Wikipedia の劣化ウランの医学的危険性の主張と反論を見ると，日本の文部科学省は劣化ウランの毒性は身の回りの海水や土砂中に存在するウランと同じ又は小さい」(平成14年11月)と発表しています．しかしこれではウランそのものの危険性がわからないので，比較になりません．一方，劣化ウランの危険性の項を短くまとめると，「劣化ウランは重金属であり，重金属中毒の原因となる．その毒性は砒素と同程度である．」という主張があります．また，アメリカ政府とWHOは危険性に対する証拠は不十分であると反対しています．ただし，子どもは口にしないよう WHO は警告しています．またアメリカの法律では劣化ウランは有害物質に指定されており管理下に置かれなくてはいけません．
We have seen the graph of “Radioactivity attenuation of vitrified waste over time.” How long time we need to keep the waste is based on the radioactive level of mined Uranium ore. I wonder if it is save. I could not find information about this. I know that not everything in nature is safe. A venomous snake, poisonous mushroom, volcano gas, … they are all natural, but not always safe. Uranium ore is in nature, but is it safe?
A Wikipedia article entitled “Depleted uranium,” Japanese MEXT (Ministry of Education, Culture, Sports, Science and Technology) stated that the toxicity of depleted Uranium is the same of the Uranium in the sea or rocks [1] (in November, 2002). This doesn’t tell much. I would like to know about the safty of Uranium ore in the first place, then why all of sudden am I talking about depleted Uranium? You might think that I switched subject. Unfortunately, this is the information that I found on the Internet, which is only indirect. I could only find two connected things that Uranium ore has danger similar to depleted Uranium and that depleted Uranium is dangerous. Assuming they are both correct, we could conclude that Uranium ore is also dangerous. Yet I doubt there are many different types of depleted Uranium (density and so on) and many different qualities Uranium ore. I don’t believe all the Uranium ore have exactly the same density of Uranium rocks. I didn’t understand this information completely. Some say depleted Uranium is dangerous and some say it is not. The former say, “it is not dangerous, but it’s a harmful substance, so it must be controlled under the law (US government), and children should not take it (WHO).” Then, Uranium ore from nature is as safe as depleted Uranium (Japanese MEXT), or less safe. You could check this out yourself in Wiki [1]. Personally I cannot judge any risk based on such low quality information. Then my decision can only be to avoid them. Without information, we cannot decide on anything. We need more information.
Also it is not clear that “the corresponding radioactivity of 1t fuel is 1000GBq.” The value is 1GBq/kg (=1,000,000,000 Bq/kg) which is quite large. Common food has a safety threshold at 100 Bq/kg. 1,000,000,000 Bq/kg which does not sound safe at all to me.
Only one thing is certain, nuclear waste needs 100,000 years to get to this 1,000,000,000 Bq/kg state.
Most of the articles about final disposal mentioned this number, 100,000 years, and they usually ask: can we keep it for that long? I think that is a wrong and meaningless question, because even if we can manage nuclear waste for 100,000 years, it is apparently not safe. As far as I can understand, Uranium ore is not safe and 100,000 years is too short of a time for final disposal.
I realized that the dosage depends on how you measure it. This also depends on the measurement location. The measurement depends on the height of the measurement device. (Thus, it is mandated to be placed at the height of 1.5m above ground level.) If you clean up the measurement point, the dosage at that point becomes lower. We should care about the hotspot (a relatively small spot that has high radiation), however, we cannot say that the whole region has high radiation just because of one hotspot. In an extreme example, if we can avoid all the hotspots and the rest of the place has no radiation, we can just avoid all the hotspots and live safely. The existence of hotspots is just telling us that there could be some dangerous spots. When we observe some hotspots we usually need more measurement points. It could be that the measurement points are too sparse and we might miss some of the high radiation points. We should care about how the radiation is measured and we need to think about the risk ourselves.
For example, if the measurement point is located near a station, it is good since a lot of people are passing by. If the location point can not be accessed by anybody, it usually doesn’t matter. But such a place is usually cleaned up frequently, thus the dosage may be lower, however only in the surroundings of the measurement point. There might be higher dosage 20 meters away from the measurement point. When we are told about the values of today’s radiation, we should also know where are the measurement points. If the whole area has not been cleaned up and only some spots were, we cannot use the cleaned up spot as a measurement point.
