Andsetinn said:
What mostly scares me about Nuclear is that 3 Mile Island and Chernobyl accidents happened because of human error, in spite of the best safeguards, and best training, available money could buy. Also check Windscale and Kyshtym. Us humans are not more intelligent now than we were back then.
Let's talk about Chernobyl.
That was a first generation design that had a positive void coefficient. In other words, if bubbles started to form due to too-high thermal output, then reactivity would INCREASE - leading to a guaranteed runaway thermal event. Plus which, the control rods were tipped with a moderator - which increased reactivity as they were inserted. It was an incredibly bad design. Imagine a car design that, under some conditions (say, overly aggressive braking) would accelerate harder and harder the harder you pressed on the brake. You could say "just have drivers avoid those conditions" - but you'd still see a lot more fatalities if that car was actually on the road.
Now let's talk about Three Mile Island.
They had a SCRAM due to a problem with the coolant polisher (mechanical failure #1). No biggie. Reactor shut down automatically. Pressure increased due to reactor decay heat, and the PORV vented excess pressure into a containment pond as designed. But the PORV did not close. (Mechanical failure #2.)
Operators noticed the event. They did not notice the backup sensor in the PORV outflow pipe indicating a very high temperature due to the venting coolant (human failure #1.) Had they noticed they could have closed the backup isolation valve. Still no biggie. The backup cooling water started flowing through the heat exchanger, keeping the core cool. Someone closed the valves that allowed that water to flow (human failure #2.) Still no biggie. The HPI system detected a low pressure/high temperature condition and started injecting water directly into the core to prevent overheating. Someone shut down the HPI system (human failure #3.)
At this point there were no remaining safety systems to prevent core overheat. The core overheated and part of it melted. Everything was contained within the containment.
It would be hard to imagine a more egregious series of failures. Two simultaneous mechanical failures and three consecutive human failures. And what happened was - a melted core and no significant release of radiation. If that's the worse case failure, then nuclear - specifically the Gen 2 design that includes all US reactors - is pretty dang safe.