| Radiation, health and nuclear safety | | Print | |
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Reflections If it were not for concerns about the health effects of radiation, nuclear energy would have developed quite differently. However, it is inherent in the nature of energy that it can be used in constructive or destructive ways, and each source of energy therefore has its advantages and disadvantages. It is a difficult task to balance the dangers of climate change and acid rain, the local environmental impact of some renewables such as hydropower and the possible health effects of radiation. However, all of these issues must be judged against a world with a growing need for energy as populations increase, as people in less developed regions start to demand lives as long and as comfortable as ours in the rich North, and as fossil fuels start to run short. A mix of different energy sources would seem to be the most flexible way of dealing with all of the uncertainties in this equation, but deciding the precise levels of investment to be devoted to the various options represents an enormous challenge to decision-makers.
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Modern science has allowed us to develop safe ways of using radiation, and of exploiting its properties for the benefit of mankind. Many valuable applications have been developed:
These are just a few examples. We benefit from a large range of products and services made possible by radioactive materials, often without even realising it. [ back to top ] What
is radiation?
There are different types of radiation, and not all involve radioactive decay. The type of radiation we are looking at in this booklet is ‘ionising radiation’, and includes artificially made X-rays (which do not arise from radioactive decay). When this type of radiation bumps into other matter, it produces electrically charged particles, known as ‘ions’.
When we refer elsewhere in this booklet to ‘radiation’ we mean ionising radiation. However, there are non-ionising radiations, such as low energy electromagnetic radiation (which includes light, infra-red, radio waves and electrical and magnetic fields from power pylons), which are not normally dangerous. There are several types of ionising radiation, of which the most important are:
It is also possible to have radiation consisting of neutrons, though this is rare in the natural environment. [ back to top ]
How long does
radioactivity last?
Each radioactive material or ‘radioisotope’ has its own ‘half-life’ – the time taken for its radioactivity to fall to one half of its original value (which is also the amount of time it takes for half of the material to decay away). After two half-lives it would be a quarter of its original level, after three one-eighth and so on. By the time ten half-lives have passed, the radioactivity of the material will have fallen to less than 1% of its original intensity. Half-lives vary enormously in terms of time, from less than a thousandth of a second (e.g. polonium-214) to eight days (iodine-131), 28 years (strontium-90) or 4.6 billion years (uranium-238). If a substance has a very short half-life, this means it is giving out its total amount of radiation very quickly. If on the other hand, it has a very long half-life, it is giving out its radiation very slowly, so the amount given out in a particular period of time will be far less. Uranium-238 is the commonest form of uranium, but because it has such a very long half-life, it is not very radioactive, and can safely be held in our hands (indeed, it is used for purposes such as ballast in ships). [ back to top ]
How do we measure
radioactivity?
To describe the effect which radioactivity has on living tissues, we use a different unit. The amount of radiation absorbed by a living organism is called the dose or exposure. The unit which estimates the effect a dose of radiation has on living matter is the Sievert (Sv). It is a very large unit – a dose of 4 or 5 Sv received in one exposure would probably lead to death – so we usually talk of millisieverts (mSv). A millisievert is one thousandth of a Sievert. As most of us receive a dose of less than three millisieverts a year from natural sources, the mSv is often called the basic background unit. Confusingly, in the USA another set of units is commonly used, namely ‘rads’ and ‘grays’. However, in the rest of this booklet one unit equals one mSv. [ back to top ]
Where does
radiation come from?
