三里岛事故文档格式.docx

1、 In 1979 a cooling malfunction caused part of the core to melt in the # 2 reactor at Three Mile Island in USA. The reactor was destroyed. Some radioactive gas was released a couple of days after the accident, but not enough to cause any dose above background levels to local residents. There were no

2、injuries or adverse health effects from the accident. The Three Mile Island power station is near Harrisburg, Pennsylvania in USA. It had two pressurized water reactors. One PWR was of 800 MWe and entered service in 1974. It remains one of the best-performing units in USA. Unit 2 was of 900 MWe and

3、almost brand new.The accident to unit 2 happened at 4 am on 28 March 1979 when the reactor was operating at 97% power. It involved a relatively minor malfunction in the secondary cooling circuit which caused the temperature in the primary coolant to rise. This in turn caused the reactor to shut down

4、 automatically. Shut down took about one second. At this point a relief valve failed to close, but instrumentation did not reveal the fact, and so much of the primary coolant drained away that the residual decay heat in the reactor core was not removed. The core suffered severe damage as a result.Th

5、e operators were unable to diagnose or respond properly to the unplanned automatic shutdown of the reactor. Deficient control room instrumentation and inadequate emergency response training proved to be root causes of the accident.The chain of eventsWithin seconds of the shutdown, the pilot-operated

6、 relief valve （PORV） on the reactor cooling system opened, as it was supposed to. About 10 seconds later it should have closed. But it remained open, leaking vital reactor coolant water to the reactor coolant drain tank. The operators believed the PORV had shut because instruments showed them that a

7、 close signal was sent to the valve. However, they did not have an instrument indicating the valves actual position. Responding to the loss of cooling water, high-pressure injection pumps automatically pushed replacement water into the reactor system. As water and steam escaped through the relief va

8、lve, cooling water surged into the pressuriser, raising the water level in it. （The pressuriser is a tank which is part of the reactor coolant system, maintaining proper pressure in the system. The relief valve is located on the pressuriser. In a pressurized water reactor like TMI-2, water in the pr

9、imary cooling system around the core is kept under very high pressure to keep it from boiling.）Operators responded by reducing the flow of replacement water. Their training told them that the pressurizer water level was the only dependable indication of the amount of cooling water in the system. Bec

10、ause the pressuriser level was increasing, they thought the reactor system was too full of water. Their training told them to do all they could to keep the pressuriser from filling with water. If it filled, they could not control pressure in the cooling system and it might rupture.Steam then formed

11、in the reactor cooling system. Pumping a mixture of steam and water caused the reactor cooling pumps to vibrate. Because the severe vibrations could have damaged the pumps and made them unusable, operators shut down the pumps. This ended forced cooling of the reactor. （The operators still believed t

12、he system was nearly full of water because the pressuriser level remained high.） However, as reactor coolant water boiled away, the reactors fuel core was uncovered and became even hotter. The fuel rods were damaged and released radioactive material into the cooling water.At 6:22 am operators closed

13、 a block valve between the relief valve and the pressuriser. This action stopped the loss of coolant water through the relief valve. However, superheated steam and gases blocked the flow of water through the core cooling system. Throughout the morning, operators attempted to force more water into th

14、e reactor system to condense steam bubbles that they believed were blocking the flow of cooling water. During the afternoon, operators attempted to decrease the pressure in the reactor system to allow a low pressure cooling system to be used and emergency water supplies to be put into the system.Coo

15、ling RestoredBy late afternoon, operators began high-pressure injection of water into the reactor cooling system to increase pressure and to collapse steam bubbles. By 7:50 pm on 28 March, they restored forced cooling of the reactor when they were able to restart one reactor coolant pump. They had c

16、ondensed steam so that the pump could run without severe vibrations.Radioactive gases from the reactor cooling system built up in the makeup tank in the auxiliary building. During March 29 and 30, operators used a system of pipes and compressors to move the gas to waste gas decay tanks. The compress

