Isotope Fractionation of Uranium in the Process of Leaching of Nuclides ofDispersed Fuel of RBMK of the Chernobyl NPP
German N. BONDARENKO and Emlen V. SOBOTOVICHState Scientific Center of Environmental Radiogeochemistry NAS,
Palladin av. 34, 252680 Kiev, Ukraine, tel. (044) 444-12-15, fax (044)444-00-60
SUMMARY
The particles of fragmented nuclear fuel containing
uranium of different degree of oxidation were found as
the parts of conglomerates in the premises of the IVunit and in soils of the Chernobyl NPP Industrial site.
When leaching uranium from contaminated soils,
fractionation of the nuclides of 235U and 238U between
solid and liquid phases may take place. Fractionation
was not indicated in leaching of uranium from theparticles patched into graphite fragments.
Fractionation of uranium nuclides is due todifference in energies of these nuclides bonds in crystal
lattice of solid phase of the products of fuelfragmentation. Two hypotheses are accepted. The firstone supposes heterogeneous composition of nuclear
fuel of RBMK reactor consisting of fractions withdifferent enrichment by 235U, uranium isotopes of themore enriched fraction having higher mobility due tofeatures ofpreparation technology. Anotherhypothesis bases on the assumption of selective
loosening of bonds of separate 235U atoms takes placein the solid body under action of slow neutrons.

1. Fuel-containing products of accident on the IVunit core of Chernobyl NPP.
Unit cores of the Chernobyl NPP are equipped withreactors of channel type (RBMK), that use graphite asreaction moderator and light water as heat-carrier.
Reactor core is an vertical cylinder 11.8 m in diameterand 7 m in height. It contains fuel assemblies,
moderator, heat-carrier, technological channels,
neutron absorber rods. Graphite brickwork makes up
vertical columns with through holes for placement oftechnological and control channels. Graphite density is
1.65 g/cm3. Fuel assembly consists of two sequentiallyconnected sections 3.5 m each in length. Each section
includes central tube from zirconium alloy, steel
distanced grid, 18 fuel rods.
Initial enrichment of fuel is 2%. If the designburn-up is taken as 18.6 MW/ton, then nuclide contents
are: 235U — 4.1; 236U — 2.1; 238U — 969.3; 239Pu —2.4; 240Pu — 1.7; 241 Pu — 0.5; and 242Pu — 0.3 kg per
tone of initial uranium [5]. So spent nuclear fuel should
contain 0.42 % of 235U.
On the moment of the accident the reactor active
zone of IV block of the Chernobyl NPP contained 1659
fuel assemblies (FA). Their nuclear fuel total mass was
190.2 tons [4]. The bulk of the FA was enclosed in thefirst charge cassettes with the burn-up of 11 to 15
MW⋅day/kg of U. Some amount of “fresh” nuclear fuelwas in the active zone too. Calculations of mass andactivity of fission products and transuranium elementsthat took account of factors of irregularity of the fieldof energy emission in RBMK-1000 were carried out foreach cassette. The total mass of each accumulated
nuclide was estimated by summing over all fuelassemblies in the reactor core. According to theseestimates 235U content conformed to effectiveenrichment of 1.1% [2]. Plutonium nuclides contents
were estimated in: 239Pu — 2.166, 240Pu — 0.925, 241Pu— 0.257, and 242Pu — 0.074 kg per ton of uranium [8].
More than 180 tons of nuclear fuel are considered to
be enclosed in the object “Ukrytiye” (“Sarcophagus”).
Irradiated fuel inside “Sarcophagus” has the following
modifications:- fragments of the reactor core - fuel tablets, fuelassemblies and their fragments, radioactive graphite,fragments of technologic channels;

INTRODUCTION
Works on study of uranium isotopic composition in
the products of accidental ejection from the Chernobyl
NPP by its scope take up rather modest place among
other studies on consequences of 1986 accident in spiteof obvious connection between 235U distribution in theprocess of fuel destruction and nuclear safety ofSarcophagus. Studies on the uranium isotopes
distribution in dissolution, leaching of dispersed fuelgave discrepancy results: some experiments showed235
U enrichment of uranium in water solutions relative
to the solid phase [1,2], others showed an identity of
the isotopic composition of the both of phases [2-4].
The results were explained by the heterogeneouscomposition of fuel component of Chernobyl ejection.
Detailed analysis of the performed studies has enabled
to establish the regular connection between thepossibility of uranium isotopes fractionation andconditions of dispersed fuel formation and to propose
the version of rise of weakening of 235U bond in thesolid phase of fragmentation products of fuel.

