No scaremongering, you got to be kidding. Is that an excerpt from the same documentary that claimed the three engineers died directly afterwards from radiation poisoning? The ones that turned out to be alive and healthy and still working in the nuclear energy industry?
You keep pretending Leatherbarrow made these estimations himself, but that's obviously not the case. He conducted interviews with those involved and various experts. These are obviously more qualified than him, you or I. You need to stop pretending you know better than someone who did proper research and various actual nuclear experts.
This might come as a surprise to you but when it comes to technical subjects it often turns out I know better than journalists and it's also common for them not to do proper research. I'm sure many others on this forum have had the same experience. Leatherbarrow can easily have made up, exaggerated and/or misunderstood what others told him and can also have misjudged the credibility of the people he interviewed. I still know nothing about his background. If you want to be taken seriously you need to provide some report/study from some credible institution. A TV-documentary and a book by an activist isn't credible basis for a serious discussion.
I don't see why I should be impressed by what politicians say either, although I can't disagree with this sentence: "It is difficult to even imagine what would happen if the molten core of the reactor touched water", indeed it is. If Gorbachev believed that it was so dangerous, do you not find it strange he didn't choose to shut down the remaining RBMK reactors. The Russians still have 11 in operation today.
Note the quote by Vassili Nesterenko, a top nuclear physicist who was present at the site.
That's the first expert comment you have found supporting what you claimed, unfortunately what he's saying sounds like hyperbole. It's hard to comment on it since we don't know what analysis he based that statement on. And this snippet from his Wikipedia page doesn't exactly instil a lot of confidence "As an expert on the subject and with his experience as a fire fighter, he threw liquid nitrogen containers from a helicopter on the burning reactor core."
The soviet government clearly panicked and didn't respond completely rationally at the time, that is well documented. But who can blame them, it was a terrible disaster, about 100 people died during the accident, it must have been a nightmare. Trying to assess and react to the development of the meltdown must have been very difficult. I do not doubt they considered the water in the basement a very serious threat at the time and maybe it could have been worse. It would be very hard to accurately determine what the probability of that actually happening or what the actual consequences would have been. It's not something that can be done on the back of an envelope. It's also strange that most thorough post accident analyses do not mention this "near miss" disaster that allegedly would have made most of Europe uninhabitable don't you think.
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For balance, here is a documentary about the risk of sudden methane release caused by the GHG emissions from coal power plants:
"Countdown to Doomsday Methane Release"
All this scaremongering isn't really helping anyone, is it, it just makes everyone look stupid.