As the title says.

Works fine with Nuclear Waste, Polonium and Plutonium

Is this a feature or a bug?

Playing Cafetoria 1.3.2 .

This is a feature for the sake of balancing. You can selectively enable support for Spent Nuclear Waste via a config option.

As the title says.

Works fine with Nuclear Waste, Polonium and Plutonium

Is this a feature or a bug?

Playing Cafetoria 1.3.2 .

If you want to deal with your spent nuclear waste, the best way to do it is to have a storage bus with max priority, partitioned for spent nuclear waste, and attached to the top or bottom of a Mekanism radioactive waste barrel. Then use some form of chemical transfer to distribute this waste evenly to a bunch of other barrels to increase the rate of decay (Gas Pipez are the best at this in my experience)