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はてなキーワード: chain reactionとは
飛田展男氏の声で、緩急（躁鬱）つけて読んでください。所々、大幅に意訳してます。一人称が僕なのも、カミーユ風ということで……。
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……伝えたいのは、あなたがたを見てるってことです。
こんなのは、絶対に間違ってるんだ……。僕はね、こんなところに立ってる人間じゃあないんです。本当は海の反対側で学校に戻っているべきなんですよ。それなのにあなたがたは、僕のような若者のところに、希望なんてものを求めてやってくる。よくもそんなこと……！
お前たちが、繰り言を弄して僕の夢や、子ども時代を奪い去ったんだ！ それだけじゃない、僕なんて運が良い方なんだ！ たくさんの人が苦しみ、死にかけて……生態系全体が崩壊しかけてるんだぞ！ 僕たちを絶滅のふちに追い込んでおきながら、それなのに話すのはカネのこと！ 永遠の経済成長だとか、おとぎ話じゃあないんだぞ！ よくも！
これまで三十年以上、科学はこれ以上ないぐらい明瞭だったんだ。必要な政策だって解決策だって、どこにもないんですよ！ それに目を背けたままノコノコとここに来て「十分やっている」だなんて、どうして言えるんだよ！
……僕らの声を聞いて、あなたがたは緊急性を理解したと言ってみせる。悲しいですよ。腹も立ちますよ。でもね、僕にはやっぱり信じられないんだ。だってそうでしょう、もしあなたがたが状況を理解していたとして、それでも何もしないなら、それは悪だ。悪人の言うこと、信じられるわけないでしょうが……！
10年で温室効果ガスの排出を半分にしても、気温上昇を1.5度に抑えられる可能性は5割しかない。それが定説なんですよ。人の手に負えない連鎖反応が起こって、環境が暴走するリスクだってある。
なのにあなたがたは、5割の勝率で十分だというんでしょう。でもね、この数字は、暴走が始まる一線も、変化を加速させるフィードバックループも、大気汚染による隠れ温暖化も考えに入れちゃあいない。公平性だってなければ、正義すらないんだ。なのに、まともに存在すらしない技術で、僕たちの世代がなんとかしてくれると当てにして！ 何千億トンもの二酸化炭素をバラまいてるのは、お前らなんだぞ！
5割の勝率だなんて、受け入れられるわけないんだよ！ 結果を抱えて生きてかなきゃなんないのは、僕たちなんだぞ！
この惑星（ほし）の気温上昇を1.5度に抑える確率を67％にするには、今後のCO2の排出量をトータルで4,200億トン以下にしなくちゃならない。これが、2018年1月1日時点で、IPCCが出したベストの数字です。いまはね、3,500億トン以下なんですよ。
それなのに、今まで通りのやり方と技術で、何とかできるだなんて、どうかしてるだろ？！ 現状の排出レベルじゃあ、あと8年半で限界が来るってわかってるのに！
いまこの数字に基づいた解決策なんて、どこにもありはしない。計画だってない。この数字がね、都合が悪すぎるからなんですよ。お前ら、ありのままを語る勇気だってないじゃないか！
失望させないでほしい。そう思います。でもね、若い人たちは分かり始めているんです。あなたがたの裏切りに。未来の世代の全員の目が、あなたがたを見てるんです。だから、もし判断を誤って、失望させたのなら、僕たちは許しませんよ、絶対に。
この問題から逃げるだなんて、そんなことは絶対にさせない。いま、ここで、やり直さなきゃならないんだ……。世界の覚醒が見えるんです。否応なしに、変化は、来る……。
"My message is that we'll be watching you.
"This is all wrong. I shouldn't be up here. I should be back in school on the other side of the ocean. Yet you all come to us young people for hope. How dare you!
"You have stolen my dreams and my childhood with your empty words. And yet I'm one of the lucky ones. People are suffering. People are dying. Entire ecosystems are collapsing. We are in the beginning of a mass extinction, and all you can talk about is money and fairy tales of eternal economic growth. How dare you!
"For more than 30 years, the science has been crystal clear. How dare you continue to look away and come here saying that you're doing enough, when the politics and solutions needed are still nowhere in sight.