93

Sampling of hot particles from the soils in thefloodplain of the Prypiat (5-7 km distant from the unit)
and those from soddy-podzolic soils from st.Yanov(2-3 km from the unit) was presented by
microconglomerates and fragments of reactor graphite[2]. Several types of particles can be separated by theircomposition and structure:- quartz-feldspar, clastogeneous, having grains
predominantly of sharp-angle profile cemented bymicaceous-kaolinic mass;- clastogeneous substantially micaceous-silicate,consisting of entangled fibrous aggregates of sheet
crystals;condensation-clastogeneous,consistingof
microgeodes, spherules of condensation genesis
with admixture of clastogeneous matter.
Predomination of small sharp-angle clastic
fragments, cementation, presence of
implanted
uranium particles, condensation formations haveenabled to assume that the microconglomerates as theradioactivity carriers were formed mainly in result of
agglutination of particles of fluid materials used during
fire fighting, dispersed materials of reactor unit
constructions, fuel and graphite at high temperatures[10, 12, 15].
One of the fragments, 2x3 mm in size, sampled in
the premise of the IV unit consisted of grains ofgraphite and small fragments of concrete. Grains ofquartz and microcline were cemented with fineCa-containing material. Particles with high atomic
mass that had substantially uranium composition were
found in that fragments by vision of preparates inreverse reflected electrons. Uranium particles of 1-50
µm in size were placed on the surface of graphite
aggregates or were slightly implanted into them.
Two types of particles were separated by themorphology:- shperules and their fragments of 3-90 µm,
irregular-shaped
particles,
predominantly
sheet-shaped, with dimensions of fractions of
micron to 50 µm.
Spherical particles showed microdifraction picture
corresponding to UO2 structure with diminishedparameter of elemental cell (à0 = 0.535 ± 0.005 nm). It
was attributed to the partial oxidation of U(IV) to
U(VI), that increased number of defects in crystals anddecreased effective dimensions of blocks of coherent
scattering of electrons. Mixture of uranium dioxide andelemental uranium was identified by characteristic
X-ray emission in number of plate-shaped particles.
Hypothesized process of UO2 sublimation at hightemperatures in the authors view could be accompanied
by formation of pre-stoichiometric oxide UO2—õ andfollowing disproportionation to UO2 and U.
Among the synthesized uranium oxides used as anuclear fuel in the modern energetic units the series of- finely dispersed fuel - “hot” fuel particles that werethe main radioactive components of dispersed phaseof aerosols and solid precipitates on different
surfaces;- lava-like masses containing nuclear fuel;- crystalline nova-formations that are bright-yellow
spots and figures;- water solutions.
Finely dispersed fuel of micron dimensions wascharacterized form of accidental ejection of theChernobyl NPP into the environment. In the earlymonths and years after the accident they were theobjects of numerous studies. “Hot” particles were
sampled in the premises of the IV unit, in close
proximity to the Chernobyl NPP, on the area of theChernobyl NPP industrial site and sanitary zone, on the
territories of Ukraine, Byelaruss as well as
Scandinavian countries, Central and West Europe.
The most of works were limited by studies ofradionuclide content of particles. Studies concerningwith radionuclide state in particles are not numerous.
Works on study of chemical and structural compositionof particles, on kinetics of dissolution and leaching ofradionuclides, and on distribution of radionuclides inthe system “solid phase - solution” [1-3, 12-14] areamong them.

2. Elemental and structural composition of hot
particles.
Some particles contained radioactive substancesthat had low boiling temperature (131I, 137Cs and 134Ñs),others contained radionuclides with rather high boilingtemperature (isotopes of Sr, Ce, Zr, Pu, Am, Cu, Npetc.) as the parts of uranium matrix [3]. At morphology
fibre-like and spherical microconglomerates and that ofirregular habitus. Density of the heaviest of theparticles reached 8 g/cm3 [2, 6].
In one of samplings hot particles of two varieties 30 µm and 1 to 2 µm in size - predominated. Particlesof the first variety were identified as fuel ones, those of
the second variety were recognized to becondensational particles formed during burning ofgraphite. In the sampling of particles that deposited in
the premises of “Sarcophagus” maximum of dimension
distribution was in the region of 15-20 µm [6].
At a distance of more than 1 km from the accident
unit, separate hot particles were found on the surface ofthe earth, buildings and constructions as the parts ofconglomerates of particles of different substantial
composition. Oxides of U(IV) and U(VI) as well as
blobs of carbonaceous matter, small fragments ofquartz, aluminosilicate matter, ferrous minerals,
rounded titanic fragments were detected in these
conglomerates by the methods of microscope probing
and X-ray-structural analysis [19].

