matrices for isolation of long-lived radionuclides

ISSN 0012�5008, Doklady Chemistry, 2010, Vol. 431, Part 2, pp. 102–108. © Pleiades Publishing, Ltd., 2010.Original Russian Text © N.P. Laverov, S.V. Yudintsev, E.E. Konovalov, T.O. Mishevets, B.S. Nikonov, B.I. Omel’yanenko, 2010, published in Doklady Akademii Nauk, 2010,Vol. 431, No. 4, pp. 490–496.

102

Safe isolation of highly radioactive waste is a mostimportant condition of successful development ofnuclear power engineering. The main source of suchwaste is facilities that process irradiated nuclear fuel.This fuel is processed for extracting fissionableactinide isotopes for their reuse and reducing wastevolume. The principal difficulties of safe isolation ofhighly radioactive waste are related to the presence oflong�lived radioisotopes with half�lives of thousandsyears and more (actinides and some fission products).It is universally accepted that such radioisotopes mustbe included in slightly soluble matrices capable ofholding radionuclides throughout their hazard time.Matrices with waste are proposed to be disposed underreducing, subneutral to slightly alkaline, hydro�geochemical conditions, under which leakage of long�lived radionuclides is unlikely. Such conditions arecharacteristic, in particular, of crystalline rock forma�tions at depths of 500 m and more [1]. Storages ofhighly radioactive waste under these conditions canprotect the Earth’s biosphere from radionuclides formany millions of years, which will be sufficient fortheir virtually complete decay.

Separation of waste into fractions containing radi�onuclides with different properties simplifies thechoice of optimal matrices. Special attention shouldbe paid to a fraction of rare�earth elements (REEs)and actinides and also long�lived fission products, inparticular, 99Tc, which are extracted by the conven�tional PUREX (Plutonium and Uranium Recovery byEXtraction) process for processing irradiated nuclearfuel and its new variants: TRUEX (TRansUranicEXtraction), DIAMEX (DIAMide EXtraction),UREX (URanium EXtraction), etc. [2–5]. In theREE–actinides fraction, light lanthanides (Nd > Ce >

La > Pr, Sm > Gd, Eu) and Y dominate and actinidesare represented by Am, Cm, and residual amounts ofU and Pu. The ratio of the amounts of Am and Cmdepends on the time of storage before processing. Thisratio is about 10 immediately after extraction from thereactor and rapidly increases with time because of dif�ferent half�lives and additional generation of 241Amfrom 241Pu [4, 5]. The percentages of REEs andactinides in the fraction are 90–95 and 5–10 wt %,respectively. The volume of such waste can be esti�mated from the weights of lanthanides (10 kg) andtransplutonium actinides (about 1 kg) in 1 t of irradi�ated nuclear fuel and the throughput of plants for itsprocessing (5000 t/year). The 99Tc concentration is anorder of magnitude lower than that of REEs and isabout 1 kg in 1 t of irradiated nuclear fuel.

From the standpoint of radioecological hazard ofactinides, which depends on their concentration inwaste and half�lives, the main danger is 241Am (half�life T1/2 is 432 years), 242mAm (150 years), 243Am(7370 years), 245Cm (8500 years), 246Cm (5500 years),and 244Cm (18 years). The last isotope rapidly trans�forms into 240Pu (T1/2 = 6600 years). These actinidesconstitute four families ending with stable isotopes ofPb and Bi. In nature, only their long�lived representa�tives with half�lives of hundreds of millions to billionsof years (235,238U, 232Th) persist.

Among REEs, 151Sm has a relatively long half�life(90 years). The most hazardous fission product is 99Tcbecause of high concentration in irradiated nuclearfuel, considerable half�life (213 000 years), and high

solubility in waters (as Тc ). Modern regulations [6]impose very strict requirements on the concentrationsof these radionuclides in drinking water. The limitingconcentration, or intervention level, is less than1 Bq/kg for the above actinides, 210 Bq/kg for fissionproducts (99Tc), and 1400 Bq/kg for 151Sm, where oneBq (becquerel) is the activity of an amount of radioac�tive material in which one nucleus decays per second.