"You say you hear us and that you understand the urgency. But no matter how sad and angry I am, I do not want to believe that. Because if you really understood the situation and still kept on failing to act, then you would be evil. And that I refuse to believe.
"The popular idea of cutting our emissions in half in 10 years only gives us a 50% chance of staying below 1.5 degrees [Celsius], and the risk of setting off irreversible chain reactions beyond human control.
"Fifty percent may be acceptable to you. But those numbers do not include tipping points, most feedback loops, additional warming hidden by toxic air pollution or the aspects of equity and climate justice. They also rely on my generation sucking hundreds of billions of tons of your CO2 out of the air with technologies that barely exist.
"So a 50% risk is simply not acceptable to us — we who have to live with the consequences.
"To have a 67% chance of staying below a 1.5 degrees global temperature rise – the best odds given by the [Intergovernmental Panel on Climate Change] – the world had 420 gigatons of CO2 left to emit back on Jan. 1st, 2018. Today that figure is already down to less than 350 gigatons.
"How dare you pretend that this can be solved with just 'business as usual' and some technical solutions? With today's emissions levels, that remaining CO2 budget will be entirely gone within less than 8 1/2 years.
"There will not be any solutions or plans presented in line with these figures here today, because these numbers are too uncomfortable. And you are still not mature enough to tell it like it is.
"You are failing us. But the young people are starting to understand your betrayal. The eyes of all future generations are upon you. And if you choose to fail us, I say: We will never forgive you.
"We will not let you get away with this. Right here, right now is where we draw the line. The world is waking up. And change is coming, whether you like it or not.
"Thank you."
新型コロナウイルス(COVID-19)の流行で頻出するようになった言葉。
日本語の生成が追い付いてない。
たとえ訳語がなくても呼びやすい略語があれば良いが、それもない。
例えば「PCR検査」を毎度毎度「ポリメラーゼ連鎖反応検査」とか“polymerase chain reaction test”とか表記したり発話してたら面倒臭いじゃん。
ソーシャルディスタンス、「ソーディス」はなんか語呂が悪いし「SD」では伝わりづらい。
「密です。」が近い使われ方だが「ソーシャルディスタンス」と1対1では置き換えられない。
エッセンシャルワーカーもそう。「エッワー」では語呂悪いし「EW」ではわかんない。
日本語だと「医療従事者、介護や保育の仕事にかかわる人、宅配業者、スーパーの従業員、公共交通機関で働く人、ゴミ収集業者など
市民の生命と財産を守る為、社会を支える必要不可欠な仕事に従事している人達」と長ったらしすぎる。
PCRってなんの略だ？「核酸増幅法」の事らしい。正確にはPCRと言うのはPolymerase Chain Reactionで核酸を増幅させるやり方らしい。なるほど分からん。核酸ってなんだ。増幅させたからなんになるんだ。
https://www.mhlw.go.jp/stf/seisakunitsuite/bunya/kenkou_iryou/dengue_fever_qa_00004.html
https://www.bs.jrc.or.jp/hkd/hokkaido/process/m3_01_01_02_00000138.html
PCRは難しい事を言ってると言うことだけが分かった。メカニズムも問題点も検査手順も何も分からない。
そもそもPCRが核酸を増幅させるやり方の事なら別の方法じゃいけないのか。核酸を増幅させなきゃいけないのか。そんな事も分からない。
世間では全自動検査機が騒がれているが、手順が分からないから何が全自動なのか分からない。うちの全自動システムもエクセルファイルの体裁を整えてやらないと全自動にしてくれない。いつも全部は自動じゃないじゃんと思う。
小耳に聞いた話では生物系でもPCR検査をするらしい。医療系のPCRと何が違うんだろうか。もしかして一緒なんだろうか。そもそもPCR検査だけで済むのか。うちの「全（然）自動（じゃない）システム」も結局体裁の整え方のマニュアルを作らなきゃいけなかった。
騒いでる人はPCRについてちゃんと分かってるんだろうか。それともうちの全自動（）システムを導入した時の上司みたいに「これで一瞬で終わるな！ハッハッハ」って感じなんだろうか。導入後の上司は「これじゃ全然自動じゃないシステムじゃないか…ハッハッハ…」って言ってお客さんにフォーマット統一してくれるようにかけあってくれてたので、すごくいい上司だと思う。でも全自動システムはまだ全自動じゃない。
グレタ・トゥーンベリ氏のU.N. Climate Action Summit 2019におけるスピーチが話題になってるわね。
ブコメとかに言いたいこともあるけれど、それよりNHKの全訳(https://www3.nhk.or.jp/news/html/20190924/k10012095931000.html)がなんかしっくりこないので、自分で訳してみたわ。
ごめん嘘。バズると思って訳し始めたらNHKの方がずっと早かったの(よくもそんなことができますね！)。せっかくだから書き上げたわ。でもしっくりこないのは本当よ。
私のメッセージ(※1)はこうです。”私たちはあなた方を見ています”
My message is that we'll be watching you.
