The majority of problems with nuclear come from people. Yes, in a couple cases, you have design failure, but that isnt too common.(in fact really rare actually) In most cases, the causes are entirely due to people being lazy or ignoring proper procedures.
In the case of 3 mile island, the people there were not properly trained. Once things started happening, the operators didnt know what half the blinking lights on the consoles meant.
There is one case from russia where they were pouring nuclear material into a container and this set off a nuclear reaction in the fluid and caused alarms to start blaring. The shift supervisor decided at some point to simply pour the stuff down the drain instead. He ended up being the only casualty.(you can find the full story on the darwin awards)
In other cases, lax inspections etc allow piping to deteriorate and leak etc.
In 99.9% of cases, its always human "error" that causes problems.
To climb the mountain, you must believe you can.
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