The only thought I've had to justify it so far has to do with the presumption of some 'sealant' plaster like material. My current thought is that over time they were tight and with the lower block resting against the 'sealant' plaster, (or what ever material was used to seal the gaps at the bottom), since its the last block that may have shoveled more of the 'sealant' down towards the bottom, that sealant over time degraded and lost integrity, allowing that bottom plug to shift down. Of course this then jives with the 'prism stone' idea a bit, (which I don't like because of the unearned respect for such an unlikely story), because with that opening at the bottom, if there were a sealant applied to the surfaces at the bottom, (every time they were about to replace the plugs), a prism stone would benefit the system so the sealant wouldn't all just toothpaste tube out the bottom. But that would be the sole purpose of the prism stone which allows for its very rudimentary temporary fit on some bulging nobs to hold it in place, as I've read.
I understand the concept of a gasket, but I don't think that makes sense with the existing materials. A gasket system for pressure relies on a seal, which can only work with a non porous smooth flawless material, and even with that, it relies on a lubricant and exact tolerances, which I can't see being a part of a system that was meant to function for as long as it would have. I base the time duration of its functioning period between plug removals on general instinct and respect for the design. Were it a system who's lubricant could survive the heat and pressure to be available to the gasket, it would imply a much shorter period of time, and that would imply a much more frequent plug removal. This to me seems like the most glaring inefficiency of a system in terms of work to remove, down time, and general clumsiness of a system that doesn't fit with the presumed 'prefection' of the concept.
Also, were there metal lining in the AP, it wouldn't have just been perfectly fit plates that fit inside the passage. I presume there would be groves along the corners for a plate overlap into the course of stones to keep it from warping under heat. There is not evidence of this especially considering the surround of the girdle stones. And on top of that, the AP is designed to be a gradually reducing space which kills any gasket function.
No, I'm sticking with the lose hypothesis that those plugs were meant to be removed, but only maybe once every 10 years, (to randomly pick a time frame that still allows for the respect of 'perfection' of the system).
And come to think of it, I'm now kind of thinking the idea of plutonium removal falls apart, because if the purpose of removal of a beneficial material as fuel for something else, means they have to wait far too long for this system to be as beneficial as I hope it to be. So perhaps what is being removed is a spent catalyst of some kind, that requires replacement to continue the production of 'something' that they have access either by microwave collection, or something like that.
I don't know. Many gaps in my knowledge so its getting dicey for me. lol