Atomic Energy, Vol. 84, No. 1, 1998

CESIUM AND STRONTIUM RADIOACTIVE

SORPTION

SEPARATION

WASTES AND IMMOBILIZATION

FROM LIQUID

IN GEOCEMENTS

N. G. Bogdanovich, i~. E. Konovalov, O. V. Starkov, E. A. Kochetkova, E. A. Grushecheva, V. D. Shumskaya, V. P. Emel'yanov, and M. P. Myshkovskii

UDC 621.039.7

One way to immobilize medium-level liquid wastes is to solidify the wastes by cementing [1, 2]. The main drawbacks of this method are the low filling of the cement with radionuclides from the liquid wastes and the large volume of the radioactive products obtained. To solve these problems it is desirable to extract radionuclides from the wastes by sorption followed by inclusion of the sorbent into a water-resistant cement. In the present paper we report the results of investigations of sorption extraction of cesium and strontium from liquid wastes by natural zeolite and immobilization of the radioactive sorbent, using alkaline slag concrete (geocement) [2]. The stability of this material is largely determined by the similarity of the sorbent and the regenerations of the binding system with respect to the elemental composition and cross-linking groups. Inorganic materials, including natural materials, are attracting attention for removal of radionuclides from contaminated water because of their radiation resistance and high sorption characteristics [3, 4], in contrast to synthetic ion-exchange resins which are suitable for extracting radionuclides only from low-salt-content, low-level solutions [5]. The possibility of immobilizing inexpensive sorbents containing low-lived cesium and strontium radionuclides in mechanically strong and waterresistant matrices makes it possible to reduce substantially the volume of radioactive wastes to be buried. The initial materials for the process being developed are the natural mineral clinoptilolite as the sorbent, and the natural and technogenic components of the binding system - - granular slag from metallurgical production, clays, and solutions of alkali-metal compounds. Experimental Conditions. The sorption characteristics of clinoptilolite from the Dzegvi-Tedzami deposits were investigated. Model solutions of wastes from nuclear power plants with BBER and RBMK reactors were used together with real solutions from the Main Science Center of the Russian Federation -- Physics and Power Engineering Institute with inorganic content up to 200 g/liter and organic content up to 135 g/liter, 134,137Cs, 9~ specific activity up to 7.107 Bq/liter, and nonradioactive strontium and cesium concentrations 2.10 - 4 - - 10 g/liter. The ratio of the sorbent mass (kg) to the volume of the liquid phase (m 3) during investigation of sorption under static conditions was equal to 1:0.2. Sorption under dynamic conditions was studied using 5 - 1 0 mm in diameter adsorption columns with 5 0 - 5 5 0 mm high sorbent layers. The rate of passage of the liquid phase was equal to 5 - 1 0 0 column volumes per hour (linear velocity 1 - 10 m/h). Static Sorption. Clinoptilolite in sodium form exhibits the highest sorption capacity with respect to cesium and strontium (Table 1). For this reason, this form of the sorbent was chosen for studying the process parameters and all results are presented for clinoptiiolite chemically modified with sodium ions. The investigations showed that the highest rate of cesium and strontium sorption is observed during the first 30 min after sorbent is introduced. Complete saturation of the sorbent with radionuclides occurs after 3 h (Fig. 1). This character of the process must be taken into account for sorption extraction of radionuclides from liquid wastes. Cesium and strontium sorption efficiency increases as the solution pH increases from 8 to 14 (see Table 1). This phenomenon could be due to facilitation of the breakdown of water complexes of Cs + and Sr 2+ in a strongly alkaline medium, necessary for localizing these cations in voids in the structure of clinoptilolite [6]. The increase in sorption with pH Main Science Center of the Russian Federation - - A. I. Leipunskii Physics and Power Engineering Institute. Translated from Atomnaya I~nergiya, Vol. 84, No. 1, pp. 16-20, January, 1998. Original article submitted September 3, 1997. 14

