Measures of radioactivity and the radiation: Becquerel and Sievert

The purpose of this section: Understand the science to determine your own risk

Radioactivity is an ability to emit radiation. Honestly I didn’t know the difference between radioactivity and radiation a while ago. If you also don’t, this article is for you.

As a citizen I am most interested in “safety.” But the problem of radiation is that we cannot see it directly. Also there is not so much available experimental data on safety of radiation. If we want to know it in a scientific way, we need to perform an experiment. That means we would probably need several thousand subjects and we would have to put them into some controlled environments with different radiation for many years. Then we could know what kind of dosage affects the human body. But this kind of experiment raises a lot of issues. Since we cannot make such an experiment, we cannot know for sure. Each specialist has different opinions. It is hard to make a constructive discussion about radiation safety. What we could do is basically avoid the radiation. However, if that is difficult we need to judge the risk on our own based on available information, although we saw what happened when we just believed some authorities in Fukushima as they told us “atomic reactors are safe”. We just didn’t know what they meant by “safe.” I think this time we should understand the problem on our own.

Here I can only provide some information from the government and from other sources. First we would like to understand it, because if we cannot understand what it means we cannot judge the risk. For example, what the Becquerel means. What is the difference between radioactivity and radiation. Let’s try to understand these words.

The goal of this appendix is to learn how to read and understand the information. Let’s get started.

The knowlege of basic words: Radiation, radioactivity, and so on.

The question “Whet is radioactivity?” is related to the one “what are atoms?” The concepts of atom and molecule is very common in nowadays industry, and we learn them in public school. When I learned them, I did not realize that these concepts are related to everyday life and to our surrounding industry, or I just missed it, and you might miss it too. What is electricity is also deeply connected with atoms. You use electricity everyday, but can you answer the question, “What is electricity?” if your children asked you? I learned it when I was in a junior high school. If you also forgot them, but suddenly the newspaper started saying, Cesium 137, Cesium 134, and so on, you can look up your old physics textbook. If you have a child in junior high or a high school, you can ask them. You can also find this information in the Internet.

One way to start to know what is an atom is looking up Wikipedia [1]. You can also find online chemistry courses (e.g. [2]). Usually such courses do not provide information about radiation safety, but at least you can find what is radiation. That is a good start to understand the issue of safety.

Japanese officials provide some information about radiation safety. For instance, “How to be safe for all of you (みなさまの安全確保) [3].” You can find some criteria for the radiation safety in it. However, it is written as “Assuming 1mSv/y internal exposure, drinking water should be less than or equal to 10Bq/kg [4].” You can also find a terminology list at [4]. If you can understand that, you can stop reading this article here. In case you do not understand that, I will try to explain how I understand it. For example, what is Sievert (Sv), what is Becquerel (Bq), why we see sometimes a unit like Bq/kg, what is the difference between mSv/y and mSv? Most importantly, why should I care? Even if we know the meaning of their terms, it is still difficult to determine the risk, but at least we can understand what it means. This could be a foundation of your own decision for the risk evaluation.

I tried to make this article as precise as possible, but there is a limit. Please do not just believe this article blindly. It is important to understand the problem yourself.

References

1. Wikipedia En, Ionizing radiation, https://en.wikipedia.org/wiki/Ionizing_radiation, [Online; accessed 2016-1-13(Wed)
2. Khan academy, Chemistry, https://www.khanacademy.org/science/chemistry, [Online; accessed 2016-1-18]
3. 首相官邸, みなさまの安全確保 http://www.kantei.go.jp/saigai/anzen.html/, 「計画的避難区域」及び「緊急時避難準備区域」の設定について, http://www.meti.go.jp/press/2011/04/20110422004/20110422004-2.pdf, 2011, [Online; accessed 2014-12-21(Sun)]
4. 厚生労働省: 原子力被災者生活支援チーム, 原子力発電所外に適用されている放射能に関する主な指標例, http://www.meti.go.jp/earthquake/nuclear/pdf/120427_01a.pdf, [Online; accessed 2014-12-26]

We talked about what is the nuclear waste, how it is generated and what kind of process we need to do in order to put it in a final disposal storage. Since we still do not have a practical (cost effective) nuclide conversion technology, the most practical method to render nuclear waste innocuous is its long term management. In the next section, we will review the property of nuclear waste. That will bring up what kind of problems we have with the final disposal of waste.

