Radiation: its effects and uses
Radiation: its effects and usesTo understand the dangers of nuclear energy you don't need to know how nuclear fission works, nor the details of nuclear reactors (though a basic explanation is given here). But it is helpful to know why radiation is a problem because many people in the nuclear industry and government do not seem to see it as such (or see only one problem: the waste), and there is much disagreement among scientists as to how great the hazards are.
To understand radioactivity, we have to go into some technical detail and jargon. and have tried to reduce it to what is relevant. If it's too much for you then read the parts of most interest. There is a summary of key points at the end.
It is also useful to have some understanding of probability because many of the arguments on the hazards of radiation are based on estimates, in areas where firm knowledge is lacking. For example, no one knows whether nuclear waste can be securely contained for hundreds of years (let alone tens of thousands) because these kinds of material did not exist a hundred years ago. The only sure evidence we have is the partial record of accidents so far. Probability and risk are discussed elsewhere as a separate topic.
There are many forms of energy radiation, including heat, light and microwaves. The radiation that concerns us here is called "ionizing radiation" because it creates ions (electrically charged particles) in other materials. You can't smell, hear, see, taste or feel such radiation - except at levels which will most likely kill you. How then can we think about radiation?
One analogy is heat: a hot object continuously radiates heat, and if we bring a hot object near to a cold one, it too becomes hot. One difference between radioactivity and heat is that it is virtually impossible to make something un-radioactive. Another difference is that we need warmth, but all radioactivity is harmful. Two words which help convey the meaning of radioactivity are 'instability' (radiation is caused by unstable chemical isotopes) and 'contamination'. Anything which comes near to a source of radioactivity is likely to also become radioactive, and even if we grind it into a powder and distribute it in a larger mass, it will still remain radioactive for the same length of time. So dilution does not help to reduce radioactive pollution.
All ionizing radiation is harmful to life. It can harm the individual or, by damaging genes, harm future offspring. Genetic mutation has been found in plants and animals around several nuclear sites, not only Chernobyl. There is no minimal or 'threshold' level below which it is safe. The hazard is cumulative: many small doses over a long time have as much effect as one large dose. The more there is the worse it gets. The only safe dose is zero.
There are two main kinds of ionizing radiation: atomic particles and electromagnetic waves. Alpha particles quickly give up their energy and may be stopped by a sheet of paper, which means all their energy is absorbed by the paper. The nuclear industry would have us believe that this proves alpha is no hazard, indeed they sometimes use this as a demonstration. In fact this proves how dangerous alpha-emitters are inside the body, since all the energy of these particles can be absorbed by a single cell. If an alpha-emitter becomes lodged within the body then all its emitted alpha particles will be absorbed by the relatively few surrounding cells. Outside the body alpha presents little risk; but if swallowed or inhaled it can be deadly. Radon gas and plutonium emit alpha particles.
Beta particles can penetrate flesh, but are stopped by a thin sheet of metal. They are smaller than alpha and give up their energy less quickly, travelling further but doing less damage. Neutron particles can pass through steel and concrete. So it is untrue to say that less energetic particles are less dangerous - it depends on where they are.
Electromagnetic waves are more dangerous at higher frequencies: infra-red, ultra-violet, X-rays, gamma rays. Gamma rays are so energetic that some of them penetrate the 14 inches of lead and steel containing the spent fuel rods transported by rail. They lose their energy much more gradually. Thus if a gamma emitter is lodged within the body the surrounding cells will absorb only a fraction of the total energy emitted. The nearer cells are more at risk but cells in other parts of the body are also at risk, as are the cells of any other people in the vicinity. Some workers involved in nuclear accidents have become so radioactive that they are a hazard to others; and when they have died they have had to be buried in lead coffins; some even had the hands cut off and buried separately.
The materials most significant for health are those that emit alpha particles (only when inside the body, because of their short range), and those that emit gamma rays or X-rays (either inside or outside the body).
Each element may have a variety of 'isotopes'. These are chemically similar (having the same number of electrons and protons) but differ in the number of neutrons. To distinguish them the chemical symbol is followed by this number; for example two isotopes of uranium are U-235 and U-238. Some isotopes spontaneously break down into simpler isotopes and emit radiation in the process (alpha, beta, gamma); such isotopes are called 'radio-active' and this process is known as "radioactive decay". Different isotopes decay at different rates. The mechanism causing decay is unknown.
