It is a bit of a long post, but here are most of my thoughts on the lecture given by Ben Monreal. The following comments are the thoughts and comments of a chemistry academic.
Recently a Prof (Ben Monreal) at Santa Barbara gave a lecture on what has happened in Japan at Fukushima. I have watched the lecture and thought about it and here are my thoughts as a chemist about what he said.
If you want to know all about the normal operation of a nuclear power plant then this lecture is not for you, Ben choose to concentrate on what happens during a malfunction. I think that this is a perfectly reasonable choice for him to make when writing the lecture.
Ben is right a leak of radioactivity has occurred from the reactor site, I am sure you know that it has happened already. I have to agree with Ben that for many people radioactivity is scary, I hold the view that radioactivity is scary to many people because it can not be detected with the normal human senses, it can cause serious harm to human health, many people feel that they do not understand it and finally people feel that they have no control over it.
I do not want to enter into a detailed debate about what risks humans typically accept and which ones they refuse, but I do hold the view that when people think that they are in control they tend to be more accepting of risks. I stress that when people think that they control a risk they consider it more acceptable, many of us drive cars and ride bicycles.
When you ride a bicycle or drive a car you are exposed to possible harm, you could drive into a lamp post (or fire hydrant like Tiger Woods did) and hurt yourself or ride your bicycle into the path of a tram and be crushed to death but we all try by careful and skilful use of our cars and bicycles to avoid lamp posts, fire hydrants and fast moving trams. This is true we have an element of choice, we can choose to abstain from dangerous behaviours such as reading the newspaper while cycling but we have little if any control over the actions of others. There is always the risk that the perfectly sensible and careful cyclist will be run down and killed at a road junction by the drunken driver. Rather than telling you what to think, I would like you my reader to consider for a moment the question of “is my control of a risky situation true control or is it only an illusion of control”. This question may change the way you consider risks in your life.
Ben asks the questions of “is this radioactive release important to us?” and “how does the radioactivity move from one place to another?”
These are good questions, I have to say that depending on where you live the answer to the first question will differ, and the answer for a family whose home is about 1 km from the plant will be very different to the answer for a family living in china. The question about the movement of radioactivity from point A to point B is very important; this will control the effect on the health of the general public.
Ben then concentrates on four elements (or groups of elements). He concentrates on tritium, iodine, cesium and the actinides. I will say what I think of each of these in turn.
Tritium this is an isotope of hydrogen which forms inside nuclear reactors, it is formed by neutron activation of boron, lithium and water. The boron is present in control rods, while the lithium is present because it is used as the hydroxide to adjust the water chemistry in the reactor. By using lithium-7 the tritium production can be greatly reduced, this is because lithium-6 is converted by neutrons into alpha particles and tritium while lithium-7 can not do this reaction. The production of tritium from water in a light water reactor like the Japanese plant will be much less than what I would expect in a heavy water plant such as a CANDU.
I think that as soon as the reactors started to leak that any tritium in the cooling water would have started to escape. I think that this isotope is a minor isotope in this accident.
While tritium in the form of water is very mobile, I hold the view that is one of the least dangerous isotopes. Tritium only emits a very low energy beta particle, it is so low in energy that it can not be detected with a Geiger counter and it is so unable to pass through human tissue that tritium (outside the body) is not able to reach a fetus growing inside a woman. As a result some radiochemical workers are able to carry on working while carrying their unborn baby inside them. Internal contamination with tritium (getting it inside your body) is not good, but when compared with most isotopes it is not that bad a deal. The tritium is rapidly washed out of your body; it has a short biological half life. So it will not concentrate in the human body, as long as you drink non tritium contaminated water it will be soon washed out of your body. I am aware that in cases of humans who have been very strongly contaminated with tritium at work that one treatment was to give the person about 5 litres of water (or beer) to drink. This method of tritium removal was used many years ago at Harwell (nuclear research centre near Reading in England). I do not think that it is likely that any member of the general public will get contaminated with tritium to the point at which they need the 5 L of beer treatment, I will also say that for mothers to be (and the operators of cars & bicycles) that 5 L of beer is a very bad idea.
Also the beer trick only works for people who have been contaminated with tritium; it does not work for any other isotopes. Also if the normal drinking water is contaminated with tritium then it will not work, all the beer drinker will do is to swap one group of tritium atoms for some others, waste a lot of money, get very drunk and get a nasty hangover the next day.
