Nuclear Accidents. William M. Murphy. Professor of Geological and Environmental Sciences California State University, Chico

Transcription

1 Nuclear Accidents William M. Murphy Professor of Geological and Environmental Sciences California State University, Chico A Month After the Earth Moved: The Science Behind the Japan Disaster April 11, 2011 The German philosopher Immanuel Kant regarded earthquakes as natural phenomena and, reflecting on the earthquake that destroyed Lisbon on Easter Day 1755, he suggested that people note where earthquakes occur and then not build their cities there. Well thank you very much. It's my pleasure to be here. I'm Professor Murphy, and I'm delighted to see such a big and attentive crowd. I have a background in geologic disposal of nuclear waste, and I've been involved in nuclear issues for a long time. And one of my primary objectives today is to put the nuclear accident that occurred as a consequence of the earthquake and the tsunami into the context of nuclear accidents in a larger picture. One of the things that has distressed me somewhat as a consequence of this accident is how much misinformation there has been in the media, even by alleged experts about the details. And so maybe I'll clarify some of those points to the extent that I can. 1

2 Fission, Radioactive Decay, and Radiation Fission: splitting of large, atomic nuclei including uranium-235 and plutonium-239, releasing much energy. Radioactive Decay: radioactivity, decay of unstable atomic nuclei at systematic rates characterized by measured half lives. Radiation: energy released during fission and radioactive decay (and other processes) as energetic alpha particles, neutrons, beta particles, gamma rays, etc. First of all, let's talk about some of the basic concepts that we're concerned about. Fission is the splitting of large fissile nuclei, like uranium and plutonium. This is the splitting of the atom, and I'll show an illustration of that. Radioactive decay is another nuclear process where unstable nuclei systematically decay at a rate that's very much like a perfect plot, and it's characteristic of age individual unstable nucleus. And it's been extraordinarily useful for geologists to do things such as dating rocks, and dating the age of the earth. Radiation is the energy that's released by these processes in various forms, and by other processes. Radiation, if it has -- if it's energetic enough, it can do damage to biological systems. So these are distinct concepts, and I'll use them, and so there's the definition. 2

3 Here s a slide illustrating nuclear fission. Most nuclear reactors work on the fissile or fissionable uranium 235. If it absorbs a slow neutron, it becomes uranium 236, which is extraordinarily unstable, and it explodes into two sub-equal fragments. And it's so violent that there's a whole distribution of these fragmentsin various pairs. This releases an enormous amount of energy, and more neutrons. And if those neutrons go on to make more uranium split, that releases more energy and more neutrons. And if there's a large enough mass of this fissionable material, you can generate gigantic quantities of energy. One splitting releases an average of 215 million electron volts of energy. An electron volt is a unit of energy. In contrast, if you look at a chemical reaction, down in the lower right, one atom of methane or natural gas oxidizing the way we oxidized it this morning in our hot water heater to take a shower, releases eight electron volts. 215 million electron volts for one splitting versus eight electron volts for one atom of methane. That's why a coal-fired electric power plant requires a trainload of coal a day, and a nuclear power plant requires a few truckloads of fuel a year. 3

4 In contrast, radioactive decay is this systematic decay of unstable nucleus, here on the top right, illustrated by americium 241. This is not a natural element on earth, but it's generated in nuclear reactors. And it's radioactive, and it decays with a half life. If you have a large enough number, every 432 years half of all the unstable americium 241 decays away. And in the second half life half of that decays away, and in the third half life half of what's left decays away, and one arrives at an exponential decay curve for these radioactive isotopes. This particular decay also releases millions of electron volts of energy million electron volts. So this is a lot of energy too. Uranium 238 exists in nature because it has a very long half life, 4.5 billion years, coincidentally about the same as the age of the earth. It creates a whole series of radioactive products that eventually and in stable led 206. Among these is radon 222, which we'll see illustrated again in a moment or two. 4

5 If we control the neutrons that are released in -- in the fission process, we can get the nuclear reaction humming along at just the right speed, so that just enough neutrons can be absorbed by just the right number of fissile nuclides that -- to maintain that fission process. And we can capture the energy, and boil water, and run a nuclear reactor. This is how a nuclear reactor works. And here's a site. There's a nuclear renaissance, or there was a month ago in the United States. There are dozens of new nuclear power plants that have been licensed to be built in the United States by the Nuclear Regulatory Commission, and additional dozens in the works. There's a nuclear renaissance in place for a variety of reasons. But things may be changing. This is the site where one of them is proposed. Ground's actually been broken on at least one of these new nuclear power plants. 5

6 If one has enough of this fissionable material all jammed together very closely in a critical mass, this explosion goes until it blows itself apart, and one can make an atomic bomb in this manner. And this is the picture of the mushroom cloud over the atomic explosion at Hiroshima. This was a uranium 235 bomb. Nuclear power plants cannot have nuclear explosions like this. The physics and the geometry, the engineering is completely different. So we don't need to worry about nuclear power plants blowing up like a nuclear bomb. 6

