CH<W0W)<f Paul Scherrer Institut Labor für Chemie Migration Chemistry and Behaviour of Iodine Relevant to Geological Disposal of Radioactive Wastes A Literature Review with a Compilation of Sorption Data Y. Liu.H.R.von Gunten Paul Scherrer Institut Villigen/Würenlingen Telefon 056/992111 Telex 827417 psi ch MITTEILUNG Die Fusion des Eidg. Institutes für Reaktorforschung (EIR). Wurenlingen.und des Schweiz. Instituts für Nuklearforschung (SIN), Villigen, zum PAUL SCHERRER INSTITUT ist per 1 .Januar 1988 erfolgt. Bis zum Entscheid der PTT über die Neuregelung der Postzustellung erreichen Sie uns ab diesem Datum unter Paul Scherrer Institut oder Paul Scherrer Institut vormals EIR vormals SIN 5303 Würenlingen 5234 Villigen Telefon 056/992111 Telefon 056/993111 Telex 827417 psi ch Telex 827419 psi ch Telefax 056 / 98 23 27 PSI CH Telefax 056 / 99 32 94 PSI /IMP CH PSI - Bericht Nr. 16 September 1988 Migration Chemistry and Behaviour of Iodine Relevant to Geological Disposal of Radioactive Wastes - A Literature Review with a Compilation of Sorption Data Yuanfang Liu1 Hans R. von Gunten2 'On leave from Peking University, Beijing, China 2Paul Sclierrer Institut CII-5303 Wiirenlingen and Laboratorium für Radiochemie, Universität Bern, CII-3000 Bern 9, Switzerland 9 Contents Seite General 6 Basic properties of radioiodine G Iodine isotopes 6 Chemical states 7 Thermodynamic data 10 Distribution and behaviour of iodine in nature 13 Iodine and its speciation in aquatic systems 15 Iodine in minerals and soils 20 Iodine in the atmosphere 22 Global circulation of iodine 22 Radiological impact of l29I 25 129I in hazard assessment 25 Annual dose limit for 129I 2S Summary of Swiss scenarios for intermediate-activity wastes 29 Waste sorts and technical barriers 29 Host rock and underlying sediments 30 Geochemistry of groundwater 30 Radionuclide transport through geomedia 31 Calculated annual doses in the safety assessment 33 Analogue studies of iodine in nature 35 Validation of natural analogue models 35 Iodine migration at Oklo 35 Iodine migration in Scottish marine sediment 36 Iodine redistribution in the Alligator Rivers uranium deposit 38 Sorption and migration behaviour of iodine in the geosphere 41 Materials with limited sorption of iodine 41 Field studies on iodine transport in geomedia 45 Significant sorption of iodine by particular minerals 45 Sorption of iodine on soils 45 Sorption of iodine on hydroxides 47 Residence times of 129I in geomedia 47 Sorption and diffusion of iodine in cement and concrete 51 Cement and concrete in repositories 51 Chemical composition of cement 51 Sorption and diffusion of iodine in concrete 52 3 5 Discussion 53 5.1 Conclusions on the sorption and migration behaviour and mechanisms of 129I in geomedia 53 5.1.1 Sorption of iodine by gcomedia 53 5.1.2 Diffusion studies 55 5.1.3 Field studies on the migration behaviour of iodine in geomedia 56 5.1.4 Sorption and retardation processes of iodine 57 5.2 Retardation of iodine by cement barriers 59 5.2.1 Retardation of iodine in cement and concrete 59 5.2.2 Diffusion of iodine in cement and concrete 60 5.2.3 Summary of cement sorption 60 5.3 Iodine retardation by organic compounds, humic substances and microorganisms 61 5.3.1 Enzymatic formation of iodo-organic compounds 61 5.3.2 Iodo-organic compounds in soils 61 5.3.3 Natural organic substances in geomedia 61 5.3.4 Microorganisms in deep geologic formations 63 5.3.5 Influence of anthropogenic organic compounds on iodine mobility 63 5.4 Iodine sorption by selected minerals and chemical compounds 64 6 Recommendations for further studies on iodine sorption 67 6.1 RD measurements 67 6.2 Diffusion measurements 67 6.3 Proposed research topics on near-field chemistry 67 Aknowledgements 69 References 70 Appendix (Tables I - XVI) 92 4 Summary This report reviews the literature on iodine migration, chemistry and behaviour in the environment up to November 19S7. It deals mainly with 129I released from a land repository, with, particular consideration of the Swiss scenario for the disposal of low- and medium-level radioactive waste. As a background to this rcvieu*, the basic properties of radioiodine, its distribu tion, circulation in nature and radiological impact are presented. A large number of sorption and diffusion data for iodine on rocks, sediments, minerals, cements and other materials have been compiled from many different laboratories. Based on this information, an assessment of the sorption and retar dation of radioiodinc in geomedia is made and methodologies for obtaining sorption distribution ratios (Rp values) are discussed. The review also «rovers natural analogue stvidies of mI, retardation of iodine by cement barriers aivl the possible influences of organic compounds and microorgan isms on the behaviour of iodine. Some possibilities for fvirthcr research on diffusion measurements and near-field chemistry of radioi' >dine are outlined. 