NUCLEAR POWER - How it works/ 原子力発電所の仕組み

(English version is after the Japanese one)

「原子力」

原子力は、中性子がウラン235原子核に衝突し、原子核が分裂する時に発するエネルギーを使う。核分裂はさらに中性子を吐き出し、核分裂の連鎖反応を促す。中性子の速度は通常早すぎて核分裂を起こさないので、中性子減速材として水とか黒鉛を使う。これが原子力の原理である。西側諸国の原子力発電所の原子炉は大きく分けて沸騰型(BWR)と加圧型(PWR)に別れる。カナダ独自の原子炉(CANDU)はPWRに属する。

沸騰型は主にジェネラル・エレクトリック(GE)社が開発した。ウラン燃料棒の核分裂の熱から水を直接沸騰させ、その水蒸気で発電タービンを回す。構造が簡単で建設費が低い。原子炉内の水が沸騰してしまっても中性子が早すぎて自然に核分裂が止まるという安全性もある。ただし発電タービンを回す水蒸気は放射性である。日本の多くの原子力発電所は沸騰型である。

加圧型は主にウエスティング・ハウス(WH)社が開発した。原子炉に圧力をかけ、水が高温(300度程度)になっても沸騰しない。その高温高圧の水を熱交換器を通して別の水循環系統を加熱して水蒸気を作り発電タービンを回す。高圧を支える為に原子炉容器は堅固なものが必要だ。熱交換器や2次水循環系統の余計な設備が必要となる。ただし放射性は原子炉と熱交換器の一部だけに限られるという安全性がある。カナダ独自の原子炉(CANDU)はPWRであるが、巨大堅固な容器の代わりに400本から500本の直径15cmほどの長い筒型の「原子炉」から成り立っている。製造上特殊な技術を必要としない。筒型容器にかかる圧力も低い。運転中に燃料交換が出来る長所もある。

BWRもPWRも濃縮ウラン燃料と通常の水(軽水)を使う。CANDUは天然ウラン燃料と重水を使う。重水の水素原子は陽子に加え中性子をすでに一つ含んでおり、中性子を吸収しにくい。従って飛び回る中性子を十分に活用出来、ウランを濃縮する必要がない。天然ウラン燃料は使用前は手に持つことが出来る。ウラン1グラムは石油2000リットル、石炭3トンのエネルギーに相当する。

福島第一(6基、1970年代)・第二(4基、1980年代)原子力発電所はすべてBWRである。福島第一原子力発電所の地震・津波による破壊過程はいくつかの段階に分かれる。(以下は推測を含む)。

運転中だった1-3号機はM9の地震に、制御棒が作動し、核分裂は止まり、自動停止した。非常発電が始動し熱い燃料棒の冷却が順調に始まった。約45分後に津波が到来、非常発電が失われた。燃料棒の温度が運転中に出来た副産物 (アイソトープ) の自然崩壊熱で上昇、やがて原子炉内の水が沸騰し始めた。原子炉内の水位も下がり始めた。(配管のどこかに亀裂が出来た？)。水蒸気による原子炉内の圧力を下げる為、安全弁を開け、セシウム137 (半減期30年)やヨウ素131 (半減期8日)のアイソトープや水素などを含む放射性水蒸気が建物内に放出された。(この事故全体を通して半減期7億年のウラン燃料は外部に全く出ていない。またCANDUではこの場合放射性水蒸気は外部へ出ない構造になっている)。建物の天井に溜まった水素が酸素と接触し水素爆発が起こり、建物を次々と破壊した。(原子炉容器ではない)。

一方使用済みの燃料棒貯蔵槽の冷却も非常発電停止と共に止まった。自然崩壊熱で水槽の水が蒸発、一部が露出し始めた。維持の為に運転停止中で、熱い燃料棒が貯蔵してあった4号機貯蔵槽が一番問題になった。消防車やヘリコプターからの放水でこれを冷やした。

1-3号機原子炉内の冷却はうまく行かず、ついに海水の注入に踏み切った。(4-6号機は運転停止中でこの問題はなし)。しかしながらそれまでに燃料棒の金属ケースが溶け(2000度程度)、中の燃料(融点は3000度程度)が崩れて原子炉容器の底に溜まったと思われる (メルトダウン)。ちなみにこの状態では中性子は早すぎて核分裂は起こらない。この大量注水と使用済み燃料棒貯蔵槽からの漏れが海に流れた放射性汚水の源と想像しているが定かでない。放射性配管が発電タービンまで延々と続くBWRの弱点が関係しているように思う。

4月上旬に福島第一への電力が復旧した。これを書いている5月下旬の時点で冷却努力は功を奏しており、温度問題は収束へ向かっていると思える。問題は大量に増えつつある放射性汚水の処理をどうするかに移っている。それと平行して発電所周辺の放射線汚染に対する対策が進んでいる。

