What makes modern nuclear reactors so safe comparatively to historical reactors?
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One of the major differences is the emergency shut down procedures. Nuclear reactors generate electricity by turning water into super hot steam that drives a turbine. The heat comes from the nuclear fuel undergoing nuclear decay. That always happens, all the time. The reactor makes it go faster with a moderator, but without that, the fuel will still heat up. That means the fuel will *always* need to be cooled.
A meltdown is when the fuel heats up, uncontrolled, and literally melts into a pile of superheated goop. The explosion at Chernobyl came from the water pipes meant to cool the fuel. When the water pumps stopped, the water heated up, boiled off, and had nowhere to go until the pressure built up enough to blow up the pipes.
During normal operation, the pumps that cool the reactor can be powered *by* the reactor. Easy peasy. But what happens in an emergency when you need to shut the reactor down? The fuel is still super hot from the normal operation, so you need to cool it down and then *keep* it cool, but without power from the reactor.
Chernobyl was supposed to be able to use the residual heat and steam to power the pumps until backup diesel generators could come online. All of the things that went wrong there is its own ELI5.
Fukushima broke down because the diesel backups to power the water pumps were located next to the reactor, which was below the water line. When the plant flooded, the generators couldn’t be turned on. The location of the reactor was a feature, not a bug – designed so that in an emergency they could destroy the wall holding back the ocean and let the ocean cool the fuel. The unprecedented destruction from the earthquake and tsunami overwhelmed the facility’s emergency safeguards.
With every disaster or near disaster (like Three Mile Island) lessons are learned. Modern reactors have procedures to power the cooling pumps as the plant shuts down, procedures to borrow power back from the grid to run the pumps, back up generators, backup batteries, etc. Monitoring systems and computers have gotten better so that even in the absence of human intervention, the plant can safely shut itself down and activate all the backup systems (to a point). It’s very difficult to get a runaway meltdown.
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