If someone decides to live in a low radiation place, they should think what is the important place for them. If you almost never go to a station, the radiation measured there is not so important. For you, the important place could be a school, your working place, and so on.
Radiation is not just one kind (What is Sievert?)
If you check an introductory text of chemistry, you notice that the radiation is not just one kind. There are many kinds of radiation: alpha radiation, beta radiation, gamma radiation, neutron radiation, and so on. The different kind of radiation affects differently the human body even at the same absorption dosage (gray in unit). For example, alpha radiation is more dangerous than beta radiation at the same energy level. Therefore, there is a unit called Sievert that adjusts for this difference (Figure 8). It seems scientists agreed that we need some adjustment, but how much adjustment is needed has raised some discussions. For example, how much more dangerous alpha radiation is compare to beta radiation is difficult to determine. This adjustment is called “radiation type weighting factor.” This currently differs from country to country. In Japan, this factor is 20 for alpha radiation, 5 for proton radiation, based on a gamma radiation is value of 1. According to IPRP report 103 (Wikipedia, Sievert), the factor is 20 for alpha radiation, 2 for proton radiation, based on a gamma radiation is value of 1. This coefficient is multiplied by gray to get the value as Sievert. Sievert considers the effect to the human body, but this weighting factor is hard to determine, because this depends on many other factors like person’s age, sex, health status. It is also difficult to make detailed experiments with the human body.
The following analogy is only understandable for someone who likes computer games. But I think this is a good analogy, so I will try to use it here. In many fantasy games, usually a character has some attribute, like fire or water. If a character has a water attribute, he or she can resist more to the water magic. When a character is hit by some fire magic, the damage this character gets depends on his/her fire attribute. At the end, how many health points the character lost is the most important effect. The damage has been adjusted according to the attribute. For radiation there are different types, and the damage to the human body depends on the type. In a similar way, Sievert is an adjusted value of the absorbed energy (gray).
Figure 8. There are many kinds of radiation
If a human body gets the same amount of energy but in a different amount of time, the effect of radiation would be different. For example, whether a person gets 10mSv of dosage in a day or in a year it is a different thing, we cannot add yesterday’s radiation dosage and today’s radiation dosage. However, we still have no better measurement than Sievert. It seems Sievert is the best approximation to measure the effect of radiation on the human body. The effect of radiation is not linear, but the Sievert unit assumes that the effect is linear. If you are not familiar with the word `linear’, it is somewhat similar to say that `you can add that up’.
For example, a person cannot eat 500g of salt in an hour, that probably causes death. However, if the same person uses 10g salt per day for 50 days, the danger is drastically less (yet, it still is too much salt). We usually cannot simply add up the amount to difference the effect on a human body.
However, we assume the radiation dosage (Sievert) can be added as an approximation. We should remember that this is an assumption. The Sievert unit is not like the meter unit, which can be added up.
I would like to clarify make clear the difference between 1 mSv/y and 1 mSv since I read many news articles about them. 1 mSv/y means that if you stay at a location where your measured radiation is 1 mSv/y for one year, you get 1 mSv dosage. If you stay in such a location for two years, you get 2 mSv. The current criterion of the evacuation counsel for disasters is 20 mSv/y in Japan (2014). If you stay in a location where you have 20 mSv/y location for five years and assuming that the radiation stays the same, the dosage is 100 mSv. Please notice this difference. If you decide to take the 10 mSv risk, you can only stay at a 1 mSv/y location for 10 years. Especially young children usually have higher risk for the same dosage, so you also need to consider long term exposure. The difference between mSv/y and mSv is similar to you need to pay 1000 Euro every year and you need to pay 1000 Euro only once. Please do not confuse every year payment and one time payment.
