I saw this on New Scientist – Nuclear reactions at Chernobyl are spiking in an inaccessible chamber: https://www.newscientist.com/article/2277195-nuclear-reactions-at-chernobyl-are-spiking-in-an-inaccessible-chamber/#ixzz6uyN12l00. Longer article, but here’s some basic information. Hopefully, it stabilizes as it sounds dangerous enough to get into the chamber with humans or robots.
Scientists monitoring the ruins of the Chernobyl nuclear power plant in Ukraine have seen a surge in fission reactions in an inaccessible chamber within the complex. They are now investigating whether the problem will stabilise or require a dangerous and difficult intervention to prevent a runaway nuclear reaction.