字幕表 動画を再生する 英語字幕をプリント The following content is provided under a Creative Commons License. Your support will help MIT OpenCourseWare continue to offer high quality educational resources for free. To make a donation or to view additional materials from hundreds of MIT courses, visit MIT OpenCourseWare at ocw.mit.edu. MICHAEL SHORT: We've actually got a special guest today. It's Jake Hecla, one of the seniors at NSE who's gone on to Chernobyl for the second time, just returned from there two weeks ago. So if you remember on Tuesday, we went through all of the physics and intuition about why Chernobyl happened. And we left off on what does it look like today. So Jake is going to tell you what does it look like today. JAKE HECLA: All right, so first off I'm actually going to go over a bit of the reactor physics involved with the Chernobyl accident. I realize you guys have already covered this to some extent. But I didn't plan for that. So it's in my presentation. MICHAEL SHORT: It'll be a good review. JAKE HECLA: Yes, also I am a little sick. So I'm probably going to start coughing, apologies. I'm not dying. It's just a cold. AUDIENCE: Radiation poisoning. JAKE HECLA: I have heard that joke about eight times in the last two days. And I'm so done with it. But yes, it's not radiation poisoning. AUDIENCE: [INAUDIBLE]. JAKE HECLA: Yeah, all right, where is Chernobyl? Ah, dang it. Come on, no, go the other way, the other way, yep. OK, there. OK, so one of the first questions I got when I said I'm going to go and visit Chernobyl is wait, isn't that a war zone? Not quite. So the Ukrainian, the war in Ukraine is mostly in this portion over here. It's not entirely under Rebel control in that area. And I say "rebel" in quotation marks because rebel means Russian. However, if you notice those arrows, Russian forces are built up all along that border. So while it's not an active war zone, it's certainly not a place to be spending a large amount of time. That said, Chernobyl is north of Kiev by about, I don't know, let's see, 200, 250 kilometers. So it's not completely out in the sticks, right. Hopefully this gives you good sense of roughly where it is. All right, so what is the Chernobyl nuclear power plant look like? It consists of four finished reactors. There are two unfinished reactors, unit 5 and 6, that are not shown in this image. Units 1 and 2 are located at the right. Those were constructed in the 1970s and early 1980s. All of these reactors or the RBMK type. Units 1 and 2 operated with some success-- I'll go into that later-- for a number of years before the accident that happened in 1986. We also had some call outs up here that show the, some of the incidents that I'll talk about here a little bit later in the presentation. But this just gives you a general idea of the layout. So it's two separate buildings for units 1 and 2. And then units 3 and 4 are in one building, all connected by this turbo generator hall. So this is where the generators that turn the steam from the RBMK into power, r. This is one giant-- well, before the accident, this was one giant, not separated hallway, basically. So you could walk from one end to the other, theoretically. All right, so what is an RBMK? An RBMK is a light water-cooled, graphite-moderated, channel-type reactor. This means that it does not have a giant pressure vessel like you would see in a VVER or an equivalent American light water reactor. Why does that mean anything? Well, building giant pressure vessels is very difficult. If any of you've done research on manufacturing of nuclear reactors, you'll find out that the equipment necessary to construct a reactor pressure vessel is not actually something we even have in the US anymore. Is it Korea that does it for us now? MICHAEL SHORT: Japan Steel Works. JAKE HECLA: Japan that does it now. In the Soviet times, it was very, very difficult for the Soviet Union to produce such pressure vessels at any kind of reasonable rate. So the RBMK got around this by using individual channels that were their own pressure vessel, so to speak. So the way this works is, let's just start on the cold side. You take in cold water, goes here through these things. These are main circulating pumps-- MCPs, as you'll see them referred to later in the presentation-- goes up through the bottom up the core. These are the hot fuel rods. The water goes from liquid to steam phase as it's flowing through the channels, comes out the top, goes to the steam water separators. Steam goes to the turbines, turns the turbines, makes electricity. The important thing to remember here is that we've got a giant graphite core. The graphite is what is doing the moderating in this circumstance. It is not the water. This allows you to run very low-enriched uranium. So you could theoretically run an RBMK on I believe it was 1.2 percent was as low as they could go. But regardless, extremely low-enriched uranium, which is convenient if you don't want to waste a lot of time enriching uranium. The problem with this is that you have a giant core. If you recall the scattering cross-section for graphite, it's pretty small. And the amount of energy lost per collision is likewise also fairly small. So the core on this thing is, let's see, 