94radioactive isotopes of Sr and Cs in the outer layers anddecreasing of their activities up to the centers ofparticles. Plutonium contents in the upper layers of
spherical particles were from 1×105 to 1.5×105 Bq/g
when mass concentrations of uranium were of 0.5 to
7.5%. Plutonium contents in other layers of theparticles were less by some factors and in some casesby several orders of magnitudes. 238Pu/ 239+240Pu ratios
were stable in all these cases and corresponded to thoseof the “mean” nuclear fuel of the IV unit.
Above-mentioned data testify that the processes ofhigh-temperature atomizing of elements andsubsequent spontaneous solid condensation of vapour
resulted in the formation of spherical particles,
proceeded in the active stage of the accident at least insome point of the IV unit. Recrystallization of thenuclear fuel material accompanied with establishing ofcontents of Pu, 144Ñe, 241Am, 90Sr, and U in different
layers of spherical particles that rather differed by theirratios from the “mean” nuclear fuel of the IV unit. The
growth of fiber-like particles proceeded obviously inthe active stage of the accident too from the gas phaseat the temperatures higher than 3,000 0Ñ.

phases were established that transfer one into anotherfrom UÎ2 to UÎ3 gradually or abruptly as uraniumoxidation occurred from U(IV) to U(V) and U(VI).
Synthetic UÎ2 oxide has cubic crystal structure withelemental cell dimension of 0.548 nm. As uraniumoxidation occurs, at first diminishing of this dimensionthat is recognized as a result of isomorphous
replacement of U(IV) (ion radius is 0.1 nm) by U(VI)(r=0.08 nm), and than decreasing of oxide symmetry totetragonal U4Î9 are observed. More oxidized phase U3Î8
is known to have two polymorphous modifications(rhombic and hexagonal), maximally oxidized phaseUÎ3 - five modifications, one of them is amorphous.
Oxidation series consists of the next discrete phases:
UÎ2 — U4Î9 — U3Î7 — U2Î5 — U3Î8 — UÎ3.
In the sampling of particles aspirated from theaerosols from the object “Sarcophagus” about 10% of
the total activity were contained in fraction < 1 µm,
about 70% of it - in fraction 1-5 µm, and about 20% ofactivity were concentrated in the rough-dispersed
fraction. The main input into activity of the dispersed
matter deposited on the surfaces fraction 1-10 µm has
made (~70%), rough-dispersed fraction (>10 µm)
contained about 20% of the total activity,
fine-dispersed one - about 10% [8]. The finest particles
sampled from the soils of the south Byelaruss had thehighest specific activity [15].
All the long-lived products of nuclear fuel fission
were detected in the dispersed phase of aerosols. 238Pu
and 239+240Pu were found in all the separated fractionsof dispersed phase of aerosols. 238Pu /239+240Pu ratios
were close to those of the “mean” nuclear fuel of the IV
unit. There was no correlation between 144Ce and241
Am [4,7]. Activities of 144Ce in the separate particlesof the sampling of 17 to 55 µm in dimension were (8.1
- 9.6) × 109 Bq/g that was close to “mean” radionuclide
content in the fuel at the moment of the accident.
Rather large-sized spherical particles up to several
millimeters in diameter were collected on the surfacesof steam-dumping valves in the steam-distributing
premise of the accidental unit [6]. Study ofradionuclide distribution with depth in these particles
showed the maximal contents of plutonium as well as