7 This is a diagram from General Electric of the boiling water reactor that exists at Fukushima, where the accidents that have occurred due to the earthquake and the tsunami. There's a core that contains the nuclear fuel, those are the red rods. That water circulates through that core, and it boils. The heat from the nuclear reaction boils that water, that creates steam that leaves the containment vessel, which is the heavy black line. It leaves the secondary containment, the reactor building. It goes into turbines, the steam spins the turbines, which spin generators, which generate electricity. The steam is then condensed and recycled through the reactor. This is General Electric's schematic of the kind of nuclear reactors that exist at Fukushima, and elsewhere in the world. Another aspect of the problem you see is the containment. You see a very strong containment vessel surrounding the core, you see the Taurus [phonetic], which is a reservoir of water designed to absorb the energy if there's excess steam produced. You see how very high integrity building surrounding all of that. Outside of that containment structure in the lower right, here's a picture. This is from a different site, this is a picture of a spent nuclear fuel pool. After the fuel is used, it has lots of radioactive fission products that have been generated. It's very highly radioactive, extremely dangerous. You can't go anywhere near it, it has to be handled remotely, and it much of it is stored in pools such as this under tens of meters of water that act as a radiation shield, and also act as a cooling agent. Typically these are outside the containment vessel. 7

8 The very first GE reactor of this site was located at Humboldt Bay, very close to here. And it's one of my favorite examples, because one often thinks that nuclear technology is very modern and advanced, and that geology is this ancient science. But when the Humboldt Bay nuclear power plant was built, it's located right here at the triple junction between three tectonic plates of the earth's surface. When that reactor was built, geologists had not yet figured out plate tectonics. They put it just at the wrong spot. And it didn't last very long, because of seismic concerns, and it was a prototype in a way. There were problems with corrosion, as a result of using sea water in the cooling system. And it's been shut down. It was shut down in 1976, even before Three Mile Island. But the spent fuel is still there. We don't have a solution to the spent fuel problem yet in the United States, in fact we're going backwards in that effort. This reactor is located right on the Cascadia subduction zone. We've heard in the media lots of comparisons to the San Andreas fault. But it's the Cascadia subduction zone that is geologically very similar to the subduction zone off the coast of Japan. And at the time that this reactor was built, it was unknown. And until a couple of decades ago, people weren't even aware of earthquakes on the Cascadia subduction zone. And finally it was discovered that the last one occurred in January of 1700, which in the Pacific Northwest was prehistoric. There was no historic record in North America. But forests were discovered that had died at that time, sediments were observed of that age that had been disturbed. And the precise timing was determined from the tsunami records in Japan. And Humboldt Bay is sitting right there. This was a magnitude 9 earthquake. And the recurrence interval has been determined something like 300 years on this subduction zone. 8

9 Nuclear Accidents: Three Mile Island (near Harrisburg, PA): March 28, 1979 Malfunction of water supply to steam generators Failure of backup system due inadvertently to closed valves Automatic reactor shut down; radioactive heating Pressure release valve opened, then failed to close Emergency core cooling water pumps started and stopped Cladding started to oxidize and fuel started to melt Radioactive isotopes released to cooling water and to gases Hydrogen gas generated by cladding oxidation Operators closed pressure release valve Emergency cooling system turned on, system stabilized About $1 billion cleanup cost No detectable adverse radiation effects on public health So let's talk about a few accidents. There only are three. One often wonders about the risk of nuclear accidents, and that's a very hard thing to judge, because risk is a statistical probability of there being harm from some hazardous thing. And with only three accidents, it's hard to do statistics. The first accident was in Three Mile Island, which was in eastern Pennsylvania, a power plant near Harrisburg. And it was a perfect storm of mechanical failures and operator failures. It was a malfunction of the water supply system to the steam generator. The backup system had inadvertently been turned off by the operators, so the backup system didn't work. The reactor shut down, just like at Fukushima. The reactor shut down. The fission reactions shut down upon the earthquake. That worked just as it was supposed to work. At Three Mile Island the reactor shut down, but it continued to get hot because of the radioactive decay of the fission products that are unstable nuclides. They continue to decay radioactively and release lots of heat. The emergency core cooling water pumps started up automatically, but the operators thought the problem was there was too much water, so they turned them off. The cladding [phonetic] started to oxidize, and this generated hydrogen gas. The fuel started to melt. Ultimately about half of the fuel at Three Mile Island melted. It was a big meltdown. There were radioactive isotopes released to steam and to the atmosphere. The operators finally figured out that they needed to close the pressure valves, and they needed to turn the emergency cooling system on, and the system stabilized. Now 30 or 40 years later -- this is in '79, there have been no detectable adverse radiation effects on the environment, or on people because of this. 9