If you want to read about tritium and reactor water then see http://www.nap.edu/openbook.php?record_id=9263&page=113
Cesium is an important element, in a reactor accident the cesium isotopes are often the isotopes which will have the greatest medium term effect on the public’s health and farming, the cesium isotopes are made by nuclear fission (Cs-137) and by neutron activation of fission products (Cs-134 and Cs-136). I have to agree with Ben that the cesium is very important.
It is interesting that a Cs-134 measurement is an easy way to distinguish between fallout from a recently detonated atom bomb and the radioactivity from a nuclear power plant accident.
Iodine is more important in the short term than cesium, this is because the radioactive iodine from a nuclear reactor accident is more mobile than cesium, and it also concentrates into a single small organ (thyroid) in the human body while cesium is spread over the whole body. This concentration into a single small organ makes exposure to iodine-131 a greater health hazard than exposure to the same amount of cesium-137. Ben is right the iodine is very important, after Chernobyl the iodine-131 exposure of the general public was responsible for the vast majority of the health effects which have been seen so far.
For pregnant women iodine is of special concern, radioactive iodine is more toxic to neonates and the foetus than it is to an adult. As a result women who are radiation workers are often strongly advised not to work with radioactive iodine if they become pregnant. Also the breastfeeding woman is often advised not to work with radioactive iodine, if a lactating woman has to be treated with radioactive iodine it is wise precaution for her to stop breast feeding her baby. If this ever happens to you, I suggest that you discuss it with your doctor as some actions will be needed to enable a woman to return to breastfeeding.
Ben also concentrates on the actinides; these are elements such as neptunium, plutonium and americium. While these elements are much more toxic to humans, thankfully they are far less mobile than the tritium, iodine and the cesium. While I think Ben was right to talk about them, I think under the conditions of the reactor accident they are much less important. However in the case of a fire in the used fuel storage pond, the actinides are very important.
Ben says that the different elements will behave in different ways according to their chemical properties, this is very true. The chemical properties of the elements determine their mobility and how they behave when they enter living things (grass, trees, rice plants, onions, sheep, cows, cats, dogs, humans etc). I think that the chemical effects on the mobility of the elements are very important.
Imagine we have gone down to the zoo to take our minds off the nasty reactor accident and relax by looking at the pretty animals. Some animals are quite harmless (bunny rabbits and tortoises), other animals can threaten our wellbeing and comfort (goats which chew the toggles off your coat), some can cause injury such as the aggressive monkey while some deadly animals like the cobra and the tiger can kill people with ease. It should be clear to you that both the snake and the big cat are very bad for your health, while the tiger could eat you the transport properties of the tiger are different to a small but deadly snake. If the tiger’s enclosure has a hole the size of a man’s fist the tiger can not escape through this hole. As long as you are so super stupid that you stick your arm into the tiger’s den the hole does not allow the tiger to hurt you.
However a snake can pass through the small hole, if the snake finds the hole it might slither out through the hole into the part of the zoo where the public are. This snake has now escaped and poses a threat to the general public. Both the tiger and the snake might be equally able to hurt people who stumble across them in the wild, but in the zoo setting the transport properties of the snake make it much more dangerous to the visitor.
While the hole on its own did not allow the tiger to be dangerous to visitors, the use of a single barrier to protect the general public from the tiger is not good enough. A prudent zoo owner will add additional layers of protection to increase the safety of even the most stupid of visitors. For example a fence can be used to keep the public two meters away from the surface of the tiger cage. This idea of a series of defences is known as “defence in depth”. “Defence in depth” is a common policy in the nuclear industry, the idea is that a series of different barriers should protect both plant workers and the general public. The barriers are chosen in such a way that if one fails then another one will then ensure the safety of the humans.
A further idea in the nuclear industry is “defence in diversity”; if we imagine the zoo we could add a water filled moat could be used to improve the separation of some of the animals from the humans. This is a different type of barrier. It works in a different way, but we would have to make sure it is suitable. I think that tigers can swim, so this barrier is not going to stop the tiger. The idea of defence in diversity is that if some event occurs which causes one type of protective layer to fail then this event could also undermine the next layer.
Imagine the “bad attitude monkey”; this is a very attractive looking and clever species of monkey who through many years of being locked in zoos have developed an intense hatred of humans. If a chance exists that the monkey will learn how to climb the chain-link fences then placing two chain-link fences around the monkey enclosure will not be twice as good as protecting the public as having a chain link fence and some other barrier (such as an electric cattle fence). The reason for choosing diverse defences is that while it is possible for an event to undermine many identical defences it is less likely that the event will undermine two defences which are very different in design. This is why it is better for a nuclear power plant to buy two diesel generation sets from different companies rather than getting a cheap deal on two identical sets. Imagine if brand X is very sensitive to the vibrations from earthquakes while brand Y sets tend to choke and die if insects fly into the air intake. It is less likely that a plague of insects descend on the plant on the same day as an earthquake.
