Naturally occuring radioactive materials

naturally-occurring-radioactive-materials

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.

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)