Natural sources of radiation 85%
Artificial sources of radiation 15%
[ back to top ]
What does radiation
mean to us? In small doses, like those discussed above from background sources, radiation does not seem to be harmful. (Typically, we get over 10,000 bits of radiation, most of it from natural sources, going through our bodies every second.) An adult human is made up of about 60 billion cells, several million of which die every day and are replaced naturally. The few that are killed or damaged by low or moderate doses of radiation are usually replaced or repaired quickly. But large doses of radiation could be fatal or could change cells permanently. The medical effects of radiation exposure are split into two categories: early and delayed effects. When a cell is irradiated (given a radiation dose), it may die soon afterwards. This is called an early effect. In some cases, the cell may survive but be changed permanently. This may cause health problems later, some of which could be passed on to the next generation. These may take the form of cancer or genetic deformations. These are known as the delayed effects of radiation exposure. Early effects
Delayed effects There is no evidence of genetic effects among children of the Hiroshima and Nagasaki bomb survivors who were conceived after the bombs were used. Indeed, genetic effects have never been observed in humans, though they have been in some animals. Recent research has shown that effects of quite low-level radiation exposure (above whole-body doses of about 50 mSv) can be detected at a cellular level even among cells which are not directly struck by a bit of radiation, and that these changes can sometimes skip some generations before being expressed in the cells’ DNA. However, there is no evidence that this ‘genomic instability’ results in any damage to the organism as a whole. Indeed, it may be part of the mechanism that helps the body to resist and repair radiation damage. It is known that cancers, including leukaemia, can be caused by changes in human cells. Certain industrial chemicals, viruses and environmental agents can cause these changes, one of the best known being tobacco smoke which causes lung cancer. Radiation is recognised as another environmental agent which can cause cancer. We know that people who have received large radiation doses have a slightly higher risk of getting cancer. Much of our understanding about the health effects of radiation has derived from comprehensive research carried out over the course of more than half a century among survivors of the Hiroshima and Nagasaki bombs. The survivors show slightly higher cancer rates than are found in people in similar Japanese communities which did not suffer from either nuclear explosion. There is a debate about how appropriate study of this particular population is in trying to understand different types of radiation exposure. The bomb survivors were subjected to a single (‘acute’) very large dose of radiation from outside their bodies, and though some people subsequently breathed in or swallowed considerable amounts of radioactive material which delivered long-term ‘chronic’ radiation doses from inside their bodies, this aspect of their exposure was not measured. Since then, other groups of people who have received chronic radiation doses have been studied. These include people who have received repeated radiation doses over long periods as part of medical treatment programmes; early uranium miners working in badly ventilated mines, who breathed in significant amounts of alpha-emitting radon leaving deposits in their lungs; nuclear power plant workers; workers who painted luminous dials on clock and watch dials with paint containing radium (these workers would often lick the brush to get a fine point, so swallowing some of the radium); and people who received injections of a substance called thorotrast which circulated in the blood emitting alpha particles. The estimates of the risk associated with radiation, whether internal or external, acute or chronic, largely agree with one another for these significant dose levels. There is still some uncertainty about the effects of radiation at very low doses. Some scientists believe it may be good for us in very low doses, keeping the body’s cancer-fighting mechanisms in good working order in the same way that vaccination against disease does. It seems, for example, that people living in areas of high radon exposure may actually be less likely to develop lung cancer, rather than more likely, than people living in low-radon areas, as long as levels are not too high. (This is referred to as ‘hormesis’.) Others believe that below a certain dose level – the ‘threshold’ – radiation has no effect. But other scientists believe that it remains proportionally just as dangerous in very low doses as in moderate doses, while some claim that it is actually proportionally more dangerous at very low dose rates.
A major study looking at cancer rates in the areas immediately surrounding all nuclear installations in the UK in 1987 could find no increase in levels when compared to similar areas with no nearby nuclear installations. However, there has been concern over excesses of leukaemia round the Sellafield nuclear plant in Cumbria. Five children born in the nearby town of Seascale between 1950 and 1985 developed the disease, when on average no more than one case would have been expected. A less pronounced excess was reported near the Dounreay plant on the north coast of Scotland. A great deal of research has been carried out in an attempt to understand why these leukaemia cases may have arisen. It is difficult to link them to discharges of radioactive material into the environment from the plants themselves, since the total doses to people in the surrounding areas are still very much less than are received by people in places like Cornwall who seem to suffer no adverse health effects. (Indeed, clusters of the disease occur throughout the UK and Europe in locations nowhere near nuclear facilities.) The possibility of a link to doses received by the children’s fathers before the children were conceived has also been ruled out by the relevant research body, the Committee on Medical Aspects of Radiation in the Environment (COMARE), which was set up in 1984. The issue was scrutinised in the High Court in 1992–3, and after taking evidence and considering judgment for a year, the judge decided it was ‘overwhelmingly’ unlikely that radioactivity from Sellafield was responsible for the deaths. An alternative theory – that the leukaemia was caused by a virus brought to the very isolated Sellafield and Dounreay areas by people moving from more populous towns when the plants were first set up – has received a lot of attention, but this is also as yet unproven. [ back to top ]
How can we make
use of radiation? Industry Radioactive tracing techniques are used to check the cement grouting which fixes offshore oil platforms to the sea bed, and geologists also use radioactive tracers to follow the flow of water through underground systems.
