Acta Chim. Slov. 2003, 50, 597-600.
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IMMOBILIZATION OF     Cs IN CONCRETE Ilija Plecas
Vinca Institute of Nuclear Sciences P.O. Box 522, 11001 Belgrade, Serbia and Montenegro
Received 29-03-2003
Abstract
To assess the safety for disposal of radioactive waste-cement-bentonite composition, the leaching of 137Cs from a waste composite into a surrounding fluid has been studied. Leaching tests were carried out in accordance with a method recommended by IAEA. Determination of retardation factors, KF and coefficients of distribution, kj, using a simplified mathematical model for analyzing the migration of radionuclides, has been developed. Results presented in this paper are examples of results obtained in a 10 year mortar and concrete testing project, which will influence the design of the engineered trenches system for a future central radioactive waste disposal center.
Introduction
In order to preverit widespread dispersion of radionuclides into the human environment, radioactive waste produced in nuclear facilities has been incorporated in several kinds of matrices. " The objectives of immobilization of radioactive waste is to convert the waste into forms which are:
-    Leach resistant so that the release of radionuclides will be slow even though they may come into contact with flowing water;
-    Mechanically, physically and chemically stable for handling transport and disposal.
Concrete are widely used in low-level waste management both as a means of solidifying waste and for containment of dry or liquid wastes. At present there is also widespread interest in the use of near-surface concrete trench system for the disposal of radio-waste materials. Typical concrete is a mixture of cement, sand, granulate and water in various proportions that together determine the structural properties and tightness of the poured material. Cement is porous, continuously hydrating material whose actual surface area greatly exceeds its geometric surface area. In leaching, the rate of dissolution varies as a function of phase chemistry and this dissolution exposes or enlarges pores; thus the leaching behavior must be related to pore structure and the composition of the pore solution. Although cement has several unfavorable characteristics as a solidifying material, i.e. low volume reduction and relatively high leachability, it possesses many practical advantages: good mechanical characteristics,
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low cost, easy operation and radiation and thermal stability.
Radionuclide migration through porous materials
The dispersion of radionuclides in porous materials, such as grout or concrete, is described using a one dimensional differential model.1,5
^32A   ^T  dA   (     1 —f        ^3A
D-----7_VV-------H--------Pt^d    -----
3X          3X   t        f           J dt
0    (1)  or    D
32A         3A   T^   3A
-----F _ *v--------^-f-----
3X           3X         3t
0     (!')
where:      KF  - retardation factor (=)l                                     f
D   - diffusion coefficient (cm2/d) or (cm2/s)              pT
A    - concentration in liquid (mol/L) or (Bq)                k<j
X    - length (cm)                                                               t
Vv   - velocity of leachant fluid (cm/d)
porosity (=)1  bulk density (g/cm3)  distribution coefficient (mL/g)  time variable (d)
Using Laplace transformation method, Eq. (V) becomes:
Vt
An
A
1 = — erf z
2
VVX
1-
KFX
4D
\
Vvt
KFX
(2)
from which we can calculate a retardation factor, Kf. The coefficient of distribution, ka, can be calculated:
 (Kp —l)f kd =—---------(=) (mL/g)
(l-f)pT
(3)
in which: Vv, X, px, t and Ao are known. An and De can be determined experimentally using a leaching test procedure.
Determining the effective coefficients of diffusion
For the interpretation of the results of leach tests shown in the following figures and tables, leach coefficient D, is used, and it is defined as:
D
71       2  V             2
š2
m
(cm2/d)
where:
D   - leach coefficient (diffusion) (cm2/d) or (cm2/s)
m - (EAn/A0)(l/Vxt), slope of the straight line (d-12)
Aq - initial sample activity at time zero (Bq); (Table 1)
An - activity leached out of sample after leaching time t, (Bq)
t    - duration of leaching renewal period (d); (1,2,3,4,5,6,7,15,30,60)
V  - sample volume (cm3) S   - sample surface (cm2)
(4)

I. Plecas: Immobilization 137Cs in Concrete
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Experimental
Materials and mortar composition
Concrete samples were made of: Portland cement PC-20-Z-45 Mpa, Sand, fraction 0-2 mm, granulate, fraction 2-4, 4-8, and 8-15 mm, Water, Additive Superfluidal. More than 150 different formulations of concrete formulations were examined to optimize their mechanical and sorption properties. In this paper we discuss four representative formulations. Concrete compositions are shown in Table 1.