Transition of used fuel radioactive decay

A chart of radioactive decay over time

When used fuel is taken away from a reactor it is highly radioactive. Thus we need to store it in somewhere and wait for the radioactivity to become lower. At the meeting we saw a radioactivity attenuation chart, which shows that it takes a long time.

Yamauchi’s memorandum

How long do we need to store the used fuel? If we only need to store the used fuel for a few months and the radioactivity level became low enough, nuclear waste management might not be a big issue. However, it is not so easy.

Figure 3 shows the radioactivity attenuation of used fuel over time. You can find this chart at the web site of Research organization for Information Science & Technology (RIST) [1], that refers to the web site of Japan nuclear cycle development institute asthe source of the data.
Figure 3: Radioactivity attenuation of vitrified waste over time. The x-axis is time in years, the center is 0 years. The y-axis is labeled `1 ton nuclear fuel (MTU) corresponding radioactivity [GBq]’
[Original figure] Japan nuclear cycle development institute: 2nd period of research and development of deep geologically repository, summary report I-4 (1999-11-26)

How to read Figure 3:

Let me show you how to read this chart. First, please notice the graph scale is log-log. What does this mean? Both axes of a log-log graph are described by exponent. You may be familiar with a linear graph. In a linear graph, one tick of an axis means usually +1 (or +n), but one tick of this chart is × 10. For example, one tick of this chart means ‘times ten’ instead of ‘plus one’. If you go two ticks, the value increases 100 times more.

The vertical axis shows the radioactivity in Becquerel. When the used fuel was taken from a reactor, the radioactivity indicated 10¹⁰ GBq/t.

10¹⁰ means 10 zeros after 1. Therefore, it is 10000000000, which is 10 billion. The character ‘G’ (Giga) before the unit `Bq’ is a SI-prefix, and it means 10⁹. If you are familiar with computers, you know the memory size is represented as GBytes, which is 1000 times larger than MBytes. This means, 10¹⁰ GBq is 10,000,000,000 GBq = 10,000,000,000,000,000,000 Bq = 10 quintillion.

According to the criterion of the Japanese government, though it depends on nuclide and food, we should avoid eating food with an activity of more than 100 Bq per kg. Compare to this number, radioactivity of the used fuel is enormous. It seems beyond my imagination.

There is the 0 on the x-axis at the center of the graph. This is the time when the fuel is first used. To make Uranium useful as fuel, it is enriched, i.e., its radioactivity is increased. As we go to the right, time passes. The axis ticks increases 10⁰ years, 10² years, and so on.

The exponent number of 10 has the same meaning as in the last example about Bq, how many zeros after the 1. This means, 10⁰ years is one year (zero zeroes after 1), 10² years is 100 years, 10⁴ years is 10,000 years, and 10⁸ years is 100,000,000 years.

The graph shows a horizontal blue dotted line. This line shows the radioactivity when Uranium ore is in a mine. The cross point of the red line and the blue line is between 10⁴ and 10⁵ years. Therefore, it takes about 10000 to 100000 years until the radioactivity of used fuel reaches the level of naturally occurring Uranium ore.

By the way, it is unfair to compare the 100 Bq/kg for food and 10 quintillion Bq/t for used fuel. Note that the units are different: Bq/kg and Bq/t. This means that radioactivity of 1 Bq/kg is 1000 times higher than 1 Bq/t. Therefore a fair comparison is between 100 Bq/kg for food and 10 quadrillion Bq/kg for used nuclear waste. We sometimes miss the unit difference. Sometimes we could find these units are not consistent in some news articles. The different units are not technically a mistake, but it is still misleading. When we use the same unit here, 100 and 10 quadrillion are quite different.

These large numbers are hard to see. We do not know what 10 quintillion or 10 quadrillion are. Scientist usually use the scientific notation to write these numbers. The scientific notation uses exponent. In other words, this is how many zeros are in the number. 100 has two zeros after one, in exponent writing this is 10². 1 quadrillion is 10,000,000,000,000,000. This has 16 zeros after 1, it is 10¹⁶. Many advertisements use large numbers since they have a high impact when heard. For example, a sport drink in Japan claimed that was an 1000 mg effective medicine dosage, but this has the same effect as 1g of medicine dosage. Still, 1000 has makes an impression on us, so it is a good advertisement strategy. This is fine for an advertisement, however, I recommend to be careful about units an article about radioactive contamination for the news.

How long shall we keep the used nuclear fuel?

