説明を受けてわかったことは，廃炉における除染作業とは，原発で汚染されていると見られるものの全てを1m x 1m x 0.7 m程度の箱に入るように切断し，そのうちのできるだけ多くが10m Sv/年の基準値以下となるように汚染度を下げる作業です．(この箱のはっきりした大きさはわかりませんが，写真で拝見した所，人との比較でこの程度の大きさのようでした．) 配管，ポンプ，容器，壁，全てをこま切れにして除染し，汚染を測定し，そして不合格ならまた除染作業をする．その作業は汚染が広がらないようにそれぞれが特定の小部屋の中で防護服に守られながらの手作業です．それを全ての箱が合格するまで繰り返す作業です．20年たった今の進捗状況は70から80%と推測されています．汚染度の高いものは，50年間中間貯蔵場に保管した後に切断され，さらに除染されます。
Fig1. Let’s talk about nuclear decommissioning: meeting snapshot
This blog article is a summary of my understanding of the discussion.
1.1.1 Lecturer: Masao Fukumoto
Fukumoto is a free journalist. He lived in East Germany (Deutsche Demokratische Republik) in the mid 80s. There was the Chernobyl nuclear incident. For him, that was an on-going disaster experience and that was the clue to follow the nuclear energy problems. He is the author of a book [Fukumoto 1] dealing with the radioactive contamination problem of Chernobyl. He is also one of the creators of a TV documentary about nuclear decommissioning [Fukumoto 2].
1.1.2 Example of nuclear decommissioning
In the discussion meeting, Fukumoto suggested to talk about Greifswald nuclear power plant as an example of a nuclear decommissioning since it is currently the largest nuclear decommissioning project in the world.
1.2 What is nuclear decommissioning?
1.2.1 Nuclear reactor and nuclear decommissioning
A nuclear reactor is fundamentally the same as a huge kettle. We generate high pressure steam by the huge kettle and drive a steam turbine to generate electricity. Since we used nuclear fuel to boil the water in the kettle, we call the resulting energy nuclear energy. This huge kettle is called a nuclear reactor. Since this nuclear reactor uses highly radioactive dangerous substances, a nuclear reactor should be processed and appropriate safety measures have to be taken before trashing it. This process is called nuclear decommissioning.
1.2.2 The start and the end of a nuclear decommissioning
A nuclear decommissioning is a process to restore the site’s “greenfield” status. Radioactivity surveillance for the area should not be required after the decommissioning. However, a nuclear decommissioning is only of the cleanup. Nuclear decommissioning produces radioactive waste and processing radioactive waste was beyond the scope of the meeting. We may have another discussion meeting for the topic.
When the area is back to a greenfield status, then that is the end of nuclear decommissioning. So, when does a nuclear decommissioning start?
The nuclear decommissioning started in the design phase of the plant and the reactor. To make the decommissioning process safe, we need to design a reactor that can be safely decomposed. For instance, we need to build a reactor that can be carried by a crane (or by some kind of machine) to remove it from the building. Otherwise the process would become difficult.
In Germany today, the government cannot give permission to build a reactor if the designer of the reactor didn’t consider the concept of nuclear decommissioning (East Germany (Deutsche Demokratische Republik) might not have had this regulation.) However, including the decommissioning concept in the reactor design seems not necessary to build a reactor in Japan (Recorder’s footnote: [NHK special document 2009.10.11] shows that a Japanese company lost some of the design plan of their reactor, and the remaining design plan is too old to read it. The regulations didn’t require to keep the plans.)
1.3 The methods of nuclear decommissioning
There are mainly two methods of nuclear decommissioning:
Safe enclosure (Sometimes this method is further divided into Safe enclosure and Entombment.)
Let’s see them one by one.
1.3.1 Immediate dismantling method
This method starts decomposing the reactor directory after the power plant shuts down. The pros and cons of this method are:
Pros: There still are the workers who know the plant details. This is an advantage for the decomposing operation.
Cons: The radioactive contamination level is high
Nuclear reactors are not standardized. Each plant is highly customized. Therefore, availability of the workers who know the plant is an advantage to operate the decommission.
On the other hand, immediately after the shutdown of the reactor, the radioactive dosage is still high. This makes the decommissioning operation difficult. We need to develop some kind of technology, for instance, using a special robot.
1.3.2 Safe enclosure method
This method first keeps the reactor in storage and waits for some period (it is usually more than 30 years). The pros and cons of this method are:
Pros: The radioactive dosage is relatively low when operating the decommission
Cons: The workers who know the plant details are no longer available
The process is relatively easier than the immediate dismantling method since the radioactive dosage will be relatively low. But we can never completely remove the radioactive contamination in several decades, the difficulty is only relatively lower.
On the other hand, this method needs more than thirty to fifty years of storage time, we usually cannot expect the help from the workers who know the plant details.
1.4 Part 1 Summary
Everything on the earth has a limited lifetime. A nuclear reactor is not an exception. However, I haven’t heard what happens when a reactor reaches the end of its design lifespan. I can easily imagine that we cannot trash it like a piece of paper.
In this article, we talk about what is a nuclear decommissioning and how could we do that. My main concern is actually “Is it really possible and how?”, “How much does it cost? Who pays that?”, and “What should we do about the nuclear waste?” The first question has been answered in this article: It might be possible, but it is difficult.
Before the Fukushima disaster, I was less concerned about the energy problem. I simply thought, “Anyone should have thought through it.” However, what we saw in the Fukushima disaster was “No one thought about it.” and “No one wants to take responsibility.” The responsible people told us, “it’s unexpected.” One of the strangest logic was that “It’s unexpected, therefore no one takes responsibility. It is a natural disaster.” I can hardly understand that a nuclear reactor explosion is a natural disaster. I personally thought, I don’t want to die with the reason of “unexpected.” The Fukushima disaster opened my eyes, it said “Hey you, wake up!” But I don’t know what I can do about this. Maybe it is better if I keep sleeping, ignoring the Fukushima disaster. However, I also know that if everyone keeps sleeping, the next disaster is unavoidable. Whether I am ignoring the problem or not, Japan has around 60 nuclear reactors. That fact always remains. I asked myself, “Why did this happen to us?” A part of me still wants to go back to be a person who doesn’t think about this problem. On the other hand, I cannot stop thinking what Fukushima really means: What does it mean to use an energy source that forces us to lose a part of our country? What does it mean to use air conditioning and to access the internet powered by nuclear reactors which endanger the life of people, especially children’s lives.
Then, what can I do about it? Maybe I can do nothing. I still would like to ignore the problem since thinking is hard. However, I also said to myself, “If I cannot change anything, does it really matter? It isn’t hard to try to think about this problem. If I can achieve something, that’s pure luck, but if not, so what?” So I started researching the energy problem. It’s fine even if I can only understand just a part of it. I just don’t want to hear the excuse again, “Sorry, that is a unexpected natural disaster, we can do nothing for you.”
In part two of this article, I would like to look into my next questions, “How much does the nuclear decommissioning cost, and how long does it take?”, “What should we do with the nuclear waste?”
[Fukumoto 1] ふくもと まさお, “ドイツ・低線量被曝から28年 –チェルノブイリはおわっていない”, 言叢社, 2014 (Masao Fukumoto, “Germany, 28 years after the low level dosage exposure — Chernobyl has not been ended”, Gensou publishers, 2014, ISBN-13: 978-4862090478)