三里岛核事故分析英文DOC.docx

1、三里岛核事故分析英文DOCHow Is Japans Nuclear Disaster Different?Fukushima Daiichi may be no Chernobyl, but it has overshadowed Three Mile Island.The control room at Chernobyls Reactor No. 4 is shown here. Reactor design, wind patterns, communication and other factors can cause differences in the severity of n

2、uclear accidents.Photograph by Gerd Ludwig, National GeographicJosie GarthwaiteForNational Geographic NewsPublished March 16, 2011This story is part of aspecial seriesthat explores energy issues. For more, visitThe Great Energy Challenge.For decades, Three Mile Island and Chernobyl have served as sh

3、orthand for the nightmare of nuclear power generation gone awry. In the wake of Japans deadly earthquake and tsunami last week, the still-unfolding disaster of Fukushima Daiichi has come closer than any nuclear crisis in history to making it a fearsome trio.(Related Story: Japan Tries to Avert Nucle

4、ar Disaster)It remains to be seen how much damage will be caused by the crisis at the Fukushima nuclear power complex, where four of the six reactors have seen a range of woes including three explosions in four days, damage to two containment vessels, possible overheating from spent fuel rods, and m

5、ounting peril for the last remaining 50 workers due to dangerous spikes in radiation emissions.Yet it is already possible to outline key differences that set the current Fukushima situation apart from the 1979 Three Mile Island emergency near Harrisburg, Pennsylvania, and the disaster in Chernobyl,

6、Ukraine, that unfolded seven years later.Reactor TypeJapans Fukushima Daiichi nuclear power complex, which began operating in the 1970s, is made up of six boiling-water reactors, or BWRsa type of Light Water Reactor. (Using ordinary water, it is distinguished from heavy water reactors, which usedeut

7、eriumoxide, or D2O, instead of H2O.) Three Mile Island used another type of Light Water Reactor known as a pressurized-water reactor, or PWR.Both of these reactors use water for two purposes. It acts as a coolant, carrying heat away from the nuclear fuel, and as a moderator, slowing down the release

8、 of neutrons during fission reactions, explained Neil Wilmshurst, vice president of the nuclear sector at the U.S. Electric Power Research Institute, the industrys nonprofit research organization.In a PWR, the water is kept under pressure. This means the temperature can be higher than the boiling po

9、int of water without generating a significant amount steam (a less efficient coolant), said Wilmshurst. So the reactor core operates at a higher temperature in these systems, and heat can be transferred more efficiently. Boiling-water reactors operate at lower temperatures, and they tend to be simpl

10、er, with fewer parts, said Wilmshurst.Chernobyls reactors were a type called RBMK (for the Russian, reaktor bolshoy moshchnosty kanalny), which also used water for the coolant. But unlike the Light Water Reactors, the RMBK used graphite as a moderator. According to the World Nuclear Association, an

11、industry trade group based in London, no other power reactor in the world combines a graphite moderator and water coolant as Chernobyl did, although Russia does have several RBMK reactors in operation.Most nuclear reactors in the United States today use either BWR or PWR technology, which Wilmshurst

12、 and EPRI say are equally safe. Both types of reactors have a kind of self-regulation or negative feedback loop: As the reactor gets hotter, the fission reaction slows down, decreasing power, said Wilmshurst. The RMBK design, on the other hand, could go into positive feedback, where higher temperatu

13、re begets more power, which in turn increases the temperature, and so on.Accident CauseAt this point in the Fukushima disaster, Wilmshurst said, the tsunami appears to be the immediate culprit, since the plants shut down as they were designed to do following the earthquake. When the tsunami hit an h

14、our later, it damaged the site infrastructure, he said. So while the earthquake had cut the reactors external power supply, which is needed to keep coolant pumps doing their job, the tsunami killed the diesel backup generators needed to provide power for the cooling system. Batteries provided power

15、for only up to eight hours. Mobile generators were brought in to take over.(Related Photos: Japan Tsunami: 20 Indelible Images)Still, its too early to know for sure what sequence of events led to what outcome, said David Lochbaum, who directs the Union of Concerned Scientists Nuclear Safety Program

16、and has worked at three nuclear plants in the United States similar to the General Electric plants in Japan.According to the 1979Kemeny Commission reporton Three Mile Islandthe definitive document of that disasterequipment failures initiated the event, but operator error was the fundamental cause of

