三里岛核泄漏事故
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三里岛核泄露事故，通常简称“三里岛事件”，是1979年3月28日发生在美国宾夕法尼亚州萨斯奎哈纳河三里岛核电站（Three-Miles Island Nuclear Generating Station）的一次部分堆芯熔毁事故。这是美国商业核电历史上最严重的一次事故[1]。该事件被评为国际核事件分级的7级系统中的第5级：事故带有广泛后果[2][3]。反核运动积极分子表达对事故造成的区域健康影响的担忧[4]。 然而，流行病学研究分析自事故发生以来该地区及其周围地区的癌症发病率，确定该数据在统计上没有显着增加，因此没有因事故与这些癌症相关联的因果关系得到证实[5][6][7][8][9][10]。但处理所需要的经济损失很大，场内污染清理工作开始于1979年8月，并于1993年12月才正式结束，总清理费用约为10亿美元[11]。
| 日期 | 1979年3月28日 |
|---|---|
| 时间 | 4:00 (北美东部时区 UTC-5) |
| 地点 |  美国宾夕法尼亚州萨斯奎哈纳河三哩岛 |
三哩岛核泄漏事故是美国至今最为严重的核事故，但与之后发生的切尔诺贝利核能电厂事故与福岛第一核电站事故相比，三哩岛核泄漏事故仍然在可以控制的范围内，在该核电站周遭的居民以及邻近的几个州也都没出现像乌克兰或是日本福岛那样大规模的污染，另外该事件的知名度也不如切尔诺贝利核能电厂事故与福岛第一核电站事故那样被世界广为人知。
反应堆简介 编辑
二号机组为压水堆，1978年12月30日开始商业运行。额定电功率880MW。堆芯为177束燃料组件，共计37000根燃料棒，含二氧化铀100吨。二氧化铀的浓度为2.57%。
反应堆有两套回路。每套回路包含2台主泵、3台辅助泵（2台电动泵、1台汽动泵）、1台蒸汽产生器（SG）、1台布设在一次侧热段的调压槽。调压槽压力达到15.5MPa时，调压槽卸压阀自动开启，将冷却剂排放到调压槽卸压箱。
事故简述 编辑
当天凌晨4时0分0秒，三哩岛核电站95万千瓦压水式二号反应堆一次侧的给水主泵停转，汽轮机停机。此时备用泵应按照预设的程序启动，但是由于辅助给水系统中隔离阀在此前的例行检修中没有按规定打开，导致辅助给水系统没有动作。
二号机组的一次侧冷却水没有按照程序进入蒸汽产生器，热量在反应堆中心处持续聚集，堆芯压力上升，导致调压槽卸压阀于4时0分03秒开启，放出堆芯内的部分汽水混合物。当下反应堆于4时0分08秒自动停堆，当反应堆内压力下降至正常时，卸压阀又由于故障未能自动关闭，使堆芯冷却剂以45m3/s继续外流。压力降至正常值以下，却由于发生机械故障，在堆心压力回复正常值后堆芯冷却水继续注入减压水槽，造成减压水槽水满外溢；4时2分2秒主系统压力继续下降至11.3MPa，“堆芯紧急冷却系统(RCIC)”的高压注水自动启动，向堆芯注入冷却水。但反应堆操作员未判明卸压阀没有关闭，反而于4时3分13秒关闭应急堆芯冷却系统，停止向堆芯内注水。
一次侧冷却水大量排出造成堆芯上部失水，堆芯上部燃料棒的温度超过2760度，堆腔上部形成蒸汽。堆芯燃料棒因大量高温水蒸气开始像玻璃一样破裂，堆芯坍塌。堆芯90%的燃料棒包壳破损，47%的核燃料已经融毁并发生泄漏，系统发出放射性物质外漏的警报，但由于警报响起时并未引起运行人员的注意，甚至现时的纪录报告都指出没有人注意到警报。
事故细节 编辑
二回路停摆 编辑
1979年3月28日凌晨，三里岛二号机(TMI-2)输出达最大功率之97%，一号机处于停机状态。上午4时0分37秒(东部标准时间)，二号机二回路给水主泵停转，被认为是三里岛事故一系列连锁事件的起点。
时间倒退回十一小时以前，二回路的八组凝结水除矿器中，七号除矿器发生了堵塞问题。除矿器含有树脂滤芯，能够捕捉循环水的矿物质与杂质，降低二回路管线腐蚀速率，但也容易在水流通过一段时间后逐渐被压缩，最终产生堵塞。通常而言，对滤芯使用压缩空气便能排除此类堵塞。但在技工维修七号除矿器的过程中，压缩空气将有瑕疵的止回阀顶开，一部分水因此进入了送气管线。经过十一小时循环后，积水在四点抵达共同歧管，造成八组除矿器同时停机，同样在二回路上的给水主汞，压缩泵被迫停机。两秒后，二回路涡轮进入停转状态(turbine trip)。
泄压阀故障 编辑
随着二回路停止与一回路的热交换，一回路反应堆冷却液系统(Reactor Coolant System)温度迅速上升，冷却剂膨胀产生压力，并涌入与RCS相连的稳压器(pressurizer)。涡轮停转四秒后，压力来到2255 Psig (155.5 bar)，与一回路相连，稳压器顶端的引导式泄压阀(Pilot-operated relief valve, PORV)自动打开，将蒸气排入位于核反应堆安全壳地下室的冷却剂排水槽。
汽轮机停转八秒后，压力进一步来到2355 Psig (162.4 bar)，反应堆保护装置(Reactor Protection System)随之启动，借助重力插入所有控制棒，立即停止了链式反应。此时反应堆内持续产出相当于停机前百分之六的衰减热。尽管不多，但在一回路无法与二回路进行正常热交换的情况下，反应堆芯仍会持续累积热量。
