Nuclear power plant safety has come on a long way. In the first two generations of plants, the engineers were constantly running around to keep the plant running safely. In the newer generation, the engineers main task is to prevent the power plant from shutting itself down. In other words, the plants are designed to shut themselves down safely if someone isn't there telling it not to every few minutes. Sorry, no pictures and no math this time!
Nuclear power plants have melted down. Chernobyl was a disaster. It didn't contain safety measures like a secondary containment vessel. Three mile island and Fukushima are metldowns that were contained. No one died in either three mile island or fukushima, and that no one suffered from radioactivity damage from either event. The protective measures worked. More on this later, though. This post is about these protective measures.
The first safety measure of a nuclear reactor is the control rods. In the prior post, I mentioned that a nuclear reaction occurs when a neutron is given off, which then hits another uranium molecule, causing it to split and give off more neutrons. Control rods moderate this chain reaction. If a neutron hits a control rod molecule instead of another U-235 molecule, that neutron will not participate in and prolong the chain reaction. The control rods can be moved in and out of the reactor. The further in they are, the more likely that they will interfere with the chain reaction. Dropping them in fully can shut down the chain reaction. Pulling them out fully lets the reaction occur rapidly. Control rods can be made of several different materials, each with different properties. This becomes important, because Chernobyl used a type that burned, and Fukushima used a type that makes hydrogen from water under high heat.
The next safety measure is the nuclear pressure vessel. These behemoths have nearly 7 inch thick steel walls. They contain the pressure of the heated water (or other heated material) in the core. In the event of a reactor meltdown, it can contain a low-level meltdown.
The next layer of safety is a giant concrete containment vessel. If the pressure vessel ruptures or melts (yes, a runaway nuclear reaction can melt through the containment vessel), the concrete will contain the blast. It also protects the power plant from outside threats, like small airplanes and jet fighters crashing into it.
A point to ponder: Protecting against a fully loaded passenger aircraft is not in the cards. That being said, most coal fired power plants have 30 day supplies of coal. Or, you know, 280,000 tons of coal. This would be easier for a large airplane to hit, being a giant pile as opposed to a small reactor. This coal is meant to be burnt in controlled conditions where it is entirely burnt all the bad stuff is scrubbed. Burning it outside would be an environmental disaster, and would surely cause more deaths than Fukushima and Three Mile Island (again, 0 for these two incidents). So yeah, a nuclear plant is a target, but so is a coal plant. I really hope writing about this doesn't get me added to some list somewhere.
Getting back on track, most nuclear power plants have an emergency cooling supply that can drown the reactor and cool the reaction. It renders the reactor inoperable, and the reactor will never produce power again, but it can prevent a meltdown. Older generations relied on a series of pumps to pump water in. In the event of total power failure, these won't work. Newer generations have changed this. There is a cistern of water, large enough to drown the entire reactor core, seated above the core. As long as there is electricity applied to the valve, it stays closed and the water stays where it is. In the event of a complete power failure, the valve no longer receives the signal to stay closed. It opens. The cistern of water drains into the reactor, melting it.
The next level of protection is in case of a full meltdown of the core, and a breach of the pressure vessel. Should this happen, there is a massive concrete slab that will catch the molten material and contain it. As in the case above, a massive quantity of water will drop on the material to help cool it. Some of the newest designs even have cooling pipes in the concrete that catches the molten core.
Finally, in the event of too much pressure building up in the concrete protective structure, all new nuclear power plants are required to have filtered vents to release pressure. In other words, if water starts boiling in the reactor and pressure becomes too high, the extra pressure will be released through a vent that will filter our all of the radioactive material.
Clearly all this is very expensive. In fact, the major cost of a nuclear power plant is building things that prevent any problems in the worst-case scenarios. And, as I mentioned before, they work pretty darn well in the case of epic fail meltdown.
So that's about it. The rest of the safety stuff is all related to non-proliferation to terrorist groups, and that is not science stuff, so I am going to ignore it for this post.
Thanks for reading!