To immobilize actinides, various matrices wereproposed, including those of garnet structural type [1,7–10]. They are characterized by high capacity for

O4–

CHEMISTRY

Matrices for Isolation of Long�Lived RadionuclidesAcademician N. P. Laverova, S. V. Yudintseva, E. E. Konovalovb, T. O. Mishevetsb,

B. S. Nikonova, and B. I. Omel’yanenkoa

Received November 16, 2009

DOI: 10.1134/S0012500810040026

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

b 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. 431 Part 2 2010

MATRICES FOR ISOLATION OF LONG�LIVED RADIONUCLIDES 103

actinides and accompanying elements of highly radio�active waste (REEs, Zr, Fe), and also corrosion resis�tance in water. These matrices were obtained by sinter�ing, melting, and self�propagating high�temperaturesynthesis (SHS).

For technetium immobilization, titanates with thespinel or pyrochlore structure and also iron�basedalloys are promising [8, 11, 12]. Previously [13], forthis purpose, we proposed metal ceramic matricesproduced by SHS. The process occurs at a tempera�ture above 2000°C for 1–3 min. This ensures comple�tion of reactions in the synthesis and decreases evapo�ration of elements, which is particularly importantfor Tc.

The purpose of this work was to substantiate thepossibility of coimmobilization of the REE–actinidesfraction with Tc in matrices based on garnet andmetallic alloys using SHS. The radionuclides wereimitated by elements that are closest in chemical prop�erties to them: Re was taken instead of Tc and stableSm or Nd was taken instead of the REE–actinidesfraction.

The initial mixture for synthesizing matrices con�tained KReO4, Nd2O3, and Sm2O3, with concentra�tions of each of the elements being 10–15 wt %(Table 1). The weight of the initial mixture in theexperiments was 50 g, with one exception where it was100 g. In SHS, the reducer was metallic Al and the oxi�dizer was МоО3 or a mixture of oxides of Fe, Cr, andNi. The composition of the initial mixture was suchthat it was expected that, as a result of the reaction, thelanthanides (Sm, Nd) should enter the Y–Al garnetand Re should form a metallic alloy with Mo or withFe, Cr, and Ni. To form a silicate phase and fix potas�sium after KReO4 decomposition, a small amount ofSiO2 was added to the initial mixture. The scheme ofSHS in the case of stoichiometry of the desired phases(REE–Al garnet and Re�containing alloy) can be rep�resented as follows:

Reducer (Al) + Oxidizer (MoO3 or mixture ofoxides of Fe, Cr, and Ni) + Waste (KReO4 andLn2O3) + Garnet Components (Y2O3 and CaO) +Additives (SiO2) Re�Containing Alloy + REE–AlGarnet + Silicate Glass Phase

The samples obtained had the shape of disks 50 mmin diameter and to 15 mm in height. They are formedform a dark material containing pores and sphericalmetallic inclusions to 3 mm in diameter. The sampleswere studied by X�ray diffraction analysis (XRD) andscanning electron microscopy (SEM) for determiningthe compositions of the phases. The ratio of theamounts of the oxidizer and reducer in the initial mix�ture determined the heat release intensity and theheating of the mixture during synthesis. Table 1 pre�sents brief characterization of the initial mixtures, spe�cific features of reactions, and data on the productsobtained. The data are chosen to be the intensity andangular position of the main reflection (hkl = 420) ofthe garnet—the desired phase for elements of theREE–actinides fraction.

RESULTS OF XRD AND SEM STUDIESOF MATRICES

Garnet dominates in most of the samples; addi�tional peaks in their X�ray diffraction patterns corre�spond to the phases of alloy, mullite, and sometimesperovskite (Fig. 1). The positions and intensities ofgarnet reflections in different samples are close andcorrespond to standard values for Y3Al5O12 from theJCPDS�ICDD database. All the diffraction patternsexhibit a broad reflection in the 2θ range 22°–36°,which indicates that the samples contain an amor�phous glass phase.

