@MontereyJack,
That can be avoided with intelligent reactor design and intelligent reactor operation. Of course, there is no guarantee that we'll always act intelligently.
If we do suffer a catastrophic meltdown, we can always remove and replace the topsoil of the impacted area. It'll certainly be a big mess to clean up though.
The PRISM variants of those extremely-high-temperature helium-cooled carbon-moderated reactors are meltdown proof and 100% safe. The problem is they use fuel pellets that are impossible to reprocess. Even if we designed them for high burnup we'd still have to dispose of some valuable fuel as long-lived waste. I'm not a big fan of doing that.