紧急给水泵进水阀自动启动，试图向热交换器输水以移除衰减热。然而，两条紧急给水管线上的进水阀在两天前的例行检测中双双关闭，事后又没有开启，导致紧急系统毫无用武之地。这违反了美国核能管理委员会(NRC)所制订，在备援给水系统全数关闭时反应堆必须停机的规定，事后被NRC官员认定为重大违规。控制室内的操作员也未能于第一时间察觉进水阀误关问题。
汽轮机停转十五秒后，反应堆冷却液系统的压力随着PORV排出蒸气而下降，回到了2205 Psig (152 bar)，PORV重置点。此时PORV控制线圈自动断电，阀门理应关闭停止泄压。不料阀门此时却卡住了，蒸气持续溢出，导致一回路的冷却剂不断流失。另一方面，控制室面板的PORV指示灯设计错误，指示灯只是解释线圈通电与否，而非反映阀门实际位置。操作员因此错误地认为PORV没有卡住，浪费了数小时诊断其他问题。
关闭高压注水系统 编辑
在卡住的PORV形成冷却剂流失事故后，紧急堆芯冷却系统(Emergency Core Cooling System)自动介入，启用高压注水系统替炉芯补充冷却剂。另一方面，PORV不断泄出稳压器上方的蒸气，促使RCS压力下降，稳压器水位因替代上方蒸气而上升。
在操作员的训练中，冷却剂流失事故的症状被描述为压力水位双双下降，没有提及压力降水位升的可能性。反应堆操作员因此没有想到冷却剂流失的可能性，反而更加关注稳压器水位问题。水位过高将使调整槽失去压力缓冲能力，是操作指南中注明需要避免的状况。两分钟后，操作员判定水位已经够高，高压注水系统毋须继续向RCS注水，因此手动关闭了高压注水系统。
上午4时11分，PORV连结的冷却剂排水槽已被填满，冷却剂开始溢出，排入安全壳建筑的废液池并触发警报。此一警报与PORV管线异常高温(超过华氏200度)已是极为明显的冷却剂流失提示，现场操作员仍然未能发现。
堆芯熔毁 编辑
上午5时20分，随着一回路不断流失冷却剂，剩下的冷却剂开始随着压力下降，温度上升而沸腾，在回路内生成蒸气气泡。当蒸气通过主循环泵时，空穴现象产生连控制室都能感受到的震动。根据作业指南，主汞必须关闭以避免烧坏。操作员决定逐渐关闭四座主汞。由于调压槽水位够高，冷却剂被认为是足够的，高压注水系统仍然维持关闭状态。
上午5时44分，四座主汞全数关闭，一回路完全失去循环能力，蒸汽产生速度更加上升。
上午6时，随着炉内的反应堆燃料棒上半部开始暴露于越来越多的蒸汽中，燃料棒的锆合金外壳与水蒸气反应产生二氧化锆，氢气与热，使得外壳本身逐渐融化，露出里面的核燃料颗粒。控制室同时也开始换班，交接过程中，一位员工终于注意到了PORV管线的异常高温，关上了管线上的备用阀，至此已有超过32000加仑冷却剂流出一回路。
上午6时44分，核燃料颗粒飘逸出的同位素开始被厂房各处的辐射侦测器捕捉并发出警报，控制室终于发现正在发生的堆芯熔毁。
紧急状态 编辑
上午6时56分，厂区主管宣布厂区紧急状态。30分钟后，厂长Gary Miller宣布进入全面紧急状态。大都会爱迪生公司（Met Ed）通知宾州紧急状态管理局（PEMA）后，PEMA迅速通了地方政府，宾州州长索伯格（Richard L. Thornburgh）。索伯格旋即指派副州长史堪顿（RWilliam Scranton III）负责处理三里岛事故。
The uncertainty of operators at the plant was reflected in fragmentary, ambiguous, or contradictory statements made by Met Ed to government agencies and to the press, particularly about the possibility and severity of off-site radioactivity releases. Scranton held a press conference in which he was reassuring, yet confusing, about this possibility, stating that though there had been a "small release of radiation...no increase in normal radiation levels" had been detected. These were contradicted by another official, and by statements from Met Ed, who both claimed that no radioactivity had been released.[53] In fact, readings from instruments at the plant and off-site detectors had detected radioactivity releases, albeit at levels that were unlikely to threaten public health as long as they were temporary, and providing that containment of the then highly contaminated reactor was maintained.[54]
