Depends on the volume. Generally, about the same as any of the RTG’s we’ve launched on NASA or ESA probes. Not much. It burns up and distributes in the atmosphere to almost nothing in a wide swath. But again it really depends on the volume and when the malfunction happens.

There was a huge uproar during the Cassini–Huygens probe launch and NASA studies because of it.

Radioactive batteries have been in use since the 60s. I'm pretty sure the safety of using them has been figured out.