Radiation readings deep inside Chernobyl's ruined Reactor 4 are climbing for reasons nobody can explain, which is bad news for Ukraine and the engineers meant to keep the site safe for the next century.

• Neutron counts in Room 305/2, a buried chamber packed with melted nuclear fuel, have roughly doubled over several years.
• The cause appears to be dryness: the new steel dome keeps rain out, and drier fuel seems to make neutrons better at splitting uranium, the opposite of what physics predicted.
• The room is walled off by concrete and lethal debris, so no one can take samples; the only plan on the table is radiation-proof robots that drill in and inject neutron-absorbing material.
• Robots are not a sure fix — at Fukushima, machines built for extreme radiation had their electronics fried within hours.
• A Russian drone punched a hole in the Chernobyl dome in February 2025, and Ukraine says the structure's ability to seal in radioactive dust was damaged, raising the long-term risk of collapse.

Outlook: A 1986-style explosion is off the table, but if neutron levels keep rising the fuel could heat up and spread radioactive dust inside a shelter that is already patched and leaking.