Table 1. Representative formulation of concrete composition as grams for 1000cm3 of concrete.
	Portland cement	Sand 0-2 mm	Aggregate 2-4 mm	Aggregate 4-8 mm 463	Aggregate 8-15 mm 724	Water 150	Admixture
Sample 1	440	672	85				8
Sample 2	420	750	83	417	734	140	8
Sample 3	400	858	95	536	496	140	8
Sample 4	400	626	69	576	715	140	8
Results and discussion
The results are obtained after 60 days. Using equation (4), coefficients of diffusion are calculated for four experimental samples.
Using equation (2) and (3), retardation factors, Kf(=)1, and distribution coefficients, ka(mL/g) are calculated. Table 2 gives Cs, leach coefficients in different concrete samples.
Table 3 gives the results of retardation factors, Kf and coefficients of distribution, kd(mL/g), for four mortar formulations for each radionuclide, during 60 days.
Table 2. Leach coefficients De(cm2/d) in different grout samples after 60 days, using Eq.(4).
Formula Leach coeff.         Sample 1               Sample 2               Sample 3               Sample 4
De,     Cs              4.62xl0"5               5.62xl0"6              4.73xl0"6               8.44xl0"6
Table 3. Retardation factor KF and coefficients of distribution kd(mL/g), after 60 days, pT=2.5 (g/cm3), f=0.15-0.30.
Formula
Mi                            M2                        M3                        M4
41.6                         87.5                       98.4                      65.5
1.6-3.8                   6.8-16.7                 6.9-16.9                6.7-16.3
Coeff. KF, 137Cs
kd, 137Cs
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Conclusions
The analysis of the results presented in Table 2 and Table 3 shows that the values of retardation factors and coefficients of radionuclides Cs, are similar to the literature data, and prove that the one-dimensional model can be used for calculating parameters of the migration process. The system of concrete engineered trenches final disposal system for radioactive waste permits secure preservation of radionuclides for more than 300 years in a future disposal system, with multiple safety barriers.
Acknowledgements
Work supported by the Ministry of Science, Technologies and Development of the Republic Serbia under Contract No. 1985,"Research and Development of processes and materials for treatment of radioactive and hazardous waste and environmental hazard assessment".
References
1.  R. H. Burns, Atom. Energy Rev. 1971, 9, 547-552.
2.  I. Hashimoto, K. B. Deshpande, S. H. Thomas, I&EC Fundamentals 1964, 3, 213-217.
3.  E. D. Hespe,^4fo/w. Energy Rev. 1971, 9, 195-207.
4.  A. H. Lu, Health Physics 1978, 34, 39^4-3.
5.  N. Moriyama, S. Dojiri, H. Matsuzuru, Health Physics 1977, 32, 549-552.
6.  I. Plecas, Lj. Mihajlovic, A. Kostadinovic, Radioactive Waste Management and Nuclear Fuel Cycle
1985, 6, 161-175.
7.   I. Plecas, J. Drljaca, A. Peric, A. Kostadinovic, S. Glodic, Radioactive Waste Management and the
Nuclear Fuel Cycle 1990, 14, 195-205.
Povzetek
Za oceno varnosti odlaganja utrjenih radioaktivnih odpadkov v betonskih zmeseh cementa in bentonita smo študirali izluževanje 137Cs iz teh kompozitov v tekočinskem okolju. Izluževalni preizkusi so bili usklajeni s priporočeno metodo IAEA. Na osnovi poenostavljenega modela migracije radionuklidov v trdni snovi smo določili zadrževalne faktorje KF in koeficiente porazdelitve kd. Raziskave parametrov imobilizacije 137Cs v betonu so del 10 letnega projekta preizkušanja betonskih in maltnih materijalov za imobilzacijo odpadnih radioaktivnih snovi in so med drugim podlaga za načrtovanje varnostnega sistema inženirskih pregrad v bodočem centralnem odlagališču radioaktivnih odpadkov.
I. Plecas: Immobilization 137Cs in Concrete