Radioactive substances can make anything they come in contact with also radioactive. So radioactivity can be regarded as a kind of contamination. A vessel or building in which radioactive materials are stored will itself need to be treated as radioactive waste. Dispersing radioactive material does not reduce the total radioactivity but rather spreads the contamination - and thus puts more people at risk..
The time of decay of any particular atom cannot be predicted - it is not as if each atom had a little clock inside. Atoms do not age the way larger scale things do. Any given atom has the same probability of decay as any other atom of the same isotope, no matter how long they have been in existence. Only the statistical average decay rate is known for large numbers of atoms of a given isotope. This is expressed as the time it takes for the activity to decay to half its value, and is called the 'half-life' of that isotope. The term 'half-life' is misleading because the full-life of an isotope is not twice its half-life. (Scientists are not particularly good at language nor inventing appropriate terms for things.)
Some isotopes have a half-life of only seconds. Plutonium-239 has a half-life over 24,000 years; thus after this time a ton of it will contain only half a ton of Pu-239, after another 24,000 years it will contain only a quarter ton, after a total of ten half-lives (240,000 years) it will still contain about 1/1000 of a ton (about 1 kilogram). This may seem a small quantity but the "acceptable body dose" is one millionth of a gram. There are now many tons on plutonium in existence, all of it made in nuclear reactors, and more is produced every day. Plutonium used to have a high value, it now has an increasingly negative value (except perhaps for terrorists) as a highly toxic waste.
In nature there is normally a mixture of isotopes of an element. Due to radioactive decay the less stable isotopes reduce in number. Hence even though plutonium-239 might have existed at the birth of the Earth, over the geological time scale it would no longer be present. (Other radioactive isotopes have even longer half-life - that of U-238 is about 4.5 billion years - longer than the Earth's age.) There are now well over a thousand different isotopes in existence, most of them unstable. The evolution of life has had the time to adapt to only a fraction of these, the rest have been artificially made, in the last century.
The resulting isotopes of radioactive decay may themselves be radioactive. For example Plutonium-239 decays to U-235 (half-life 700 million years), which decays to Thorium-231 (half-life 25 hours), which decays to Proactinium-231 (half-life 34,000 years). As the half-life of U-235 is much shorter than that of U-238, it is not surprising that when uranium is mined the ore contains much more U-238 than U-235. The U-235 must be separated out by centrifuge as the isotopes are chemically similar.
Radium-226 has a half-life of 1,622 years and emits both alpha and gamma when it decays to the gas Radon-222, which has a half-life of 3.8 days and emits alpha when it decays to Polonium-218. Although the half-life of radon is relatively short, it is a significant hazard because it is created by certain rocks (from Radium, and Thorium) and is a gas and an alpha-emitter - causing damage to the lungs. This decay sequence ends, after eight more events, when Polonium-210 decays to Lead-206, which is a stable isotope.
There is no chemical way of making radiation harmless. Mixing it with other materials will not reduce the total toxicity, it will just spread the radioactivity to that material. In contrast, toxins such as mercury can be attenuated by chemical binding (eg. as in dental amalgams). All that can be done is to put the waste where it will do least harm and wait - often for a very long time. The extremely long half-life of some isotopes means that a completely new and much more thorough approach must be taken to its containment, unlike other toxic wastes. It is no good just dumping it at sea or on land, and hoping that it will go away. The people who ordered such dumping into the environment have blood on their hands, because the deadly effects have been known by scientists for decades. Already millions of gallons of radioactive waste have accidentally leaked into water tables.
As more radioactive scrap metals arise due to decommissioning, there is increasing likelihood that companies will use the tactic of "dilute and pollute". By mixing radioactive metal in with other metal they can get below the regulatory limits. Governments have not yet banned this anti-social activity.
