[MUSIC] [SOUND] I'd like to tell you about TMI. That's not too much information. It's a Three Mile Island. Three Mile Island was a power plant. And still is a power plant that's on an island which is three miles away from something, a bridge. I think there's another island a little further down the river called Nine Mile Island. It was a great place to put a power plant. You see, you want some security, people have to cross the river before they come to the fence, that's good. And you need water, you need cooling water. And you can see here that this island has plenty of natural water around it. Because you can use that water to exchange with a condenser. And since you don't want to put warmer water back into the river, you've got the cooling towers. Now, Three Mile Island became famous because in 1979 there was a nuclear accident up there. By the standards of Chernobyl or even Fukushima this was a very minor accident. But it was a very important turning point in nuclear power in the United States. Mostly because of economics, you see nobody got hurt. I mean, no one even ever would get cancer in addition because of this, and we'll describe that in more detail. But who did get hurt were the investors. This is an almost brand new power plant. It cost $4 billion to make. And in a few short hours it went from a $4 billion asset, where all the money was still owed. Because it had not yet gone its 30 years it would take to pay it off, to a negative $1 billion liability. Let's talk about what happened. So, Three Mile Island is a pressurized water reactor. And like anything, this whole part is designed to make the steam. And then the rest of the power plant are your standard turbine, generator makes electricity, condenser with another cooling loop, and the cooling tower. What we're going to concentrate on today is what happened in the reactor vessel itself. And the things that associated with it that make steam. So, any nuclear power plant has a variety of safety systems. They call it defense in depth. And what your defending against is keeping the reactor core from getting too hot. If it gets to hot the fuel itself and the claticing around the fuel could melt. That's called a melt down. And if you have a meltdown, the fission products, the things that uranium splits into, the highly radioactive material, can now get loose. That's the worry, that's what you're trying to defend against. One of the defenses is making the fuel itself. The fuel is solid, it's a ceramic fuel, it takes an extremely high temperature. And that's really good. But in this generation of power plants, the generation to power plants. Certainly the ones built in the 70s, the 80s. These power plants always needed to have some water over the reactor even when its turned off. So how do we make sure, how do we ensure, how do we have the fence in depth that makes sure that this always happens? So, the design is to have water pumped through the reactor, and that water is its coolant. So one of the very first important defense in depths is routine maintenance. Do something such that the pumps will always continue to work. What happened at Three Mile Island? It wasn't done. Someone signed off on the tag but much easier to instead of doing the work, oiling it, checking it, that's a brand new pump, what's going to go wrong? But instead of doing that someone just signed off on it. The first thing that went wrong. But don't worry let's say the pump breaks like this demo. So a pump is is going a long a pump is going a long right and the pump [SOUND] breaks. >> [LAUGH] >> All right, okay. Broken pump. Don't despair, there's a second level of protection, of defense in depth. There is a backup pump. And within seconds, when it's sensed that there's a slight drop and change in pressure because the pump failed. The backup pump turns on. And the backup pump is supposed to be connected with open pipes. So that if exactly this system happens, the backup pump goes on. Nobody, well they notice, but there's no problem, the reactor just keeps running. Someone had shut the valves. So backup pump, valve left closed, very bad. So now no water is coming to the reactor. Of course, the reactor has sensors, and what's going to happen next is that the reactor will heat up. It will create more pressure inside the reactor. And this pressure going up could be dangerous. Too high of a pressure, you could burst the entire reactor vessel. So there is another safety system. There is a relief valve. So that if the pressure goes too high, the relief valve will automatically open. And this water that's been too high in pressure and steam will now be able to go into a holding tank. Wonderful, all of this has happened. And all of this takes 30 seconds. Very short amount of time. Once of course the relief valve opens the reactor has sensed that that's great. Now, my pressure has gone back down but clearly something is going wrong. Nuclear reactors are supposed to just operate. You don't even have to change power very often. It just runs. And when it run continually, everything's happy. You make lots of electricity. You can sell the electricity. But in a case like this, the reactor senses and it says, something isn't right. I'm going to turn myself off. And this is a very important system, and there's an acronym for it, an interesting one. It's called a SCRAM. It stands for safety control rod. And get this, when this acronym was first created the AM was