Chernobyl's scram rods system was defective by design. When inserted into the core, these rods briefly increased the reaction before shutting it down. As it turned down, that brief stage was enough.
Almost everyone involved in Chernobyl is to to blame to some extent — it was a combination of gross mal-operation as well as major design flaws. This was all compounded by Soviet political doctrine, which had classified much information the party had deemed a 'State secret.'
The story in a nutshell: the reactor was due to stop for a maintenance and this was considered a good opportunity to test if cheap-and-dirty diesel and regular water pumps work when specialized bells-and-whistles SAOR (system for automatic reactor cooling) fail. The experiment was planned, everyone prepared, but a different reactor in the same plant suffered some troubles and there was not enough power in the energy system during the day. So the experiment was delayed and was to be conducted not by the experienced people from the day shift but by a night shift of "young specialists". At first they made a mistake and and dropped the reactor from 1600 MWt to almost zero (instead of the 700-1000 MWt planned). They did not shut down the reactor at that point as they should have; instead, they removed nearly all control rods to increase power and stabilized the reactor at ~200 MWt, leaving about 6-8 control rods in the active zone (not just a safety violation but a crime). And then they continued with the experiment, disabling one of the main generators and the SAOR (not just a crime but a suicide attempt). To take the SAOR offline, they also had to disable several auxiliary safety devices designed specifically to halt reaction in case of a LOCA (Loss Of Cooling Accident), in other words prevent the exact accident that happened. As expected, water stopped circulating in the reactor and heat started to rise while the diesel pumps slowly started up. Suddenly realizing that the reactor was again gaining power at a rapid rate, the staff pulled the SCRAM. The lower part of the control rods was made from graphite to decrease latency when operated properly, but pulling them back into the active zone briefly resulted in increased reactivity of the reactor. Guess what happened when two hundred rods were dropped into an already overheating unstable reactor?
There is an alternate recount by one of the surviving techs in which the final SCRAM - the one that blew up the whole thing for good - was not actually initiated because of any kind of worry, or sudden realization; nobody had figured out anything was amiss, and the SCRAM was simply how the inexperienced techs would routinely shut down the reactor (instead of using the proper procedure).
Non-technical summary: A reactor that was designed and built with disgustingly obsolete safety measures was then operated entirely ass-backwards from everything known about safe reactor procedure by inexperienced personnel who had also deliberately disabled every single one of the automatic safety systems designed to prevent people from doing what they were doing. In order for Chernobyl to occur, every single possible thing that could go wrong had to go wrong, in each and every step from laying out the original blueprints to the final button push. In other words, not remotely likely to happen ever again, especially since nobody else designs and builds reactors according to the Chernobyl specs.
The fact that the Chernobyl disaster is physically impossible with modern reactors doesn't stop today's opponents of nuclear power from citing Chernobyl in their reasoning. It's the equivalent of Godwin's Law in nuclear power debating circles.
Three Mile Island gets a lot of this too. In that one the safeties actually worked. Despite a partial core meltdown occurring, the total amount of radiation exposure to anyone outside the plant was less than what you'd pick up from a chest X-ray.
Fukushima. Which should be taken as an example of why containment buildings won't allow another Chernobyl, seen as they did exactly that - preventing a local disaster from becoming a global catastrophe. But you try explaining that to the media.
You'll get some supposedly genuine documentaries about Chernobyl throwing out ridiculous numbers either for the sake of drama or some vague anti-nuclear message. Example 1: Saying the thermal explosion that could have occurred if the melted reactor material had come in contact with the water under the core would have be equivalent to a multi-megaton detonation. Not unless a cubic kilometer of water somehow magically flashed to steam. Example 2: Saying radiation in the town nearby was 10,000 roentgens per hour!. This is the equivalent of sitting in the heap of slag that formed under the reactor's cooling units, right after the reactor had melted down.
Most people assume that because of the Zone of Alienation surrounding Chernobyl, any kind of nuclear accident will make nearby areas uninhabitable for centuries. However, the real reason the Chernobyl site is blocked off is because pieces of the reactor core were scattered around the country side and then buried in a hurry after during the cleanup. Much of the area around Chernobyl is livable though, as radiation levels have fallen dramatically over the years, as evidenced by the recent appearance of native wildlife and people to the region. The real danger is the unknown burial sites for the core parts, which could release a lot of radiation if disturbed. The biggest long term ecological concern for the area is about radioactive isotopes that have settled on the bottom of a nearby lake, which while harmless now, could make the water deadly if a dam broke.
The real danger is the sarcophagus that currently encases the blown reactor; having been built in somewhat of a hurry it never was structurally very sound in the first place, and it only currently stands thanks to a heroic and relatively recent strengthening effort. Climate and hard internal radiation keep embrittling its structure, though, and there's a very real danger that even a small earthquake, or freak climatic phenomenon, or simply failing materials, might cause it to collapse. That would cause a massive debris cloud and expose the internals to the atmosphere, with disastrous results reaching far beyond the Zone of Alienation.