これは何もかも間違っています。私はここにいるべきではありません。私は海の向こうの学校に帰るべきです。しかし、あなた方はみんな、私たち若者に希望を求めてやってきます。よくもそんなことができますね！
This is all wrong. I shouldn't be up here. I should be back in school on the other side of the ocean. Yet you all come to us young people for hope. How dare you!
あなた方は私の夢を、私の子供時代を、その空虚な言葉によって奪い去りました。それでも私は幸運な方です。人々は傷ついています。人々は死んでいます。生態系は完膚なきまでに崩壊しつつあります。我々は大量絶滅の始まりにいるのです。なのにあなた方が話すことと言えばお金のことや永遠に続く経済成長というおとぎ話ばかり。よくもそんなことが言えますね！
You have stolen my dreams and my childhood with your empty words. And yet I'm one of the lucky ones. People are suffering. People are dying. Entire ecosystems are collapsing. We are in the beginning of a mass extinction. And all you can talk about is money and fairy tales of eternal economic growth. How dare you!
過去30年以上、科学的事実はずっと明確なままでした。よくも目をそらし続けられたものですね。よくもここに来て、”私は十分にやった”などと言えたものですね。必要な政策も解決策もいまだに見えてこないというのに。
For more than 30 years, the science has been crystal clear. How dare you continue to look away and come here saying that you're doing enough, when the politics and solutions needed are still nowhere in sight.
あなた方は言います。私たちの言葉を聞いていると。緊急性は理解していると。しかし、どれだけ私が悲しみ、怒っていたとしても、私はその言葉を信じたくはありません。なぜならば、もしもあなた方が本当に状況を理解しており、それにもかかわらず行動を起こしていないとすれば、あなた方は邪悪な人々ということになってしまうからです。だからこそ、私はそう信じることを拒絶します。
You say you hear us and that you understand the urgency. But no matter how sad and angry I am, I do not want to believe that. Because if you really understood the situation and still kept on failing to act, then you would be evil. And that I refuse to believe.
一般的な考えとして、世界の(二酸化炭素)排出量を10年間で半分にするというものがあります。これによって気温上昇を1.5℃に抑えられる確率はわずか50％に過ぎず、人類の手に負えない不可逆的な連鎖反応が始まるリスクは依然としてあります。
The popular idea of cutting our emissions in half in 10 years only gives us a 50% chance of staying below 1.5 degrees [Celsius], and the risk of setting off irreversible chain reactions beyond human control.
50％という数字はあなた方にとっては許容できるものかもしれません。しかし、この数字は転換点(※2)の存在や、多くのフィードバックループ、大気汚染に隠れたさらなる温暖化、公平性や気候正義(※3)の観点を含んでいません。それらはまた私たちの世代が、あなた方の出した数千億トンの二酸化炭素を、ほとんど実現すらしていない技術で以て大気から取り除くことをあてにしているのです。
Fifty percent may be acceptable to you. But those numbers do not include tipping points, most feedback loops, additional warming hidden by toxic air pollution or the aspects of equity and climate justice. They also rely on my generation sucking hundreds of billions of tons of your CO2 out of the air with technologies that barely exist.
