Naturally-Occurring Radioactive Materials (NORM)
(Updated December 2016)
- Radioactive materials which occur naturally and where human activities
increase the exposure of people to ionising radiation are known by the
acronym 'NORM'.
- NORM results from activities such as burning coal, making and using
fertilisers, oil and gas production.
- Uranium mining exposes those involved to NORM in the uranium orebody.
- Radon in homes is one occurrence of NORM which may give rise to
concern and action to control it, by ventilation.
All minerals and
raw materials contain radionuclides of natural origin. The most important for
the purposes of radiation protection are the radionuclides in the U-238 and
Th-232 decay series. For most human activities involving minerals and raw
materials, the levels of exposure to these radionuclides are not significantly
greater than normal background levels and are not of concern for radiation
protection. However, certain work activities can give rise to significantly
enhanced exposures that may need to be controlled by regulation. Material
giving rise to these enhanced exposures has become known as naturally occurring
radioactive material (NORM).
NORM is the acronym
for Naturally Occurring Radioactive Material, which potentially includes all
radioactive elements found in the environment. However, the term is used more
specifically for all naturally occurring radioactive materials where
human activities have increased the potential for exposure compared with the
unaltered situation. Concentrations of actual radionuclides may or may not have
been increased; if they have, the term Technologically-Enhanced (TENORM) may be
used.
Long-lived
radioactive elements such as uranium, thorium and potassium and any of their
decay products, such as radium and radon are examples of NORM. These elements
have always been present in the Earth's crust and atmosphere, and are
concentrated in some places, such as uranium orebodies which may be mined. The
term NORM exists also to distinguish ‘natural radioactive material’ from
anthropogenic sources of radioactive material, such as those produced by
nuclear power and used in nuclear medicine, where incidentally the radioactive
properties of a material maybe what make it useful. However from the
perspective of radiation doses to people, such a distinction is completely
arbitrary.
Exposure to naturally
occurring radiation is responsible for the majority of an average person’s
yearly radiation dose (see also Nuclear Radiation and Health Effects paper) and is therefore not usually considered of any special
health or safety significance. However certain industries handle significant
quantities of NORM, which usually ends up in their waste streams, or in the
case of uranium mining, the tailings dam. Over time, as potential NORM hazards
have been identified, these industries have increasingly become subject to
monitoring and regulation. However, there is as yet little consistency in NORM
regulations among industries and countries. This means that material which is
considered radioactive waste in one context may not be considered so in
another. Also, that which may constitute low-level waste in the nuclear
industry might go entirely unregulated in another industry (see section below
on recycling and NORM).
The acronym TENORM, or
technologically enhanced NORM, is often used to refer to those materials where
the amount of radioactivity has actually been increased or concentrated as a
result of industrial processes. This paper addresses some of these industrial
sources, and for simplicity the term NORM will be used throughout.
Excluding uranium mining
and all associated fuel cycle activities, industries known to have NORM issues
include:
- The coal industry
(mining and combustion)
- The oil and gas
industry (production)
- Metal mining and
smelting
- Mineral sands (rare
earth minerals, titanium and zirconium).
- Fertiliser (phosphate)
industry
- Building industry
- Recycling
Another NORM issue
relates to radon exposure in homes, particularly those built on granitic
ground. Occupational health issues include the exposure of flight crew to
higher levels of cosmic radiation, the exposure of tour guides to radon in
caves, exposure of miners to radon underground, and exposure of workers in the
oil & gas and mineral sands industries to elevated radiation levels in the
materials they handle.
NORM sources
The list of isotopes
that contribute to natural radiation can be divided into those materials which
come from the ground (terrestrial sources – the vast majority) and those which
are produced as a result of the interaction of atmospheric gases with cosmic
rays (cosmogenic).
NORM levels are typically expressed in one of two ways: Becquerels per kilogram (or gram) indicates level of radioactivity generally or due to a particular isotope, while parts per million (ppm) indicates the concentration of a specific radioisotope in the material.
NORM levels are typically expressed in one of two ways: Becquerels per kilogram (or gram) indicates level of radioactivity generally or due to a particular isotope, while parts per million (ppm) indicates the concentration of a specific radioisotope in the material.
Terrestrial NORM
Terrestrial NORM
consists of radioactive material that comes out of the Earth’s crust and
mantle, and where human activity results in increased radiological exposure.