10 Nuclear Accidents: Chernobyl (near Kiev, USSR, now Ukraine) April 25, 1986 Emergency cooling and automatic control rod systems turned off for a test Fission reactor power surge Explosions caused by high steam pressures Graphite core caught fire (no containment vessel) Radioactivity spread over Ukraine, Belarus, etc. Thirty-one deaths in a short time Thousands (?) of deaths anticipated Here's Chernobyl. Chernobyl was a completely different story. People were doing experiments. They turned off the safety system, there was an explosion, the reactor while it was still fissioning caught on fire. There was no container vessel, and radiation spread around the world. 10

11 It causes one of the hazards is iodine 131, which it has a short half life, eight days. It gets into the food chain, it gets into people and into the thyroid, it causes thyroid cancer. This is a graph of thyroid cancer occurrences among children and adolescents who were in the vicinity of Chernobyl. You see the curve is still going up. Of 5,000 excess cases of thyroid cancer in Belarus, Ukraine, and Russia, there have been 15 fatalities so far. How many there will be we don't know. 11

12 Fukushima Daiichi, Japan Six GE design boiling water reactors: The reactors came online from 1970 to From 2002 to 2005 the reactors were shut down for a time due to the TEPCO data falsification scandal. In February 2011 TEPCO admitted that it previously submitted fake inspection and repair reports on components including temperature control valves, water pump motors, and emergency diesel generators. In 2008, the IAEA warned that Fukushima was built using outdated safety guidelines and could be a serious problem during a large earthquake. Wikipedia, April 9, 2011 Fukushima. Six GE designed boiling water reactors. The reactors came online from 1970 to '79, they're old. They were shut down for a time due to the Tokyo electric power company data falsification scandal. In 2008 the International Atomic Energy Agency warned that Fukushima was built using outdated safety guidelines, and could be a serious problem during a large earthquake. 12

13 Nuclear Accidents: Fukushima Daiichi Commencing March 11, 2011 Units 1, 2, 3 operating; units 4, 5, 6 shut down for maintenance M 9 earthquake (about 0.5 g at Fukushima) and resulting tsunami Automatic shut down and emergency cooling following earthquake Tsunami takes out emergency diesel power; batteries have 8 hour charge Core and spent fuel pool overheating due to radioactive decay Steam releases from containment vessel to relieve pressure Partial meltdown in reactors 1, 2, and 3 Hydrogen explosions damaging containment buildings 1, 3 and 4 Hydrogen explosion damaging containment vessel in Unit 2 Fires at reactor 4; Water and seawater used for cooling Elevated radiation detected locally, regionally, in Tokyo, and worldwide Radioactive water from Unit 2 released to the ocean Efforts continue to restore power, cool reactors and spent fuel, and limit releases of radioactivity Units 1, 2, and 3 were operating at the time of the earthquake. There was an earthquake and tsunami. The reactors shut down, the emergency cooling system came on. The tsunami took out the emergency cooling power supply. It started heating up, there was steam generated, the steam was released. That contained some radio nuclides. One of the reactors caught on fire, there were releases of material from spent fuel in reactor 4. The explosions we saw on TV were due to the ignition of hydrogen gas that had accumulated in the containment buildings. There are high levels of radiation detected inside the reactors, some radiation in the immediate vicinity, small amounts of radiation at greater distances. So far there were two real radiation problems. Two workers who waded into high radiation containing water got radiation burns on their feet. There were two workers killed by the tsunami. But so far there are a lot of uncertainties still about the health effects, and it's been reported with such great uncertainty, and it's not over yet. So I hesitate to make any predictions. But we're not at risk at present in any case. 13

14 There are 104 operating power plants in the US, mostly in the east. 14

15 Ionizing radiation is everywhere, we're all radioactive. Most ionizing radiation that we receive is from natural radon. 15

16 Transportation issues are a big concern of people, and it's a total red herring. One thing we know how to do is to drive trucks and trains around. Don't worry about transportation of nuclear waste, it's a waste of time. The National Academy of Sciences agree. 16

17 Nuclear wastes have to be disposed permanently in a geologic setting. Every expert in the world agrees. 17

18 This is why there's a nuclear renaissance in part. The carbon dioxide in the atmosphere is racing upward, and that's a problem. Nuclear fission produces no carbon dioxide. 18

19 Huge and rapidly increasing electric energy consumption in the US is met by few major sources. Thousand Megawatthours 4,500,000 4,000,000 3,500,000 3,000,000 2,500,000 2,000,000 1,500,000 1,000,000 US Electricity Generation Total Coal Nuclear > 21 percent increase in 11 years Natural Gas 500,000 Hydroelectric Data from US Energy Information Administration Year This is really I think the context for the whole problem. Huge consumption of electric energy. This is the thousands of megawatt hours consumed per year in the United States. 21% increase in this 11 year period, half of it comes from coal, about 20% from nuclear energy in the US, about 20% from natural gas, about 7% from hydroelectricity, and all those other great things that we talk about: biofuel, solar, wind, tidal energy, ethanol. They don't even show up on this graph yet. But it's an enormous demand to fulfill. And there's a huge amount of coal in the world, in the United States. We're not going to run out of fossil fuels for hundreds of years. We better figure out what to do with the carbon from the coal. Thank you. 19