17、ors leaked, and some radioactive gas was released to the environment.The Hydrogen BubbleWhen the reactors core was uncovered, on the morning of 28 March, a high-temperature chemical reaction between water and the zircaloy metal tubes holding the nuclear fuel pellets had created hydrogen gas. In the

18、afternoon of 28 March, a sudden rise in reactor building pressure shown by the control room instruments indicated a hydrogen burn had occurred. Hydrogen gas also gathered at the top of the reactor vessel. From 30 March through 1 April operators removed this hydrogen gas bubble by periodically openin

19、g the vent valve on the reactor cooling system pressurizer. For a time, regulatory （NRC） officials believed the hydrogen bubble could explode. However, such an explosion was never possible since there was not enough oxygen in the system. Cold ShutdownAfter an anxious month, on 27 April operators est

20、ablished natural convection circulation of coolant. The reactor core was being cooled by the natural movement of water rather than by mechanical pumping. The plant was in cold shutdown.Public concern and confusionWhen the TMI-2 accident is recalled, it is often in the context of what happened on Fri

21、day and Saturday, March 30-31. The height of the TMI-2 accident-induced fear, stress and confusion came on those two days. The atmosphere then and the reasons for it are described well in the book Crisis Contained, The Department of Energy at Three Mile Island, by Philip L Cantelon and Robert C. Wil

22、liams, 1982. This is an official history of the Department of Energys role during the accident.Friday appears to have become a turning point in the history of the accident because of two events: the sudden rise in reactor pressure shown by control room instruments on Wednesday afternoon （the hydroge

23、n burn） which suggested a hydrogen explosion - became known to the Nuclear Regulatory Commission that day; and the deliberate venting of radioactive gases from the plant Friday morning which produced a reading of 1,200 millirems （12 mSv） directly above the stack of the auxiliary building. What made

24、these significant was a series of misunderstandings caused, in part, by problems of communication within various state and federal agencies. Because of confused telephone conversations between people uninformed about the plants status, officials concluded that the 1,200 millirems reading was an off-

25、site reading. They also believed that another hydrogen explosion was possible, that the Nuclear Regulatory Commission had ordered evacuation and that a meltdown was conceivable. Garbled communications reported by the media generated a debate over evacuation. Whether or not there were evacuation plan

26、s soon became academic. What happened on Friday was not a planned evacuation but a weekend exodus based not on what was actually happening at Three Mile Island but on what government officials and the media imagined might happen. On Friday confused communications created the politics of fear. （Page

27、50）Throughout the book, Cantelon and Williams note that hundreds of environmental samples were taken around TMI during the accident period by the Department of Energy （which had the lead sampling role） or the then-Pennsylvania Department of Environmental Resources. But there were no unusually high r

28、eadings, except for noble gases, and virtually no iodine. Readings were far below health limits. Yet a political storm was raging based on confusion and misinformation. No Radiological Health EffectsThe TMI-2 accident caused concerns about the possibility of radiation-induced health effects, princip

29、ally cancer, in the area surrounding the plant. Because of those concerns, the Pennsylvania Department of Health for 18 years maintained a registry of more than 30,000 people who lived within five miles of Three Mile Island at the time of the accident. The states registry was discontinued in June, 1

30、997, without any evidence of unusual health trends in the area.Indeed, more than a dozen major, independent health studies of the accident showed no evidence of any abnormal number of cancers around TMI years after the accident. The only detectable effect was psychological stress during and shortly

31、after the accident.The studies found that the radiation releases during the accident were minimal, well below any levels that have been associated with health effects from radiation exposure. The average radiation dose to people living within 16 kilometres of the plant was 0.08 millisieverts, with n

32、o more than 1 millisievert to any single individual. The level of 0.08 mSv is about equal to a chest X-ray, and 1 mSv is about a third of the average background level of radiation received by U.S. residents in a year. In June 1996, 17 years after the TMI-2 accident, Harrisburg U.S. District Court Judge Sylvia Rambo dismissed a class action lawsuit alleging that the accident caused health effects. The plaintiffs have appealed Judge Rambos ruling. The appeal is before the U.S. Third Circuit Court of Appeals. However, in making her de