Nuclear energy accounts for 30% of the electricity generated in Japan.





Fukushima #1 (Daiichi) Plant

Unit	Type	First Criticality	Electric Power
Fukushima I – 1	BWR	March 26, 1971	460 MW
Fukushima I – 2	BWR	July 18, 1974	784 MW
Fukushima I – 3	BWR(MOX)	March 27, 1976	784 MW
Fukushima I – 4	BWR	October 12, 1978	784 MW
Fukushima I – 6	BWR	October 24, 1979	1,100 MW

9 = Turbine
13 = cooling system

From Wiki: After the March 11, 2011 earthquake, Nuclear Engineering International reported that units 1 to 3 were automatically shut down, and units 4 to 6 were already in maintenance outages.[2] Diesel generators installed to provide backup power for the cooling systems for units 1–3 were damaged by the tsunami;[3] they started up correctly but then stopped abruptly about 1 hour later.[4] Because cooling is needed to remove residual reactor heat, in Japan a nuclear emergency is declared upon cooling problems and therefore a nuclear emergency was declared—for the first time—when the diesel engines failed. Batteries, which last about eight hours, were being used to power the reactor controls and valves during the electrical outage.[5][6][7] Japanese ground forces were said to be trucking generators and batteries to the site.[8]

Past midnight local time, it was reported that The Tokyo Electric Power Company was considering venting hot gas from the reactor vessel number 1 into the atmosphere, which could result in the release of radiation.[13] The Tokyo Electric Company reported that radiation levels were rising in the turbine building for reactor 1.[14] At 2:00 JST, the pressure inside the reactor was reported to be 600kPa (6 bar or 87 psi), 200 kPa (2 bar or 29 psi) higher than under normal conditions.[4] At 5:30 JST the pressure inside Reactor 1 was reported to be 2.1 times the "design capacity."[15] At 6:10 JST, the IAEA reported that unit 2 was also experiencing cooling problems.[16]

To reduce mounting pressure potentially radioactive steam has been released from the primary circuit, into the secondary containment.[17] On March 12, 2011 at 6:40 JST, Chief Cabinet Secretary Yukio Edano stated that the amount of potential radiation would be small and that the prevailing winds are blowing out to sea.[18] Measured radiation levels inside the plant control room were reported to be 1000 times greater than normal.[19] Radiation levels measured at a monitoring post near the plant's main gate were reported to be more than eight times above normal.[20][21] In a press release at 7 AM (local) March 12, TEPCO stated "Measurement of radioactive material (Iodine, etc.) by monitoring car indicates increasing value compared to normal level. One of the monitoring posts is also indicating higher than normal level."[12]

不知道这个应急用的柴油发电机，当初设计时有没有考虑到会被海啸给废了？

现在说要加注海水和硼酸的办法来冷却中止核反应，却偏偏遭到又一个大的余震，真是祸不单行。

为什么用氢气做发电机的冷却剂？

Hydrogen is frequently the gas of choice for removing heat from high power generators. Hydrogen has a high heat capacity and, therefore, removes excess heat efficiently. Hydrogen also has a very low viscosity (or windage), thus allowing higher capacity operation of the generators while maintaining efficient cooling.

From Wiki:

The flammability limits (4-75% of hydrogen in air at normal temperature, wider at high temperatures[12]), its autoignition temperature at 571°C, its very low minimum ignition energy, and its tendency to form explosive mixtures with air, require provisions to be made for maintaining the hydrogen content within the generator above the upper or below the flammability limit at all times, and other hydrogen safety measures. When filled with hydrogen, overpressure has to be maintained as inlet of air into the generator could cause a dangerous explosion in confined space. The generator enclosure is purged before opening it for maintenance, and before refilling the generator with hydrogen. During shutdown, hydrogen is purged by an inert gas, then the inert gas is replaced by air; the opposite sequence is used before startup. Carbon dioxide or nitrogen can be used for this purpose, as they do not form combustible mixtures with hydrogen and are inexpensive. Gas purity sensors are used to indicate the end of the purging cycle, which shortens the startup and shutdown times and reduces consumption of the purging gas. Carbon dioxide is favored as due to very high density difference it is easily displaced by hydrogen.

Hydrogen is often produced on-site in electrolyzers, as this reduces the need for stored amount of compressed hydrogen and allows storage in lower pressure tanks, with associated safety benefits and lower costs. Some gaseous hydrogen has to be kept for refilling the generator but it can be also generated on-site.

As technology evolves no materials susceptible to hydrogen embrittlement are used in the generator design. Not adhering to this can lead to equipment failure.



沸水堆简介：Intro to BWR.pdf

核电站安全设计讲究多道设防“Defense-in-depth”，辐射性物质要从核燃料中逃到外界环境，需要经过很多重关卡。这次福岛核电站一号堆发生 氢气爆炸，并让铯137外泄，从目前网上可以搜寻到的信息看，整个事故链，最关键的一环，很有可能竟然是被人忽视的防波墙(seawall）。

地震来临，反应堆自动停堆，电网破坏，失去厂外电源。这个是Design Basis Accident，在设计考虑范围内。按照设计，紧急冷却系统（ECCS）启动，电源由柴油发动机提供。接下来，海啸来临，将柴油发动机毁坏。这个事故估 计在设计考虑范围之外——A Black Swan！此后，备用电池启动，通常只能持续4-8小时。此后，军方似乎提供了移动发电机和电池，未凑效。爆炸前，似乎又出现了一次泵的失效问题。亦有专 家怀疑，有可能在释放解压阀时解压速度太快，导致最后的爆炸。 


BWR的安全系统：

BWR的安全系统分主动与被动两类。被动系统从外到里有核安全壳(containment structure)，压力壳(Pressure vessel)，以及燃料包壳（fuel cladding）以及各种压力管道和设备所形成的被动压力边界（Safety Boundary)。 主动安全系统又分停堆系统和紧急堆芯冷却系统（ECCS）。停堆系统有控制棒（control rods）和堆芯喷芯系统。BWR 的四套ECCS系统：HPCI, ADS, LPCI, LPCS， 其中LPCI属于RHR的一部分功能。








如果containment structure比安全壳还坏得早，怎么办？有没有这种可能？如何评估？

>>>>>>>>>>>>>>>>>
update 03/23/2011
中国经济网北京3月24日讯（记者 李雨思 实习生 李方）
中国核电之父欧阳予揭秘真实核电：究竟安不安全欧阳予院士分析说，在这次日本大地震中， 福岛核电站的管道、设备受到强烈地震而断裂、破坏，致使给水送不进反应堆。再加上应急柴油机发电的供油管线也被海啸破坏，致使应急安全注射水的水泵断电而 不能运转,安全注射水注不进反应堆。反应堆堆芯燃料棒失掉冷却而烧坏，乃至熔化，从而发生严重事故。蒸汽管道破裂又使放射性核素碘－131和铯－137等 泄露出去。而堆芯中密封核燃料的锆包壳管，在温度超过400°C后产生锆-水反应，放出大量氢气，氢气泄漏到环境中燃烧，发生化学爆炸，致使某些厂房倒塌。

看来地震本身还是对设备产生的破坏！继续跟踪。


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