17、 the accident. Emergency cooling systems were shut down, with dire consequences. Three Mile Island would have been a relatively insignificant incident, the commission found, if the plant operators (or those who supervised them) had kept the emergency cooling systems on through the early stages of th

18、e accident.At the Chernobyl reactor in Ukraine, an ill-conceived, badly executed safety test initiated the disaster, said Wilmshurst. A sudden surge of power triggered a steam explosion that ruptured the reactor vessel, according to a recent report from the United Nations. This allowed further viole

19、nt fuel-steam interactions that destroyed the reactor core and severely damaged the reactor building.Understanding the ProblemThe level of access to information about what is going on inside a reactor has increased in the decades since Three Mile Island and Chernobyl.As PeterBradford, who served on

20、the U.S. Nuclear Regulatory Commission at the time of Three Mile Island, said this week, At Three Mile Island, much of what we thought we knew on the third day turned out to be incorrect. The extent of fuel melting, and even the fact that a hydrogen explosion had occurred in the containment on the f

21、irst day, he said, did not become clear for years. There was all kinds of information . . . we didnt know, he said.Related Story: Eyeing Japan, Countries Reassess Nuclear PlansDuring the first few minutes of the accident at Three Mile Island, more than 100 alarms went off, and no system was in place

22、 to filter out the important signals from the insignificant ones, according to the 1979 Kemeny report. Overall, little attention had been paid to the interaction between human beings and machines under the rapidly changing and confusing circumstances of an accident, the commissioners wrote.By contra

23、st, said Bradford, the level of computerization and information transfer available today could give Japanese officials much more insight to what happens in the four troubled reactors at Fukushimaat least in theory. Theyve got so much more going on in terms of the earthquake and the tsunami that we d

24、idnt have at TMI, that Im sure that the situation is every bit as confused, Bradford said.Radiation ContainmentLike the Three Mile Island plant, the Fukushima reactors have three barriers designed to prevent radiation leakage, including metal cladding surrounding the nuclear fuel, a reactor pressure

25、 vessel, and the primary containment vessel. Chernobyl lacked a containment vessel, said Wilmshurst.Once radiation is released into the environment, it can contaminate vast areas. Contamination levels are not linear, said Lochbaum. Further away you dont necessarily get lower doses, he explained. Amo

26、ng other factors, prevailing winds can influence what areas are affected. In the Chernobyl accident, some areas 100 miles away from the facility had radiation levels higher than areas just 10 or 20 miles away.(Related: Nuclear Reactors, Dams at Risk Due to Global Warming)The Chernobyl pattern was qu

27、ite erratic, said Lyman. Radiation was released very, very high because of the nature of the reactor and graphite fire. Weather changed over a prolonged emission period, as a graphite fire burned for 10 days. So radioactive gases and particles were picked up by wind and carried high in the atmospher

28、e over long distances before raining down on communities far from the source, he said.Ultimately, the radiation released as a result of Three Mile Island was not high enough to present detectable health effects in the general population. That accident rated as a level 5 of 7 on theInternational Nucl

29、ear Event Scale, an accident with wider than local consequence.At Chernobyl, a level 7 major accident, radiation exposureaffected thousands of people.Fukushima Daiichi has been elevated to level 4accident with local consequences. But it remains to be seen how much higher on the scale this incident w

30、ill go. In Tokyo, 180 miles away from the plant, peak radiation levels wererecorded at 23 times above normalat one point on Tuesday, but they reportedly dropped to about 10 times above normal later in the day.Exposure in PerspectiveIn the United States, the average radiation dose from natural backgr

31、ound andman-made sources, such as medical procedures and consumer products, is 620 millirems (mrem) per year,according to the NRC.One millisievert (mSv) is equal to 100 millirems. The Japanese Ministry of Health Labor and Welfare on Wednesday lifted the maximum allowable exposure for nuclear workers

32、 to 250 mSv, from 100 mSv, theAssociated Press reported. According to theNuclear Energy Institute, radioactivity at the plant hit a dose rate of 1,190 mrem per hour Tuesday evening, but dropped to 60 mrem per hour six hours later.The Chernobyl accident caused acute radiation sickness in 134 of the 600 workers who were at the site on the morning of the initial explosion and received high doses of radiation80,000 to 1.6 million mrem, according to theUN reportand theU.S. Nuclear Regulatory Commission. Of this group 28