Besides radioactive decay, some isotopes may forced to split by being hit by neutrons. This is nuclear fission, and results in two or more simpler isotopes as well as the release of other particles. For example U-235 readily splits into Strontium-90 and Xenon-143 and releases two or three neutrons. If there is enough density of U-235 these neutrons will split other nearby atoms and so on, causing a "chain reaction". If a mass of U-235 is less densely packed then the neutrons may escape without hitting other U-235 atoms, or if the U-235 is mixed with some material that absorbs neutrons, the reaction will be damped down. When the conditions for a chain reaction occur the system is said to 'critical'. Once a chain reaction is started the number of neutrons released either:
A certain minimum amount of U-235 (or Pu-239), called the "critical mass", is needed for a nuclear explosion - one or two kilograms, depending on its purity. Nuclear reactors contain many times this amount, but it is mixed with other materials (called 'moderators') in an arrangement called an atomic 'pile'. The aim is to keep the reaction going at a fixed rate, where the number of neutrons released is equal to the number escaping from the pile plus the number absorbed by other material. To sustain the right conditions and to prevent a runaway reaction, there are "control rods" which absorb neutrons and can be inserted and withdrawn from the pile. The fuel is also fabricated in the form of rods, so that they can be taken out of the pile when much of the uranium is decayed ('spent') and replaced with new fuel rods without dissassembling the pile.
The nuclear reaction creates heat, which if left to continue would melt the pile (a "melt down"). The heat is transferred and used to create steam to drive a turbine, producing electricity. The whole process is relatively inefficient - about 65% of the heat is lost, contributing to global warming.
There are several possible isotope combinations that can be used as fuel, several ways to transfer the heat (gas, water, liquid sodium), and various physical arrangements of components. Thus there are literally hundreds of different reactor designs in existence, each one of which is a complex system.
A density of a few percent U-235, in uranium which is mostly U-238, is sufficient for a chain reaction. The neutrons from the U-235 turn some of the U-238 to Plutonium-239. The only real use for Pu-239 is nuclear weapons. It is more deadly than U-235 if ingested, because it is a more energetic alpha-emitter.
To make a fission bomb it is not necessary to have pure U-235. Bombs have been made and tested with material from civil nuclear reactors. The phrase "weapons grade" is used misleadingly if it implies that there is some essential difference between military and civil nuclear material. Any country with the technology to build a reactor can build a nuclear bomb, or it can buy its uranium direct on the black market.
The bomb dropped on Hiroshima used the crude but effective technique of keeping apart two lumps of uranium, each of half the critical mass. The bomb was detonated by conventional explosive packed around the uranium and forcing the two halves together. The bomb dropped on Nagasaki was a different design, and it is likely that it was used, within just two days of Hiroshima, to compare its destructive effect. (Whatever you may think of the rationale for dropping a second bomb, this cynical testing of a larger explosion cannot be excused.)
Nuclear fusion, when two atoms combine, is very much rarer because it takes a tremendous amount of energy to force them together. Only the simpler isotopes produce a net energy when they fuse, and in practice only the isotopes of hydrogen are used: deuterium and tritium. Two example conditions of fusion are the high pressures in stars such as the sun, and nuclear (fission) explosions. So far, all attempts at sustainable fusion for power generation have failed.
The main natural sources of radiation are from rocks (eg. granite) and outside the Earth: cosmic radiation. The Earth's atmosphere shields some of the cosmic radiation, so frequent flyers and aircraft crew receive more radiation. The term "background radiation" includes these natural sources and also the man-made ones such as fallout from nuclear bomb tests, dumping of waste, accidents, and routine emissions from power stations. Thus background radiation varies from place to place and has increased due to man's technology.
If we make the reasonable assumption that all radiation is harmful, then clearly we are making matters worse by adding to it. But it is not just a matter of quantity. From the facts above we can see that new and different sources of radiation have been created. (The main question is whether the benefits of nuclear power are worth the real costs and the future risks. These costs include not only radiation effects on health but also weapons of mass destruction, civil liberties, and other issues.)
An assumption of some radiobiologists - usually those chosen and paid by government agencies - is that since humans have adapted to background levels of radiation, it therefore has no discernable ill-effects. This is an unwarranted assumption: for example, species can tolerate a certain level of premature deaths, and evolution will favour adaptations which reduce mortatlity. But if the effects only occur after the age at which individuals reproduce, then evolution is far less likely to adapt to prevent serious harm. And what do we find? Cancer is increasingly prevalent in old age. (The proponents of nuclear power cannot have it both ways: if some cancer or leukemia is due to radiation, then it must be due to natural background radiation or man-made radiation - or both. If it's due to background radiation then the assumption of no ill-effects is false; if it's not due to background radiation then the nuclear industry is responsible for its effects.)