If you search about radioactivity attenuation of nuclear waste (Figure 3) you find many similar figures on the Web. I later realized that this figure only shows it takes more than 10 thousand years for the used fuel radioactivity to decay to the same level of an Uranium mine. I found nothing about the safety of the level of an Uranium mine. My first impression was, “wow, at least 10 thousands years, that’s a long time!” But I never asked the question: “is it safe after 10 thousands years?”

The blue line in the figure indicates 1000GBq/t, which means 1GBq/t (one billion Bq/t). 1 GBq/t is quite a large radioactivity value. Though one thing is not clear here that in the explanation, “1 ton nuclear fuel (MTU) corresponding radioactivity [GBq]”, I assume that this unit means GBq/t. If I convert it to Bq/kg, this is 1000 MBq/kg, that is 1GBq/kg, and this is 10 billion Bq/kg. As we mentioned before, the recommendation of safety for food is 100 Bq/kg according to Japanese government, and a value that is 10 million times larger does not safe to me.

I do not know what is blue line in Figure 3 means. Can I safely get near to some radioactive substances if the level is lower than this blue line? As a citizen, I am more interested in safety, so I try to connect this figure to safety, but this is not even labelled as radioactive dosage. I think this figure only shows the relationship between time and radioactivity attenuation, but that is not related to safety.

Progress of radioactive decay and waste management

Figure 3 shows the progress of radioactive decay over time. When the fuel is taken out of the reactor, its radioactivity is 10 billion GBq/t. After three to five years of cooling down in the fuel water tank, it becomes a few tens of million GBq/t. Then, after 30 to 50 years the radioactivity of vitrified waste in interim storage becomes a bit less than ten million GBq/t. Until the radioactivity drops to the level of a Uranium mine, we need to wait approximately ten thousand years. The final disposal process starts after the interim storage stage. Let’s see what is planned for that.

References

1. Research Organization for Information Science and Technology (RIST: 高度情報科学技術研究機構), “Safety problems of high level nuclear waste and its processing (高レベル放射性廃棄物と処分対策の安全問題), note: the decay graph of vitrified waste can be accessed from this page)”, http://www.rist.or.jp/atomica/data/dat_detail.php?Title_Key=05-01-01-03, [Online; accessed 2015-7-1]

Procedures of handling the nuclear waste

Until now, we do not have a practical technology to render the nuclear waste innocuous. Therefore, what basically we can do is to wait for the nuclear waste to become low enough in radioactivity due to decay. To do so, we need to store the nuclear waste in a safe storage for a long time until it does not harm the environment.

The low level and middle level nuclear waste do not generate much heat, so it would be possible to store them in the final disposal storage immediately. But, what happened is that it gets stored in a middle term storage due to either of the following reasons:

• There is no such final disposal storage (Most of the countries that have nuclear reactors have no final disposal storage. Ex. Japan.)
• The process would take too long, even when there is a final disposal nuclear waste storage.

Since high level nuclear waste generates high temperature heat, we first need to cool it down. We store them in an interim storage facility to wait for it to cool down. Used nuclear fuel is a high level nuclear waste. First we usually store it in a fuel storage water pool for three to five years to cool it down. Next, we store it in a interim storage facility for 30 to 50 years. If it has cooled down enough, then we store it in a final disposal facility. There are some issues with this interim storage facility, for example where should we store and how should we store.

Japan also reprocesses used fuel, it is one of a few exceptional counties since most of the countries do not any more. In this case, the used fuel is stored in a fuel storage pool in the nuclear reactor building until it is sent to a reprocessing facility. This process of cooling down takes approximately 10 years in Japan. Reprocessing generates nuclear liquid waste as a by-product. One way to stabilize this waste is through vitrification. In the vitrification process, the high level waste is mixed with fragmented glass and melted. This is stored in a steel container called “canister.”

There are two interim storing strategy: 1. geologically distributing them among the nuclear power plants, 2. concentrating them in a few interim storage sites. In Germany, the used fuel is distributed among the nuclear power plants and the vitrified waste is stored in one storage facility.

There are two storing methods: 1. a used fuel pool method, where the used nuclear fuel is put in a water pool and cooled down with water. 2. A dry cask storage method, where the used fuel is put in a canister which in turn is put in a storage place on the ground and cooled down with air. The used fuel pool method needs less space than the dry cask storage method. It does not need special storage cases. Therefore, the cost of pool method for the used fuel lower than the dry method. However, if the water pool loses water for some reason, e.g., because of an earthquake, it is quite dangerous. The dry method needs more space but cooing only needs natural air, therefore this method can avoid the problems the pool method can have. But this method might still pollute the environment with neutron radiation. Despite Japan having relatively frequent earthquakes compare to other countries, the pool method is used. This means that even if all the nuclear reactors would not run, Japan is always under danger because of earthquakes.

