RADIONUCLIDE FACT SHEET Natural Uranium and the environment 238 92U Olivier Seignette/Mikaël Lafontan/IRSN Contents Characteristics 3 Origins 4 Environmental concentrations 4 Metrology, analytical techniques and detection limits 6 Mobility and bioavailability in terrestrial environments 8 Mobility and bioavailability in continental aquatic environments 9 Mobility and bioavailability in marine environments 10 Mobility and bioavailability in semi-natural ecosystems 11 Environmental dosimetry 11 Environmental toxicity 11 Summary 13 Review of measurement methods 14 Usual radioecological parameters: terrestrial environment 14 Usual radioecological parameters: freshwater environment 16 Usual radioecological parameters: marine environment 17 Radiotoxicological parameters 18 Selected bibliography 19 Radionuclide fact sheet NATURAL URANIUM This fact sheet describes the behaviour of the chemical element in the principal compartments of terrestrial and aquatic ecosystems using the following assumptions. Isotope discrimination is negligible, which is verified for most of the elements considered. When the element has stable isotopes, the behavioural analogy between the stable and radioactive isotopes is accepted implicitly, with the understanding that for naturally- occurring elements, the chemical form and emission environment for anthropogenic discharge may involve pathways and transfer processes different from those identified for the stable natural element. The radioactive isotope designated in the title of the fact sheet has major radioecological importance with regard to quantity and persistence in the environment, with other isotopes, both radioactive and stable, being cited as well. The information, which has been intentionally simplified, is intended to reflect the level of knowledge on the topic as of publication and provide values for the main usual radioecological parameters for estimating transfer in the environment and the food chain in particular. Written by: J. Garnier–Laplace, C. Colle and M. Morello Verified by: B. Bonin Date of first issue: 14 May 2001 Revision: Terrestrial ecosystem: L. Février Continental aquatic ecosystem: O. Simon Marine ecosystem: D. Boust and J. Pommier Metrology: C. Augeray, B. Boulet, X. Cagnat, J. Loyen, J.L. Picolo and R. Vidal Environmental concentrations: P. Renaud Verified by: K. Beaugelin-Seiller Date of revision: 18 December 2010 This document is the intellectual property of IRSN and may not be reproduced without its consent. Radionuclide fact sheet NATURAL URANIUM Characteristics Chemical Uranium, a chemical element with atomic number of 92 is part of the actinide family. A pyrophoric grey metal that is very dense in its pure state, uranium is always found combined with other elements, especially oxygen. It has four possible valences (+III to +VI), with valences 4 and 6 the most common in ore. The conditions for passing from valence 4 to valence 6 depend on the reduction potential of the environment; they resemble the conditions for passing from ferrous iron to ferric iron. Hexavalent uranium is much more soluble than tetravalent uranium; it forms complexes, with the most common being uranylcarbonates and uranylsulfates. Nuclear Natural uranium is composed of three main isotopes (234U, 235U and 238U), all of which are radioactive. The two most abundant isotopes on earth are 238U and 235U, which have existed since the planet’s creation. The 234U isotope is produced by alpha disintegration of 238U and represents only a minute share of all uranium. On the other hand, it is more radioactive than the other isotopes and makes up approximately half of all radioactivity from natural uranium. The 235U isotope is the only natural fissile isotope. The enriched, depleted and reprocessed forms of uranium correspond to variable percentages of these isotopes. Other radioactive isotopes of uranium exist, but they are produced artificially (see table below). Natural uranium such as it is extracted from ore contains by weight 99.275% of the isotope 238, 0.719% of isotope 235 and 0.0057% of isotope 234. Thus for 1 g of natural uranium and without taking into account radioactivity of decay products, the chemical element is distributed as follows: 0.99275 g of 238U, or approximately 12,311 Bq; 0.00719 g of 235U, or approximately 576 Bq; 0.000057 g of 234U, or approximately 12,880 Bq. Isotope 232U 233U 234U 235U 236U 238U Natural abundance - - 0.0057 0.719 - 99.275 (% by weight) Half-life (years) 69.8 1.6 × 105 2.5 × 105 7.04 × 108 2.37 × 107 4.47 × 109 Specific activity 8.17 × 1011 3.56 × 108 2.30 × 108 8.00 × 104 2.36 × 106 1.24 × 104 (Bq.g-1) Précursors 232Th 233Pa 234mPa 235mU 236Pa 238Pa 232Pa 233Np 234Pa 235Np 236mNp 242Pu 232Np 237Pu 234Np 239Pu 236Np 236Pu 238Pu 236Pu 240Pu Decay products 228Th 229Th 230Th 231Th 232Th 234Th Principal emission(s) through disintegration (keV) (% emission probability) α 5,320 4,824 4,775 4,397 4,495 4,198 (68.8) (82.7) (71.4) (54) (76) (77.5) γ 57.8 42.5 53.2 185.7 49.4 49.6 (0.2) (0.065) (0.12) (57.1) (0.072) (0.068) X 13.41 13.41 13.41 13.41 13.41 13.41 (10.2) (7) (9.3) (23.2) (7.9) (7.3) (Nucleonica; CE, 2009) 3 Radionuclide fact sheet NATURAL URANIUM Radionuclide fact sheet NATURAL URANIUM Origins Natural The origin of this radioelement is exclusively natural with redistribution related to human activity (Paulin, 1997). Four principal sources of industrial activity enrich certain compartments, including soil, sediment and water, of the biosphere with uranium: - nuclear fuel cycle from uranium mining to waste processing: France has only 3% of reserves and no more domestic production of uranium. In general, nuclear fuel is an alloy of uranium, either uranium dioxide, a mix of uranium and plutonium oxide, or uranium carbide; - military use of depleted uranium (natural uranium in which the 235U content has been reduced by 0.7 to 0.2%). The metal is used for its pyrophoric properties and sites subject to bombardment by this type of weapon are enriched with fine particles of UO2(solid) deposited near the explosion; - use of coal, which emits uranium into the atmosphere during combustion; - agricultural use of phosphate fertilisers produced from natural phosphates that are particularly rich in uranium 238. Artificial Not applicable. Environmental concentrations Conversion rule: radioactivity/weight conversions given below are performed using the assumption of the measurement of uranium 238 in the expected ratio for natural uranium, with 1 Bq of 238U corresponding to 0.08 mg of natural uranium. These conversions are for purposes of information and comparison. In fact, due to possible imbalances between uranium isotopes in the sampled compartments, only measurement is valid. Uranium-related radioactivity for the principal components of the continental environment – air, plants, animals, surface water and sediments, underground water – is related to that of soil, which is itself related to adjacent geological formations. Overall, soils in sedimentary basins and limestone formations have less uranium than those of granite massifs. On a more local level, specific geological conditions may also lead to unusual levels of uranium, particularly in rivers and ground waters (IRSN, 2009). In Europe, median uranium concentration in soil is estimated to be approximately 2 mg.kg-1, with a range of variation from less than 0.1 to more than 50 mg.kg-1 (De Vos and Tarvainen, 2006). The highest concentrations in France are found in the Massif Central in conjunction with secondary hydrothermal alteration of granite domes, vein deposits or the presence of Autunian black schists. These results are coherent with measurements of radioactivity from uranium 238 for the majority of French soil, which vary between several Bq.kg-1 dry weight (d.w.) and several hundred Bq.kg-1 d.w. (several tenths to tens of mg.kg-1 d.w.). They may reach a thousand Bq.kg-1 d.w. (a hundred mg.g-1) in uraniferous granitic soils. Le Roux (2007) proposes an overall average (including all regions, sites and soil types) of 40 Bq.kg-1 d.w. (or approximately 3 mg.kg-1 d.w.). Nevertheless, such an average is necessarily relative because there is significant influence from the representation of soils analysed with regards to various soils found in France. In this instance, samples of soil found on sedimentary substrate are probably overrepresented. In a compilation of French data from various sources, Picat et al. (2002) indicate that an average of 98 Bq.g-1 d.w. (8 mg.kg-1) around facilities upstream of the nuclear cycle, for a range of variation from 25 to 173 Bq.kg-1 d.w. (2 to 14 mg.kg-1). The United Nations Scientific Commission on the Effects of Atomic Radiation (UNSCEAR) assesses radioactivity in the air from 238U to be around 1 µBq.m-3 (UNSCEAR, 2000) based on some references that measure uranium outside the influence of nuclear facilities, as in Poland (1 to 18 µBq.m-3; or 0.08 to 1.5 ng.m-3) and in the USA (0.9 to 5 µBq.m-3; 0.07 to 0.4 ng.m-3). At the Cadarache site in France, where the substrate is sedimentary by nature, a lower value of 0.1 µBq.m3 (0.008 ng.m-3) corresponding to local background was recently measured. In Europe, median uranium concentration in sediments is estimated to be approximately 2 mg.kg-1, with a range of variation from less than 1 to more than 90 mg.kg-1 (De Vos and Tarvainen, 2006). As with soils, sediments from rivers in the Massif Central may have high uranium concentrations of up to 59 mg.kg-1. These results are consistent with those obtained from measuring 238U. Indeed, according to Le Roux (2007), 4 Radionuclide fact sheet NATURAL URANIUM radioactivity from uranium in sediments from rivers in France resembles that in soil: from several Bq.kg-1 d.w.