ISSN 0012�5008, Doklady Chemistry, 2010, Vol. 434, Part 1, pp. 214–218. © Pleiades Publishing, Ltd., 2010.Original Russian Text © N.P. Laverov, S.V. Yudintsev, Yu.I. Korneiko, E.E. Konovalov, T.O. Mishevets, B.S. Nikonov, B.I. Omel’yanenko, 2010, published in Doklady AkademiiNauk, 2010, Vol. 434, No. 1, pp. 60–64.

214

Radioactive waste of nuclear power engineering is aconsiderable environmental hazard. Liquid highlyradioactive waste (HRW) of irradiated nuclear fuelreprocessing is most hazardous. Such HRW with com�plex chemical composition is solidified into glassyconfinement matrices to improve its storage safety [1].For this purpose, aluminophosphate glasses are usedin Russia and borosilicate glasses are applied in othercountries. Such glasses are chosen because of the sim�plicity of industrial production and high solubility ofmany elemental substances from HRW in them. At thesame time, there are some doubts in the longevity oftheir properties. The glass will crystallize, especiallyon heating accompanying the decay of Cs and Sr iso�topes. This increases the rate of leaching of radionu�clides from such matrices and, thus, the hazard ofthese radionuclides. First of all, this concernsactinides and some of their fission products, for exam�ple, 99Tc, with half�lives of hundreds of thousands ofyears and more.

As an alternative to glasses, crystalline matriceswere proposed, the most known of which are mul�tiphase ceramics Synroc [2]. Elemental substancesfrom waste are distributed between different compo�nents: actinides enter into zirconolite and perovskiteand Tc is incorporated into a metallic alloy. The latteris determined by the method of synthesis of matricesby hot pressing in a reducing medium. Because ofhigh cost, using this method was restricted to synthe�sis of Synroc matrices on a pilot plant (ANSTO, Aus�tralia).

Another solution of the challenge of handling long�lived radioisotopes is to separate fractions, such as

rare�earth elements (REE)/actinides and Tc, fromHRW and incorporate them into oxides with thestructures of garnet, pyrochlore, perovskite, etc.[1⎯5]. Matrices for 99Tc are metallic alloys or titanateswith the structure of spinel, pyrochlore, or rutile[4⎯7]. They are produced by hot pressing, cold press�ing–sintering, and also melting with subsequent meltcrystallization.

A method for producing matrices for long�livedactinides is self�propagating high�temperature syn�thesis (SHS) [8, 9]. SHS has recently been proposedfor immobilizing Tc and also fractions of Tc andactinides (or REE/actinides) into a single matrix[10, 11]. Such a matrix consisting of (REE, Al)�garnet(phase of actinides) and an alloy (matrix for Tc) wasstudied previously [12]. At the first stage of the study,radionuclides were imitated by Sm or Nd (represent�ing REE/actinides fraction) and Re (representing Tc).The main phases in the samples were aluminum garnet(to 70 vol %), glass (20 vol %), and alloys (10–15 vol %).Sm and Nd enter into garnet and glass, and Re formsalloys with iron group elements or molybdenum.Although temperature was high, Re did not evaporateduring SHS. A similar behavior is also expected for Tc.In this work, for the first time, we consider matrices fortechnetium. For comparison, previously obtained dataon an inactive sample with similar composition arepresented.

PRODUCTION OF A MATRIX WITH TECHNETIUM

For SHS, the reducer was metallic Al and the oxi�dizer was a mixture of oxides of Fe, Cr, and Ni. Thecomposition of the initial mixture (Table 1) was cho�sen so that the reaction should result in incorporationof HRW imitators into Y–Al garnet (Sm) and alloys(Re, Tc). To bind potassium after KReO4 decomposi�tion, SiO2 was added to the initial mixture. Theassumed SHS process can be represented as follows:

Al (reducer) + Fe, Cr, and Ni oxides (oxidizer) +Sm2O3, KReO4, and KTcO4 (HRW imitators) + Y2O3

and CaO (garnet components) + SiO2 (K2O�binding

CHEMISTRY

Matrix for Isolation of Technetium and ActinidesAcademician N. P. Laverova, S. V. Yudintseva, Yu. I. Korneikob, E. E. Konovalovc,

T. O. Mishevetsc, B. S. Nikonova, and B. I. Omel’yanenkoa

Received March 17, 2010

DOI: 10.1134/S0012500810090028

a Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academyof Sciences, Staromonetnyi per. 35, Moscow, 119017 Russia

b NPO Khlopin Radium Institute, Vtoroi Murinskii proezd 28, St. Petersburg, 194021 Russia

c Leipunskii Institute of Physics and Power Engineering, Russian Federation State Research Center, Federal State Unitary Enterprise, ul. Bondarenko 1, Obninsk, Kaluga oblast, 249033 Russia

DOKLADY CHEMISTRY Vol. 434 Part 1 2010

MATRIX FOR ISOLATION OF TECHNETIUM AND ACTINIDES 215

additive) (Re, Tc)�containing alloy + (REE, Ca)�Al�garnet + glass phase.