Radiation has also been used to induce genetic modification in plants and so to develop new varieties of crops. Some, such as drought-resistant crops, have proved to be life-saving in very poor countries. Energy
Radioactive sources called radioisotopes can be swallowed by patients and their journey through the body photographed by a special camera, so that the function of organs and tissues can be examined without surgery. This is particularly useful for examining bones, the heart, liver, thyroid gland and brain. The ability of radiation to destroy living cells can save lives. Some forms of cancer can be cured this way, while others are slowed down or the pain removed. Doctors can use computer technology to focus high levels of radiation on the cancerous cells and so avoid destroying healthy tissue alongside. These high doses may result in unpleasant side-effects like fatigue, nausea and sore skin, but many patients accept this in return for a longer life. Food irradiation [ back to top ]
Protection from
radiation However, we should take steps to avoid all unnecessary exposure to radiation. Protection against direct radiation is not difficult: significant sources are shielded in appropriate ways, alpha particles cannot even penetrate a sheet of paper, and X- and gamma-rays are stopped by a thick layer of metal, concrete or water. It is very important, though, to minimise the amount of radioactive material that we absorb into our bodies which can therefore give us ‘internal’ doses. Legislation, based on the recommendations of the International Commission on Radiological Protection (ICRP), requires that any exposure to radiation must be justified by resulting benefits, and that all exposures and doses are as low as reasonably achievable. This legislation has affected our lives considerably. Jewellery and toys which contained radioactive materials have been banned, as their only benefit was to give pleasure and non-radioactive materials could be used instead. Smoke detectors, on the other hand, are allowed because they save lives and give only very small radiation doses. There are also ‘dose limits’, that is, levels of exposure which cannot be breached under any conditions except in an unforeseen emergency. In the UK, the National Radiological Protection Board (NRPB) advises the government on radiation safety, and controls are enforced through Acts of Parliament, regulations and licences. Airline crews and workers in the nuclear industry and healthcare must wear ‘film badges’ or other detectors so that their radiation doses can be carefully monitored. Similarly, the risks of giving a patient X-rays must be balanced against the benefit of possible diagnosis and treatment. In most cases, the benefit is clearly greater than the risk, but one exception is the X-raying of pregnant women where it is thought there may be a risk to the unborn child. [ back to top ] Safer
by design Safety is therefore top priority in our nuclear power industries. Nuclear safety strategy involves identifying and assessing all of the hazards which might lead to an accident, and then setting up fail-safe systems and multiple barriers to combat them. For a large amount of radioactive material to escape from a modern reactor, a whole chain of failures would have to take place:
With existing safety measures, such an accident is extremely unlikely and a significant escape of radioactivity even more so, but it is especially important that nuclear power stations are designed and constructed to extremely high standards. The safety record of nuclear power reactors in the UK, and indeed in most of the world, is excellent. There has never been a major accident at any electricity-generating station in the UK. There was an accident at an early military reactor at Windscale, Cumbria, in 1957. It should be borne in mind however, that this reactor had been designed less than ten years after nuclear fission had first been demonstrated by Fermi in 1942. Modern power reactors are much safer by design, and benefit from changes in safety procedures, some of which were developed as a response to the Windscale fire.
The world’s worst nuclear accident happened at Chernobyl, in Ukraine, in 1986. The Chernobyl reactor was a Soviet-designed water-cooled, graphite moderated reactor known as RBMK, a design unique to the Former Soviet Union which would never have succeeded in gaining a license for use in the UK, or many other countries. This design had several weaknesses, including fatally a risk of the power output ‘running away with itself’ under certain circumstances. There were operator errors and crucial safety regulations were ignored. (The plant operators were carrying out a safety test at the time, which involved running the reactor at very low power, something which was absolutely forbidden under operating instructions.) The company town of Pripyat had to be evacuated, and in the weeks following the accident over 30 people died from burns or radiation exposure. A further 300 workers and firefighters suffered radiation sickness at the time and there have been several thousands of cases of thyroid cancer among young people living in the region. These could have been avoided if the Soviet authorities, anxious to keep the accident secret, had distributed iodine tablets in the first few hours after the accident to the regional populations. Fortunately, thyroid cancer is relatively easy to treat, and fewer than ten of the affected individuals actually died, but there is no denying that this was a major industrial accident. A zone 30km wide around the plant was declared unfit for habitation, although a few people have returned and wildlife is now flourishing despite high levels of radioactive contamination in some ‘hotspots’. Radioactive material from the accident could be detected in many areas of northern Europe, including the UK where sale of sheep from a number of farms was banned. In a few cases, these bans are still in force today. However, since radioactivity can be detected at very low levels, it is an easy task to monitor these sheep to prevent any danger of potentially harmful materials entering the food chain.
