What makes modern nuclear reactors so safe comparatively to historical reactors?

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What makes modern nuclear reactors so safe comparatively to historical reactors?

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Imagine burning a bunch of different things. Some stuff burns consistently, and other logs snap, crackle, and pop unexpectedly. Some river stones have pockets of water and might explode if you make a fire pit with them.

Nuclear was a very new technology, and we had no idea certain rocks explode if you put them in a fire.

We learn from every breakdown. The safety regulations are written in blood.

Early reactors (assuming research and power) varied in design quite a bit so lessons learned were more difficult to apply to others. Luck and hope played a large role. In modern times the big diff is just operating time. Like anything, the more you work with something the more you learn about it. Experience for dealing with waste, maintenance, training, environmental impacts, worker safety, quality, etc.

A nuclear reactor exists to generate heat. By placing a bunch of nuclear fuel close together, a lot of energy is released. In a nuclear power plant, the heat is used to create electricity – the heat created by the nuclear reaction is used to boil water, which turns into steam. The steam spins a turbine connected to a generator.

Running a nuclear power plant is about keeping the reactor hot enough for power generation, while also keeping the reactor cool enough to prevent overheating. Basically, it’s a balancing act to keep the reactor within operating temperature.

**Old nuclear power plants required supporting systems to cool the reactor. Basically, the systems that controlled the reactor keep it from overheating. If the supporting systems fail, the reactor overheats.** This may lead to a situation where radioactive material is released to the environment. This type of problem happened at Fukushima Daiichi Nuclear Power Plant. The systems used to cool the reactors failed, which lead to three reactors overheating.

**New nuclear power plants require supporting systems to keep the reactor running. Basically, the systems that control the reactor keep it working. If the supporting systems fail, the reactor stops by itself.** The reactor immediately goes cold and the plant shuts down, but no danger.

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.

So I just want to clarify that most nuclear reactors operating today _are_ “historical” reactors. [The global nuclear fleet is pretty old](https://www.statista.com/statistics/517060/average-age-of-nuclear-reactors-worldwide/). For individual countries, it is even older (for the US, the average age is [40 years](https://www.eia.gov/energyexplained/nuclear/us-nuclear-industry.php#:~:text=At%20the%20end%20of%202021,commercial%20operation%20in%20December%201969.)).

So while one can talk about how new (e.g. Gen IV) reactor designs are safer, one should not draw as much of a contrast between “modern nuclear reactors” on the whole an “historical reactors.” They are pretty much the same, albeit sometimes they have been given upgrades.

The “old” reactors were not, I would point out, inherently dangerous, except perhaps for the RBMKs, but even those were capable of being operated safely most of the time (Chernobyl was a very bad set of strange circumstances). That does not mean they were perfectly safe, or that current reactors are perfectly safe.