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Radiation and the Nuclear Fuel Cycle March 2004 Printer friendly version is also available
SOURCES AND EFFECTS Radiation can arise from human activities or from natural sources. Most radiation exposure is from natural sources. These include: radioactivity in rocks and soil of the earth's crust; radon, a radioactive gas from the earth and present in the air; and cosmic radiation. About one quarter of natural radiation comes from the human body itself. The human environment has always been radioactive. Radiation arising from human activities typically accounts for up to 25% of the public's exposure every year. This radiation is no different from natural radiation except that it can be controlled. X-rays and other medical procedures account for most exposure from this quarter. The rest comes from coal burning, appliances, and sundry sources. Less than 1% of exposure is due to the fallout from past testing of nuclear weapons or the generation of electricity in nuclear, as well as coal and geothermal power plants. Radiation exposure is measured by the Sievert (Sv) or milliSievert (mSv) which takes into account the particular biological effects of different types of radiation (see below). Natural background radiation exposure averages about 2 mSv a year but this varies depending on the geology and altitude where people live. Our knowledge of radiation effects derives primarily from groups of people who have received high doses. Radiation protection standards assume that any dose of radiation, no matter how small, involves a possible risk to human health. However, available scientific evidence does not indicate any cancer risk or immediate effects at doses below 100 mSv a year. At low levels of exposure, the body's natural repair mechanisms seem to be adequate to repair radiation damage to cells soon after it occurs. Some comparative radiation doses and their effects:
TYPES OF RADIATION Nuclear radiation arises from hundreds of different kinds of unstable atoms. While many exist in nature, the majority are created in nuclear reactions. Ionising radiation which can damage living tissue is emitted as they change ('decay') spontaneously to become different kinds of atoms. The principal kinds of ionising radiation are: Alpha particles are intensely ionising but cannot penetrate the skin, so are dangerous only if emitted inside the body. Radon gas, given out by many volcanic rocks and uranium ore, has decay products that are alpha-emitters. This is why radon can be dangerous. People everywhere are typically exposed to up to 3 mSv/yr from inhaled radon without apparent ill-effect. However, in industrial situations its control is a high priority. Beta particles are fast-moving electrons emitted by many radioactive elements. They are more penetrating than alpha particles, but easily shielded. Gamma rays are high-energy beams much the same as X-rays. They are very penetrating and require shielding. Radiation dose badges are worn by workers in exposed situations to detect them and hence monitor exposure. All of us receive about 0.5 - 1 mSv per year of gamma radiation from cosmic rays and from rocks, and in some places, much more. Neutrons are mostly released by nuclear fission - the splitting of atoms in a nuclear reactor, and hence are not normally a problem outside nuclear plants. However, fast neutrons can be very destructive to human tissue. DOSE LIMITS Public dose limits for exposure from uranium mining or nuclear plants are usually set at 1 mSv/yr above background. In most countries the current maximum permissible dose to radiation workers is 20 mSv a year averaged over five years, with a maximum of 50 mSv in any one year. This is over and above background exposure, and excludes medical exposure. The value originates from the International Commission on Radiological Protection (ICRP), and is coupled with the requirement to keep exposure as low as reasonably achievable - taking into account social and economic factors. Radiation protection at uranium mining operations and in the rest of the nuclear fuel cycle is tightly regulated, and levels of exposure are monitored. LIMITING EXPOSURE There are four ways in which people are protected from identified radiation sources: Limiting time: In occupational situations, dose is reduced by limiting exposure time. Distance: The intensity of radiation decreases with distance from its source. Shielding: Barriers of lead, concrete or water give good protection from high levels of penetrating radiation such as gamma rays. Intensely radioactive materials are therefore often stored or handled under water, or by remote control in rooms constructed of thick concrete or lined with lead. Containment: Highly radioactive materials are confined and kept out of the workplace and environment. Nuclear reactors operate within closed systems with multiple barriers which keep the radioactive materials contained. Rooms have a reduced air pressure so that any leaks occur into the room. NUCLEAR FUEL CYCLE The average annual radiation dose to employees at uranium mines (in addition to natural background) is around two mSv (ranging up to 10 mSv). The natural background radiation is about 2 mSv. In most mines, keeping doses to such low levels is achieved with straightforward ventilation techniques coupled with rigorously enforced procedures for hygiene. In some Canadian mines, with very high-grade ore, sophisticated means are employed to limit exposure. (See also: Occupational Safety in Uranium Mining paper) Occupational doses in the US nuclear energy industry - conversion, enrichment, fuel fabrication and reactor operation - average less than 3 mSv/yr. Reprocessing plants in Europe and Russia treat spent fuel to recover useable uranium and plutonium and separate the highly radioactive wastes. These facilities employ massive shielding to screen gamma radiation in particular. Manual operations are carried by operators behind lead glass using remote handling equipment. In mixed oxide (MOX) fuel fabrication, little shielding is required, but the whole process is enclosed with access via gloveboxes to eliminate the possibility of alpha contamination from the plutonium. Where people are likely to be working alongside the production line, a 25mm layer of perspex shields neutron radiat-ion from the Pu-240. (In uranium oxide fuel fabrication, no shielding is required.) Interestingly, due to the substantial amounts of granite in their construction, many public buildings including Australia's Parliament House and New York Grand Central Station, would have some difficulty in getting a licence to operate if they were nuclear power stations. NUCLEAR ACCIDENTS The March 1979 accident at Three Mile Island in the US caused some people near the plant to receive