SEM confirmed the domination of the garnetphase, which forms aggregates of skeletal crystals andisometric grains to 30 μm in size (Figs. 2, 3). The shapeof the grains clearly indicates crystallization frommelt, and their sizes depend on the cooling rate: they

Table 1. Characterization of the initial mixture, intensities of SHS reactions, and specific features of the product obtained

Sample OxidizerElement, wt %* SHS intensity

(I420, d420)*** Ln** Re

Re�4 Fe, Cr, and Ni oxides 7.6 (Sm) 10.6 High (180counts/s, 2.68 Å)

Re�5 Fe, Cr, and Ni oxides 9.2 (Sm) 12.3 The same (180 counts/s, 2.68 Å)

Re�6 MoO3 10.1 (Sm) 13.4 Very high (32 counts/s, 2.68 Å)

Re�8 MoO3 11.6 (Sm) 15.5 High (147 counts/s, 2.69 Å)

Re�9 MoO3 10.8 (Nd) 15.0 The same (140 counts/s, 2.68 Å)

Re�10 Fe, Cr, and Ni oxides 9.1 (Nd) 12.7 The same (200 counts/s, 2.68 Å)

Re�13 MoO3 10.6 (Sm) 14.1 Very high (46 counts/s, 2.69 Å)

* Calculated concentration in the SHS product.** Lanthanide.

*** I420 is the intensity of the main reflection for garnet in the X�ray diffraction pattern (Fig. 1); d420 is the interplanar spacing in garnet.

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LAVEROV et al.

differ by a factor of tens in different samples and evenwithin a single sample.

The garnet composition (Table 2) was recalculatedto a standard formula with 12 oxygen anions (Table 3).The primary elements are Al and Y, and the secondaryelements are Sm (or Nd) and also Ca and Si. In thegarnet of samples Re�5 and Re�10, Cr and Fe weredetected, which is related to specific features of the

composition of the initial mixture. Note that neitherNi, nor Mo was found in the garnet composition. Thisis likely to be because they predominantly occur in themetallic phases.

The concentration of the lanthanides in the garnetdecreases from 10–13 wt % (Sm) to 5–6 wt % (Nd).This decrease by half is caused by the difference inradii between Nd3+ (1.11 Å) and Sm3+ (1.08 Å). The

64564840322416 682θ, deg

Inte

nsi

ty

Re�8

Re�13

MG

PB B

B

B

B

B

B

B

B

B

B

B

M

M

MM

M

A

M

M

M

A

AP

G

G G

G

GG G

G GG

G

G

G

G G

G

GG

G

G

G

G

G

G

G

G

Glass phase

(a) 50 μm 1 μm

2

3

2

1

3

(b)

Fig. 1. X�ray diffraction patterns of samples Re�8 and Re�13. M indicates mullite; G, garnet; P, perovskite; and A and B, metallicalloys.

Fig. 2. (a) General and (b) detailed views of SEM image of sample Re�13: (1) garnet, (2) glass, and (3) molybdenum–rheniumalloy. Black regions in panel (a) are pores.



Y3+ cation radius is 1.02 Å and is closer to the Sm3+

size, which explains why the amount of samarium thathas entered the garnet is larger than that of neody�mium. The radii of Am3+ (1.09 Å) and Cm3+ (1.08 Å)suggest that the distributions of these radionuclides inthe samples should be similar to that of Sm3+.

Space between garnet grains is filled with a ratheruniform material, in which darker and lighter regionssometimes occur. This material is mostly glass, whichis suggested by a broad peak in X�ray diffraction pat�terns (Fig. 1). No crystalline phases, except rare alloyinclusions, were detected in this material even at highmagnification (Fig. 2).

The glassy material occupies not only the spacebetween garnet grains but also entire regions in sam�ples with maximal intensity of SHS reactions, e.g.,Re�13 (Fig. 3). In this sample, the glass phase domi�nates over the garnet, which forms skeletal crystalsseveral microns in size across. The garnet composition(40.2% Al2O3, 8.9% SiO2, 1.1% K2O, 1.7% CaO,33.4% Y2O3, 14.5% Sm2O3) differs from the data for

the other samples by the presence of potassium and anincreased silicon content. Obviously, the compositionof these grains under the electron microscope is deter�mined inaccurately since, because of their fine size,the surrounding glass is captured. The intensities ofthe garnet peaks in the X�ray diffraction pattern ofsample Re�13 are very low (Table 1), and the reflectioncharacterizing the glass phase is most noticeable(Fig. 1).

