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Use of Isotopic Signature of Radionuclides Released from Uranium Mines and Mills to Discriminate Low Levels of Environmental Impact Against Natural Background Levels

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XA9745957 USE OF ISOTOPIC SIGNATURE OF RADIONUCLIDES RELEASED FROM URANIUM MINES AND MILLS TO DISCRIMINATE LOW LEVELS OF ENVIRONMENTAL IMPACT AGAINST NATURAL BACKGROUND LEVELS P. ZETTWOOG, N. LEMAITRE Office de protection contre les rayonnements Ionisants, Le Vesinet S. BERNHARD, Y. VAUZELLE ALGADE, Bessines sur Gartempe France Abstract In France, uranium ores have been exploited in rural areas with a low population density. The critical population group which is identified for radiological impact studies lives close to the uranium facilities, at distances from a few hundred metres to 1 kilometre. Within this range, the radioactivity of the environment is still detectable amongst the natural background. Mining companies manage surveillance networks according to strict specifications laid down by the government authorities. For active mining operations it is the exposure to daughter products of radon 222 that forms the bulk of the total effective dose. After closure of a mine and rehabilitation of the site, products such as uranium 238 and radium 226 in the water can become the major components of the total effective dose. Surveillance networks are built to allow direct measurement of the radon daughter products critical to the alpha energy and measurements of the water activities for uranium 238 and radium 226. Annual limits for the effective individual doses are given and determined by the authorities. The industry must manage effective annual doses of the order of 1 mSv. The natural exposure which is not part of the regulation is of the same order (1 mSv) as the exposure created by the industry. The results given are therefore lacking in clarity for the public. The Office de Protection contre les Rayonnements Ionisants (OPRI) and ALGADE are developing methods which allow differentiation between natural phenomena and man-made phenomena. It has been recognised that for a region where mining activities have taken place, the isotopic signature of uranium, radium and radon can clearly be recognised from the same product of natural origin. In the case of radon, for example, the industry produces only radon 222 while natural emanations are composed of radon 220 and radon 222. 1. FRENCH REGULATIONS RELATING TO ENVIRONMENTAL PROTECTION AT URANIUM MINES 1.1. Historical background In France, the legal document specifically covering environmental protection around uranium mines, ore treatment plants, storage plants for mining waste and solid waste from treatment is Decree no 90-222 of 9 March 1990. The decree was published in the Official Journal of the French Republic of 13 March 1990, and was introduced into the general regulations governing the extractive industries brought into force by Decree no 80-331 of 7 May 1980. Arrangements for monitoring waste disposal and measuring the exposure of the general population to ionising radiation were progressively put into place by the producers from the beginning of the 1970s under their own initiative, the design of these arrangements being based on the recommendations of international organisations in force at the time. For the most part, the above document ratifies these arrangements. Among the documents used as reference material by those drafting Decree no 90-222 were AIEA SS no 43 of 1975 [1] (which laid down the objectives of surveillance networks) and ICRP Publication no 43 [2] of 1984. The latter publication confirmed that the objectives of document SS 337 no 43 were sound, clarified a few important concepts, among them that of "critical population groups" already introduced in ICRP Publication no 26 [3], and endorsed two types of monitoring strategy: those aimed at qualifying environmental impact via a determination of sources and transfers, and those aimed at direct estimates of the exposure of populations. 1.2 The key points in Decree no 90-222 Decree no 90-222 is based on the concept of "critical population groups". Its objective is to limit the fraction of their annual exposure attributable to the "industrial" discharges of mining companies and plants that depend on them. The "critical population groups" are identified during the radiological study of the site carried out prior to the establishment of the surveillance network, taking into account the sources and critical transfer routes of the radionuclides significant for radiological protection. Specific Prefectorial Orders specify the frequency and siting of the sampling points in the environment and the methods of measurement to be used. As regards the radioactivity contained in drainage water discharged from mines and in liquid effluent from industrial plants, the decree introduces the principle of optimising radiological protection. The limits for discharge are fairly severe as regards both the dilution provided by the waterway receiving it and the possible transfer routes. If there is a demand that a radium or uranium purification station is necessary, the operator must use the best available technology leading to the smallest possible discharge. 