From this unwarranted assumption, natural background radiation is taken as a baseline for setting standards for controlling man-made radiation (eg. by the International Commission for Radiological Protection, and the NRPB).
The truth is that no one knows the full extent of the health effects of natural background radiation. It has been estimated that in the UK alone it causes several thousand deaths per year by cancer, more than the number of road deaths. The average background level dose, from both natural and man-made sources, is about 2500 microSieverts per year.
The level of background radiation is portrayed by the nuclear industry as innocuous, but although more background radiation is due to natural causes than artificial ones, in this case natural does not mean good for you - all radiation is hazardous. For an organisation to set an 'acceptable' level of additional exposure at average background level means they are willing to expose people to double the hazard.
If a government or corporation is determined to introduce some hazardous process, they will set some standard of safety that is attainable and acceptable to them, rather than a standard that is acceptable to the workers or the public. In the nuclear industry, background radiation is taken as the norm. Most of the current level of background radiation is from natural sources. To most people the word 'natural' implies harmless or beneficial. Thus the authorities may dismiss a radiation risk as "similar to the natural background". What they omit to say is that this is an additional risk (not a replacement risk) and that background radiation is not insignificant. Since it would be incorrect to say that any radiation level is a "safe dose" they use the term "acceptable dose" - even though this has never been democratically accepted and its meaning has never been properly explained to the public. The only people who have accepted it are the experts who proposed it, who are largely dependent on the nuclear industry or government. It is not these experts, nor industry, nor even the current government, who will have to suffer the major health costs (in several years time) of their decisions.
Having set a figure for the public 'acceptable' dose, the authorities find this too low to be practical for workers in the industry. So they set a figure several times higher for workers. Thus even if we regard the acceptable dose as just acceptable, they are authorizing un-acceptable doses for a proportion of the population. It is a common practice in the industry to employ many workers for short periods in radioactive areas, so that each worker uses up their annual 'allowed' dose. This despicable action treats radiation dosage as if it had a threshold below which one is safe, whereas all the evidence indicates there is no safe dose.
It is significant that over time, as more is learned about radiation, and information gets publicised, that the acceptable dose figure has been reduced - usually with many years delay. This tells us several things: the experts did not know all the answers (and they still don't); the industry can operate at better safety standards (but will only do so if forced to - because safety cuts profits); the regulators listen to the industry experts more than independent experts (unless there is loud enough public protest); and they are slow to make changes even when the evidence is accepted..
Radiation is an unusual hazard, though it shows some of the features of the risks from asbestos: workers in the industry are more at risk, but there is a general risk to the population at large since asbestos has been mined, fabricated and spread about in the environment. The ill effects of asbestos may not be experienced until many years after exposure. It has taken decades and many deaths to get industry to accept responsibility for the problem and start to clean up the environment.
(There are a lot of numbers and new terms in this section; you don't need to remember them, but you might need to refer to them sometime.)
One gram of radium emits one Curie; this is measure of its radioactivity: the amount of energy emitted. A related measure is the Becquerel (one atomic decay event, or click on a geiger counter, per second); one Curie = 37,000 million Becquerel.
The effect of radiation depends not only on total energy but on how much of that energy is absorbed. The Rad (Radiation Absorbed Dose) is defined as the amount of radiation depositing 100 ergs of energy in one gram of tissue. A related measure is the Gray = 100 Rad.
This measure is not good enough, as different forms of radiation cause different amounts of damage. The Rem (Roentgen Equivalent Man) is defined as one rad of alpha or ten rads of beta or gamma. This is a crude attempt to take account of the greater damage of alpha particles. A related measure is the Sievert = 100 Rem. The most common units are Becquerel (Bq) and Sievert (Sv).
Even the Rem and Sievert are inadequate measures of damage: different tissues or organs absorb radiation differently. Any such gross figure ignores reality; to accurately estimate the damage/hazard we must know the isotopes involved, their physical form, and the manner of exposure to them. A one gram piece of plutonium is hazardous outside the body, but nowhere near as hazardous as that same gram ground up and dispersed in the atmosphere that people breathe, or ingested in food.