50％のリスクは私たちにとって到底受け入れられるものではないのです。私たちはその結果と共に生きていかなければならないのですから。
So a 50% risk is simply not acceptable to us — we who have to live with the consequences.
[気候変動に関する政府間パネル]による最も分のよい試算では、67％の確率で気温上昇を1.5℃以下にするために、世界全体で許される二酸化炭素排出量は2018年1月1日以降で420ギガトンまでです。今日(2019年9月24日)、既にその数字は350ギガトンを割っています。
To have a 67% chance of staying below a 1.5 degrees global temperature rise – the best odds given by the [Intergovernmental Panel on Climate Change] – the world had 420 gigatons of CO2 left to emit back on Jan. 1st, 2018. Today that figure is already down to less than 350 gigatons.
よくも”今まで通りのやり方”や何かしらの技術で解決できるなどと嘯けますね。今の排出レベルでは、残りの350ギガトンの猶予も8年半以内で使い切ることになります。
How dare you pretend that this can be solved with just 'business as usual' and some technical solutions? With today's emissions levels, that remaining CO2 budget will be entirely gone within less than 8 1/2 years.
今日ここにいたるまで、これらのデータに沿った解決法も計画もまったくありません。なぜなら、これらの数字は非常に不愉快であり、あなた方はそのことをありのままに伝えられるほど成熟していないからです。
There will not be any solutions or plans presented in line with these figures here today, because these numbers are too uncomfortable. And you are still not mature enough to tell it like it is.
あなた方は私たちを裏切り続けています。そして若者たちはあなた方の裏切りに気付き始めています。未来の世代の目はすべて、あなた方に注がれています。そして、もしあなた方が私たちを裏切ることを選ぶのなら、私は言います。”私たちはあなた方を許さないでしょう”
You are failing us. But the young people are starting to understand your betrayal. The eyes of all future generations are upon you. And if you choose to fail us, I say: We will never forgive you.
私たちはあなた方を逃がしません。今この場所、今この時から、私たちは線を引きます。世界は目覚めつつあります。そして、あなた方の好むと好まざるとにかかわらず、変化もまた訪れるのです。
We will not let you get away with this. Right here, right now is where we draw the line. The world is waking up. And change is coming, whether you like it or not.
ありがとうございました。
Thank you.
※1 これは「世界の指導者たちへ向けてのメッセージはありますか？」という司会からの質問を受けて始まるスピーチなので、話の相手方は大人たち全般ではないことに注意が必要よ。
※2 Tipping point：NHKの方にも注釈があるけれど、気候変動が急転するポイントのことよ。具体的には正のフィードバックループが止められなくなったりする気温だったりするわね。例えば、気温が上がれば水蒸気が増える、水蒸気が増えれば温室効果がアップ、そしてさらに……、といった具合。演説中で触れられていた1.5℃の温度上昇もtipping pointの一つよ。
※3 Climate justice：「先進国が出した二酸化炭素のせいで温暖化してるのに、途上国にしわ寄せが来すぎるのはおかしいだろう。」的な話。先進国(や富裕層)は途上国(や貧困層)に対して温暖化被害の点で責任があるし、対策はそれを踏まえて両者に公平な形で進められるべきであるという考え方よ。多少人権周りの話も絡んでくるので詳細はもっと複雑ね。こっちにも注釈付けた方が良かったんでないのNHKさん。
When the diesel generators were gone, the reactor operators switched to emergency battery power. The batteries were designed as one of the backups to the backups, to provide power for cooling the core for 8 hours. And they did.
Within the 8 hours, another power source had to be found and connected to the power plant. The power grid was down due to the earthquake. The diesel generators were destroyed by the tsunami. So mobile diesel generators were trucked in.
This is where things started to go seriously wrong. The external power generators could not be connected to the power plant (the plugs did not fit). So after the batteries ran out, the residual heat could not be carried away any more.
At this point the plant operators begin to follow emergency procedures that are in place for a “loss of cooling event”. It is again a step along the “Depth of Defense” lines. The power to the cooling systems should never have failed completely, but it did, so they “retreat” to the next line of defense. All of this, however shocking it seems to us, is part of the day-to-day training you go through as an operator, right through to managing a core meltdown.