Ben was correct when he said that the number of protons defines the element, the number of neutrons is important. The number of neutrons often defines the radioactivity / non radioactivity for elements lighter than lead. This was perfectly correct in the lecture.
Any nucleus which has too many neutrons or too few neutrons (a proton rich nucleus) will tend to decay by beta decay. Two main forms of beta decay exist; there is the conventional emission of an electron (which tends to occur for neutron rich nuclei) and the emission of a positron (antielectron) which occurs for the proton rich nuclei. As an alternative to positron emission some nuclei such as beryllium-7 undergo beta decay by electron capture.
During beta decay neutrons are converted into protons (or the other way around). See my post of the nuclear U bend for some more details. (https://markforeman.wordpress.com/2010/10/23/the-nuclear-u-bend/)
Proton rich nuclei are not very relevant for a reactor accident, very few proton rich isotopes are formed in nuclear reactors. Ben is right on this point. The proton rich isotopes tend to be formed by cyclotrons and are typically used for medical purposes. For example PET scanning often uses F-18 which decays by positron emission to form O-18.
In a nuclear reactor both the neutron activation and the nuclear fission tends to form neutron rich isotopes which decay by electron emission towards the line of stability. Ben is right on this point; a valley of stability does exist. This valley of stability for light elements such as oxygen, chlorine and carbon exists where the numbers of protons and neutrons are equal, but for the more heavy elements such as gold, lead and mercury the number of neutrons in an atom of a stable isotope is greater than the number of protons in the atom,
As in general the neutron rich isotopes will decay by beta, he is right that the majority of the radioactivity which we are currently dealing with is beta. But he did say that in the future (depending on fires) that this might not be true for ever. We will get onto the fuel pond fire later.
This statement that we are mainly dealing with beta emitters is correct; you might ask why I have not mentioned gamma emitters so far. Gamma emission occurs when a radioactive decay event occurs and the altered nucleus finds its self in an excited state. The nucleus will lose energy by the emission of a gamma ray. Gamma decay can be linked to both alpha and beta decay.
It so happens that each radioisotope has different nuclear properties, as a result those which emit gamma rays normally can be distinguished from each other by equipment which can measure the energy of the individual gamma rays. This is important as some gamma rays are very low in energy (can hardly go through a car door or a phone book) while other gamma emitters are super penetrating. For example some of the lower energy lines emitted by americium-241 such as the 26.3 keV line is not much higher in energy than the X-ray photons used to look for breast cancer in a mammogram. Typically Mo-K X-rays are used for looking at breasts, the photon energy is about 17.5 keV,
While at the other extreme the gamma rays from a sodium-24 source are very high in energy at about 2.8 MeV. These gamma rays are able to pass through a thick steel or lead block.
(For details go and see http://atom.kaeri.re.kr)
Fission is the conversion of uranium to two parts; he is right about the fission process.
The fission products are neutron rich; this is because a uranium atom has 92 protons and about 144 neutrons. If the nucleus was to split into two equal parts (a rare splitting) then we would have two atoms of palladium with masses of 118. This is far more neutron rich than the most stable neutron rich isotope of palladium which is palladium-110. Normally fission forms two unequal sized nuclei.
Ben rightly said that the fission product radioactivity has a finite life, this is correct the radioisotopes will decay away to stable atoms. However horrible a radioisotope is, the good news is that it will one day decay away, the bad news is that for some radioisotopes you need to wait a very long time.
But the good news is that if a radioisotope has a very long half life then the radioactivity for a given number of atoms will be much smaller than a radioisotope with a shorter half life. The radioactivity level is given by the following equation.
Activity (Events per second, Bq) = Number of atoms x (ln 2)/half life (seconds)
As a result a radioisotope with a super long half life such as 40K or 99Tc has a low radioactivity level per gram, while a short-lived isotope such as 8Be has a very high radioactivity per gram.
If the half life is super short then the radioisotope will decay away before they have a chance to contaminate a human or get into a troublesome place. I have to be careful on this issue as if a very short lived beta or gamma emitter is formed inside a sealed source (closed source) near a human it is possible for the decay of the radioisotope to expose a human during its short lifetime. For example 137mBa is a short-lived gamma emitter which is formed by the beta decay of 137Cs. The vast majority of human exposure to a “Cs-137” source is due to the short-lived daughter.
