The answer would largely depend on the immortal being you use. However, if you ask me, the immortal being would probably be more concerned about whatever apparatus you are using to expose it to the highly radioactive substance than the substance itself. Here's what the Icelandic Waters made of the thought experiment:

[strained breathing]
...help-

Hmm?..

If you put a human in a glass box with a million hungry ants, they would be in more danger from the glass box than they would from each other. And if the immortal being managed to escape the apparatus, it would be- *cough* -Pariki implores me not to think about that.