Paul Scherrer Institut

184 Pages·2007·2.86 MB·English

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 [phone] Telex 827417 psi ch
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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
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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
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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
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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.
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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.
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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
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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, ( ).
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