It was at this stage that people started to talk about core meltdown. Because at the end of the day, if cooling cannot be restored, the core will eventually melt (after hours or days), and the last line of defense, the core catcher and third containment, would come into play.
But the goal at this stage was to manage the core while it was heating up, and ensure that the first containment (the Zircaloy tubes that contains the nuclear fuel), as well as the second containment (our pressure cooker) remain intact and operational for as long as possible, to give the engineers time to fix the cooling systems.
Because cooling the core is such a big deal, the reactor has a number of cooling systems, each in multiple versions (the reactor water cleanup system, the decay heat removal, the reactor core isolating cooling, the standby liquid cooling system, and the emergency core cooling system). Which one failed when or did not fail is not clear at this point in time.
So imagine our pressure cooker on the stove, heat on low, but on. The operators use whatever cooling system capacity they have to get rid of as much heat as possible, but the pressure starts building up. The priority now is to maintain integrity of the first containment (keep temperature of the fuel rods below 2200°C), as well as the second containment, the pressure cooker. In order to maintain integrity of the pressure cooker (the second containment), the pressure has to be released from time to time. Because the ability to do that in an emergency is so important, the reactor has 11 pressure release valves. The operators now started venting steam from time to time to control the pressure. The temperature at this stage was about 550°C.
This is when the reports about “radiation leakage” starting coming in. I believe I explained above why venting the steam is theoretically the same as releasing radiation into the environment, but why it was and is not dangerous. The radioactive nitrogen as well as the noble gases do not pose a threat to human health.
At some stage during this venting, the explosion occurred. The explosion took place outside of the third containment (our “last line of defense”), and the reactor building. Remember that the reactor building has no function in keeping the radioactivity contained. It is not entirely clear yet what has happened, but this is the likely scenario: The operators decided to vent the steam from the pressure vessel not directly into the environment, but into the space between the third containment and the reactor building (to give the radioactivity in the steam more time to subside). The problem is that at the high temperatures that the core had reached at this stage, water molecules can “disassociate” into oxygen and hydrogen – an explosive mixture. And it did explode, outside the third containment, damaging the reactor building around. It was that sort of explosion, but inside the pressure vessel (because it was badly designed and not managed properly by the operators) that lead to the explosion of Chernobyl. This was never a risk at Fukushima. The problem of hydrogen-oxygen formation is one of the biggies when you design a power plant (if you are not Soviet, that is), so the reactor is build and operated in a way it cannot happen inside the containment. It happened outside, which was not intended but a possible scenario and OK, because it did not pose a risk for the containment.
So the pressure was under control, as steam was vented. Now, if you keep boiling your pot, the problem is that the water level will keep falling and falling. The core is covered by several meters of water in order to allow for some time to pass (hours, days) before it gets exposed. Once the rods start to be exposed at the top, the exposed parts will reach the critical temperature of 2200 °C after about 45 minutes. This is when the first containment, the Zircaloy tube, would fail.
And this started to happen. The cooling could not be restored before there was some (very limited, but still) damage to the casing of some of the fuel. The nuclear material itself was still intact, but the surrounding Zircaloy shell had started melting. What happened now is that some of the byproducts of the uranium decay – radioactive Cesium and Iodine – started to mix with the steam. The big problem, uranium, was still under control, because the uranium oxide rods were good until 3000 °C. It is confirmed that a very small amount of Cesium and Iodine was measured in the steam that was released into the atmosphere.
It seems this was the “go signal” for a major plan B. The small amounts of Cesium that were measured told the operators that the first containment on one of the rods somewhere was about to give. The Plan A had been to restore one of the regular cooling systems to the core. Why that failed is unclear. One plausible explanation is that the tsunami also took away / polluted all the clean water needed for the regular cooling systems.
The water used in the cooling system is very clean, demineralized (like distilled) water. The reason to use pure water is the above mentioned activation by the neutrons from the Uranium: Pure water does not get activated much, so stays practically radioactive-free. Dirt or salt in the water will absorb the neutrons quicker, becoming more radioactive. This has no effect whatsoever on the core – it does not care what it is cooled by. But it makes life more difficult for the operators and mechanics when they have to deal with activated (i.e. slightly radioactive) water.