The materials may be original (such as uranium and thorium) or decay products
thereof, forming part of characteristic decay chain series, or potassium-40.
The two most important chains providing nuclides of significance in NORM are
the thorium series and the uranium series:
Another major source of
terrestrial NORM is potassium 40 (K-40). The long half-life of K-40 (1.25 billion
years) means that it still exists in measurable quantities today. It beta
decays, mostly to calcium-40, and forms 0.012% of natural potassium which is
otherwise made up of stable K-39 and K-41. Potassium is the seventh most
abundant element in the Earth’s crust, and K-40 averages 850 Bq/kg there. It is
found in many foodstuffs (bananas for example), and indeed fills an important
dietary requirement, ending up in our bones. (Humans have about 65 Bq/kg of
K-40 and along with those foods are therefore correspondingly radioactive to a
small degree. A 70 kg person has 4400 Bq of K-40 – and 3000 Bq of carbon-14.)
Cosmogenic NORM
Cosmogenic NORM is
formed as a result of interactions between certain gases in the Earth’s
atmosphere and cosmic rays, and is only relevant to this paper due to flying
being a common mode of transport. Since most cosmic radiation is deflected by
the Earth’s magnetic field or absorbed by the atmosphere, very little reaches
the Earth’s surface and cosmogenic radionuclides contribute more to dose at low
altitudes than cosmic rays as such. At higher altitudes, the dose due to both
increases, meaning that mountain dwellers and frequent flyers are exposed to
higher doses than others. For most people, cosmogenic NORM barely contributes
to dose – perhaps a few tens of microsieverts per year. By contrast,
terrestrial NORM – especially radon – contributes to the majority of natural
dose, usually over 1000 microsieverts (1 mSv) per year. Some of the main
comsogenic nuclides are shown in Table 1, carbon-14 being important for dating
early human activities.
NORM and cosmic
radiation account for over 85% of an ‘average individual’s’ radiation exposure.
Most of the balance is from exposure related to medical procedures. (Exposure
from the nuclear fuel cycle - including fallout from the Chernobyl accident -
accounts for less than 0.1%.)
Industries producing NORM
Coal Energy – combustion and ash
Over the years there
have been many occasions when it was asserted that coal-fired power stations
emitted more radioactivity into the environment (from NORM) than was released
anywhere in the nuclear fuel cycle. While having some basis in fact, the claim
is generally not correct now where deployment of emission reduction
technology – scrubbers, filters and flue gas desulphurization – acts
to capture solids from this material. More volatile Po-210 and Pb-210
still escape. In China, coal-fired power plants are a major source of
radioactivity released to the environment and thus contribute significantly to
enhanced NORM there. (Wu et
al in NORM VII)
Most coal contains
uranium and thorium, as well as their decay products and K-40. The total levels
of individual radionuclides typically are not great and are generally about the
same as in other rocks near the coal, which varies according to region and
geology. Enhanced radionuclide concentration in coal tends to be associated
with the presence of other heavy metals and high sulfur content. Table 2
presents some characteristic values,* though coal in some areas can contain notably
higher levels than shown. For comparison, the average radioactivity of the
Earth’s crust is about 1400 Bq/kg, more than half of it from K-40.
The amounts of
radionuclides involved are noteworthy. US, Australian, Indian and UK coals
contain up to about 4 ppm uranium, those in Germany up to 13 ppm, and those
from Brazil and China range up to 20 ppm uranium. Thorium concentrations are
often about three times those of uranium.
During combustion
the radionuclides are retained and concentrated in the flyash and bottom ash,
with a greater concentration to be found in the flyash. The concentration of
uranium and thorium in bottom and flyash can be up to ten times greater than
for the burnt coal, while other radionuclides such as Pb-210 and K-40 can
concentrate to an even greater degree in the flyash. Some 99% of flyash is
typically retained in a modern power station (90% in some older ones). While
much flyash is buried in an ash dam, a lot is used in building construction.
Table 3 gives some published figures for the radioactivity of ash. There are
obvious implications for the use of flyash in concrete.
At a coal-fired
power plant in China the amount of polonium-210 aerosol emitted from a coal
plant stack was measured and found to be 257 MBq/GW/yr. (Liu et al in NORM VII)
