Explainer-What is a dirty bomb and why is Russia talking about one now?

LONDON (Reuters) - In Russia's latest advocacy campaign over its invasion of Ukraine, Moscow has focused on accusations that Kyiv might be planning to use a so-called "dirty bomb" - a conventional explosive device laced with toxic nuclear material.

Kyiv and its Western allies say there is no truth at all to the accusation, and that the idea that Ukraine would poison its own territory is patently absurd. They say Moscow could be making the allegation to justify an escalation of its own.