1.3. Annual dose limits introduced in the circulaire d 'application (memorandum of application) A circulaire d 'application lays down the limits to be complied with as regards the effective dose equivalent added by industry to that of natural origin for the same radionuclides and the same exposure routes. Except for special cases, the effective dose is determined by taking into account the exposure to gamma radiation, the inhalation of short-lived daughter products of radon isotopes and of long-lived alpha emitters in radioactive dust, the consumption of water at the site and the local food produced. At present (May 1996), the annual limit for the effective dose equivalent for people belonging to the critical population group is 5 mSv. It is assumed in the calculation that 5 mSv corresponds to the inhalation of a potential alpha energy (PAE) of 2 mJ due to the daughter products of radon 222, 6 mJ due to those of radon 220, 170 Bq due to ore dust, and to the ingestion of 7000 Bq of radium 226 and 2 g of natural uranium. In the near future, the circulaire a"application will in all probability have to legalise a limit on the effective dose equivalent of 1 mSv per year averaged over 5 years, a limit that the EU is preparing to introduce officially into its directives. The value of 1 mSv will correspond in particular to the ingestion of a PAE of 0.73 mJ from radon 222 daughter products. This quantity is inhaled by a person exposed for a year to an equilibrium radon 222 concentration of 18 Bq m'3, the concentration expressed as a PAE being about 100 nJ m3. 338 2. PROBLEMS ENCOUNTERED BY THE OPERATORS IN APPLYING DECREE NO 90-222 2.1. Current situation After discussion, operators have accepted and implemented the arrangements included in the Prefectorial Orders for the monitoring of discharges and the surveillance networks. It turns out that the value of the effective dose equivalent corresponding to the combination of "industrial" and "natural" exposures is in general less than the annual limit of 5 mSv. Everything is thus fine, and there are no problems still to be solved. In order to recover the contribution of "industrial" origin, the operator subtracts a fixed value for the "natural background" from the total. Since even the total dose falls within the legal limit, the operator is not very particular about the way in which this natural background is determined. To ease matters, he adopts values measured relatively distant from the industrial sources, which are more representative of the regional background than of the local background at the mine workings being monitored and thus possibly too low. This could lead to an overestimate of the industrial component. 2.2. The current annual limit of 1 mSv The total exposure at mines leads in general to an effective dose for populations of more than 1 mSv per year. With the present arrangements for monitoring discharges and the release of solid waste by containment facilities, it might be impossible at some sites to comply with a limit on the industry-related annual effective dose equivalent of 1 mSv (with the above-mentioned corresponding value for radon 222) for all critical groups. The operator should clearly improve his containment facilities, and he is in fact quite prepared to do this provided, however, he is convinced that in his case the limit is not complied with. The operator sees that the natural background is of the same order as the statutory limit and is often greater than the legal industrial contribution. He will have to produce sound arguments for the natural background values that, at the moment, he will have calculated at most just before subtracting them from the total values. The introduction of a stricter limit reveals a problem which has not existed so far: that of discriminating between contributions from natural sources and those from industrial sources in the total effective doses. In the case of mines, using the regional background is not in general adequate. This is because the radioactive atmospheric load of radon 222 due to its exhalation from local soils varies considerably on a local scale, depending as it does on the ventilation capacity and the weather conditions. High values obtained at a station do not necessarily indicate that it is marked by radon of industrial origin. It could simply be a case of a station installed in a poorly ventilated location. Similarly, the radioactive load of natural origin in underground and surface waters, observed in the catchment area of the mine, corresponds to a geological formation which contained an anomalously high uranium concentration. Such a radioactive load of natural origin may be very different from that observed in another catchment area nearby having the same geology but never having contained a uranium deposit. It is therefore essential to find a method of discriminating between the two contributions which is based on measurements made on the scale of the site being monitored and not on any measurements made elsewhere.
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