Printer friendly version is also available
- Natural sources account for most of the
radiation we all receive each year. Up to a quarter of that
received is due to human activity and originates mainly from
- The nuclear fuel cycle does not give rise to
significant radiation exposure for members of the public.
- Radiation protection standards assume that
any dose of radiation, no matter how small, involves a possible
risk to human health. This deliberately conservative assumption is
increasingly being questioned.
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
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
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
||Typical background radiation experienced by
everyone (av 1.5 mSv in Australia, 3 mSv in North America).
|1.5 to 2.0 mSv/year
||Average dose to Australian uranium miners,
above background and medical. |
||Average dose to US nuclear industry
|up to 5 mSv/year
||Typical incremental dose for aircrew in
middle latitudes. |
||Exposure by airline crew flying the New
York - Tokyo polar route. |
||Maximum actual dose to Australian uranium
||Current limit (averaged) for nuclear
industry employees and uranium miners. |
||Former routine limit for nuclear industry
employees. It is also the dose rate which arises from natural
background levels in several places in Iran, India and Europe.
||Lowest level at which any increase in
cancer is clearly evident. Above this, the probability of
cancer occurrence (rather than the severity) increases with
||Criterion for relocating people after
Chernobyl accident. |
||Would probably cause a fatal cancer many
years later in 5 of every 100 persons exposed to it (ie. if
the normal incidence of fatal cancer were 25%, this dose would
increase it to 30%).|
|1,000 mSv/single dose
||Causes (temporary) radiation sickness such
as nausea and decreased white blood cell count, but not death.
Above this, severity of illness increases with dose.|
|5,000 mSv/single dose
||Would kill about half those receiving it
within a month. |
|10,000 mSv/single dose
||Fatal within a few
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
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.
Public dose limits for exposure from uranium
mining or nuclear plants are usually set at 1 mSv/yr above
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.
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
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
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.
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
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
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
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
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
Uranium Information Centre (2002) Radiation and
NRPB Radiation Protection Bulletin # 167, July
1995, pp 13-16