for Ax Man. I'm not kidding you. The very first nuclear core, the first controlled fission nuclear reaction was done in 1942 under a stadium, Stag Field in Chicago. And Enrico Fermi and his crew were trying to learn what it would take to make nuclear chain reactions. They realize that these things could get out of control, so they needed something, they needed a control rod, something that would absorb neutrons. So like this demo shows, they suspended a control rod over the reactor so it could go down a hole. And they stationed a guy with an ax so that if anything goes wrong, he cuts the rope. And the control rod falls into the core, absorbs all the neutrons, and everything's great. Well here at Three Mile Island, this was great. And we actually don't call it ax man anymore. We now have a much more enlightened description, Activation Mechanism. So, there's no more man with an ax. It all happens automatically. The reactor SCRAM's the control rods go in, the chain reactions stop, the awesome heat production a nuclear reactor makes stops. And you might say wonderful, there should be no problem, so let's go home. Generation III reactors, the new ones that are being built today, absolutely true. You don't need to keep water over their cores, convection can take care of all the heat It just can go home. Here in the generation II reactors of the 70s, scramming was wonderful because it would cut the 3,000 megawatts of thermal power being produced. This would go down instantaneously after you SCRAM to 2%. Only 60 million watts of thermal power. That's still a lot of watts. That's still enough to in time heat the reactor up too much if you did not keep water over it. So the reactor is SCRAM in the control room, the alarms are going off, it's always the middle of the night when anything goes wrong. And they're trying to figure out why in the world did the reactor SCRAM? And they can see that clearly the water was going through. And that a relief valve was indicated that it had it open. But now that the reactor's scrammed and the power level has dropped so dramatically. The indication is that the relief valve sticks and worse than that it sticks open. So even though my heat level in the reactor has gone down I'm not boiling things off as much. But the water that's supposed to be covering the reactor core is still going out to the holding tank. There's another safety system, just in case that you don't have water on the core we need a backup. We need a big, giant pool of water we can throw onto the reactor. It has an acronym, the Emergency Core Cooling System. And at two minutes, into the accident, two minutes from when this pump breaks. After all that was the first thing that initiated all this that pump breaks, stops working. Two minutes later, the Emergency Core Cooling System turns on, water into the reactor. Excellent. If we had just stopped right now, everything would be fine. Let's go back to those operators in the control room. They figured out at this point that, hah, the veil to the backup pump was closed. No wonder the relief valve opened. SCRAM the reactor our power level when down, that makes perfect sense. My indicator in the control room tells me the relief valve is closed again, even though that's not true. The relief valve is stuck open, but they don't know that. Emergency Core Cooling Systems goes on. Wonderful, we've put more water into the core at four and half minutes. The operators make a decision to turn off the ECCS. After all our level is way down. They have an indication not an actual reality, but an indication in the control room that this relief valve, Is closed again. So why keep pumping more water in we have the problem? We've noticed that this valve was left closed. In fact at eight minutes into the accident they opened backup pump. And I can imagine them patting themselves on the back. These are smart, dedicated people they're trying very hard, figuring out what's wrong, it looks like we're in good shape, right? The reactor is scrammed, it's off, it's down and low-powered. It'll eventually go to lower power as the fission products decay. We added more water, [COUGH] we found what was wrong, the valve was closed. We've open the close, we've turned off the ECCS. Everything should be fine. Man we got some paperwork but that's all right the investment's safe, the people are safe, the reactor will probably turn back on in a couple days. We'll be generating electricity again. They didn't know the relief valve was stuck open. So here they sit figuring out stuff, filing reports, thinking, calling in supervisors, I'm sure, the valve's open. The water which is no longer being made up for are just circulating with this pump is still going out through this valve into the holding tank. The reactor core is here, that water level is going down, and down, and down. Up to 100 minutes, an hour and 40 minutes later, if they had done the right thing, it should be very difficult to find out. If they had turned off someone manually closed that relief valve we would not be standing here talking about. But what happened at this point is that the reactor core starts to become uncovered. When that happens, the uncovered fuel starts getting very hot. You start getting some hydrogen production. Water is H20, you get it hot enough, you have the right types of catalytic materials around, you start producing hydrogen gas. Still our reactor is fine and our reactor is intact. [MUSIC]
