Naturally-occurring radioactive materials (NORM)
This EOE article is adapted from an information paper published by the World Nuclear Association (WNA). WNA information papers are frequently updated, so for greater detail or more up to date numbers, please see the latest version on WNA website (link at end of article).
NORM is an acronym for Naturally Occurring Radioactive Material, which includes all radioactive elements found in the environment. 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 within tissues of all living beings.
Many natural materials contain radioactive elements (radionuclides). The Earth's crust is radioactive and constantly leaks radon gas into our atmosphere. However, while the level of individual exposure from all this is usually trivial, some issues arise regarding regulation, and also perspective in relation to what is classified as radioactive waste.
Although the concentration of NORM in most natural substances is low, higher concentrations may arise as a result of human activities. For example radium may be precipitated out in scale that forms in a natural gas processing pipe or radon decay products may concentrate on the turbine blades of a natural gas pump. This enhancement of natural radioactivity has been found in:
- Petroleum and natural gas production
- Mineral extraction and processing
- Metal recycling
- Forest products
- Thermal electric power generation
- Water treatment facilities
- Tunneling and underground workings
Over the years there have been many occasions when it was asserted that coal-fire power plants emitted more radioactivity (from naturally-occurring radioactive materials in coal) than was released in any phase of the nuclear fuel cycle. While having some basis in fact, the claim is generally not correct now. Today the contention regarding NORM tends to be in relation to steel and other materials released from demolished industrial facilities, and whether the clearance level for this should be at or below naturally-occurring levels.
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, and exposure of miners generally and workers in the oil and gas and mineral sands industries to elevated radiation levels due to radionuclides in the Earth.
A characteristic of NORM is that because of their wide distribution from many sources, they give rise to a very much larger radiological effect to the public (by about four orders of magnitude) compared with that caused by the nuclear industry.
Most coal contains uranium and thorium, as well as potassium-40, lead-210, and radium-226. The total levels are generally about the same as in other rocks of the Earth's crust. Most emerge from a power plant in the light flyash, which is fused and chemically stable. Some 99% of flyash is typically retained in a modern power station (only 90% in some older ones), and this is buried in an ash dam. Some is sold for making concrete.
The amounts of radionuclides involved are noteworthy. In Victoria, Australia, 65 million tonnes of brown coal is burned annually for electricity production. This contains about 1.6 ppm uranium (U) and 3.0-3.5 ppm thorium (Th), hence about 100 tonnes of uranium and 200 tonnes of thorium is buried in landfills each year in the Latrobe Valley. Australia exports 235 Mt/yr of coal with 1 to 2 ppm U and about 3.5 ppm Th (Dale & Lavrencic 1993) in it, hence up to 400 tonnes of uranium and about 800 tonnes of thorium could conceivably be added to published export figures.
Other coals are quoted as ranging up to 25 ppm U and 80 ppm Th. In the USA, ash from coal-fired power plants contains on average 1.3 ppm of uranium and 3.2 ppm of thorium, giving rise to 1200 tonnes of uranium and 3000 tonnes of thorium in ash each year (for 955 million tonnes of coal used for power generation). Applying these concentration figures to world coal consumption for power generation (7800 Mt/yr) gives 10,000 tonnes of uranium and 25,000 tonnes of thorium per year.
It is evident that even at 1 ppm U in coal there is more energy in the contained uranium (if it were to be used in a fast breeder reactor) than in the coal itself. At 25 ppm U and used in a conventional reactor it would be half as much as the coal.
The actual radioactivity levels are not great. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) estimated that average concentrations in coal worldwide were 50 Bq/kg K-40 and 20 Bq/kg each U & Th, though Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO) puts Australian figures at average 830 Bq/kg total radioactivity, related to 1.8 ppm U and 7 ppm Th in the coal, contrasted with some 1400 Bq/kg average in the Earth's crust. The U.S. National Council on Radiation Protection (NCRP) figures give 174 Bq/kg average total radioactivity in US coal. Cooper (2003) gives 100-600 Bq/kg range for New South Wales (NSW) coals and Misha (2004) 145 Bq/kg average in Indian coal.
