Even if Chernobyl would have had a containment vessel, what would have been the best case scenario? I'm not an expert but the blast threw the multi-ton slab of steel and concrete lid into the night sky, surely the containment vessel would have had a giant hole in it, albeit saving some of the radiation from the atmosphere of course but not all of it. One reason I think Three Mile Island wasn't as bad is because nobody in the west was crazy enough to build an RBMK.
The whole point of the containment building is to contain a pressure vessel failure. American containment buildings are built to withstand impact of a fully loaded passenger airliner. That's why the containment vessels are reinforced concrete more than a meter thick.
If Chernobyl had secondary containment, the burning fuel rods would not have been exposed directly to atmosphere. Basically, if you have a fire emitting toxic soot it's a lot better to have this fire happen in a concrete dome versus totally exposed.