As the garnet crystallizes, Nd and, to a smallerdegree, Sm are accumulated in the residual melt andtheir concentrations in the glass exceed that of Y. Thisexplains the difference in composition between theinterstitial material and the glass far from the garnet(Table 4). The garnet composition is close to the com�position of the entire sample, except for the elementsthat form alloys (Fe, Cr, and Ni or Mo). The compo�sition of the bulk of the sample depends on the loca�tion. The Y concentration near garnet grains is lowerthan that far from them. This is because, after the gar�net crystallization, the melt is depleted in yttrium andenriched in elements that are less prone to enter the

1

100 μm(a)

2

2

1

50 μm(b)

Fig. 3. (a) General and (b) detailed SEM images of sample Re�13: (1) glass and (2) aggregate of garnet grains. Black regions inpanel (a) are pores.

Table 2. Garnet composition in samples (average over 3–5 analyses)

OxideSample, wt %

Re�5 Re�8 Re�9 Re�10

Al2O3 41.2 43.7 44.6 40.9

SiO2 3.0 2.6 2.3 3.5

CaO 1.0 1.1 1.3 0.9

Cr2O3 2.4 – – 3.7

FeO 0.4 – – 0.3

Y2O3 39.1 42.2 46.7 44.8

Ln2O3 12.9 (Sm) 10.4 (Sm) 5.1 (Nd) 5.9 (Nd)

Table 3. Garnet composition recalculated for standard for�mula

CationNumber of atoms in garnet formula*

Re�5 Re�8 Re�9 Re�10

Al3+ 4.81 5.04 5.05 4.67

Si4+ 0.30 0.26 0.22 0.34

Ca2+ 0.11 0.11 0.14 0.10

Cr3+ 0.19 – – 0.28

Fe2+ 0.04 – – 0.03

Y3+ 2.06 2.19 2.39 2.30

Ln3+ 0.44 (Sm) 0.35 (Sm) 0.17 (Nd) 0.21 (Nd)

Total of cations 7.95 7.95 7.97 7.93

Note: Dash means that the corresponding element was not added.* Per 12 O2– anions.

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LAVEROV et al.

lattice. The bulk of some of the samples is nonuni�form, namely, contains regions of different colors andcompositions.

The glass is unstable and will start to crystallize withtime. One of the most probable devitrification prod�ucts is perovskite. This is caused by the fact that theperovskite, rather than garnet, structure is stable forsamarium and neodymium aluminates [8, 14]. TheSEM study failed to detect perovskite, although anumber of reflections, first of all, the main one(d121 = 2.63 Å), are clearly seen in X�ray diffractionpatterns. This is probably because of small grain size ofperovskite, which prevents its identification.

The third most abundant phase, after garnet andglass, are spherical alloy inclusions several microns to3 mm in size. As was expected, they include all of therhenium. The most characteristic are inclusions con�sisting of two alloys (Fig. 4) of contrast compositions.The rhenium concentration in them varies from28⎯30 to 62–74 wt %, with its average concentrationin the entire inclusion being 42–47 wt % (Table 5).Along with Re, the alloy phases contain Fe, Cr, and Nior Mo. Unexpectedly, they also hold Si and (in sam�ples with МоО3) Y.