UNSCEAR (1993) gives 3645 Bq/kg average in flyash. The above US data at 15% ash give 1200 Bq/kg in flyash. Dale (1996) quotes CSIRO figures of 2630 and 3200 Bq/kg from a high-ash NSW coal. Cooper (2003) gives up to 1500 Bq/kg for flyash and up to 570 Bq/kg for bottom ash in NSW. There are obvious implications for the use of flyash in concrete, and the data also may be compared with levels of 1.0 or 3.7 MBq/kg sometimes taken as threshold levels for classifying material as low-level radioactive waste, or with 25 MBq/kg for uranium metal.
With increased uranium prices the uranium in ash becomes significant economically. In the 1960s and 1970s, some 1100 tU was recovered from coal ash in USA. In 2007 China National Nuclear Corp commissioned Sparton Resources of Canada with the Beijing No.5 Testing Institute to undertake advanced trials on leaching uranium from coal ash out of the Xiaolongtang power station in central Yunnan. It and two nearby power stations use lignite with high ash content (20-30%) and very high uranium content, from a single open-cut mine. The coal uranium content varies from about 20 to 315 ppm and averages about 65 ppm. The ash averages about 210 ppm U (0.021%U) - above the cut-off level for some uranium mines. The power station ash heap contains over 1000 tU, with annual arisings of 190 tU. (Recovery of this by acid leaching is about 70%.) Sparton also has an agreement to extract uranium from coal ash following germanium recovery in the Bangmai and Mengwang basins in Yunnan. This ash ranges from 150 to over 4000 ppm U (0.40 %U), averaging 250 ppm U (0.025%). Then Sparton was commissioned by WildHorse Energy to assess the potential for recovering uranium from European coal ash having 80 - 135 ppm U.
Mineral sands, mined chiefly for titanium minerals and zircon, often have a significant proportion of monazite, a rare earth mineral containing thorium and other elements of economic significance. The minerals in the sands are subject to gravity concentration, and some concentrates are significantly radioactive, up to 4000 Bq/kg. Dust control in mineral sand plants is the main means of limiting radiation doses to personnel.
Radioactivity in Australian mineral sands (Bq/g)
Tantalum ores, often derived from pegmatites, comprise a wide variety of more than one hundred minerals, some of which contain uranium and/or thorium. Hence, the mined ore and its concentrate contain both these and their decay products in their crystal lattice. Concentration of the tantalum minerals is generally by gravity methods (as with mineral sands), so the lattice-bound radioisotope impurities, if present, will report with the concentrate.
While this has little radiological significance in the processing plant, concentrates shipped to customers sometimes exceed the Transport Code threshold of 10 kBq/kg, requiring declaration and some special documentation, labeling and handling procedures. Some reache 75 kBq/kg.
Phosphate rock used in the production of fertilizer is a major source of naturally-occurring radioactive materials (NORM), containing uranium and thorium. Australian phosphate rock contains up to 900 Bq/kg and imported sources contain about twice this, yielding about 1000 Bq/kg in phosphogypsum waste streams and up to 3000 Bq/kg in the superphosphate product. In the U.S., some 50 million tonnes per year are produced and state figures average up to 10,000 Bq/kg of total radioactivity. Processing this sometimes gives rise to measurable doses of radioactivity to people. Phosphate rocks containing up to 120 ppm U have been used as a source of uranium as byproduct – some 17,000 tU in USA, and are likely to be so again.
European fertilizer manufacturing gave rise to discharges of phophogypsum containing significant quantities of radium-226, lead-210 and polonium-210 into the North Sea and North Atlantic. This has been overtaken by offshore oil and natural gas production in Norwegian and UK waters releasing some 10 TBq/yr of radium-226, radium-228, and lead-210—contributing 90% of alpha-active discharges in those waters (two orders of magnitude more than the nuclear industry, and with this NORM having higher radiotoxicity).
Oil and Gas production
In the oil and natural gas industry, radium-226 (Ra-226) and lead-210 (Pb-210) are deposited as scale in pipes and equipment. If the scale has an activity of 30,000 Bq/kg it is 'contaminated' (Victoria, Australia regulations). This means that for Ra-226 scale (decay series of 9 progeny) the level of Ra-226 itself is 3300 Bq/kg. For Pb-210 scale (decay series of 3) the level is 10,000 Bq/kg. These figures refer to the scale, not the overall mass of pipes or other material (cf Recycling, below). Published data (quoted in Cooper 2003) show radionuclide concentrations in scale of up to 300,000 Bq/kg for Pb-210, 250,000 Bq/kg for Ra-226 and 100,000 Bq/kg for Ra-228. In Cooper 2005, the latter two maxima are 100,000 and 40,000 respectively.