1063-4258/98/8401-0014520.00

9

Plenum Publishing Corporation

TABLE 1. Effect of the Chemical Form of Clinoptilolite and the pH and Salt Content of the Solutions on the Degree of Sorption of Cesium and Strontium Chemical form of the

Degree of sorption of radionuclides (%) from NaNO3 solution with concentration, g/liter pH of the medium

sorbenl Na:ural Sodium

I00

l0 Cs

Sr

Cs

Sr

Cs

Sr

75,0

70,0

--

45

27

<0.t

8

93,8

86,0

81,0

79

37

18.5

12

95.8

99,8

89,8

--

91

99,9

8

0,8

0,6 o,4

0,2

~, .I. . . . . . . . . . . . . . . . .

0 2

t,h

4

Fig. 1. Kinetic curves of cesium (1) and strontium (2) sorption by clinoptilolite (A0, A r - - specific activity of solutions before sorption and after r h of sorption).

is especially large in solutions with high salt content ( ~ 100 g/liter). The concentration of inorganic salt in a strongly alkaline medium does not strongly influence the cesium and strontium sorption efficiency of clinoptilolite. Conversely,-the degree of cesium and strontium sorption in the low-alkalinity region (pH 8) depends strongly on the concentration inorganic sodium salt. Analysis of the isotherms of sorption of cesium and strontium by clinoptilolite from sodium nitrate solution shows that these cations are most effectively absorbed when their concentration is <0.02 g/liter. This is confirmed by the distribution factor (Table 2). The static capacity of clinoptilolite under these conditions is - 0.1 g of strontium and - 1.4 g of cesium per gram of sorbent. Dynamic Sorption. The adsorption properties of clinoptilolite under dynamic conditions were investigated according to the output curves. One can see (Fig. 2) that for solutions with high (109-10 l~ Bq/liter) and low initial 137Cs activity (_< 104 Bq/liter) a high relative content of this nuclide is observed in the solution after sorption ( 3 - 5 % of the initial content versus - 0 . 1 % for solutions with an initial activity 107 - - 108 Bq/liter). The steps on the output curves reflect adsorption where several types of adsorption centers of the crystalline structure of clinoptilolite with different interaction energy participate [6]. The dynamic capacity of clinoptilolite is - 0 . 3 g cesium per 1 g sorbent. The data in Fig. 3 permit estimating the number of adsorption columns required to e x t r a c t 137Cs from the liquid wastes up to its admissable concentration in water and the number of times the sorbent in the columns is used. The dynamic distribution factor of 137Cs for model solutions with different chemical composition and initial activity 7-107 Bq/liter and salt content I 0 - 1 5 g/liter with pH 8 ranges from 5-103 in the presence of a large quantity of competing K § up to 5-105 in the presence of deactivating substances and up to 1-106 in sodium nitrate solutions. The one-step purification coefficient under these conditions equals 100-2000. The height of the working clinoptilolite layer, calculated using the Michaels-Treible formula, can reach 0.6 m, depending on the conditions of sorption. Investigations of 9~ sorption under dynamic conditions showed that after a model solution with pH 8, containing 13 g/liter sodium nitrate, 20 t~g/liter cesium in the form of nitrate and a specific activity of 1.47.107 Bq/liter is passed through three adsorption columns with clinoptilolite in a sodium form, the activity in the solution decreases to 13.9 Bq/liter, i.e. below the admissable concentration in water according to the NRB-96 standards (Table 3). The total purification coefficient was equal to - 106, and the degree of sorption was equal to 99.9999%. 15

TABLE 2. Distribution Factor of the Radionuclides Versus Their Concentration in Solution with Salt Content 1 - 10 g/liter, pH 8 (Sr) and 13.7 (Cs) Distribution factor, ml/g Concentration, g/liter

Cs 5.103

I 9 104

0,2--1

2.109

3.102

5--10

5 102

20

(0,1 - - 2 ) 10 -z

Sr

1

Ae, Bq/liter

E. Bq/g

Or8 o~6

IOt:l

IO,a

9

2 <

0,4.

ioI

I

0,2

2

0

!

