ENVIRONMENTAL PROBLEMS POSED BY WASTES FROM THE URANIUM MILLING INDUSTRY
4. ENVIRONMENTAL EFFECTS OF SOLID WASTES 1. Mill tailings
The tailings release radioactive material to the air as radon gas and airborne particulates, to waterways as particulates that may settle out in certain locations, and as radionuclides that may be leached out by precipitation, surface run-off and the waste solutions. The tailings are sufficiently radioactive to create a weak field of gamma radiation in their immediate vicinity. Chemical pollutants including various heavy metals, sulphates and sulphuric acid may also be released from them.
Dust is referred to in Section 2. Wind and water erosion of tailings heaps depends on the physical properties of the waste and the climate of the area.
The sulphide tailings released from some uranium mills will produce acid seepage and oxida-tion of the surface layer. This surface oxidaoxida-tion will hinder the development of vegetaoxida-tion.
The decay of radon gas within the tailings and the atmospheric dilution of radionuclides released are usually sufficient to keep the radon concentrations, at some distance from operational and non-operational waste retention systems, sufficiently low so as not to constitute a significant health hazard.
IAEA-AG/33-14 121 TABLE I. URANIUM MILLING: AIRBORNE
RELEASES OF RADIOACTIVE MATERIALS
„ , Site boundary _ Release rate
Radionuclide , . , , , air concentration
Natural uranium
Distance to site boundary assumed to be 600 m.
However, in houses built in the immediate vicinity of or directly on tailings heaps, an elevated radon concentration can be expected. Significantly elevated radon daughter concentrations as well as increased dose rates have been observed in houses built over fill material containing uranium mining and milling waste [5].
The estimated maximum release rates of airborne radionuclides and conservative estimates of air concentrations at the site boundary for a model uranium mill producing 960 t U/a are given in a recent survey [6].
The source of the radionuclides is the plant and the ore pile and tailings system (see Table I).
The radionuclide concentrations constitute only a fraction of those considered acceptable for individual members of the public.
Radon-222 dosimetry is a complex and difficult subject. The radiation dose caused by radon is a function of the equilibrium state between radon and its daughters at the time of exposure and therefore it is not enough to know the radon-222 concentration alone. Long-term exposure occurs because mill tailings contain long-half-life 226Ra, the parent of radon, so that radon will be con-tinually produced and will emanate from the pile indefinitely.
A 226Ra concentration of 1 pCi/1 above background has been used as the recommended limit for continuous exposure to individuals in the general population [7]. It should be noted that background levels may range from 0.1 to 1.0 pCi/1 in the free atmosphere and from 1 to 10 pCi/1 in granite dwellings.
Wind-borne particulate material from waste retention systems may also be of some significance because of the 230Th and 226 Ra content. It appears that radon releases, which are the most difficult to control, are of most significance but that radioactive particulate releases also may sometimes be significant [8].
The United Nations Scientific Committee on the Effects of Atomic Radiation [9] found that the contribution of uranium mining and milling operations to the radiation dose commitment is probably small but should be evaluated.
Radon emanation and the release of radioactive particulates from a given amount of tailings is reduced when:
(a) The tailings area exposed is decreased;
(b) The tailings are under water, ice or snow;
(c) The tailings are covered with a thick cover of earth or an impermeable membrane.
122 BONHOTE The radiation dose to the public can be reduced by:
(a) Siting tailings dams as far as possible from established residential areas;
(b) Restricting the development of new residential areas close to operating or non-operating tailings areas.
In spite of some uncertainty regarding the actual health effect of radon and particulates released from tailings, it appears prudent to utilize all the above measures to the maximum extent possible to limit a potential future detriment.
Radium and heavy-metal accumulation on the surface of sulphide and non-sulphide wastes may hinder or inhibit seed germination and plant establishment. The impact of these accumulated metals is widespread and not clearly defined. Several plant species can tolerate a high accumulation of metals with no apparent damage, but they may be toxic to grazing animals. Other reactions in the food chain to man are not clearly known. When the chemical substances that inhibit vegetation establishment are treated, more waste results with similar detrimental properties.
Owing to the size of some large tailings areas the complete isolation of the waste from water courses will be impossible. Filling a lake or valley with tailings will result in a greater moisture reserve than previously existent whereas constructing a tailings dam on a flat surface and raising it by perimeter dams will maintain a drier surface.
When tailings are deposited near population centres or in a scenic area, it is essential to fit them aesthetically into the surroundings. These considerations will influence the extent of recla-mation to be carried out.
4.2. Sludges
Precipitated sludges will normally remain insoluble and will not present an immediate hazard.
However, as the operations proceed, or if the nature of the liquid flowing over the precipitate changes - either because of process changes or simply because of abandonment of the operation — problems may be encountered caused by slow release of the 226 Ra content.
The accumulated sludge is considerably more radioactive than solid tailings and may become a source of radium contamination. Under unusual conditions, such as a 10-year storm or in the case of a main dam or settling dam failure, the sludge could be washed downstream into public receiving waters. Should this occur, it will present a radiological hazard both as regards the ingestion of particulate matter and the possibility of dissolution because of the composition of the receiving water.