But Plan A had failed – cooling systems down or additional clean water unavailable – so Plan B came into effect. This is what it looks like happened:
In order to prevent a core meltdown, the operators started to use sea water to cool the core. I am not quite sure if they flooded our pressure cooker with it (the second containment), or if they flooded the third containment, immersing the pressure cooker. But that is not relevant for us.
The point is that the nuclear fuel has now been cooled down. Because the chain reaction has been stopped a long time ago, there is only very little residual heat being produced now. The large amount of cooling water that has been used is sufficient to take up that heat. Because it is a lot of water, the core does not produce sufficient heat any more to produce any significant pressure. Also, boric acid has been added to the seawater. Boric acid is “liquid control rod”. Whatever decay is still going on, the Boron will capture the neutrons and further speed up the cooling down of the core.
The plant came close to a core meltdown. Here is the worst-case scenario that was avoided: If the seawater could not have been used for treatment, the operators would have continued to vent the water steam to avoid pressure buildup. The third containment would then have been completely sealed to allow the core meltdown to happen without releasing radioactive material. After the meltdown, there would have been a waiting period for the intermediate radioactive materials to decay inside the reactor, and all radioactive particles to settle on a surface inside the containment. The cooling system would have been restored eventually, and the molten core cooled to a manageable temperature. The containment would have been cleaned up on the inside. Then a messy job of removing the molten core from the containment would have begun, packing the (now solid again) fuel bit by bit into transportation containers to be shipped to processing plants. Depending on the damage, the block of the plant would then either be repaired or dismantled.
Now, where does that leave us?
・The plant is safe now and will stay safe.
・Japan is looking at an INES Level 4 Accident: Nuclear accident with local consequences. That is bad for the company that owns the plant, but not for anyone else.
・Some radiation was released when the pressure vessel was vented. All radioactive isotopes from the activated steam have gone (decayed). A very small amount of Cesium was released, as well as Iodine. If you were sitting on top of the plants’ chimney when they were venting, you should probably give up smoking to return to your former life expectancy. The Cesium and Iodine isotopes were carried out to the sea and will never be seen again.
・There was some limited damage to the first containment. That means that some amounts of radioactive Cesium and Iodine will also be released into the cooling water, but no Uranium or other nasty stuff (the Uranium oxide does not “dissolve” in the water). There are facilities for treating the cooling water inside the third containment. The radioactive Cesium and Iodine will be removed there and eventually stored as radioactive waste in terminal storage.
・The seawater used as cooling water will be activated to some degree. Because the control rods are fully inserted, the Uranium chain reaction is not happening. That means the “main” nuclear reaction is not happening, thus not contributing to the activation. The intermediate radioactive materials (Cesium and Iodine) are also almost gone at this stage, because the Uranium decay was stopped a long time ago. This further reduces the activation. The bottom line is that there will be some low level of activation of the seawater, which will also be removed by the treatment facilities.
・The seawater will then be replaced over time with the “normal” cooling water
・The reactor core will then be dismantled and transported to a processing facility, just like during a regular fuel change.
・Fuel rods and the entire plant will be checked for potential damage. This will take about 4-5 years.
・The safety systems on all Japanese plants will be upgraded to withstand a 9.0 earthquake and tsunami (or worse)
・I believe the most significant problem will be a prolonged power shortage. About half of Japan’s nuclear reactors will probably have to be inspected, reducing the nation’s power generating capacity by 15%. This will probably be covered by running gas power plants that are usually only used for peak loads to cover some of the base load as well. That will increase your electricity bill, as well as lead to potential power shortages during peak demand, in Japan.
If you want to stay informed, please forget the usual media outlets and consult the following websites:
http://www.world-nuclear-news.org/RS_Battle_to_stabilise_earthquake_reactors_1203111.html
http://bravenewclimate.com/2011/03/12/japan-nuclear-earthquake/
http://ansnuclearcafe.org/2011/03/11/media-updates-on-nuclear-power-stations-in-japan/
結論：大丈夫。
MvK2010
I'm going to copy paste a full blog post of a research scientist at MIT here, who explains the situation at Fukushima much better than anyone else has, his message: no worries.
