or stacked newspapers overhead.
Where is your fresh air coming from? Is it filtered?
I have a medium-sized PortaPotty for camping, and found out that during a flood (water filled the septic tank), it would fill up in 24 hrs with 3 people. One of the problems was the flushing, which helped to fill it. One solution to this can probably be found in the online book (but it's in print as well) The Humanure Handbook by Joseph Jenkins. You can read the whole thing online (drawings and photos included) at
http://www.weblife.org/humanure/default.html and it will answer all the questions that you can come up with. This guy has been using the method for at least 20 years. The main point is that I have tried it myself, and there is NO odor. Absolutely none.
It sounds like you've put a lot of good thinking into this plan. But I hope you never need it.
Sue