Other solid NORM
Building materials can contain elevated levels of radionuclides including radium-226, thorium-232 and potassium-40, the last being most significant in published Australian data, ranging up to 4000 Bq/kg in natural stone and 1600 Bq/kg in clay bricks and concrete. Bricks can also contain up to 2200 Bq/kg of Ra-226 (Cooper 2005).
In smelting iron ore, lead-210 and polonium-210 accumulate in dust from smelter and sinter plant operations, in the latter case to 34,000 Bq/kg at Port Kembla, Australia.
Granite, widely used as a cladding on city buildings and also architecturally in homes, contains an average of 3 ppm (40 Bq/kg) uranium and 17 ppm (70 Bq/kg) thorium. Radiation measurements on granite surfaces can show levels similar to those from low-grade uranium mine tailings.
Radium-226 is one of the decay products of uranium-238, a uranium isotope widespread in most rocks and soils. When this radium decays it produces radon-222, an inert gas with a half-life of almost 4 days. Radium-224 is a decay product of thorium, and it decays to radon-220, also known as thoron, with a 54-second half-life. Because radon is so short-lived, and alpha-decays to a number of daughter products which are solid and very short-lived, there is a high probability of its decay when breathed in, or when radon daughter products in dust are breathed in. This is a problem because alpha particles in the lung are hazardous to human health.
Radon levels in the air range from about 4 to 20 Bq/m3. Indoor radon levels have attracted a lot of interest since the 1970s. In the US they average about 55 Bq/m3, with an EPA action level of 150 Bq/m3. Levels in Scandinavian homes are about double the US average, and those in Australian homes average one-fifth of those in US. Levels up to 100,000 Bq/m3 have been measured in US homes. In caves open to the public, levels of up to 25,000 Bq/m3 have been measured.
Radon also occurs in natural gas at up to 37,000 Bq/m3, but by the time the product reaches consumers, the radon has largely decayed. However, the solid decay products then contaminate gas processing plants, and this manifestation of NORM is an occupational health issue.
Recycling and NORM
Scrap steel from gas plants may be recycled if it has less than 500,000 Bq/kg (0.5 MBq/kg) radioactivity (the exemption level). This level, however, is one thousand times higher than the clearance level for recycled material (both seel and concrete) from the nuclear industry. Anything above 500 Bq/kg may not be cleared from regulatory control for recycling.
Decommissioning experts are increasingly concerned about double standards developing in Europe, where 30 times the dose rate from non-nuclear recycled materials than from those out of the nuclear industry is allowed. Norway and Holland are the only countries with consistent standards. Elsewhere, a 0.3 to 1.0 mSv/yr individual dose constraint is applied to oil and natual gas recyclables, and 0.01 mSv/yr for release of materials with the same kind of radiation from the nuclear industry.
The main radionuclide in scrap from the oil and gas industry is radium-226 (Ra-226), with a half-life of 1600 years as it decays to radon. Those in nuclear industry scrap are cobalt-60 and caesium-137, with much shorter half-lives. Application of a 0.3 mSv/yr dose limit results in a clearance level for Ra-226 of 500 Bq/kg, compared with 10 Bq/kg for nuclear material.
The concern arises because of the very large amounts of Tenorm (technologically-enhanced NORM) needing recycling or disposal from many sources. The largest Tenorm waste stream is coal ash, with 280 million tonnes arising globally each year, and carrying uranium-238 and all its non-gaseous decay products, as well as thorium-232 and its progeny. This waste is usually just buried; however, the double standard means that the same radionuclide, at the same concentration, can either be sent to deep disposal or released for use in building materials, depending on where it comes from. The 0.3 mSv/yr dose limit is still only one-tenth of most natural background levels, and two orders of magnitude lower than those experienced naturally by many people, who suffer no apparent ill effects.
- WNA paper on Naturally occurring radioactive materials (NORM)