We have two criteria that define what is nuclear waste. One is based on the Sievert value, the other is based on the Becquerel value. If the waste consists of one kind of radioactive substance, it is reasonable to define the criterion based on the Becquerel value. Since the half life are depends on the radioactive substance, you cannot really estimate what the danger is if the substance are mixed. Therefore, there are criteria based on each nuclide. However, if many kinds of radioactive substances are mixed up, which is often the case, it is difficult to determine what is nuclear waste and what isn’t. If we can separate all the nuclides, we can still use the Becquerel value criterion separately, however, it is usually not easy to do. In that case, we use a criterion based on radioactive dosage — we use the Sievert value in this case.
If we read a newspaper, it seems that these criteria are used arbitrarily. Sometimes we see a Cesium 137 Becquerel value for cleaning up bi-product waste (e.g., [1]). I wonder if there are other nuclides, for example, is there Strontium 90 in the waste?
Another problem is what kind of radiation can the measurement devices measure. Most of the measurement devices can only measure gamma radiation, but there are other types of radiation.
If you review the periodic table in chemistry, you may notice there are mass numbers in it. For example, there are several kinds of the same element, e.g., Cesium 134, Cesium 137. They are isotopes. These numbers represent their mass number. For example, there are many kinds of radioactive Cesiums. If a newspaper mentions a radioactive Cesium, I would like to know which one it is. Because their half lives differ. The half life of Cesium 134 is around 2 years. The half life of Cesium 137 is around 30 years. This means, the Becquerel value of Cesium 134 becomes 1/1000 after 20 years. On the other hand, the Becquerel value of Cesium 137 becomes only less than half after 20 years. You could learn these things in high school. I found high school science is quite sufficient to know most of these things. I understand that the information sources have only limited time and amount of information they can give. If you know more about this basic chemistry knowledge, you can understand the provided information more.
This is maybe a small detail, the safety criteria of the radioactive waste depend on organizations, countries, and the year [2],[3]. According to Japanese prime minister office (首相官邸)[4], the criterion based on which you should leave an area is when there is more than 20mSv/y. This threshold for the Ukrainian government is 5mSv/y [5]. There is no right and wrong here. Risk is evaluated based on some assumptions and these assumptions differ from government to government. The governments determine these assumptions, therefore, the criteria depend on the country.
In the end, there is always a risk when there is radioactive waste. There is no absolute safety, but there is also an acceptable risk level. Governments usually provide documents about how they consider the risk and what are their assumptions. We need to determine whether we can accept the risk and the assumptions. (Although if a government forces people to accept the risk in some way, or if people cannot decide on their own, I believe this is violation of human rights.) To judge the risk, we first need to understand the information. Then we need to think on our own. The risk is usually probabilistic, we should think through it and decide whether we can accept it or not. If people cannot agree with the government decision, then they should change the government.
The message I’m sending here is to understand this information. This is a first step. It would be not so simple at the end, but the important thing is that we understand the information and we take our decisions on our own.
References
Kahoku-shinpou (河北新報), The governor of Miyagi-prefecture has accepted for the investigation to build the final disposal repository (宮城県知事、詳細調査受け入れ 最終処分場), http://www.kahoku.co.jp/tohokunews/201408/20140805_11016.html, (Online; accessed 2014-12-26)
Ministry of Health, Labour and Welfare of Japan (厚生労働省), How we handle the radioactive substance in food (食品中の放射性物質への対応), http://www.mhlw.go.jp/shinsai_jouhou/shokuhin.html, 2014, (Online; accessed 2014-12-21(Sun))
Ministry of Health, Labour and Welfare of Japan (厚生労働省), We updated the safety criterion of radioactive substances in food (食品中の放射性物質の新たな基準値を設定しました), http://www.mhlw.go.jp/shinsai_jouhou/dl/leaflet_120329_d.pdf, 2014, (Online; accessed 2014-12-21(Sun))