Angry that Met Ed had not informed them before conducting a steam venting from the plant, and convinced that the company was downplaying the severity of the accident, state officials turned to the NRC.[55] After receiving word of the accident from Met Ed, the NRC had activated its emergency response headquarters in Bethesda, Maryland and sent staff members to Three Mile Island. NRC chairman Joseph Hendrie and commissioner Victor Gilinsky[56] initially viewed the accident as a "cause for concern but not alarm".[57] Gilinsky briefed reporters and members of Congress on the situation and informed White House staff, and at 10:00 a.m. met with two other commissioners. However, the NRC faced the same problems in obtaining accurate information as the state, and was further hampered by being organizationally ill-prepared to deal with emergencies, as it lacked a clear command structure and did not have the authority either to tell the utility what to do or to order an evacuation of the local area.[58]
In a 2009 article, Gilinsky wrote that it took five weeks to learn that "the reactor operators had measured fuel temperatures near the melting point".[59] He further wrote: "We didn't learn for years—until the reactor vessel was physically opened—that by the time the plant operator called the NRC at about 8:00 a.m., roughly half of the uranium fuel had already melted."[59]
It was still not clear to the control room staff that the primary loop water levels were low and that over half of the core was exposed. A group of workers took manual readings from the thermocouples and obtained a sample of primary loop water. Seven hours into the emergency, new water was pumped into the primary loop and the backup relief valve was opened to reduce pressure so that the loop could be filled with water. After 16 hours the primary loop pumps were turned on once again, and the core temperature began to fall. A large part of the core had melted, and the system was still dangerously radioactive.[citation needed]