While we are at it, we might as well consider the term. I suspect that the term “daughter” was used as Marie Curie and her daughter were leading early radiochemists who may have adopted the feminine rather than the masculine term for the product nuclide which is formed by a radioactive decay. In some ways you can think of a radioisotope giving birth to a new isotope by means of the radioactive event. But that is just my theory about the origin of the term “radioactive daughter”.
Ben is right about the origins of elements more heavy than iron. After a supernova occurs many neutron rich radioisotopes are formed which decay into elements such as iridium, gold, rhodium, and ruthenium.
Ben is right about the majority of the power from a reactor being fission while a little is from decay.
Ben is right that some side reactions (neutron capture on fuel does create more unstable elements in the fuel which are called minor actinides). But for some reason his arrow goes to left not the right. Most minor actinides are made by neutron capture not neutron loss.
The minor actinides are neptunium (Np), americium (Am), curium (Cm), Berkelium (Bf) and small traces of higher actinides can be found in the fuel. The concentration of the minor actinides tend to be higher in reactors which are operated using MOX fuel. Many of the minor actinides are formed from plutonium by neutron activation.
239Pu + n –> 240Pu
240Pu + n –> 241Pu
241Pu –> 241Am (beta decay)
The first of the three reactions I am showing above is very good for world peace and good will between men, if a bomb grade Pu is used to make MOX then when the MOX is used in a power reactor the isotope signature of the plutonium changes to make it less suitable for making bombs. Even if more 239Pu is made by the following three reactions
238U + n –> 239U
239U –> 239Np
239Np –> 239Pu
The neutron activation of 239Pu to form 240Pu will spoil the isotope signature of the plutonium, the reason why a trace of 240Pu in plutonium has such a good effect for world peace is that it has a much higher spontaneous fission rate and its presence makes it harder for a bomb maker to build a bomb which will detonate as intended.
It is important to always understand that plutonium is not all the same, to make a good quality bomb useable plutonium a uranium fuel needs to be used in a reactor with only a low burnup (small harvesting of energy), the higher the burnup the worse the plutonium quality will be for a bomb maker. This makes the production of bomb grade plutonium very incompatible with the production of electric power. An electricity utility company want to squeeze the most energy out of each fuel element which means they want to get the highest burn up possible from the fuel.
One particularly obnoxious plutonium isotope has is not formed from 239Pu, this is the 238Pu which is a signature isotope of power reactors and is not normally seen in large amounts in bomb fallout. It is formed from 235U in a series of reactions which require time for beta decay to occur, so as a result when neutron fluxes are very high (H-bomb detonations and supernovas) it does not normally form.
235U + n –> 236U
236U + n –> 237U
237U –> 237Np (beta decay)
237Np + n –> 238Np
238Np –> 238Pu
It is important when a release of plutonium occurs to consider the 238Pu release; while it may be a small percentage of the plutonium atoms it is much more radioactive than 239Pu. As a result the 238Pu can pose a greater threat to humans than the 239Pu.
Neutron activation of steels, water and air does occur inside reactors. Induced activity is a problem for plant operators but is not the majority of the activity. The induced radioactivity is a problem for plant operators, some of the very short lived isotopes such as nitrogen-16 causes the dose-rate in the turbine hall of a BWR to be quite high during normal operation, as the half life is short (7 seconds) it soon declines after shut down. I would suggest keeping humans out of the area near the turbine of a BWR while it is running and for some minutes after shut down.
The activation of steel and corrosion products can increase the radiation levels inside the reactor’s containment; this can make it harder for workers to perform repair work on the reactor without getting a dose of radiation. Also when the reactor is dismantled at the end of its life it can make it harder to dispose of the metal.
Ben is right about the different types of radiation
Alpha, this causes very intense damage but is very short ranged. Unless the alpha emitter has been swallowed or inhaled it is not a threat to your health, but if it has entered your body it can do a lot of harm.
Beta/gamma is able to pass through your body and damage it even if the radioactive atom has not entered your body. But the damage caused by beta and gamma is not as damaging to your body.
While neutron radiation is both very penetrating and also very damaging in this case we do not have to consider it. In criticality accidents (like the 1999 accident at JCO’s plant at Tokaimura), when working with large amounts of minor actinides such as Californium (which decays by spontaneous fission), some exotic forms of radiotherapy and when a nuclear bomb is used then we do need to consider the effects of neutrons on living people. The neutron flux inside the nuclear reactors would be declined to close to zero after the control rods were inserted into the core.
Ignore the neutrinos not important, many nuclear processes release neutrinos these particles almost never ever interact with matter so it is very unlikely that they will deposit any energy in a human body. Neutrinos can pass right through the earth without interacting with any of the atoms they pass.