The initial mixture for producing a matrix for Tcwas prepared in two stages. Initially, its inactive con�stituent without reducer (Al) was produced. To a partof this mixture, 0.48 g of KTcO4 was added, and themixture obtained was ground in a jasper mortar in aglovebox. The powder was combined with the rest ofthe initial mixture and metallic Al, placed in a flask,and mixed. This mixture was loaded in a graphite cru�cible and compacted. Then, a small amount of Al andMg metal powders was placed on the surface of thismixture and ignited by a thermite match for initiatingan SHS reaction. As a result, a sample was obtained,which was virtually identical in appearance to theinactive matrix. Radiometry showed that Tc is concen�trated in the alloy phase and its loss in the synthesisdoes not exceed 0.1% of its amount in the initial mix�ture for SHS synthesis.

RESULTS OF STUDY OF THE MATRIXWITH TECHNETIUM

The sample was studied by x�ray diffraction analysis,microprobe analysis (MPA), and scanning electron

microscopy (SEM). The main phase is garnetaccounting for 50 to 70 vol % of the sample. Garnetgrains 5–20 µm in size form skeletal�crystal aggregates(Fig. 2) and consists primarily of REE and aluminumand also of small amounts of oxides (in decreasingorder): SiO2, Cr2O3, MgO, CaO, and FeO (Table 2).All these elements are quite typical of artificial garnetphases [13] and occupy here structural positions IV(Al3+, Si4+), VI (Al3+, Cr3+), or VIII (REE3+, Ca2+,Fe2+, Mg2+), depending on the cation radius. The gar�net composition corresponds to the standard formula(Ca, Fe, Mg, REE)3(Cr,Al)2(Al,Si)3O12 (Table 3). Thephase from inactive sample Re�5 contains more alu�minum but less yttrium, than the phase from sampleRe�12. The concentrations of Sm, the imitator of theREE/actinides fraction of HRW, in these samples areclose and are 12–13 wt %.

100 µm(a) (b)

Alloy

Bulk mass

Fig. 1. (a) General view of sample Re�12 with technetium (4× magnification) and (b) its photomicrographs taken through an opti�cal microscope in reflected light. White regions are alloy; gray, garnet; dark gray, glass; and black, pores.

Table 2. Garnet phase compositions, wt % (as determined byMPA and SEM)

Oxide

Re�12 (MPA) Re�5 (SEM)

analysis 1 analysis 2 analysis 3 meanvalue

meanvalue

Y2O3 41.6 43.3 42.9 42.7 39.1

Al2O3 33.5 34.4 34.2 34.0 41.2

Sm2O3 12.1 12.1 11.9 12.0 12.9

SiO2 5.6 3.6 4.4 4.5 3.0

Cr2O3 4.7 3.2 3.3 3.7 2.4

MgO 1.2 1.1 1.0 1.1 –

CaO 0.8 0.8 1.0 0.9 1.0

FeO 0.5 1.5 1.3 1.1 0.4

Note: The total content of element oxides in the phase compositionis brought to 100 wt %.

Table 1. Specific features of the compositions of the initialmixture for SHS and the product obtained

Sam�ple

Al contentof initial mix�

ture, wt %

Imitators,wt % Main phases in sam�

ples (by quantity)Sm Re

Re�5 13.3 9.2 12.3 Garnet > aluminosili�cates ~ metallic alloyRe�12 12.8 9.5 12.6

Note: The initial mixture of sample Re�12 contains 0.48 g KTcO4(~0.3 wt % Tc).

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LAVEROV et al.

A part of Sm is in the glass filling the spacesbetween garnet grains. The glass composition with thedata for sample Re�5 in the parentheses is the follow�ing (wt %): Al2O3, 29.9 (44.1); SiO2, 29.0 (23.5); K2O,5.4 (8.0); CaO + SrO, 7.7 (3.5); Sm2O3, 14.1 (13.7);

Y2O3, 4.4 (4.1); Cr2O3, 2.1 (2.2); MgO, 2.5 (no); andFeO, 1.9 (0.9). The main differences from the similarphase in the inactive sample are the presence of Sr andMg admixtures (probably, Sr was an admixture in Y2O3

and Mg was transferred from the ignition mixture) andhigher alumina content.

The Sm2O3:Y2O3 ratios differ between the glass andgarnet approximately by an order of magnitude (3.0and 0.3, respectively). In the garnet crystallization,elemental substances are fractionated and the residualmelt is enriched with samarium. Since we expect sim�ilar behavior for actinides (Am), the glass stability is ofgreat interest. As for the inactive samples [12], it canbe assumed that the glass devitrification produces per�ovskite (Y, Sm)AlO3, which can incorporate muchactinides and is stable in solutions [14]. We detected itsemergence using x�ray diffraction patterns of the inac�tive samples.