Prime Minister’s Official Residence (首相官邸), Keeping everyone’s safety (みなさまの安全確保) http://www.kantei.go.jp/saigai/anzen.html/, How we set up “the planned evacuation region” and “preparation necessary region when the emergency “(「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))
前回紹介した Wikpedia や化学入門などの解説をご覧になられた方は原子というものが何かとか，放射線とは原子核の一部の陽子や中性子であったり，高エネルギーの電子であったり，あるいは高エネルギーの電磁波であったりということをご理解されたかと思います．これらが危険なのは，私達生命が生きていくために重要な私達の体の中の DNA 等を破壊することができるからです．どれだけ破壊するかというのは，放射線を放射する側のエネルギー，そして生体側がどれだけ吸収するか，などにかかわってきます．それは，どれだけ攻撃が強いのか，どれだけ守備が強いか，に似た感じです．これについて知るために，まずは，放射能と放射線と，その強さについての言葉について考えましょう．それを示す言葉がないと，なんだかわからない怖いもののままであったり，なんだかわからないから，権威の言うことを信じるしかないということになりかねません．そこで，ちょっと退屈かもしれませんが，放射能，放射線，ベクレル，シーベルトなどの言葉の意味をまとめてみます．
If you check Wikipedia or any chemistry course, you have some idea of what is an atom, what is radiation, what is radioactivity, and so on. Why is radiation dangerous? Because it can break the DNA in our body and that makes you sick. How much does it can break depends on the radiation energy, on how much energy our body absorbed and so on. In a way, it depends on how strong the offense is and how strong the defense is. To know more about this, first we should know about the concept of radioactivity, radiation, and their strength. Since we don’t know the meaning of those things, we are left with trusts the authorities on these matters. I would like to avoid such situation. Here I summarize the meaning of radioactivity, radiation, Becquerel, Sievert, and so forth.
We can use an analogy with light to think about these substances.
Figure 6: An analogy between a light bulb and radioactivity
Radioactivity is also known as radioactive decay or nuclear decay. It is the process of a nucleus of an unstable atom losing energy by emitting radiation to move to a more stable state. If an atom can emit radiation, it is considered radioactive. From an observer point of view, saying that something is radioactive means that a substance can emit radiation. It is an ability of emitting radiation. If you think about a model with an offense and a defense, this is the offense side. If you think about this as light, a light bulb has the ability to emit light, so I could say that a light bulb is light-active. In this analogy, the emitted light is itself radiation. Figure 6 shows how a light bulb and a radioactive emitter that correspond to each other. Radioactivity is more like a property of a substance, that is similar to the property of a light bulb. Please note, radioactivity does not correspond to emitted light, but rather to a light bulb. A light bulb has the ability to emit light, but a light but is not light per se. We can grab a light bulb and move it to a box, then we can take it out from the box, but we cannot grab light and put it in a box. We can also use the analogy of sound. A speaker has the ability to create sound, so we could say it is sound-active, but, the speaker itself is not sound. I hope by now you know the difference between radioactivity and radiation.
Radiation is the emission of energy from the nucleus. When a nucleus changes the state, an emission happens. What kind of radiation will be emitted depends on the type of nucleus. The unit this quantifies how many times the decay happens in a second per a certain unit mass is called Becquerel (Bq). This is the strength of the offense side as we were saying earlier. A certain unit mass means for example 1kg or 1t, it is just some amount of mass.
Why do we need a certain unit of mass? Because even if we only have the same Bq radioactive substance, when we have more of it, the number of decay events increases. It is same as saying that if we have more light bulbs switched on, we get more light. Therefore, usually we see this amount of number of decay per kg (Bq/kg). If 1kg of radioactive waste is 8000 Bq it means that this 1kg of radioactive waste has 8000 decays per second. If there is 2kg of this waste, you observe 16000 decays per second. When the number of decays per second (Bq) increases, the corresponding radiation energy will also be increased. However the Bq/kg in this example is the same in both cases, that is 8000 Bq/kg. If an article only said Bq without ‘/kg’, then that should be the number of decay per second. If the certain unit of mass is smaller, then the number becomes smaller even if radioactivity stays the same. 8000Bq/kg is equal to 8Bq/g. If some food has 1000 cal/kg, it is the same as 1 cal/g. Please be careful with units.
