Neutral-to-worrying: the sealed Chernobyl reactor is not dead — its melted core is still reacting and slowly crumbling into radioactive dust, and scientists don't fully know where it's headed.

• The $1.7 billion steel dome traps dust well, but the melted fuel inside is turning to sand-like powder and shedding particles the design never expected.
• Neutron activity in a buried room jumped sharply between 2016 and 2021, a sign that new nuclear reactions are flickering back to life.
• The likely cause: the new dome dried out the groundwater that had been quietly keeping the core calm, so the reaction is picking back up.
• Radioactive material has already seeped into the groundwater feeding the river that supplies Kyiv and millions of people.
• Russia's 2022 invasion sent tanks through the contaminated Red Forest nearby, stirring up decades-old fallout that has grown more toxic over time.

Outlook: Experts doubt a second explosion, but the core keeps changing, and there's still no finished plan to safely take it apart.