11, yeah, 11.5 meters across. The core for an equivalent American reactor-- so well, there is no real equivalent to this-- but for, let's say, an AP 1000 reactor of equivalent electrical output, is about four meters across. So the core is huge. As I already discussed, this is what the individual pressure channels look like. So cool water comes in the bottom, goes by the fuel rods, pops out the top. The RBMK had some serious design flaws. So as I said, the core is huge. This allows local power anomalies to form really, really easily. If you look at the core, one portion can be kind of neutronically separated from the others because neutrons just don't make it all that far when diffusing across the core. So you can have very, very high power in one corner and very low power in the other, which is not something that can develop in a physically smaller core, which has a characteristic scale equivalent to that of the neutron being free path. Further, the encore flux monitoring on the RBMK is seriously deficient. So there are a variety of neutron detectors that exist around the periphery of the core. But they're wholly insufficient to catch these local power anomalies. Chernobyl actually found out the hard way on this one. In 1982, unit 1 suffered a quote "localized core melt," not really something that can happen in LWR, really any other type of reactor. But a couple of the fuel channels actually experienced one of these local power anomalies and ended up melting. So if you go into the control room of unit 1, you can see that on the fuel channel cartogram on the wall, there are two of them that are just Sharpied out. And those are the ones that melted. Further, it has a positive void reactivity coefficient. What does that mean? Well, when the water boils in the core, the density of the water there goes down. And the power of the reactor ends up going up because the water is primarily acting not as moderator but as a neutron absorber. This is bad for a whole variety of reasons. And they found out quite catastrophically in 1986 exactly why. Further, the system is extremely unstable at low power. So how did the 1986 accident happen? It was part of this thing called a turbo generator rundown test. The general idea is that if you have an off-site power failure, and your main circulating pumps are no longer have off-site power, you somehow need to keep water flowing through the core, such that the fuel does not melt. The problem is that the backup, large diesel generators, are just that. They're large. They're diesel. And therefore they're very, very slow to come online and come up to full power. The way that you can bridge this gap is by using the energy that you've stored in the turbines to effectively power the main circulating pumps until the diesel generators can come up online. When unit 4 was fully constructed in 1983 and turned on for the first time, they had never actually done this test where they did a turbo generator rundown, despite the fact that it was required by law in the Soviet Union that all new power stations should have this test performed. It was delayed until 1986. And yeah, it was delayed until 1986 is the long story short. The test procedure-- sorry for all the text on this slide-- is basically as follows. So you would ramp the reactor down. So you would bring it from a normal thermal output of up to 2,400 megawatts thermal, down to 600 or 700 megawatts. You'd bring the turbo generators up to full speed. So you'd store as much energy in them as you possibly could, then cut off the steam supply such that now you are just extracting energy from the spinning turbo generator. This would then be used to power the main circulating pumps, each of which took about 40 megawatts. There are eight of them total. I believe six could be used for normal operation. The rundown would take somewhere in the range of 60 to 70 seconds. And hopefully by this time your diesel generators would be turned on, pumping water, and everything would be fine. What happened in the test was decidedly quite different from that. So on April 26, 1986, they attempted to begin this test about six hours behind schedule because there was an incident in another part of Ukraine, in which a coal power plant went offline. So what happened was the authority for the grid in the area ordered that Chernobyl should stay online at full power for an additional six hours. They began the test by bringing power down. But as a result of running for an extra six hours, they'd built up a significant amount of xenon precursors in the core. So when they started turning the power down, the power started going down, and down, and down. And they were unable to arrest its drop. What ended up happening was that the power dropped all the way down to 30 megawatts thermal. And the reactor operators kind of panicked. Their response to this, instead of canceling the test, was to pull out as many control rods as they could get their hands on. They did so. And this managed to rescue the thermal output of the reactor. And it bumped up to around 200 megawatts thermal. At this point, the reactor was in an extremely unstable state. Mind you, almost all of the rods that they could get their hands on were out of the reactor.