Sad stuff and it shows the dangers of nuclear power.
What danger is that? Nuclear Power has the best safety record of any means of energy production.
As Edward Teller said, "Nuclear Power is foolproof." The problem is that at Chernobyl the fools were bigger than the proof.
For starters Chernobyl was a graphite-moderated light water cooled reactor which are notoriously unstable at low power levels.
Secondly, to save money, the old Soviet Union, which built the reactors, decided they didn't need a containment building.
Thirdly, they knew the reactors were unstable. A partial core meltdown occurred in Reactor No. 1 at the Chernobyl site in 1982. But they kept it a secret. Even from their own people.
In 1986 Reactor 4 still had not passed the safety tests where the outside power grid went down shutting down the steam generator feed pumps and reactor coolant pumps.
The operators were trying an experiment to see if the natural tendency of the reactor coolant pumps to keep turning after power was lost was sufficient to get them past the 1 minute required for the backup diesel generators to start and come up to speed. So they conducted an experiment. They shut down power to the feed pumps to see if they could make the 1 minute mark.
What they failed to realize was that shutting down the feed pumps produced a positive feedback loop, in which the formation of steam voids reduced the ability of the liquid water coolant to absorb neutrons, which in turn increased the reactor's power output. This caused yet more water to flash into steam, giving yet a further power increase.
During almost the entire period of the experiment the automatic control system successfully counteracted this positive feedback, continuously inserting control rods into the reactor core to limit the power rise. However, this system had control of only 12 rods, and nearly all others had been manually retracted.
After a SCRAM (emergency shutdown - Super Critical Reactor Ax Man - the first prototypes had a man with an ax who cut the rope that held the control rods out of the reactor in the event of an emergency - Google SL-1) the insertion of control rods into the reactor core began. The control rod insertion mechanism moved the rods at 16 inches per second, so that the rods took 18 to 20 seconds to travel the full height of the core, about 21 feet.
A bigger problem was a flawed graphite-tip control rod design, which initially displaced neutron-absorbing coolant with moderating graphite before introducing replacement neutron-absorbing boron material to slow the reaction. As a result, the SCRAM actually
increased the reaction rate in the upper half of the core as the tips displaced water. This behavior was known after a shutdown of another graphite-moderated light water cooled reactor at Ignalina Nuclear Power Plant in 1983 induced an initial power spike, but as the SCRAM of that reactor was successful, the information was, once again, kept secret.
After the SCRAM the graphite rod tips entered the fuel pile. A massive power spike occurred, and the core overheated, causing some of the fuel rods to fracture, blocking the control rod columns and jamming the control rods at one-third insertion, with the graphite tips in the middle of the core. Within three seconds the reactor output rose above 530 MW, its safe operating limit, the coolant flashed off into steam, the graphite control rods caught fire, and the steam pressure got so high the reactor exploded, and the smoke from the burning graphite rods carried the radiological contamination into the air and all the way to Denmark and Finland.
The good news is that no western country uses graphite moderated reactors nor do they build reactors without pressure containment domes capable of containing the steam produced if all the water in the primary and secondary systems flashed to steam at the same time. When I was still at SONGS the containments at 2 and 3 could hold up to 60psi and were tested to that level several times before our operating license was issued.
Fukushima, as noted above, was an earthquake/tsunami problem, not poor design, poor construction, and poor operation like Chernobyl.
Insofar as other means of energy production are becoming more and more rare and costly, Nuclear Power is not the best possible solution, it is the only possible solution, at least until we manage to harness fusion.
Read
The Health Hazards of Not Going Nuclear before saying nuclear is unsafe.