4.3. Non-operating mill sites
Very few countries have any experience in the decommissioning of uranium mill sites. The USA does have certain regulations concerning the clean-up levels necessary to decontaminate abandoned buildings and the surrounding area. These criteria are also utilized at abandoned uranium ore-buying stations. The prescribed procedures are necessary especially in areas where the mill site may be utilized after the mill has been shut down. In the western USA where over twenty mills have been closed down, the local residents are often interested in utilizing the remaining buildings. Extensive decontamination procedures have been necessary to reduce the contamination levels from the uranium tailings and former ore stockpiles.
5. CONCLUSIONS
The environmental problems posed by wastes from the uranium milling industry are receiving attention by the industry and by national authorities. Air pollution from mills is not a serious
IAEA-AG/33-14 123 problem. Water pollution is potentially a greater problem, but methods are available to treat liquid effluents released from operational mills so that their impact need not be significant.
The problems posed by the disposal of tailings in engineered surface waste retention systems are not completely resolved at the present time. Some of these problems, such as contaminated seepage, revegetation and stabilization, are common to other mineral extraction operations. The unique problems arise from the long half-life of theradionuclides (notably 23OTh and 226Ra) present in the tailings, which have the potential to lead to radiation exposure of both present and future populations. The actual significance of these potential radiation exposures has not yet been convincingly demonstrated. Bearing in mind the fact that these exposures are caused by natural radioactivity to which mankind has always been exposed, it is believed that the total radiation detriment caused by the extraction of uranium from ores can be shown not to be unjustifiable in relation to the benefits from such extraction.
ACKNOWLEDGEMENT
The information reported here is drawn extensively from the work of two IAEA meetings convened to consider waste management in the uranium and thorium mining and milling industry.
The first meeting was held in Ottawa, 8-12 July 1974, under Mr. K. Hester as Chairman, the second was convened in Vienna, 12-16 May 1975, with Mr. D. Nussbaumer as Chairman. The author wishes to thank all members of those meetings for their active participation.
REFERENCES
[1 ] INTERNATIONAL ATOMIC ENERGY AGENCY, Management of Wastes from the Mining and Milling of Uranium and Thorium Ores, Safety Series No.44, IAEA, Vienna (1976).
[2] Water Pollution from the Uranium Mining Industry in the Elliot Lake and Bancroft Areas, Vol.1 : Summary October 1971, Vol.2: Detailed Findings March 1972, Ontario Water Resources Commission (1972).
[3] Radium-226, Uranium and other Radiological Data from Water Quality Surveillance Stations located in the Colorado River Basin of Colorado, Utah, New Mexico and Arizona, January 1961 through June 1972, 8 SA/T1B-24, US Environmental Protection Agency (July 1973).
[4] INTERNATIONAL ATOMIC ENERGY AGENCY, Disposal of Radioactive Wastes into Rivers, Lakes and Estuaries, Safety Series No.36, IAEA, Vienna (1971).
[5] Use of Uranium Mill Tailings for Construction Purposes, Statement of Dr. C E . Larson at the Hearings of the Joint Committee on Atomic Energy, Congress of the United States, 28 and 29 Oct. 1971, US Government Printing Office, Washington, DC (1971).
[6] Environmental Survey of the Uranium Fuel Cycle, Rep. WASH-1248, USAEC, Washington, DC (Apr.1974).
[7] US DEPARTMENT OF HEALTH, EDUCATION AND WELFARE, Evaluation of Radon-222 Near Uranium Tailings Piles, Rep. DER 69-1, Bureau of Radiological Health, New Rockville, MD (Mar.1969).
[8] SWIFT, J.J., HARDIN, J.M., CALLEY, H.W., Assessment of Potential Radiological Impact of Airborne Releases and Direct Gamma Radiation from Inactive Uranium Mill Tailings Piles, Draft, US Environmental Protection Agency, Office of Radiation Programmes (Oct.1974).
[9] Ionizing Radiation: Levels and Effects, AReportof the United Nations Scientific Committee on the Effects of Atomic Radiation to the General Assembly, Vol. 1 : Levels, United Nations (1972).
DISCUSSION
F.E. McGINLEY: I want to thank Mr. Bonhote for an excellent paper. There is a very good com-pilation of information and data accumulated throughout the years in the United States which I can recommend to those interested in this particular subject, namely the Oak Ridge National Laboratory
124 BONHOTE
Report TM-4903, Volumes 1 and 2 ("Correlation of Radioactive Waste Treatment Costs and the Environmental Impact of Waste Effluents in the Nuclear Fuel Cycle for Use in Establishing 'As Low As Practicable' Guides - Milling of Uranium Ores", by M.B. Sears et al., May 1975).
The report is available from the National Technical Information Services, U.S. Department of Commerce, 5285 Port Royal Road, Springfield, Virginia, 22161.
W.R. FLOTER: Is it related to the USA or worldwide?
F.E. McGINLEY: This is specific to the USA but it does contain a compilation of just about all the information that has ever been published in the USA on this particular subject — it is a very good reference work.