虾米护手:问与答：地震对日本核电站干了什么？

这是3月13日拍摄的日本福岛第一核电站1号（左）和3号机组（右）。明显可以看住1号机组的建筑顶部遭到爆炸损坏，露出钢架。

What kind of nuclear reactor is involved?

The reactors at the Fukushima Daiichi plant are boiling- water reactors. The reactor affected by the explosion is Fukushima Daiichi 1. It was connected to the grid in November 1970, making it about 40 years old. There are six reactors at the Fukushima Daiichi site, of which unit 1 is the oldest, according to the World Nuclear Association.

How does a nuclear reactor of this kind work?

Uranium 235 -- the fuel inside a nuclear reactor -- undergoes nuclear fission. This process emits a lot of heat energy, which produces steam, and that steam turns a turbine, generating electricity.

铀235——反应堆中的燃料——经过核裂变。这一过程释放出大量的热能，热能产生蒸汽，蒸汽驱动涡轮发电。

3月13日，在日本郡山的一个紧急避难所，身着防护服的工作人员检查从福岛第二核电站附近撤离的民众是否遭辐射影响。

What happened to the nuclear reactors during the quake?

Three of the six reactors at the site were in operation when the earthquake hit. The reactors are designed to shut down automatically when a quake strikes, and emergency diesel generators began the task of pumping water around the reactors to cool them down. However, these stopped about an hour later. The failure of the back-up generators has been blamed on tsunami flooding by the International Atomic Energy Agency (IAEA).

On Sunday an official with Japan's Nuclear and Industrial Safety Agency said a meltdown may be under way at one of Fukushima Daiichi's nuclear power reactors.

Described by one analyst as a "Hail Mary pass", Tokyo Electric Power Company began injecting sea water mixed with boron into the primary containment vessel in an operation that got under way Saturday night, IAEA said.

Boron is an element with the ability to absorb neutrons, the sub-atomic particles that occur in the nucleus of all atoms. In a nuclear reactor, it is essential that just the right number of neutrons are present. Too many neutrons can cause a fission reaction to get out of control. Too few neutrons and a fission reaction stops.

What could have caused the explosion at the plant?

The blast was not caused by damage to the nuclear reactor but by a pumping system that failed as crews tried to bring the reactor's temperature down, Chief Cabinet Secretary Yukio Edano said Saturday. 

He said the plant operator confirmed there was no damage to the steel container housing the reactor.

Malcolm Grimston, Associate Fellow for Energy, Environment and Development at London's Chatham House, said he believed the explosion had been caused by a build-up of pressure inside the inner containment of the reactor.

"Because they lost power to the water cooling system, they needed to vent the pressure that's building up inside.

"My suspicion is that as the temperature inside the reactor was rising, some of the metal cans that surround the fuel may have burst and at high temperature, that fuel cladding can react with water to produce zirconium oxide and hydrogen.

"That hydrogen then will be part of the gases that need to be vented. That hydrogen then mixes with the surrounding air. Hydrogen and oxygen can then recombine explosively.

"So it seems while the explosion wasn't directly connected with the nuclear processes, it was indirectly connected, because the hydrogen was only present because of what was going on in the reactor core."

What is a meltdown, and can it be avoided here?

Japan's nuclear agency said there was a strong possibility that radioactive cesium detected at the plant after the blast was from the melting of a fuel rod.

Robert Alvarez, Senior Scholar at the U.S. Institute for Policy Studies, explained that a meltdown could happen when the water surrounding the core of the reactor boiled or leaked away, leaving the fuel rods exposed, allowing temperatures to rise to up to 5,000 degrees Fahrenheit.

"The radiation is so intense it's impossible to deal with it. The control room would be uninhabitable," he said. "Without cooling, cladding surrounding the fuel can ignite, and the fuel itself start to melt.

"Then you have a huge amount of radioactive gases and particles, and if the primary and secondary containment fails, you have a large amount of radioactive gases escaping into the environment."

Whether a meltdown happens in this case depends on whether the pumping and cooling system can be restored in time, and whether if a meltdown starts, the secondary containment is strong enough to stay intact, according to Alvarez.

"If the pumping system is down there won't be enough pressure or water inside to cool the fuel rods down," he said.

Did something similar happen at Three Mile Island?

According to Grimston, poor instrumentation was to blame for the accident and partial core meltdown at Three Mile Island, Pennsylvania, in 1979. In that case, poor control room indicators led plant officials to switch off the circuit which would have cooled the reactor core. As a result the core was left uncovered and 30 to 40% of the core melted.

"Much was learned from this accident, with much done with the instrumentation to reduce the extent to which operators can intervene in the automatic safety system," he said.

“从这场事故中我们学到了很多东西，对仪表进行了多次修改，一定程度上避免了操作人员干涉自动安全系统”，他说。

---------------------------------

新闻背景：福岛核电站

　　新华社北京3月12日电 位于日本福岛县的核电站有福岛第一核电站和福岛第二核电站，它们都由东京电力公司负责运营。

　　两座核电站的反应堆均为以普通水作为冷却剂和中子减速剂的沸水反应堆。与压水反应堆相比，这种反应堆构造更为简单，安全性更高。本次大地震之前，福岛第一和第二核电站共有7个反应堆处于运转状态，地震发生后全部自动关闭。

　　目前，第一核电站1号机组周边检测出放射性物质铯和碘，已确认反应堆堆芯燃料的一部分熔化。反应堆容器内的蒸汽已被释放，容器内的气压已经下降。第二核电站1号、2号和4号机组也都丧失了冷却功能。日本政府已就两座核电站发布了“原子能紧急事态宣言”,并要求福岛第一核电站周围的居民撤退到距核电站半径10公里之外。

　　福岛第一和第二核电站此前也多次发生事故。其中福岛第一核电站1978年曾经发生临界事故，但是事故一直被隐瞒至2007年才公诸于众。

　　2006年，福岛第一核电站6号机组曾发生放射性物质泄漏事故。

　　2007年，东京电力公司承认，从1977年起在对下属3家核电站总计199次定期检查中，这家公司曾窜改数据，隐瞒安全隐患。其中，福岛第一核电站1号机组反应堆主蒸汽管流量计测得的数据曾在1979年至1998年间先后28次被窜改。（完）