Ben gets the units right, he also is right that a lot of steps are required to go from activity to hazard.
Ben is very right to include the sievert and the relative effectiveness of the different radiations. The events caused by gamma rays are much less able to damage cells than the intense ionization caused by an alpha particle.
A curie is a number, he is right to say that a curie of tritium is much less dangerous (external threat) than 1 Ci of Sr-90. I would keep my distance from 1 Ci of Sr-90 also. A curie is a number. I would not be willing to walk around with any radioactive source in my pocket, it is a very bad habit to start. Also if the source was damaged it could start to leak. As a general rule never put a source in your pocket.
Ben is right that a few mSv per year is not a dose to worry about. Background doses of 1.5 to 7 mSv per year to humans are typical. But mSv per hour can add up to dangerous doses.
Ben said that 1 Sv will not kill you quickly with “radiation sickness”, it is hard to work out what is the dose which will kill 50 % of a human population. One 1980s estimate by the US government was that with a poor diet and no medical care that the LD50 dose for radiation is 2.5 Sv. While the 1960s estimate of the LD50 with the best medical care which was possible in those days was 3.5 Sv.
If a population of humans was to get a dose of 1 Sv in a very short time I would expect to see some non fatal health effects in many people, the cell counts of blood cells might change. I would also expect to see some other effects in some blood tests. It would not be good for the population to get this dose. With these acute effects a minimum exposure is required to cause the effects, and the greater the dose of radiation the more intense the effects are.
The international atomic energy authority (IAEA) think a 1 Sv exposure can be dealt with as an outpatient and the US military think that at this level of exposure there will be no deaths due to the acute effects of the radiation.
A 1 Sv dose is bigger; risk of cancer becomes more important. Ben says that a habitual texter while driving person will get the same risk as 1 Sv. Ben considers the risk of death in a car wreck, his data suggests that a normal person has a probability of 1.5 x 10-4 chance of dying in a car crash each year.
If we use the assumption that a 1 Sv dose has a 5 % chance of inducing cancer and we make the pessimistic assumption that all cancers are fatal then a 3 mSv dose of radiation each year would have a 1.5 x 10-4 chance of killing you.
Ben told me that texting while driving is thought to increase the risk of a fatal car crash by a factor of 4, I suspect that it is hard to estimate how dangerous texting while driving is so I do not trust the x 4 factor totally. If we assume that the x 4 is correct then texting while driving for a year will carry a risk which is equivalent to a radiation dose of 12 mSv. I hold the view that while it would take a lot of texting while driving (111 years worth of this bad conduct, a lot of phone credit and a case of RSI) to accumulate the same level of risk as a 1 Sv dose of radiation it is still an optional risk which I would choose to avoid taking.
I hold the view that a better comparison can be made with a more obviously repulsive habit (smoking), like many things the radiation dose which is equivalent to a cigarette is estimated at different doses by different radiation protection experts. The following source suggests that 1 packet of cigarettes will have the same threat as 25 x 10-6 Sv on your health. This suggests that if you smoke 2 packs a day for a year then you will get the same risk as a dose of 18.25 mSv. If this dirty smoking was to continue for 55 years then the risk would build up to the same risk as 1 Sv dose of radiation.
While another source of data suggests that the radiation equivalent of a cigarette is higher at about 20 micro sieverts, this would make a box of 20 the equivalent to a dose of 400 microsieverts. So a 2 pack a day smoker would get the equivalent of 292 mSv per year. This would require only about three and a half years reaching a risk equivalent to one sievert. As data sources differ so much on the radiation equivalence of a cigarette I think it is best if we leave this “exposure unit” well alone.
While I am at it, I would like to slam the “banana dose”. I think it is a silly idea. The banana dose is based on the fact that bananas are quite a radioactive food due to their high potassium content. All potassium contains a trace of radioactive 40K.
Due to homeostasis (The maintenance of a constant internal state by the human body) if you go on a banana binge (oh that sounds so exotic) and stuff your face with bananas then your body will start to excrete potassium to return your potassium level to the normal level. As all potassium has the same amount of 40K (specific activity) per gram then eating more “normal” potassium will not change the body burden of 40K because the human body will dispose of the excess potassium.
So the increase in 40K in your body is short-lived compared with the increase in the 40K level in your body if one was to swallow a carrier free sample of 40K.
Whatever the degree of harm caused by each cigarette, for me the smoking is a behaviour which is optional which endangers a person, and normally brings no benefit to a person. I think that a better way to think about this is to consider the tale of Mr X. He was a man who worked for many years as a high voltage electrical engineer. He told once me a story.