On the third day following the accident, a hydrogen bubble was discovered in the dome of the pressure vessel and became the focus of concern. A hydrogen explosion might not only breach the pressure vessel but, depending on its magnitude, might compromise the integrity of the containment vessel leading to a large-scale release of radioactive material. However, it was determined that there was no oxygen present in the pressure vessel, a prerequisite for hydrogen to burn or explode. Immediate steps were taken to reduce the hydrogen bubble and, by the following day, it was significantly smaller. Over the next week, steam and hydrogen were removed from the reactor using a catalytic recombiner and, controversially, by venting straight to the atmosphere.[citation needed]
事故后果 编辑
事故后，原子能管理委员会[12]对周围居民进行连续追踪研究，研究结果显示：
- 在以三哩岛核电站为圆心的50英里范围内，220万居民中无人发生急性辐射反应。
- 周围居民所受到的辐射相当于进行一次胸部X光照射的辐射剂量。
- 三哩岛核泄漏事故对于周围居民的癌症发生率没有显著性影响。
- 三哩岛附近未发现动植物异常现象。
- 当地农作物产量未发生异常变化。
事故影响 编辑
三里岛核泄漏事故是核能史上第一次反应堆堆芯融毁的事故，此事故的严重后果反映在经济上，公共安全及周围居民的健康上则没有不良影响。究其原因在于围阻体发挥重要作用，凸显其作为核电站最后一道安全防线的重要作用。在整个事件中，人员的操作错误和机械故障是主要的原因，因此核电站运行人员的培训、面对紧急事件的处理能力、控制系统的人性化设计等细节对核电站的安全运行有着重要影响。
这场事故恰巧发生在描述核电站安全问题的惊悚片《中国综合症》（China Symdrome）上映12天后，美国公众对核电信心大受影响，美国核电产业陷入长期的不景气，也因此核电站的兴建计划锐减，美国核电相关的公司因此流失了许多资深经验的工程师。虽然说此事故并没有证明西方国家的核电站事故会造成人畜伤亡及公共危害，但也大幅提高核电站安全设施的建造成本，以免事故造成重大的经济损失。但提高安全系数后的核电站造价昂贵，因此核电站兴建数量大减，直到21世纪初的化石燃料价格大涨及全球暖化效应显现后，各国才开始重启核能计划。
1986年切尔诺贝利核能电厂事故后，核能的安全性一度被许多人质疑，但由于该苏联电厂的设计隐患，加上安全设备远落后西方核电站，因此三里岛事故后的美国的核能安全形象并没有遭到严重影响，但是在2011年福岛第一核电站事故中，美国奇异制造的反应堆最终还是出了事故，确实使许多人对核能安全再度产生不信任。
事故分类 编辑
- 紧急戒备事故（Alert）：安全状况有显著劣化或有发生的可能，尚不须执行核子事故民众防护行动。
- 厂区紧急事故（Site Area Emergency）：安全功能重大失效或有发生的可能性，可能须执行核子事故民众防护行动。
- 全面紧急事故（General Emergency）：发生堆芯严重恶化或熔损，可能丧失围阻体完整性或有发生的可能，必须执行核子事故民众防护行动。
相关条目 编辑
注释 编辑
- ^ Nuclear Regulatory Commission – Backgrounder on the Three Mile Island Accident. [2009-03-28]. （原始内容存档于2015-12-06）.
- ^ Spiegelberg-Planer, Rejane. A Matter of Degree: A revised International Nuclear and Radiological Event Scale (INES) extends its reach.. IAEA.org. [March 19, 2011]. （原始内容存档于2011年1月5日）.
- ^ King, Laura; Kenji Hall and Mark Magnier. In Japan, workers struggling to hook up power to Fukushima reactor. Los Angeles Times. March 18, 2011 [March 19, 2011].