This post is by Dr Josef Oehmen, a research scientist at MIT, in Boston.
He is a PhD Scientist, whose father has extensive experience in Germany’s nuclear industry. I asked him to write this information to my family in Australia, who were being made sick with worry by the media reports coming from Japan. I am republishing it with his permission.
It is a few hours old, so if any information is out of date, blame me for the delay in getting it published.
This is his text in full and unedited. It is very long, so get comfy.
I am writing this text (Mar 12) to give you some peace of mind regarding some of the troubles in Japan, that is the safety of Japan’s nuclear reactors. Up front, the situation is serious, but under control. And this text is long! But you will know more about nuclear power plants after reading it than all journalists on this planet put together.
There was and will *not* be any significant release of radioactivity.
By “significant” I mean a level of radiation of more than what you would receive on – say – a long distance flight, or drinking a glass of beer that comes from certain areas with high levels of natural background radiation.
I have been reading every news release on the incident since the earthquake. There has not been one single (!) report that was accurate and free of errors (and part of that problem is also a weakness in the Japanese crisis communication). By “not free of errors” I do not refer to tendentious anti-nuclear journalism – that is quite normal these days. By “not free of errors” I mean blatant errors regarding physics and natural law, as well as gross misinterpretation of facts, due to an obvious lack of fundamental and basic understanding of the way nuclear reactors are build and operated. I have read a 3 page report on CNN where every single paragraph contained an error.
We will have to cover some fundamentals, before we get into what is going on.
Construction of the Fukushima nuclear power plants
The plants at Fukushima are so called Boiling Water Reactors, or BWR for short. Boiling Water Reactors are similar to a pressure cooker. The nuclear fuel heats water, the water boils and creates steam, the steam then drives turbines that create the electricity, and the steam is then cooled and condensed back to water, and the water send back to be heated by the nuclear fuel. The pressure cooker operates at about 250 °C.
The nuclear fuel is uranium oxide. Uranium oxide is a ceramic with a very high melting point of about 3000 °C. The fuel is manufactured in pellets (think little cylinders the size of Lego bricks). Those pieces are then put into a long tube made of Zircaloy with a melting point of 2200 °C, and sealed tight. The assembly is called a fuel rod. These fuel rods are then put together to form larger packages, and a number of these packages are then put into the reactor. All these packages together are referred to as “the core”.
The Zircaloy casing is the first containment. It separates the radioactive fuel from the rest of the world.
The core is then placed in the “pressure vessels”. That is the pressure cooker we talked about before. The pressure vessels is the second containment. This is one sturdy piece of a pot, designed to safely contain the core for temperatures several hundred °C. That covers the scenarios where cooling can be restored at some point.
The entire “hardware” of the nuclear reactor – the pressure vessel and all pipes, pumps, coolant (water) reserves, are then encased in the third containment. The third containment is a hermetically (air tight) sealed, very thick bubble of the strongest steel. The third containment is designed, built and tested for one single purpose: To contain, indefinitely, a complete core meltdown. For that purpose, a large and thick concrete basin is cast under the pressure vessel (the second containment), which is filled with graphite, all inside the third containment. This is the so-called “core catcher”. If the core melts and the pressure vessel bursts (and eventually melts), it will catch the molten fuel and everything else. It is built in such a way that the nuclear fuel will be spread out, so it can cool down.
This third containment is then surrounded by the reactor building. The reactor building is an outer shell that is supposed to keep the weather out, but nothing in. (this is the part that was damaged in the explosion, but more to that later).
Fundamentals of nuclear reactions
The uranium fuel generates heat by nuclear fission. Big uranium atoms are split into smaller atoms. That generates heat plus neutrons (one of the particles that forms an atom). When the neutron hits another uranium atom, that splits, generating more neutrons and so on. That is called the nuclear chain reaction.
Now, just packing a lot of fuel rods next to each other would quickly lead to overheating and after about 45 minutes to a melting of the fuel rods. It is worth mentioning at this point that the nuclear fuel in a reactor can *never* cause a nuclear explosion the type of a nuclear bomb. Building a nuclear bomb is actually quite difficult (ask Iran). In Chernobyl, the explosion was caused by excessive pressure buildup, hydrogen explosion and rupture of all containments, propelling molten core material into the environment (a “dirty bomb”). Why that did not and will not happen in Japan, further below.