The most important result of this study is the dataon the Tc behavior. A preliminary investigation of thesample by radiometric analysis showed the presence ofTc in the metallic alloy. Quantitative determination byMPA and SEM proved this completely (Table 4).

(a)

20 µm100 µm

(b)

(d)(c)

1

1

12

2

2

3

Fig. 2. SEM images of the structure of samples (a, b) Re�12 and (c, d) Re�5: (1) garnet, (2) glass, and (3) spinel. White regionsare alloy; black, pores.

Table 3. Number of atoms in standard garnet formula with12 O2– anions

Cation

Re�12 Re�5

analysis 1 analysis 2 analysis 3 mean value

mean value

Y3+ 2.24 2.36 2.33 2.31 2.06

Al3+ 4.00 4.15 4.11 4.09 4.81

Sm3+ 0.43 0.43 0.42 0.43 0.44

Si4+ 0.56 0.37 0.45 0.46 0.30

Cr3+ 0.37 0.26 0.27 0.30 0.19

Mg2+ 0.17 0.18 0.15 0.17 no

Ca2+ 0.09 0.09 0.11 0.10 0.11

Fe2+ 0.04 0.13 0.11 0.09 0.04

Totalof cations

7.90 7.97 7.95 7.95 7.95

10 µm30 µm

DOKLADY CHEMISTRY Vol. 434 Part 1 2010

MATRIX FOR ISOLATION OF TECHNETIUM AND ACTINIDES 217

Technetium is contained in large (0.1–5.0 mm)metallic inclusions. In SEM images, three phases canbe seen, two of which are alloy phases, and the third isa phase of oxides (of Fe, Cr, and Ni). Technetium isdetected only in the alloy phases, which consist of Reand Fe (with admixtures of Cr and Ni). In the oxidephase (dark mass in the micrographs), only a smallamount of Re is found, but the Tc content of this phaseis below the MPA detection limit (0.05 wt %).

Sample Re�12 also contains small metallic inclu�sions less than 15 µm in size of uniform structure.They comprise Fe (76–96 wt %) and Cr (6–22 wt %);

no Tc is found here. They also contain no Re, whichlikens them to similar inclusions in the previouslystudied inactive samples [10].

In the sample, along with the above phases (garnet,glass, alloys), we detected spinel MgAl2O4 and also anoxide with the composition (wt %): Al2O3, 74.6; CaO,1.4; Sm2O3, 10.8; Y2O3, 5.2; MgO, 3.5, and SiO2, 5.6.The composition and grain shape are typical of a hibo�nite phase with the formula (Ln,Ca,Mg)Al12O19 + x.Silica in its composition is likely to be incorporatedfrom the glass surrounding the grains of this phase.

1000 µm(a) 100 µm

10 µm 10 µm

(b)

(d)(c)

II

III

I

II

II

I

I

I

Fig. 3. SEM images of inclusions in (a–c) sample Re�12 and (d) inactive sample Re�5. I, II, alloys; III, oxide phase (the resultsof analysis of their compositions are given in Table 4).

Table 4. Composition of phases of metallic inclusions, wt % (as determined by MPA and SEM)

ElementSample Re�12 (MPA) Inactive sample* [10] (SEM)

I (1) I (2) II (1) II (2) III I II

Re 63.4 67.7 55.7 55.0 3.9 80.2 55.2Fe 26.2 22.8 31.7 32.1 54.6 15.2 31.7Cr 7.9 7.6 8.8 9.0 4.5 0.5 1.1Ni 2.0 1.5 3.3 3.6 8.2 4.1 12.0Tc 0.5 0.4 0.5 0.3 – – –O – – – – 28.8 – –

Note: I, II, metallic alloys; III, oxide. The total content of elemental substances in the phase composition is brought to 100 wt %.

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LAVEROV et al.

The fundamental issue is the stability of the matrixin solutions. Technetium�containing stainless steel�based alloys are known to be corrosion�resistant [15].In these tests, a model solution imitating low mineral�ized underground water was used (for 14 to 90 days at90°С). With an increase in treatment time, the rate ofTc leaching from alloys decreased from 7 × 10–4 to10⎯4 g/(m2 day).

We have planned experiments for determining therates of Tc leaching from the matrix obtained. Furtherinvestigations will also be aimed at synthesizing amatrix with Tc and actinides simultaneously and atsimplifying the preparation of the initial mixture andadapting it to industrial conditions.

ACKNOWLEDGMENTS

We thank B.E. Burakov and V.M. Garbuzov (NPOKhlopin Radium Institute, St. Petersburg, Russia) forhelp in synthesizing a sample with 99Tc.

This work was supported by the Russian Founda�tion for Basic Research (project no. 09–05–13500–ofi–ts).

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