福島第一は津波の壊滅的な打撃を受けた。福島第二は安全に停止したままだ。第一では非常発電設備が通常の建物の中にあった。第二では堅固な原子炉建物の中にあった。なぜ第一も第二と同じ非常発電設備防護処置をしなかったか。起こってから言える「なぜ」要素である。

事故が起こって初めて欠陥があることに気がつくことは良くある。航空機の安全性の歴史を見ればそれは明らかである。その努力は今も続く。2009年、ブラジルからパリへ向かったエアバス自慢のA330が南大西洋上で消息を絶った。安全追求の努力は2年後の今年5月、南大西洋の3900メートルの深海についにブラックボックスを発見、引き上げた。このしつこいまでの原因追求が航空機の安全性につながっている。今回の原子力発電所の事故はすでに多くの教訓をもたらしている。世界中の原子力技術者達が技術と安全の見直しを始めている。新しいもっと安全な形式の原子炉の開発も進んでいる。

僕はCANDU炉の核の維持と検査をする機器の開発・設計をして来た。原子力が万全なものとは思っていない。火をもて遊べば不注意から火傷をする。人間が作る物に完全な物はない。原子力、航空機、自動車、電車。それらを限りなく安全なものにすることがそれらを作り出した者の義務と責任であろう。

産業革命の石炭が主役の座を降り、蒸気機関車が消えていったように、原子力に代わる安定したエネルギー供給源が現れれば、原子力は消える運命にある。現在原子力に代わる地球温暖化に寄与しない技術がない。風力発電も太陽熱発電も膨大な電力を要求する現代社会を支える発電容量と安定性に欠ける。例えば原子炉4基のダーリントン原子力発電所は発電容量が3200MWある。巨大な風力発電塔１基は2MW程度である。原子炉4基置き換えるのに風力発電塔1600基が必要となる。これらをどこへ建設するか、送電線はどうするか。しかもこの発電容量は24時間100%稼動しての話である。風がなければゼロである。太陽熱発電も他のグリーン発電源も似たような問題を持つ。

現在の最良の方策は原子力を基盤としグリーン発電源を最大に生かすことだろう。発電だけに頼るのではなく、電力消費を減らす努力も重要だろう。現在日本国内で広がっている電力節減努力が長期的・世界的な電力消費削減、原子力の必要性への軽減につながって欲しいと思う。

三浦信義 Nobby Miura, P.Eng., M.Eng.
Design and Development Engineer (Retired)
CANDU Inspection and Maintenance Systems
May, 2011 – R0


NUCLEAR POWER

The nuclear energy is generated when a neutron hits a uranium 235 atom and splits it. This nuclear fission releases more neutrons. This creates the so-called nuclear chain reactions. The neutrons are usually too fast to be captured by the uranium atoms. To slow them down, water or graphite is used as a moderator. This is the basic principle of the nuclear energy.

There are roughly two types of nuclear reactor design in the western world. One is the boiling water reactors (BWRs) and the other is the pressurized water reactors (PWRs).

The BWR was developed by General Electric. The water is boiled directly by the heat from the fission of the uranium rods. The resulting steam drives the turbine to generate the electricity. The design is simpler and the construction cost is generally lower. When water at the core boils off accidentally, the fission automatically stops as the neutrons are too fast to cause the fission reactions. A draw back is the steam to run the turbine is radioactive. The majority of the reactors in Japan are this type.

The PWR was developed by Westinghouse. The reactor is pressurized so that the water does not boil at 300C. This high pressure high temperature water is put through a heat exchanger to boil the water in another water circulation system. This steam drives the turbine. In order to withstand the high pressure, the reactor vessel needs to be strong. The additional systems such as the heat exchanger and the secondary water circulation system are required in the PWRs. The advantage of this design is that the radioactivity is confined to the reactor vessel and part of the heat exchanger.

The unique Canadian design CANDU reactor is a type of the PWR. Instead of the huge reactor vessel, it has 400 to 500 pressure tubes that contain uranium fuels. These are simpler to manufacture. Also it needs to withstand much lower pressures. The fuels can be changed without stopping the reactor.

The BWRs and PWRs use enriched uranium fuels and regular water. The CANDU uses natural uranium fuels and heavy water. The hydrogen atoms of heavy water contain an extra neutron. It does not capture neutrons easily. Thus, more neutrons are available even though the fuels contain less uranium. You can hold the natural uranium fuels on your hand before they are irradiated. One gram of uranium provides the amount of energy equivalent to 2000L of oil and 3 tons of coal.

All the reactors at the Fukushima Daiichi (#1, 6 reactors, 1970s) and Fukushima Daini (#2, 4 reactors, 1980s) nuclear stations are the BWRs.

The progress of the damages at the Fukushima Daiichi (#1) nuclear station can be divided in to a several stages. (Some assumptions are included as not enough information is available at this point).