Germany uses the dry method. The interim storage facility of Germany is located in Gorleben. In our meeting we saw a documentary film that shows the facility in Gorleben. We saw many casks with fins for cooling in the facility.

In either case pool or dry method, the waste is first stored for 30 to 50 years in the interim storage facilities, afterwords, it will be put in the final disposal storage facilities. This process flow is shown in Figure.

Yamauchi’s memorandum

Supplemental information on vitrified waste:

Vitrified waste emits high radiation right after having been generated, so high to kill a person near will die in 20 seconds. Its surface temperature is more than 200 degree C. (c.f. The web site of “The federation of electric power companies of Japan”[1])

This waste will be stored and cooled down for 30 to 50 years in a interim storage site. Reprocessed plutonium can be turned into a MOX (mixed oxides) fuel, Or, alternatively it can be reused in the nuclear cycle. A national research institute has been established to develop a feasible nuclear cycle, and this research has been going on for over 40 years (and 10 trillion yen been spent), but still there is no plan [2] and the government continues to invest on this.

Supplemental information of nuclear transmutation technology:

Nuclear transmutation technology is fundamentally a technology that is able to change the atomic elements. Nuclear fission is one of such technologies. Mankind has spent a few thousand years to efforts in a field called “chemistry” in order to be able to do precisely this. Element conversion at will is a very old dream of mankind. For example, alchemists were looking for a method to convert lead into gold. This is what nuclear transmutation technology could do, but unfortunately we still do not have a practical technology to achieve that, especially to process a large amount of nuclear waste. A national funded project in Japan, the Omega project [3] is an endeavor of such kind.

References

1. 電気事業連合会 (The Federation of Electric Power Companies of Japan), 原子力発電について (About Nuclear power plant): ガラス固化体 (Vitrified radioactive waste),  ([Online; accessed 2014-12-21(Sun)])
2. 東京新聞 (Tokyo Shinbun), 45年で10兆円投入．核燃サイクル事業めどなく (10 trillion yen been spent for 45 years, still no plan of nuclear cycle), [Online; accessed 2014-10-04]
3. Wikipedia ja, オメガ計画 (Plan Omega), [Online; accessed 2015-3-28]

The final disposal of nuclear waste consists in storing the waste until its radioactivity becomes low enough to not affect the environment. We first talk about what kind of nuclear waste there is, then what the sources of the waste are, and then how we manage them.

What is nuclear waste?

There are two criteria to define whether some waste is nuclear waste or not. The first one is based on measuring the amount of radiation that is absorbed in a certain amount of time (10 mSv/y), the second one is based on the amount of emitted radiation (radioactivity, Bq). Both have thresholds that used to define whether something is nuclear waste or not. If some specific waste goes over either one of two thresholds, it is classified as nuclear waste. One other point here is if a waste is less than this criteria even the waste has some radioactivity, such waste can be recycled in the market.

Yamauchi’s memorandum

Previously, I had the misconception that all the waste which is radioactive and which is produced by nuclear reactors is nuclear waste. However, there is a criterion that defines what nuclear waste is. In a later section, I extended my memo about the meaning of the criterion. For instance, what is Sievert and what is Becquerel, and what is the difference between radioactivity and radiation.

Classification of nuclear waste

Nuclear waste is classified in the following three categories:

• High level (TRU waste, used nuclear fuel)
• Middle level (This waste is rare)
• Low level

Germany’s classification is based on how much heat the waste generates since it is important for the waste management. TRU waste consists of TRans-Uranium elements whose atomic number is higher than Uranium. TRU waste is usually generated by a running reactor.

Source of nuclear waste

The sources of high level nuclear waste (+TRU waste) are:

• Used nuclear fuel
• Waste liquid by reprocessing, vitrified waste
• etc. …

The sources of low and middle level nuclear waste are:

• Nuclear power plant
• Decommissioning process
• Nuclear fuel fabrication facility
• Used nuclear fuel reprocessing plant
• (uranium mine, enrichment plant)(mining field)
• Medical institution
• Industrial factory
• Research and development institutions, miscellaneous

A nuclear power plant always produces nuclear waste regardless of whether the reactors are running or not, because of the maintenance process. For example, the clothes of the workers will become irradiated even if the reactor is not active. Hospitals and some industry are also sources since they use radioactive materials.