- ^ Gofman, John W.; Tamplin, Arthur R. Poisoned Power: The Case Against Nuclear Power Plants Before and After Three Mile Island Updated. Emmaus, PA: Rodale Press. December 1, 1979: xvii [October 1, 2013]. （原始内容存档于2019-06-15）.
(In 1979 Foreword:) "...we arrive at 333 fatal cancers or leukemias."
- ^ Hatch, Maureen C.; et al. Cancer near the Three Mile Island Nuclear Plant: Radiation Emissions. American Journal of Epidemiology. 1990, 132 (3): 397–412 [2019-05-09]. PMID 2389745. （原始内容存档于2013-06-08）.
- ^ Levin, R. J. Incidence of thyroid cancer in residents surrounding the Three-Mile Island nuclear facility. Laryngoscope. 2008, 118 (4): 618–628. PMID 18300710. doi:10.1097/MLG.0b013e3181613ad2.
Thyroid cancer incidence has not increased in Dauphin County, the county in which TMI is located. York County demonstrated a trend toward increasing thyroid cancer incidence beginning in 1995, approximately 15 years after the TMI accident. Lancaster County showed a significant increase in thyroid cancer incidence beginning in 1990. These findings, however, do not provide a causal link to the TMI accident.
- ^ Levin, R. J.; De Simone, N. F.; Slotkin, J. F.; Henson, B. L. Incidence of thyroid cancer surrounding Three Mile Island nuclear facility: the 30-year follow-up. Laryngoscope. August 2013, 123 (8): 2064–2071. PMID 23371046. doi:10.1002/lary.23953.
- ^ Han, Y Y.; Youk, A. O.; Sasser, H.; Talbott, E. O. Cancer incidence among residents of the Three Mile Island accident area: 1982–1995. Environ Res. November 2011, 111 (8): 1230–1235. Bibcode:2011ER....111.1230H. PMID 21855866. doi:10.1016/j.envres.2011.08.005.
- ^ Hatch, M.C.; Wallenstein, S.; Beyea, J.; Nieves, J. W.; Susser, M. Cancer rates after the Three Mile Island nuclear accident and proximity of residence to the plant. American Journal of Public Health. June 1991, 81 (6): 719–724. PMC 1405170
. PMID 2029040. doi:10.2105/AJPH.81.6.719.
- ^ Backgrounder on the Three Mile Island Accident: Health Effects. Nuclear Regulatory Commission. [January 13, 2018]. （原始内容存档于2019-05-13）.
The NRC conducted detailed studies of the accident's radiological consequences, as did the Environmental Protection Agency, the Department of Health, Education and Welfare (now Health and Human Services), the Department of Energy, and the Commonwealth of Pennsylvania. Several independent groups also conducted studies. The approximately 2 million people around TMI-2 during the accident are estimated to have received an average radiation dose of only about 1 millirem above the usual background dose. To put this into context, exposure from a chest X-ray is about 6 millirem and the area's natural radioactive background dose is about 100-125 millirem per year for the area. The accident's maximum dose to a person at the site boundary would have been less than 100 millirem above background. In the months following the accident, although questions were raised about possible adverse effects from radiation on human, animal, and plant life in the TMI area, none could be directly correlated to the accident. Thousands of environmental samples of air, water, milk, vegetation, soil, and foodstuffs were collected by various government agencies monitoring the area. Very low levels of radionuclides could be attributed to releases from the accident. Comprehensive investigations and assessments by several well respected organizations, such as Columbia University and the University of Pittsburgh, have concluded that in spite of serious damage to the reactor, the actual release had negligible effects on the physical health of individuals or the environment.
- ^ 14-Year Cleanup at Three Mile Island Concludes. The New York Times. August 15, 1993 [March 28, 2011]. （原始内容存档于2011-03-17）.
- ^ Fact Sheet on the Three Mile Island Accident (NRC). [2009-03-28]. （原始内容存档于2015-12-06）.
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