The reactors, Units 1, 2 and 3, were running when the M9 earthquake hit. The control rods were activated, the fission stopped and the reactors were shut down. The back-up generators started cooling the reactor cores. The tsunami hit the station 45 minutes later. The back-up systems were gone. The cooling systems stopped. The heat from the decay of the radioactive isotopes (by-products in the fission process) rose the temperatures of the cores. The water inside the reactor vessels started to boil. At the same time, the water level in the vessels started to drop. (I suspect that cracks exist somewhere in the water/steam circulation systems). The steam raised the pressure inside the vessels. In order to lower the pressure, safety valves were opened. The steam that contained cesium 137 (half life 30 years), iodine-131 (half life 8 days), hydrogen gas etc. was released inside the buildings. (Throughout the accident, the uranium fuel, half life 700 million years, was not released outside at all. Also, the CANDU is designed not to release the steam outside in this situation). The hydrogen accumulated in the ceiling of the buildings. It was ignited by the oxygen in the air causing the hydrogen explosions that destroyed the buildings. (Not the reactor vessels).

In the meantime, the cooling system for the spent fuel pools stopped too. The heat from the decay of the isotopes raised the temperatures of the water in the pools, exposing some of the spent fuels. This was especially rapid in the pool at Unit 4 where the new hot spent fuels were just deposited for the maintenance of the reactor. Water was poured by fire engines and helicopters to fill and cool the pools.

The cooling efforts for the core of Units 1, 2 and 3 did not go well. Thus, it was decided to pour a huge amount of sea water to the cores. (Units 4, 5 and 6 were not running). However, by the time, the metal casings (Melting point at 2000C) of the fuel rods melted and the uranium fuel pellets (Melting point 3000C) dropped at the bottom of the vessels (Meltdown). Under this condition, the nuclear fission does not happen as the neutrons are too fast. I suspect that the leaks from the piping systems and the pools were the sources of the radioactive waste water that was discharged to the sea. I feel the weakness of the BWRs where the radioactive water/steam runs all the way to the turbines contributed to this contamination.

The power supply to the Fukushima Daiichi nuclear station was restored in the early April. At present (The end of May), the cooling effort of the reactor vessels and pools started to work. The effort has shifted to the dealing of the huge amount of radioactive waste water. At the same time, the plans to deal with the contamination around the station are being developed.

The Fukushima Daiichi (#1) suffered the catastrophic damages from the tsunami. The Fukushima Daini (#2) shut down safely and stayed in that way. The back-up systems at the Fukushima Daiichi (#1) were housed in regular buildings while the back-up systems at the Fukushima Daini (#2) were housed in the strong reactor buildings. So, why the TEPCO did not do the same at the Fukushima Daiichi (#1) as at the Fukushima Daini (#2)? This is one of those “if” factors that can be said after the fact.

Often problems are revealed only after accidents. It is most evident when you look at the safety effort of the jetliners. The drive for safety continues. The modern A330 of Airbus disappeared over the southern Atlantic Ocean while flying from Brazil to Paris in 2009. Just this month (May), they found the black box at the bottom (3900m) of the deep Atlantic Ocean. This persistent effort for the safety is the reason of safer jetliners today. The nuclear accident in Japan already provided many lessons. It triggered a world wide review of the technology and safety of the nuclear stations. The development of newer and safer reactor designs will be accelerated.

I was a design and development engineer (now retired) for the machines and tools for inspecting and maintaining the core of the CANDU reactors. I never took the nuclear as a completely safe technology. We are playing with fire. We will be burnt if not careful. There is no such a thing as a perfectly safe technology as long as human designs and produces it. Nuclear, jetliners, automobiles, even trains. It is our responsibility and duty to make them as safe as possible.

The coal was the king of the industrial revolution. Now the steam locomotives are a thing of the past. The nuclear will be replaced when new and safer technologies emerge. However, at present, there are no other technologies that do not contribute to the global warming and that provide a reliable and steady supply of the electricity to the energy hungry modern society. The green energy technologies such as wind or solar power do not have the capacity and reliability of the nuclear. The four reactor Darlington nuclear station has a generating capacity of 3200MW. A huge wind mill has generally a generating capacity of around 2MW. In order to replace the four reactors, you need 1600 huge wind mills. Where do you erect them and how to connect them to grids? Besides, this generating capacity is based on 100% efficiency. If there is no wind, there is no power generated. The solar power and other green energy technologies have similar drawbacks.

The best we can do now is to use the nuclear power as the base power supply and supplement it by the green energy sources as much as possible. Also, it is important to put more effort to reduce the energy consumption at the same time. In Japan, such efforts are wide spread now that they do not have enough generating capacity for summer. I hope this effort stays as a long term strategy and spreads to the world in order to reduce the dependency on the nuclear power.

Nobby Miura, P.Eng., M.Eng.
Design and Development Engineer (Retired)
CANDU Inspection and Maintenance Systems
May, 2011 – R0