Why would that be absolutely unacceptable. It wouldn't be much worse than a rocket exploding.
You know Chernobyl deliberately disregarded and deactivated safety features, right?
Okay, so it blows up.......It's fairly simple to calculate the activity(how radioactive) of the debris.
[math]A=N \lambda[/math] where A is the activity, N is the concentration, and [math]\lambda=\frac{\ln 2}{t_{\frac{1}{2}}}[/math]. Now, the fuel has a half life of millions of years. I guess we just need to know how big the explosion is, how much drifts down into the atmosphere, and how strong the wind is.............a rough estimate is that the activity due to such explosion would be extremely negligible.
Overbuilding? What do you mean? Sure, there's redundancy, but not much "ovedrbuilding", whatever that means. However, if you provide some examples, I can tell you how wrong you are. Did I mention I operate two reactors?