He was a test pilot during world war two and he never had to fly a single combat mission, as an electrical engineer he was given the task of flying bombers after their electrical systems had been modified. He had to fly them around the UK to see if they functioned correctly. While flying an aircraft as a test pilot is not a totally safe activity, he was quite willing to fly these aircraft. I imagine that if he had got into trouble then he could have made a forced landing on farmland without fear of being captured or harmed by the Germans.
One day he was sitting around the officer’s mess when a group of men got back from having bombed Germany. They told him that he was cowardly and challenged him to join them on a combat mission. Stupidly this man agreed to join the mission, the next day he was even more stupid to not to wiggle his way out.
He was then put on board an airplane and taken on a very long combat mission from England to south east Germany, he told me is was a horrible experience of being shot at. He then explained to me that this experience had a lasting effect on him. He became much more risk averse; he then decided to think long and hard before taking risks in the work place.
He did sometimes take some risks such as being inside defective substations which were live, but he insisted that he would only ever take a risk which was outweighed by the benefits AND he would adjust his behaviour to reduce the risk. For example he told me that a common method of finding corona discharges in a substation is to enter it at night and look for the glow. He told me that the safest way is to enter a de-energised substation with a chair. Put the chair somewhere safe, sit down and then get a person outside the substation to energize the system so he could then look for the glow. He maintained that this was a safer method than trying to enter a live substation in darkness.
Here is an example of a person using the idea of ALARP, we will get onto what ALARP means later.
It is important to understand that if a cancer is induced by radiation then no relationship exists between the dose and how painful or treatable the cancer is. If you were to get cancer of X after a 100 mSv dose then your cancer is no different to the person in the next bed who got the same cancer after a 2 Sv dose. The only difference between the two of you is that the person in the next bed had a 20 times larger chance of getting cancer because their dose was 20 times larger than yours.
I have checked the UK rules on radiation limits, I could be exposed legally to doses higher than 100 mSv only under very dire conditions. The normal limit for a classified worker like me in the UK is 20 mSv.
If a nuclear or radiation emergency progressed to a stage where it endangered the general public the UK health and safety authority have suggested that emergency workers should never be exposed to more than 500 mGy.
I hold the view that for such radiation exposures to be morally acceptable then a series of five rules must be complied with.
1. The person being exposed must have given an informed consent to getting the dose. I hold the view that economically vulnerable people, people in prison, people under the age of 21, the mentally impaired and other vulnerable groups should be given special protection against exploitation.
2. The exposed person should be provided with healthcare afterwards to deal with any biological effects of the radiation
3. Where possible the exposed person should be a classified radiation worker already as such a person will have on file pre-exposure blood counts which establish a baseline for that person and thus improve the ability of the medical doctors to manage the health effects of such a person’s radiation exposure.
4. The exposure should be kept as low as is reasonably possible, one important step which should be taken is that the person should be well trained for the work.
5. The exposure should be for a good purpose
I hold the view that very few emergencies would justify such large doses, I think that in the following imaginary situation a dose of 200 mSv to a worker would be justified.
A three man bomb disposal team who each get 200 mSv while making a dirty bomb safe, if the hypothetical bomb had exploded then 10000 people would each have got a dose of 10 mSv.
Here collective dose suffered by bomb disposal team of 600 mSv prevents the general public getting a collective dose of 100 man-Sieverts, I estimate that this would prevent the 5 members of the general public getting cancer while I would expect each member of the bomb disposal team to have an additional 1 % chance of developing cancer as a result of their work. Here we have traded 5 cancer cases for 0.03 cases of cancer. I would say that it is a good trade.
However to use the words of the HSE, “Emergency dose levels should not be seen by Operators or Carriers as a panacea to deal with accidents”. This government body then says that “evidence should also be sought that further reductions in the proposed levels through planning and preparation are not reasonably practicable”
My understanding of this text is that emergency dose limits should not be used as an excuse to throw good radiological practises out the window. The work should still be planned and conducted in such a way to make the radiation dose as low as reasonable.
As low as reasonable is the idea of the ALARP, which is “As Low As Reasonably Possible”. This is should be a central guiding principle for all radiation work. The idea is that if there is an alternative method of doing a job which results in a lower exposure to humans than unless it is very unreasonable to do it using the lower exposure method then even if the higher exposure method does not break a legal exposure limit then the lower exposure method should be used.
If will give you an example.
Imagine that we need to recover a radiation source from a building site. It is a stainless steel rod which is near a pile of scarp metal
A series of different methods exist for recovering it.