5 Preface Many laboratory studies and in-situ observations have shown the potential impor tance of 12DI released in the far future to the biosphere from geological repositories for radioactive wastes. The factors which make this nuclide important are its high mobility in the geomedia and its very long hnlf-lifc of 1.57-10' years. After release to the environment, 12DI may be taken up by man, with accumulation in the thy roid gland thereby causing harm to the human body. This paper reviews the literature fairly completely up to November 10S7. The con tent is extracted partly from the computer libraries of Chemical Abstracts (CA), National Technical Information Service (NTIS), and International Nuclear Infor mation Service (INIS), as well as from many direct sources. This paper deals mainly with the chemistry and behaviour of 129I released from a land repository, following transport though the cement barriers (involving near- field chemical interactions) and then through the surrounding geological media (involving far-field chemistry). As a background to this review, the basic proper ties and the natural distributions of iodine are discussed at the outset. The sorption data for iodine on rocks, sediments, minerals and other materials obtained from both laboratory and field studies are compiled in sixteen tables. Based on this information, a critical assessment of the sorption and retardation of radioiodine in geomedia is presented. In conclusion, we make a few relevmit propositions for possible future investigations, particularly in relation to the Swiss project work on geologic disposal. Previous review articles (Andcrsson & Allard, 19S3; Black et ah, 19S0; Ball, 19S3) were very helpful in preparing this report. 6 1 General 1.1 Basic properties of radioiodine 1.1.1 Iodine isotopes Thirty three isotopes of iodine are presently known with, mass numbers ranging from 110 to 142; twenty seven of them have half-lives of less than 1 day. The only stable isotope of iodine is 127I with an atomic weight of 126.9045 AMU. 120I is the only naturally occurring radioisotope with a long half-life of 1.57xl07 y (Seclmann-Eggebcrt et al., 19S1). 129I decays exclusively by the route: 120T g-,15RkoV 129my„ 1T,39.fikcV 12SV _t_L ,, >. 1 T^dbV Xe 1.0MS Xc(stable) Only 7.52% of the 129mXe transitions involve gamma emission. The remaining are by way of conversion electrons (mainly 5 to 10 keV) and X-rays (Table 1-1). The specific activity of 129I is 1.7x10"'' Ci/g. energy, abundance keV per 100 decays 29.46 19 29.7S 36 33.6 10 34.4 2.2 39.58 7.5 Table 1-1 7- and X-rays associated with I decay (Erdtmann Sz Soyka, 1979) Although all 129I formed in the primordial nucleosynthesis has decayed to 129Xe, natural processes such as spontaneous fission of 238U, thermal neutron-induced fission of 235U and spallation reactions of Xe in the upper atmosphere contribute to a steady state concentration of 10-llg of 129I per g of 127I (Edwards, 1962). More recent arid precise analyses of samples of a natural silver iodide deposit in Australia (Srinivasan et al., 1971) have led to an estimate for the terrestrial equi librium ratio of: 2.2xl0~15 < 129I : 127I < 3.3xl0-15 The short-lived isotopes ,3,I (S.02 d), 132I (2.3 h) and 133I (20.S h) have only been detected in nuclear fallout. The formation of 129I during nuclear fission is mainly 7 the result of the decay of isotopes in the isobar-129. The independent fission yield from 235U is 0.OQ574 atoms/fission and O.0154 from 239Pu (Crouch, 1977). The natural and artificial inventory of 129I is summarized in Table 1-2 (Scheele et al., 19S4; Russell & Hahn, 1971; McKay et al., 19S4; NCRP, 19S3; Fauville, 19S5). Environmental inventory: Natural 40 Ci Total 300-1000 Ci Reactor production rate (33,000 MWd/ton U) 0.04 Ci/ton U of fuel discharged 60 Ci/y for a 1500 MTU/y reprocessing plant Total 10'' Ci estimated by 2000 a.d. (2500 Ci estimated by 2000 a.d. in U.S.) Table 1-2 Inventory of environmental and nuclear reactor production of 129I. Stable 127I is also produced in the fission process, as indicated in Table 1-3. Hence, 129I is always diluted by about 20% with the stable isotope 127I. Iodine isotope Generation (kg/GWe • y) 1-127 1.5 1-129 5.9 Table 1-3 Generation of fission product iodine in a PWR (Hebel &: Cottone, 1982). 1.1.2 Chemical states Iodine is an electronegative element and has a relatively large ionic radius of 0.22 tun (Sharpe, 1967). In aqueous solution its oxidation states are -1, 0, +1, +3, +5 and +7; in the environment the most abundant states are -1, 0 and +5. The common forms of iodine are I~, IOj, IOH, IO~, I2 and IJ (Downs Sz Adams, 1973; Sass & Grauby, 1977; Wong, 19S0). The Eh-pH diagram (Pourbaix diagram) for iodine in water at 25°C is given in Fig. 1-1 (Bowen,1979; Wong & Brewer, 1977; Ball, 19S3). s 1.5 1.0 ö °-5 .c ai ^ H 0 2 H> **> '•os s -0//^ l^ I -0.5 6 8 10 12 14 pH Figure 1-1 Pourbaix diagram for iodine in water at 25°C. Domain of stability ofH 0, ( ). 2