All nuclear waste is managed as dangerous substances by law in Germany. On the other hand, the last three items are not managed as dangerous substances in Japan. Therefore, we do not know how some nuclear waste is transported or preserved in Japan.

Yamauchi’s memorandum

In my personal opinion, Japan should also handle the last three items as dangerous substances as in Germany. This is to ensure control of these types of radioactive waste.

The classification as nuclear waste should depends on whether the radiation dose is more than 10mSv/y or not. If the radiation dose is less than 10mSv/y, we can recycle the waste in the public market. However, this threshold (10mSv/y) does not really tell whether this is safe or not. I would like to notice that by this criterion, safety is unclear. Also according to this definition, we can recycle a steel with 9.9mSv/y radiation dose to build a house or household items. This is legal. If you search with keywords “steel radioactive waste recycle”, you can find some information. However, many of the cases could be completely legal. In this article, I do not discuss the safety of this clearance criterion since the safety of radioactive waste is difficult to discuss. The best way is just to avoid it. But I would like to point out that some waste which is pass this criterion can be legally in our market.

In our last meeting “Let’s talk about nuclear decommissioning,” we realized the nuclear decommissioning produces nuclear waste. But we did not talk about what do we do about the nuclear waste. A nuclear decommissioning is about what do we do for a reactor after its lifespan ended or cannot use the reactor anymore when it is broken by any reason (e.g., an accident). What we learned was that a decommissioning costs money (e.g., 1 billion dollar/reactor) and time (e.g., 30 years). Even then the decommissioning has finished, the nuclear waste still remains. The last meeting ended here. Then some of our members asked: “What do we do about the nuclear waste?” “How much does they cost?” “Who need to pay that?” “What is the current plan?”

The members of SNB (Sayonara Nukes Berlin) originally asked both questions: nuclear decommissioning and nuclear waste management. However, two and half hours were not enough to talk about both themes. So, we have this second meeting.

There are two parts alternately in this report: Fukumoto’s lecture and Yamauchi’s memorandum. Yamauchi’s memorandum is the part of how the reporter understood.

Introduction

In the first meeting, Fukumoto pointed out the importance of “think through what I can do as a citizen,” when anyone thinks about the energy problem. He would like to add one more point in this meeting:

After the Fukushima incident, we had blackout days. When the power companies explained that this is because we do not have nuclear reactors running. Can you still be against the nuclear reactor? Please think through this once more.

He mentioned the importance of thinking about the problem first and having an opinion. It is important to understand the problem yourself and also important to find the information by yourself. It does not matter whether you are in favor of the reactor or against the reactor. But you need to find the correct information and you need to think about the problem based on that information.

Yamauchi’s memorandum

The sections of Yamauchi’s memorandum shows the reporter’s opinion and research; how we think in the current situation, how we think about our future. Under our current political system, these ideas will be shown in the vote. After the meeting, I reconsider this.

What I thought was that: Do I want to have a future with nuclear reactors? Do I agree with the nuclear reactors because the electric company said they are needed. Do I disagree with the nuclear reactors if the electricity is enough? I thought first I needed a clear vision for our future. The vision should not depend on whether the current electricity is enough or not. The reality would have some compromises, but the vision should not. I would like to think about what I could do, or what each of us could do for our future. Of course this also includes the near future, tomorrow. Shall I make a compromise tomorrow? Another idea is that there is no future without tomorrow. So we could put the highest priority on tomorrow. It is not a simple problem. Still I think I should have a clear vision for the future. Because we and our children will live in that future. In my personal opinion, I could not agree with the idea that now is important enough, so that we can destroy the future.

We see a lot of potential in our technology. I understand that we cannot stop using electricity now. However, we have some technology that generates sustainable energy. For example, a combination of solar energy and hydrogen energy, we might have a stable energy source. We could develop a technology to produce this energy at low cost in the future. Investing in such technology is also a way of working on the future. One day we could export such technology to the people who need all over the world.

We also have a lot of possibility in politics. We can provide a way to only buy natural energy for the people who want this. This means we can also provide a way to only sell nuclear energy to the people who want it. I see that the current problem in Japan is that the people cannot choose in either way. (There is an official plan to make this possible.) We can work on these political problems together.

In the end, I need to have my own opinion based on the current situation. It will be cumbersome, it will be hard, but I need to understand the current situation. I hope this report is of use to someone who would like to understand the current situation.

We tend to avoid the politics, we can leave it alone. But politics will catch you. — Richard Stallman.