very minor doses of radiation, well under the internationally recommended level. Subsequent scientific studies found no evidence of any harm resulting from that exposure. In 1996, some 2,100 lawsuits claiming adverse health effects from the accident were dismissed for lack of evidence. Immediately after the Chernobyl disaster in 1986, much larger doses were experienced. All of the 22 who received more than 6,000 mSv died. Seven of the 23 who received 4,000-6,000 mSv also died, as did one of the 158 receiving 1,000-4,000 mSv. The main casualties were among the firefighters, including those who rapidly put out the initial small fires on the roof of the building. Apart from the residents of nearby Pripyat, who were evacuated within two days, some 24,000 people living within 15 km of the plant received an average of 450 mSv before they were evacuated. In June 1989 a group of experts from the World Health Organisation agreed that an incremental long term dose of 350 mSv should be the criterion for relocating people affected by the 1986 Chernobyl accident. This was considered a "conservative value which ensured that the risk to health from this exposure was very small compared with other risks over a lifetime". (For comparison, background radiation averages about 100-200 mSv over a lifetime in most places.) Over 100,000 people were relocated away from Chernobyl. About 185,000 people received significant radiation exposure, above 20 mSv, between 1986 and 1989. These continue to be monitored. In 1995 the World Health Organisation linked nearly 700 cases of thyroid cancer among children and adolescents to the Chernobyl accident, including 10 which resulted in death. So far no increase in leukaemia is discernable, but this is expected to become evident in the next few years. There has been no increase attributable to Chernobyl in congenital abnorm-alities, adverse pregnancy outcomes or any other radiation-induced disease in the general population either in the contaminated areas or further afield. After the shelter was built over the destroyed reactor at Chernobyl, a team of about 15 engineers and scientists was set up to investigate the situation inside it. Over several years they repeatedly entered the ruin, accumulating doses of up to 15,000 mSv. Daily dose was mostly restricted to 50 mSv, though occasionally it was many times this. None of the men developed any symptoms of radiation sickness, but they must be considered to have a considerably increased cancer risk. CANCER RISKS FROM RADIATION Studies of populations exposed to radiation doses in excess of natural background have yielded information on the risk of cancer. The risk associated with large radiation doses is relatively well established. However, the risks associated with doses under about 200 mSv are less obvious because of the large underlying incidence of cancer caused by other factors. Risks for exposures under about 100 mSv are assumed rather than demonstrated. . Epidemiological studies continue on the survivors of the atomic bombing of Hiroshima and Nagasaki, involving some 76,000 people exposed at levels ranging up to more than 5,000 mSv. These have shown that radiation is the likely cause of several hundred deaths from cancer, in addition to the normal incidence found in any population. From this data the ICRP and others estimate the fatal cancer risk as 5% per Sievert exposure for a population of all ages, so that one person in 20 exposed to it could be expected to develop a fatal cancer some years later. In western countries, about a quarter of people die from cancers, with smoking, dietary factors, genetic factors and strong sunlight being among the main causes. Radiation is a weak carcinogen, but undue exposure can certainly increase health risks. In 1990 the US National Cancer Institute (NCI) found no evidence of any increase in cancer mortality among people living near to 62 major nuclear facilities. The NCI study was the broadest of its kind ever conducted and supported similar studies conducted elsewhere in the US as well as in Canada and Europe. In Britain there are significantly elevated childhood leukaemia levels near Sellafield as well as elsewhere in the country. The reasons for these increases, or clusters, are unclear, but a major study of those near Sellafield has ruled out any contribution from nuclear sources. Apart from anything else, the levels of radiation at these sites are orders of magnitude too low to account for the excess incidences reported. However, studies are continuing in order to provide more conclusive answers. LOW-LEVEL RADIATION RISKS
In the last 25 years a lot of research has been undertaken on the effects of low-level radiation. Many of the findings have failed to support the so-called linear hypothesis. This theory assumes that the demonstrated relationships between radiation dose and adverse effects at high levels of exposure also applies to low levels and provides the (deliberately conservative) basis of occupational health and other radiation protection standards. Extensive research has not supported the linear hypothesis for low-level radiation exposure. Some evidence suggests that there may be a threshold below which no harmful effects of radiation occur. However, this is not yet accepted by national or international radiation protection bodies as sufficiently well proven to be taken into official standards. In addition, there is increasing evidence of beneficial effect from low-level radiation (up to about 10 mSv/yr). This "radiation hormesis" may be due to an adaptive response by the body's cells, the same as that with other toxins at low doses. In the case of carcinogens such as ionizing radiation, the beneficial effect is seen both in lower incidence of cancer and in resistance to the effects of higher doses. However, until possible mechanisms are confirmed, uncertainty will remain. Further research is under way and the debate continues. In 1998 the world-famous health physicist Professor Bernard Cohen also presented a comprehensive paper on the question. A detailed report published early in 1999 from the Low Dose Task Group of the International Nuclear Societies' Council also suggests a better basis for radiation protection. A September 1999 paper by Prof Jaworowski, published by the American Inststute of Physics, highlights the ethical problems arising from failure to change the basis of radiation protection to adopt a "practical threshold" for exposure. See also University of Michigan Radiation & Health Physics home page and material and the Health Physics Society's Radiation factsheets and 2001 papers, as well as the Radiation Oncology Online cancer information summaries. MAIN SOURCES Uranium Information Centre (2002) Radiation and
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