0

500

1000

1500

Column volume

Fig. 2

2000 Ao/Bq/liter

Fig. 3

Fig. 2. Relative 137Cs activity in solution at the exit from the adsorption column versus the volume of the solution passed with initial activity 6.5-1010 (1), 6.5-107 (2), 4.104 Bq/liter (3) (Ae - - specific activity of cesium at the exit). Fig. 3. Dynamic capacity (E) of clinoptilolite (1) and equilibrium activity (Ae) of t37Cs in solution (2) after single purification versus the initial activity (,4o) with salt content 1 0 - 1 5 g/liter, pH 8.

An experimental check of the sorption extraction of cesium and strontium by clinoptilolite from liquid wastes, from the Main Science Center of the Russian Federation - - Physics and Power Engineering Institute, with different chemical composition (Table 4) and volume up to 7 liters confirmed the results obtained with model solutions. One can see that a high salt content increases sorbent consumption. Combining cesium and strontium extraction into several steps with maximum saturation of the sorbent at the first stages of sorption will make it possible to decrease their concentration in the salt solutions below the admissable content in water and to increase substantially the utilization factor of the sorbent and the total purification factor, and it will decrease the specific consumption of the sorbent. Immobilization of Radioactive Sorbent. Fixation of radioactive sorbent in a water-stable mechanically strong material - - alkaline slag cement (geocement) - - is used for isolation from the biosphere [2]. Geocement forms as a result of hydration solidification of the binding system, which contains granular domain slag, a solution of the alkali component, and clay additive. Radioactive sorbent in this system fulfills the function of a light filler. The processes leading to solidification of slag/alkaline binders model natural processes of mineralization of low-basic calcium hydrosilicates, alkaline and alkalinealkaline-earth hydroalumosilicates, characterized by high water stability. In the investigation of immobilization of clinoptilolite containing t37Cs a solution of sodium silicate was used as an alkaline component and kaolin was used as the additive. The optimal fill of the geocement with sorbent was equal to 30 mass %. The solidified material was characterized by high strength in compression tests ( 2 1 - 2 8 MPa) and low rate of t37Cs leaching into water ( - 10 -5 g/(cmZ-day) in the first few days and - 1 0 - 6 - 1 0 - 7 g/(cm2..day) 1 4 - 3 0 days after tests following the procedure of [7]).

16

T A B L E 3 . 9 0 S r Activity in a Model Solution Versus the Number of Purification Steps and Solution Volume Indicator

T

Number of adsorption column

Ratio of volume of the wastes for sorbent volume Activity in solution after sorption. Bq/liter

I

2

3

800 2,6.106

600 8,2 104

100 13,9

T A B L E 4. Indicators o f the Efficiency of Sorption Extraction o f 137Cs by Clinoptilolite f r o m Liquid Wastes from the Main Science Center of the Russian Federation - - Physics and P o w e r Engineering Institute with a Single Pass of the Solution through an Adsorption Column Wastes Radiochemical composition of the wastes and sorption efficiency indicators Initial activity. Bq/liter :.Total matter content, g/liter ~:Organic matter content, g/liter I Ratio of waste volume to sorbent volume i Activity in solution after sorption. Bq/liter IDegree of sorption. % ! Purification factor i Average sorbent activity. Bq/kg i Dynamic distribution factor, ml/g

Z

medium level, for

after evaporator

8.3"106 [ 6,7" 107 I 1.3 13 Not determined I, Not determined. 200 ! 70 1,4" 10 3 3,8' tO4 99.98

]

6100 2-109

concentrated

1700

2,3-107 46,5 13,2 50 3,9.104 99.8 5"10

99.9

1,5.10 a

I ]

