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.