A. A man just walks up to it with no prior planning, find it, grab it with tongs and drop it in a lead pot then he would get a 20 mSv dose. This operation might take 40 minutes; the source recovery man might charge $ 1000 for the source recovery. This is a very bad system of work.
B. A photographer with a telephoto lens photographs the scene from a great distance (cost of $ 200, I suspect that A paparazzi photojournalist who spies from afar on celebrities would have the required skill set), we spend a hours talking to the site workers finding out as much as possible about the accident which caused the radiographic source to be lost (cost $ 500). We hire a mobile crane which spends a whole day placing concrete blocks as shielding near the source. The crane driver charges $ 2000 for his time and the use of the crane. This time the source recovery man (he still charges $ 1000) can walk up to the source from behind the concrete blocks, he only has to search a small area and he has a much thicker lead pot which was delivered by the crane to drop the source into. This time he gets a dose of only 5 mSv.
I hold the view that method B is a lot more work and costs more, but it is the right way to do the source recovery. This is because through good planning and using our brains we have managed to recover the source with dose which was only 25 % of the dose from method A.
Ben is right about the idea that fractionation of the dose makes it less dangerous, experiments with cells and experience from radiation treatment of cancer and accidents suggests strongly that damage caused by radiation can be partly reversed by the self repair processes which occur in living things.
About the idea of a 5 Sv dose, I think I would have started running long before it got to that level. I think that I would start to run for my life at a lower dose.
Ben got one thing slightly wrong, the Goiânia accident in Brazil involved Cs-137 not Sr-90. I think that this is a minor error (slip of the tongue). This was when some scrap metal worker cut up a cancer treatment machine which contained a large Cs-137 source.
Ben got one thing very right that was about the difference between dose and dose rate.
Dose = dose rate . Time
For example if the dose rate in Göteborg is 1 microSv per hour, then if I stay in Göteborg for 4 hours then I will get a dose of 4 microSv.
As Ben says be careful not to misunderstand Sv per hour with dose, Sv hr-1 is a rate.
One way to think of it is that speed is not the same as the distance. Distance can be compared with the dose (Sv) while the speed relates to the dose rate. Imagine two persons.
Man A feels the urge for some cigarettes, he jumps into his car and tears through town at 40 miles per hour and drives 2 miles to the supermarket where he buys an instant curry, a chocolate bar, packet of crisps and a pack of 20 cigarettes. Twenty minutes after arriving at the supermarket he then drives home at a more sedate 30 miles per hour.
Man B likes to go for a long healthy walk with his dog each evening, he walks at 4 miles per hour for 2 hours on the disused golf course near his home before coming home with the dog.
Man A travels a total of four (4) miles while man B travels a total of eight (8) hours. The highest rate of A’s travel is 40 miles per hour (40 miles hour-1) while B travels at a rate of 4 miles hour-1.
He is right about the locations of the different radioactivities which relate to the accident in the periodic table. He is right that gases are more mobile, the noble gases krypton / xenon and the tritium are very mobile
While the group I elements (Cs) form many water soluble products, the cesium can be released from nuclear fuel when it is heated. I think that Ben was wrong about the group II metals. While water soluble forms of group II metals exist such as slaked lime (Calcium hydroxide) many water insoluble forms of calcium and the other group II metals exist. For example limestone (calcium carbonate) is water insoluble. Much of the strontium in the used nuclear fuel is in a form of a very insoluble compound (strontium zirconium oxide, SrZrO3) which is of the perovskite family of solids. As a result much of the strontium is not mobile.
The technetium present in the used fuel is mostly in the form of insoluble metallic particles, this will mean that little of it will dissolve in water. One thing which complicates the technetium behaviour is the fact that the highly radioactive technetium-99m is formed by the beta decay of molybedum-99. The Mo-99 is partly in the form of the metal particles and also partly in the form of the perovskite solid. As a result very little Mo-99 will be lost when used fuel is immersed in water.
If however the fuel is heated in air during the accident then it is possible for the uranium dioxide to be converted into triuranium octaoxide (U3O8). This will cause the fuel pellets to be converted into powder, also many elements may be converted into higher oxidation state compounds. The molybdenum may be converted into the trioxide which is much more volatile than its normal form in used fuel.
During an accident the fuel may heat up above its normal working temperature, under these conditions the fission products are released according to their boiling points. Ben got this part right.
For further information about the chemical forms of fission products see http://abulafia.mt.ic.ac.uk/publications/theses/stanek/solutioninuo2.pdf
In a normal “healthy” light water reactor (LWR) Ben explained the state of the fuel with its zircalloy coating correctly.