5,9.109 !,6. l ~

3,4.104

4

1

12

16

'. Specific consumption of sorbent, kg/m3

i

1,4- I 09

T A B L E 5. Results of Computational Assessment of the Basic Indicators of the Technological Process Leading to Decomtamination of Liquid Wastes in Order to Achieve the Admissable 137Cs Concentration in the Water-Salt Solution. Content of the main components of the wastes land indicators of the technological process Salt content, g/liter Initia 137Csactivity, Bq/Iker Number of adsorbents, necessary to lower the 13?Cs activity below the admissable level in water Service life of adsorbents in column volumes Total consumption of clinoptilolite, kg/m3 Total purification factor Geocemem volume after reprocessing of I m3 of liquid wastes, m3 Activity of geocemem. Bq/kg Factor by which the volume of liquid wastes decreases with decontamination

Low-salt Up to2

Solutions i Medium-salt [ Up to 15

106

+

10 7

2

!

3

I

Concentrates Up to 200 10~ 2

i

i

5o (lst) 300 (2nd)

0,002

1000 (lst), 1000 (2nd). 750 (3rd) 2,7 i0 -s 0,003

3 10'~ 500

3.5 10m 330

i 91o m

1000 (lst). 750 (2nd). 1,9 I O'

i

18,4 I 0:' 0,025

40

A s s e s s m e n t of T e c h n o l o g i c a l Efficiency. Table 5 gives the computational results obtained for the main indicators of the technological process of decontaminating liquid wastes on the basis of investigations of the sorption separation and immobilization of cesium and strontium in model and real wastes. It was shown that this method is most effective for solutions with specific activity 106-107 Bq/liter and salt content l - 15 g/liter. This will make it possible to reduce substantial17

ly the volume of the wastes (by a factor of 300-500) with the wastes being transferred from a liquid into a solid state with a relatively low general consumption of clinoptilolite ( 2 - 3 kg/m3) and provide high quality indicators of the solidified material with the maximum admissable filling of the geocement with radioclides. Salt solutions of liquid wastes after cesium and strontium have been extracted from them become nonradioactive or weakly radioactive. This greatly facilitates further handling of such wastes.

REFERENCES 1.

2.

3.

4.

.

6. 7.

18

A. S. Polyakov, O. L. Masanov, K. P. Zakharova, et al., "Cementing~ of radioactive salt concentrates," Atom. I~nerg., 77, No. 6, 468-470 (1994). !~. E. Konovalov, A. I. Lastov, I. V. Malymyan, et al., ~Immobilization of radioactive wastes by solidification in geocement," in: Radioecological Problems in Nuclear Power Production and Conversion of Production. Obninsk Symposium of the Fifteenth Mendelev Conference on General and Applied Chemistry, May 31-June 5, 1993, Reports of the Main Science Center of the Russian Federation -- Physics and Power Engineering Institute, Part 1, Obninsk, 1994, pp. 273-280. V. M. Komarevskii, O. V. Stepanets, L. M. Sharygin, et al., "Efficiency of purification of liquid radioactive wastes with inorganic granular sorbents," Atom. l~nerg., 79, No. 6, 419-422 (1995). B. Yu. Kornilovieh, "Protection of the water basin from radioactive contaminants using modified natural sorbents" in: Abstracts of Reports on Radioecological Problems in Nuclear Power and Conversion of Production, Obninsk, Main Science Center of the Russian Federation - - Physics and Power Engineering Institute, 1993, Vol. 2, p. 104. A. S. Nikiforov, M. I. Zhikharev, V. I. Zemlyanukhin, et al., "Handling of radioactive wastes from a nuclear power plant and regeneration of spent nuclear fuel," Atom. l~nerg., 50, No. 2, 128-136 (1981). Yu. I. Tarasevich, "Crystal-chemical principle of selectivity of natural zeolites to large cations," Khim. i Tekhnologiya Vody, I1, No. 4, 305-310 (1989).

Radioactive Wastes. Methodfor Measuring the Chemical Stability of Solidified Radioactive Wastes by Means of LongTerra Leaching, GOST 29114-91 [in Russian], Izd. Standartov, Moscow (1992).