Once inside the body, different radioactive elements have different effects. Strontium-90 (half-life 28 years) is one of the products of nuclear fission; it can enter the body by inhalation, in drinking water, through crops, through the milk of grazing cattle, or from eating animals, birds, fish, shellfish. It has chemical similarities with Calcium and settles in bone, rather than being excreted.
Cesium-137 (half-life 30 years) also gets into plants, milk, and animal muscle but is concentrated in soft tissue rather than bone. It is both a cancer risk and a genetic risk, not only because it lodges in soft tissue such as the gonads, but also because it emits gamma rays, which can travel to the gonads from inside or outside the body.
Iodine-131 (half-life 8 days) was released in the Windscale fire of 1957. It accumulates in the thyroid gland, a very small organ, eventually causing thyroid cancer. Although its half-life is relatively short, it can be passed to humans in cow's milk. It can also be concentrated in edible sea-weed, and by shellfish, hundred or thousands of times the concentration in the surrounding water. Insects can also concentrate radioactive isotopes, and they in turn are eaten by birds.
The overwhelming problem with radiation is that it's bad for your health. There are other problems, such as making metals brittle or interrupting communications, but its main effects are on living cells. Cells may be affected in three ways: killed, made cancerous, or mutated so that offspring are affected and pass on the mutation (if they survive it). The death of a few cells is of little concern to an adult, but may be significant for a foetus. The effects of a cancerous cell may not be noticed for some years, and may be more of a risk to the elderly, whose body repair mechanisms are not as strong. The effects of genetic mutation cause no problems to the person irradiated but to their offspring, their effects may not become apparent for decades.
Although different particles and rays may have qualitatively different effects on cells, their main effect is due to ionization, and the damage from this is proportional to the energy transferred from them to the cell. This depends on their mass, speed and electric charge. The technical measure is called the "linear energy transfer" (LET). The details of energy transfer and cell damage are complex and not fully understood; but there is enough data to make estimates of the effects of different particles and rays.
Most information on the health effects of radiation comes from extremely high sudden doses: serious accidents and Hiroshima and Nagasaki. The damaging effects of such doses are obvious in days or weeks (radiation burns, vomiting, death) and the link between cause and effect is clear. It is from these events that 'acceptable' doses are usually estimated.
Small doses over long periods are rarely studied, partly from lack of opportunity and partly due to the cost of such studies. It is also more difficult to link cause and effect. The latency period between such exposure and the onset of observable disease is commonly 20 years or more. In this time the person may have already died from some other cause.
An X-ray or gamma ray passing through the body may or may not cause cellular damage - it depends on the exact path it takes. So we can only make an estimate of its likely damage, based on past incidents that we have records for. A massive external dose of radiation can overwhelm any recovery mechanisms in an animal and produces obvious damage (radiation burns have similar initial effects to heat burns and ultra-violet burns). At the other extreme a single alpha particle inside the body can damage a cell with effects that are not apparent for decades or not until the next generation, and these effects may not be recognizable as radiation damage. While death from a massive dose is virtually certain, damage from a very small dose can only be guessed at and expressed as a probability.
The consequences of these facts are: (i) it is difficult to detect, let alone prove, a causal link between a particular event and a particular illness, even though the general effects of radiation are known; (ii) 'authorities' tend to take the easier option of basing 'acceptable' dose levels on acute exposure rather than on more realistic but complex low-level and chronic exposure; (iii) calculations based on massive exposure will underestimate damage at lower levels, because the effects have an upper limit (death) and are partly self-limiting (a cell mutated by one particle may then be killed by another particle, and an already killed cell cannot be mutated); contrariwise, calculations based on low exposure will underestimate damage because the damage may not be recognised within the lifetime of studies, or the lifetime of the individual. This is an indication of the complexity of the problem.
Because of the complex relation between radiation and its observable health effects, we cannot be absolutely certain that any given person's illness is due to a specific source of radiation. This is similar to the problem of determining whether a lung cancer was due to smoking tobacco or some other cause. We must rely on statistics and epidemiology to provide scientifically acceptable evidence of the connection of cause and effect. (This also creates problems for people seeking compensation through legal proceedings: a direct causal link is difficult to prove in any single case. It usually takes several court cases before companies or governments will admit to liability, even when the statistical evidence is clear.)
created: 6-Dec-06