Sometimes a criterion is defined on Bq/kg. For instance, a safety criterion defines that water is safe if it is less than 10Bq/kg. Personally, I have a problem with this criterion. Since you can add non-contaminated substances to mix with contaminated waste, you can fulfill this criterion. For example, if you have a 1kg 8000Bq/kg waste, if you mix it with a clean 1kg substance you have 2kg 4000Bq/kg waste. If you have high Bq/kg tritium water, you can just put more water and achieve low enough waste water that allows to throw it in the sea according to this criterion. If any poisonous substance has this density criterion (which sometimes make sense), we could put it in the sea and pass this safety criterion. Do you think we can continue with that? I find this is dangerous. For me this density criterion (Bq/kg) is questionable. At least I would like to know how much is the absolute value, like some “8000Bq/kg waste 10kg was trashed.” If you understand this unit (Bq/kg), I think you know what kind of information you would like to know more. If I only have the Bq/kg, I see that the information — How much waste have you actually trashed? — is hidden away.
Figure 7: An analogy between light and radiation
Amount of radiation (energy)
By now, you see that the real danger is radiation rather than radioactivity. We cannot switch off radioactive substances in order not to emit radiation. (On the other hand, we can switch off a light bulb in order not to emit light.) Thus, radioactive substances are of course dangerous. But, we could block or weaken the radiation from radioactive waste if we can put it in a thick container of lead. Therefore, if radioactive waste is well under control, we can stop the radiation. Then there is no harm to life. The problem is not like in a situation, where some radioactive substance is dispersed in the environment. In that case, how much radiation is emitted is important.
A dosage shows how much radiation is there. To say that more exactly, a dosage shows how much radiation is absorbed by something. But we could think about this as first how much radiation is there and then how much that affect a person. Figure 7 shows an analogy between light and radiation. If you go away from the light source, the brightness your receive (irradiance) becomes lower, which means it gets darker. In that Figure you see two kind of units. For radiation, we consider the energy absorption instead of radiation energy itself. You see two kind of units, Gy (gray) and Sv (Sievert). Why do we consider the energy absorption? Because if a human body is exposed to the same radiation energy, the effect also depends on how much radiation energy was absorbed by the body. For a measurement about safety, the amount of absorption is important. We measure how much energy is absorbed by gray (Gy). It is defined as the absorption of 1J of radiation energy per 1kg of matter. (If you don’t know about what is 1J, Please look up what it is.) You can imagine that thinking of the amount of energy absorption is similar to thinking of how much damage you got.
If you play some role playing video games, you know the damage depends on what is your armor or how you got the damage. That is represented by decreased your health points. In these games, what is important is how many health points you lost, not how strong the attacker is. The amount of absorption for radiation is like the amount of decreased the health points. But actually the most important value is the remaining health points, not their past decrements. However, the remaining health points for radiation depend on the person, age, and so forth. There is no simple criterion for safety, but the amount of absorbed energy could be one of such criteria.[:]
原子などについての科学的知識の資料としては Wikipedia の記事[1]からはじめるのも1つの手です．また，化学のオンライン授業を提供するサイトなどに原子の話があります．これらのサイトは原子力発電とは関係ありませんので安全性などについてはほとんど述べられていません．しかし，「放射線とは何か」などについては，初学者向けに比較的容易な解説があり参考になると思います．手前味噌ですが，化学の初学者向けの解説ビデオとして [2] を挙げておきます．ただし，これは中学から高校生向けの一般的な化学入門なので，もっと簡単なものが知りたいとか，その反対にもっと高いレベルのものが詳しく知りたいという方々は，御自身でいろいろと調べてみて下さい．
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.
[:en]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
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]
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.
Figure 2: Process flow of used nuclear fuel
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
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)])
やまうちは原子力発電所から出てきて，放射線を出すものは全て放射性廃棄物であると誤解していました．この放射性廃棄物かどうかの基準値そのものの意味，つまり，放射能の単位のベクレルと放射線量の単位のシーベルト，また放射能と放射線の違いについて別枠にやまうちが調べたことを豆知識としてまとめておきます(FIXME: 後の記事が up されたらここからリンクを作成)．
今回は放射性廃棄物というものがどのような基準で決められているのかについて考えました．次回は放射性廃棄物とされたものはどのように処分されるのかの現状について考えていきたいと思います．[:en]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.[:]
次回は最終処分とはどういうものか，放射性廃棄物とは何なのかについて考えていきたいと思います．[:en]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.
Let’s talk about nuclear waste: A meeting snapshot
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.