In the UK, before a nuclear power station is licensed, its owners must demonstrate that it is safe and prove that the likelihood of uncontrolled radioactivity escaping is, literally, less than one in a million for every year of the reactor’s life. To achieve these results everything from earthquakes and fire to crashing aircraft must be allowed for. Designers must also assume that human operators can make mistakes, so in traditional nuclear designs all protective systems must be duplicated or triplicated. Some can detect unsafe conditions and restore the plant to a safe condition automatically. Temperature and pressure are measured by several independent systems, and any variation outside the permitted range will automatically cause the reactor to shut down. All main pumps have back-up pumps, which will switch on automatically if required, because gas or liquid coolants are continually pumped through the core of reactors to remove the heat. If the coolant escapes (a ‘loss of coolant accident’ or LOCA), automatic systems will shut the reactor down and special reservoirs will supply coolant until the temperatures drop to safe levels. With such multiple safety arrangements in place, reactors can work day and night, sometimes for years, without any need to shut down except for routine inspections and refueling. More recently, the focus has moved towards ‘passive safety’. Instead of relying on valves, pumps and other engineered features, designers are increasingly using the forces of nature – gravity, or the fact that materials expand when they get hotter, for example – to ensure safety. Such an approach both reduces the cost of building the reactor, and increases the reliability and predictability of how the plant behaves under both normal and abnormal circumstances. The nuclear industry spends millions of pounds each year on safety in order to meet not only its own strict requirements but also those of several external regulating and advisory bodies, including:
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Radiation,
Health and Nuclear Safety 
Everything
in the world, from the air we breathe to the cars we drive,
is made up of atoms (for more details see
Ionisation
causes chemical changes in materials, and can affect our
bodies as well as the things around us. In high quantities,
ionising radiation can damage living tissue and even destroy
human cells. This can be beneficial in some circumstances –
for example, radiotherapy is used in the treatment of
cancer, the radiation being used to kill cancerous cells,
leaving other cells nearby relatively unharmed. In this
sense, radiation is rather like a surgeon’s scalpel. Used
carefully it can provide many benefits and can enable us to
live longer, healthier, more comfortable lives, although it
can be dangerous if misused.
14%
medicine: In most countries, medicine is the largest
source of artificial exposure to radiation. The most
common sources are X-ray examinations; the most powerful
sources are radiation doses to treat cancer. Radioactive
materials are used in many other areas of medicine to
prevent and treat diseases. Typical doses from X-ray
examinations are: dental 0.02 mSv; chest 0.05 mSv;
pelvis 1.2 mSv; spine 2 mSv.
Such
a dispute occurs mainly because the health effects of very
low levels of radiation, if there are any such effects, are
very small. We cannot detect them against the natural levels
of cancer, which is responsible for something like a quarter
of all deaths in developed countries like the UK. Sometimes
when we toss a coin three times we get three heads,
sometimes two heads, sometimes one head, sometimes no heads.
There is no real reason for this – just the workings of
chance. Every year about half a million people die in
England and Wales – the figures for the 12 months up to
April 2001, April 2002 and April 2003 were 523,541, 531,627
and 530,334 respectively. Just like the example of the
coins, there were no apparent reasons for the small
variations among these three years. If in any one of these
years there had been twenty deaths, say, caused by a release
of radioactive material from a hospital, these deaths could
not have been detected against the natural random change in
numbers from year to year.
Agriculture
Medicine
At
Three Mile Island in the USA a major reactor accident
happened in a PWR in 1979, but the built-in protective
features worked as designed. Only small quantities of
radioactive material were released, less than the amounts
which were authorised to be released each week, and nobody
was harmed.
One
positive outcome of the Chernobyl accident was the formation
of the World Association of Nuclear Operators (WANO), which
brought together the operators of plants all over the world.
Most western reactors are now ‘twinned’ with reactors in the
former Communist bloc or the developing world. Teams of
operators will spend time at each other’s plants, learning
from their hosts’ skills and offering advice on possible
improvements. This initiative works alongside projects run
by the International Atomic Energy Agency to make detailed
technical appraisals of plants in areas such as Eastern and
Central Europe and make recommendations for improvement.