In order to control the nuclear chain reaction, the reactor operators use so-called “moderator rods”. The moderator rods absorb the neutrons and kill the chain reaction instantaneously. A nuclear reactor is built in such a way, that when operating normally, you take out all the moderator rods. The coolant water then takes away the heat (and converts it into steam and electricity) at the same rate as the core produces it. And you have a lot of leeway around the standard operating point of 250°C.
The challenge is that after inserting the rods and stopping the chain reaction, the core still keeps producing heat. The uranium “stopped” the chain reaction. But a number of intermediate radioactive elements are created by the uranium during its fission process, most notably Cesium and Iodine isotopes, i.e. radioactive versions of these elements that will eventually split up into smaller atoms and not be radioactive anymore. Those elements keep decaying and producing heat. Because they are not regenerated any longer from the uranium (the uranium stopped decaying after the moderator rods were put in), they get less and less, and so the core cools down over a matter of days, until those intermediate radioactive elements are used up.
This residual heat is causing the headaches right now.
So the first “type” of radioactive material is the uranium in the fuel rods, plus the intermediate radioactive elements that the uranium splits into, also inside the fuel rod (Cesium and Iodine).
There is a second type of radioactive material created, outside the fuel rods. The big main difference up front: Those radioactive materials have a very short half-life, that means that they decay very fast and split into non-radioactive materials. By fast I mean seconds. So if these radioactive materials are released into the environment, yes, radioactivity was released, but no, it is not dangerous, at all. Why? By the time you spelled “R-A-D-I-O-N-U-C-L-I-D-E”, they will be harmless, because they will have split up into non radioactive elements. Those radioactive elements are N-16, the radioactive isotope (or version) of nitrogen (air). The others are noble gases such as Xenon. But where do they come from? When the uranium splits, it generates a neutron (see above). Most of these neutrons will hit other uranium atoms and keep the nuclear chain reaction going. But some will leave the fuel rod and hit the water molecules, or the air that is in the water. Then, a non-radioactive element can “capture” the neutron. It becomes radioactive. As described above, it will quickly (seconds) get rid again of the neutron to return to its former beautiful self.
This second “type” of radiation is very important when we talk about the radioactivity being released into the environment later on.
I will try to summarize the main facts. The earthquake that hit Japan was 7 times more powerful than the worst earthquake the nuclear power plant was built for (the Richter scale works logarithmically; the difference between the 8.2 that the plants were built for and the 8.9 that happened is 7 times, not 0.7). So the first hooray for Japanese engineering, everything held up.
When the earthquake hit with 8.9, the nuclear reactors all went into automatic shutdown. Within seconds after the earthquake started, the moderator rods had been inserted into the core and nuclear chain reaction of the uranium stopped. Now, the cooling system has to carry away the residual heat. The residual heat load is about 3% of the heat load under normal operating conditions.
The earthquake destroyed the external power supply of the nuclear reactor. That is one of the most serious accidents for a nuclear power plant, and accordingly, a “plant black out” receives a lot of attention when designing backup systems. The power is needed to keep the coolant pumps working. Since the power plant had been shut down, it cannot produce any electricity by itself any more.
Things were going well for an hour. One set of multiple sets of emergency Diesel power generators kicked in and provided the electricity that was needed. Then the Tsunami came, much bigger than people had expected when building the power plant (see above, factor 7). The tsunami took out all multiple sets of backup Diesel generators.
When designing a nuclear power plant, engineers follow a philosophy called “Defense of Depth”. That means that you first build everything to withstand the worst catastrophe you can imagine, and then design the plant in such a way that it can still handle one system failure (that you thought could never happen) after the other. A tsunami taking out all backup power in one swift strike is such a scenario. The last line of defense is putting everything into the third containment (see above), that will keep everything, whatever the mess, moderator rods in our out, core molten or not, inside the reactor.
http://anond.hatelabo.jp/20110314030613
へ続く