One more phase detected in the X�ray diffractionpattern is aluminum silicate with the mullite structure

Table 4. Composition of interstitial material and glass

Oxide

Sample, wt %

Re�5 Re�8 Re�9 Re�10 Re�13

1�т 1�с 1 2 1 1�т 1�с 1 2

Al2O3 44.1 40.9 31.8 31.3 34.9 43.6 28.5 35.8 36.6

SiO2 23.5 23.7 27.7 26.2 26.9 21.7 30.3 30.1 19.8

K2O 8.0 4.8 5.4 2.2 3.3 5.2 7.1 2.6 2.3

CaO 3.5 2.6 3.4 2.6 3.4 2.7 3.4 2.7 2.5

Cr2O3 2.2 2.2 – – – 2.5 2.7 – –

FeO 0.9 0.6 – – – 0.9 0.6 – –

MoO2 – – 1.3 1.0 1.3 – – 0.5 1.1

Y2O3 4.1 8.4 10.8 16.0 7.2 7.1 7.2 7.5 20.7

Sm2O3 13.7 16.8 19.6 20.7 – – – 20.8 17.0

Nd2O3 – – – – 23.0 16.3 20.2 – –

Note: Dash means that the corresponding element was not added; 1, interstitial material (l, light regions; d, dark regions); 2, glass far fromgarnet grains. The Ni concentration in samples Re�5 and Re�10 is below the detection limit.

Table 5. Composition of metallic inclusions

Element

Sample, wt %

Re�5 Re�13

phase�1 phase�2 raster phase�1 phase�2 raster

Re 30.3 74.3 42.2 27.6 61.7 46.6

Fe 41.5 17.8 37.7 – – –

Ni 11.0 1.5 8.2 – – –

Cr 9.6 6.4 7.2 – – –

Mo – – – 58.8 32.8 44.9

Si 7.6 – 4.7 11.6 2.2 6.9

Y – – – 2.0 3.3 1.6

Note: Dash means that the corresponding element was not added. Raster means analysis of a 300 × 400 µm region (sample Re�5) or a 100 × 100 region(sample Re�13).



(Fig. 1) and the nominal composition Al6Si2O13. Thisphase contains no imitators of irradiated nuclear fuel(Sm, Nd, Re); therefore, we did not study it.

The spherical shape of the metallic inclusionsproves their formation from melt. Using the state dia�gram of the Re–Mo system [15], one can believe thatthe temperature in the synthesis of the sampleexceeded 2500°C. Heating during SHS is sufficient forcomplete melting of the mixture since the meltingpoint of Y–Al garnet is ~1900°C. After completion ofthe synthesis and cooling, there was garnet crystalliza�tion, the degree of which depended on the heatremoval rate, composition, and initial temperature ofthe melt.

Under optimal process conditions, the garnet has acontent of up to 70 vol % (samples Re�5, Re�8, Re�9,and Re�10) and accumulates much of the imitators ofthe REE–actinides fraction (Sm, Nd). The remainingpart of them is contained in the glass and, probably,perovskite. The glass devitrification should lead toadditional formation of perovskite. Although KReO4

is low�stable (it melts at 518°C and boils at 1370°C)and the synthesis temperature is high, Re remains inthe system, binding in the alloy phases while reducingperrhenate by metallic Al. Similar behavior can also beexpected for Tc if it is added as pertechnates to the ini�tial mixture for SHS. Confirmation of this assumptionrequires separate experiments with technetium.

Thus, in this work, we used SHS to produce metalceramic matrices with imitators of radionuclides of theREE–actinides fraction (Sm, Nd) and technetium(Re). The main phases in these samples are aluminumgarnet, silicates (glass and mullite), and metallicalloys. The lanthanides are accumulated in the garnetand glass, and rhenium forms alloys with Fe, Ni, andCr or with Mo. The ratio of the garnet and the glass isdetermined by the SHS intensity, which depends onthe amounts of the oxidizer and reducer in the initialmixture. Under the optimal synthesis conditions, thesamples are dominated by the garnet (up to 70 vol %),

the concentration of the metallic alloys is about 10–15 vol %, and the content of the silicates (glass andmullite) is 15–25 vol %. Further, we intend to producematrices with 241Am and 99Tc for determining theircorrosion resistance in water.

ACKNOWLEDGMENTS

We thank L.A. Kochetkova, A.V. Mokhov, andM.I. Lapina for help in performing analytical studies.

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

REFERENCES

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(a) 100 μm 50 μm(b)

Fig. 4. Scanning electron microscopy images of metallic inclusions in samples (a) Re�5 and (b) Re�13. The results of analysis ofthe alloy phases are given in Table 5. The lighter phase contains more rhenium.

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