Simply put: Chernobyl's reactors were a design called the RBMK. The "test" that they were doing is one that all other nuclear plants with RBMK reactors absolutely refused to attempt when asked by the soviet government. So they presented the assignment to Chernobyl and said, in effect, "We're not asking you to do this. We're telling you to." Translation: Refuse, and you get an all-expense-paid trip to a gulag. One way ticket. There's a very good reason why all other RBMK operators refused to do the test: it was so dangerous that attempting it would be suicidal unless absolutely every circumstance was completely perfect. Given that the test was intended to determine the feasibility of a certain protocol for dealing with loss of power to the building, the test was for a protocol that would be unusable in the circumstance when it was ostensibly supposed to be used, which makes it pretty obvious why everyone else had refused to attempt it. The operators at Chernobyl were basically forced to do it at gunpoint.
The RBMK is physically gigantic. It's so big that construction of a true containment building would dwarf the cost to build the reactor it'd be containing. So they simply omitted the containment building. A prospective successor to the RBMK called the MKER corrects many of the RBMK's design flaws, but construction of the first prototype MKER, Kursk-5, has long since been cancelled, as have all other proposed MKER builds, which may well be for the better.
Events like the crash of the Galileo probe against Jupiter made a bunch of conspiracy theorists start claiming that there are plans to ignite Jupiter into a second sun. Similar predictions were made when the comet Shoemaker-Levy 9 was about to impact Jupiter. For reference, the minimum mass to actually get self-sustaining nuclear fusion in a ball of gas (that is, a small red dwarf star) is around 75 Jupiter masses.note even a brown dwarf takes 13 Jovian masses So, even if you did manage to initiate some fusion on Jupiter, all you'd get would be a nuclear explosion. That's it.
#4.1 was unfortunately played straight in the Goiânia accident — the misplaced radioactive source in question did apparently have a blue glow once extracted. This encouraged people to play with it, with fatal results.
The US Army's Stationary Low-Power Reactor Number One, SL-1, was the site of the most fatal nuclear accident in the United States. The main control rod needed to be manually withdrawn to a certain distance—four inches (approximately ten centimetres)—before the automatic system engaged. On the day in question, the control rod was withdrawn a full twenty inches for reasons unknown, causing the reactor to go prompt critical, resulting in a steam explosion that killed all three operators (pinning one to the ceiling with the control rod). After this the AEC wisely stopped building reactors in which one control rod's removal would cause such a disaster. The SL-1 is also proof that reactors don't simply explode like an atomic bomb when they go prompt critical, and just how hard it is to make a reactor melt down even in an emergency.
The suggestion of one official report states that the control rods and/or their robotic armature had an unfortunate tendency to become 'sticky' and not slide smoothly into and out of their housings. The suspicion was that the unfortunate operator was trying to pull the rod out without the armature, King Arthur style. He completely succeeded and crowned himself king of radiation for a few seconds before he died.
There's reason to believe the armature had been left disassembled (not unreasonable given the stage of development) and the techs were putting it back together. Due to an oversight, the reassembly directions given in the manual ("follow the disassembly instructions backwards") were physically impossible, as the rod had to be pulled farther out than needed to take the armature apart. The rod was not gradated to show how far out it was, and a ruler couldn't be placed near it. Further, a conventional reactor design can have up to a hundred rods which can be entirely removed (one at a time) with no danger, while every one of the SL-1's rods was individually critical (the manual never actually said, or laid out the effects of withdrawing a specific rod to a specific range on the reaction process) and had just one control rod. The SL-1 was intended to power semisecret posts which couldn't receive regular supply shipments, and there may have been forces leading both to sketchiness in the manual and a reluctance for the engineers to request clarification. Unsurprisingly, the official investigation chose to focus blame on those whose careers could no longer be affected by it.
For that matter, Fukushima was more of an unforeseen circumstance. The reactors itself were built virtually at the same time as the Chernobyl's one, in mid-Seventies, but to a newer design, so when the disaster struck they successfully withstood the quake and were stopped alright. They took the seashore location into account, by lowering the bluff the site was built on to utilize smaller pumps for the secondary cooling loop that pumped in seawater (To be fair, this also let the reactors sit on solid bedrock which helped their earthquake resistance). The site was designed to survive a more typical 6m tsunami, but not the monster 14m one it was hit with. Thus, the emergency generators that supplied the electricity to the cooling systems of reactors stood on the very edge of the sea and were immediately washed out when the wave came. The designers also put the main distribution point of the whole station into the basement where it was promptly inundated and taken out of service — which would bite the operators in the ass later, when it turned out that the mobile generators that were brought to help from elsewhere were incompatible with the station's systems and could be connected only through the now underwater distribution point...
The problem with Fukushima wasn't in nuclear designs... that held up rather well. It was industrial design that the Japanese botched. Western experts were called in for consultation when the plant was built. TEPCO listened, smiled and ignored everything they said - like not putting the critical systems in the basement where they could be flooded.
The reactors at Fukushima weren't even critical, they were quite sub-critical (having been shut down by safety mechanisms reacting to the earthquake), but they were still producing some heat from short-lived isotopes left over from the fission reactions. The eventual explosions were only indirectly connected to the reactors, they were due to accumulation of hydrogen resulting from overheating equipment and inadequate ventilation due to power problems. The reactors themselves suffered a quiet, slow death due to overheating from decay heat as a result of the loss of cooling systems.