In melt down Ben says that cesium, iodine and technetium will be in the water in water, while xenon, krypton and radon will be in the steam. I do not think that he is perfectly right about this.
Firstly the amount of radon in a reactor fuel is very small, while radium and radon will become a problem on a geological time scale in a used nuclear fuel store in the time since the uranium was extracted from the ore, made into fuel and then used in a reactor very little radium or radon will have formed. The radioactivity of the krypton and xenon isotopes is so much larger than the radon activity that I think that we can ignore the idea of radon in reactor accidents,
Due to its chemical form in reactor fuel I think that very little technetium will dissolve into the water inside the core. So I think we can ignore the technetium as long as the accident is only an over heating accident. If air gets in then I suspect that some of the parent of technetium-99m could be released.
Some of the iodine will be converted from the involatile water soluble form (iodide) into a volatile and water insoluble form (elemental iodine) this can escape into the air from the damaged reactor. In addition some very mobile organic forms of iodine such as methyl iodide can form inside the reactor containment; this form of iodine has a very low boiling point and is very able to escape from the plant.
We must distinguish between two types of accident, there is the loss of cooling accident (LOCA) and the reactivity initiated accident (RIA). A LOCA occurs when the cooling of the reactor fails, this type of accident is often a gentle slow burn accident where the fuel is heated up to a very high temperature without being subjected to mechanical violence. Three Mile Island (TMI) was an example of a LOCA, another example of a LOCA would be the Czech accident where the operators failed to remove a silica gel drying pack from a new fuel element. The fuel element was placed in the reactor, due to the silica gel pack the coolant could not circulate through the fuel element and as a result it overheated. This was an accident which damaged plant and equipment rather than people and the environment.
The reactor accident in Japan is a LOCA type accident, it appears to me to be like Three Mile Island but unlike TMI the accident seemed to progress further into a worse accident.
Chernobyl was a RIA type of accident, at Chernobyl a large surge in the fission rate in the core occurred which lead to a steam explosion. The steam explosion smashed up the fuel and flung both large lumps and small particles out of the core. During the surge in reactivity and afterwards the fuel overheated which resulted in further releases of radioactivity. Another example of a RIA is the SL-1 accident which killed three men many years before Chernobyl.
During both the SL-1 and Chernobyl accidents fragments of fuel where flung out of the reactors by the force of the explosions. In the case of SL-1 this radioactivity was largely confined by the reactor building while at Chernobyl the larger explosion blew the roof off the building and some of the fuel escaped from the building. These escaping fragments of fuel took the less volatile radioisotopes such as plutonium out of the reactor and into the environment.
SL-1 was a small military reactor which exploded in mysterious circumstances; no one has worked out exactly what made the men pull the control rods out so far. As all the witnesses were killed before they could give an account of what happened part of this accident is shrouded in mystery.
Ben is right that at TMI that venting did occur, this was a controlled venting of gas from the reactor containment which occurred through filters. Noble gases and a little radioactive iodine escaped. At Fukushima gases have also escaped from the reactor containments, but in addition water leaks have allowed the water soluble cesium and iodine to escape as well.
Ben is right when he said that most of the radioactivity which has escaped so far is beta emitters. If a serious fire occurs in the used fuel storage ponds then the least volatile elements such as the actinides (plutonium, americium, neptunium and curium) could escape. These actinides are the main alpha emitters present in the used fuel. Another possibility would be a steam explosion inside a reactor or a containment, as the reactors have cooled down do much since the start of the accident this is now very unlikely.
Ben is right to say that Chernobyl was more serious because of the graphite fire which followed the explosion, the fire kept the core hot for days afterwards and helped to spread the radioactivity further. Ben did not point out that the fuel is not uranium metal but instead uranium dioxide which is much harder to oxidise. It is important to understand that plutonium dioxide fuel is impossible to burn in air, if I was to heat plutonium dioxide all day with a blow torch it would not catch fire.
Ben was right to say that the fact that the Japanese reactors were shut down before the release did make the accident much less dangerous. By keeping the reactor switched off and in an OK for several days the amount of activity in the core dropped a lot.
About isotopes released,
The iodine-131 and the cesium-137 are both important. But I think that he is wrong on the plutonium. Plutonium is very involatile (hard to vaporise) so it is not very mobile unless a steam explosion or a fire in the used fuel pond occurs. So it is unlikely that much plutonium has been released from the reactor site, the bad news is that plutonium contamination can be very hard to clean up in the environment.
The good news is that plutonium tends to be very insoluble and it sticks very well to soil minerals, as a result it does not migrate quickly through soil. But the bad news is that plutonium contaminated dust is very harmful to the lungs.