3. Leaching of fission products from hot particles
in soils.
Radioactivity of soils in the industrial site of theChernobyl NPP was attributed to fallout of irradiatedfuel in the next principal forms:
1) fragmented fuel of different grain size; its particles
differed in their degree of oxidation, due tointeraction between the heated fuel and oxygen in
the course of explosive release. The U(VI)/U(IV)ratios in the fuel fallouts of near-field zone varied
from 0.6 to 1.7;
2) fuel-graphite conglomerates; the main part offragmentary nuclides in the near-field zone wasattributed to these fallouts. According to the X-ray
microanalytical data, a spatial association between
uranium and carbon was found for many particles;3) nuclear fuel embedded into the matrices of
construction materials (iron, zirconium, copper);4) extremely fine (≤ 1 µm) hot particles ofaluminosilicate composition, as a rule with a regular

Table 1. Vertical distribution of plutonium and 238U/235U atomic ratios in total uranium in the soils of
the nearest zone of the Chernobyl NPP.
239(240)
238
Pu contents in layers, %U/235U atomic ratios
Index
0-1cm 1-2 cm
2-3 cm 3-4 cm 4-5 cm 0-1 cm 1-2 cm 2-3 cm 3-4 cm 4-5 cm
1
92.0
7.4
0.4
0.2
79.9
128.2
134.7
137.4
137.6
2
91.2
7.9
0.6
0.2
0.1
80.3
129.6
129.6
137.3
137.7
3
95.9
3.8
0.3
89.6
135.8
137.3
137.8
137.8
4
96.7
2.9
0.3
0.1
96.1
126.7
134.8
137.1
137.6
5
91.8
7.8
0.3
0.1
98.4
131.5
136.9
137.8
137.8
6
96.0
3.4
0.4
0.1
0.1
103.5
132.9
137.1
137.6
137.6
95

immobilization, and remobilization of radionuclides
sequentially.
The constant of radionuclide mobilization rate k1 isa characteristic of the resistance of solid-phase fallout
in the soil. Its value first of all depends on thesolid-phase particles qualities (material composition,
degree of dispersion, way of formation) as well as on
physical-chemical characteristics of the soil. The
constants of radionuclide immobilization rate k2 andremobilization rate k3 are determined by radionuclide
chemical properties, and for each radionuclide theyessentially depend on physical-chemical characteristicsof the soil. Numerical values of constants oftransformation rates of 90Sr and 137Ñs forms were
estimated by calculations on the base of comparison of
the data on radionuclides forms in the soils in the
different moments of post-accidental period.
The kinetic curves of 90Sr and 137Ñs mobile forms
accumulation in soddy-podzolic soils in the Chernobyl
exclusion zone as well as experimental data on theircontents are presented in Figs. 1 and 2.
In the first years after the accident, sharp increasingof 90Sr mobile forms content took place, and then theirstabilization was observed. So immobilization was notsubstantial in 90Sr forms transformation. Dynamics ofchanges of 137Cs mobile forms activities insoddy-podzolic soils was characterized by maximumthat was observed in 1987-1988 and subsequent
decreasing of ion-exchangeable form content at theexpense of advancing immobilization.
The mean value of constant of radionuclide
mobilization rate was evaluated in 0.195 year-1 withvariations of 0.12 to 0.30 year-1. Hot particles that werein the air-dry state preserved fission products in theircompound for a much longer time.

carbon admixture;5) so-called condensation particles enriched withvolatile nuclides, foremost by 137Ñs and 134Ñs, morecharacterized for the distant zone.
In the first months after the accident more than 99%
of uranium and plutonium radionuclides wereaccumulated in the upper 2 cm-layer of the soil in thenearest zone of the Chernobyl NPP (Table 1).
Vertical distribution of the fission products in thefirst weeks and months after the accident was the same.
Their activity was localized in form of hot particles,that was confirmed by the autoradiography data [15].
In soils hot particles are exposed to destruction due
to effects of soil solutions and products of bacteria
action. In the result of leaching of radionuclides anddissolution of matrix, products of uranium fission werecarried out from the particles and their water-solublecompounds were formed. This process and following
transformations of radionuclide forms can be presented
by processes of mobilization, immobilization,
remobilization of radionuclides:

W
k1

k2

Ap

F
k3
E
A0 = Ap + W + E + F

where : À0 is bulk radionuclide activity; Àp - part ofradionuclide in solid-phase particles; W - part ofradionuclide in water-soluble form; Å - part ofradionuclide in ion-exchangeable form; F - part ofradionuclide in fixed form.
Mobilization is identified with carry-out ofradionuclides from solid-phase particles and formationof water-soluble and ion-exchangeable forms ofradionuclides that are subjected to water migration.
Immobilization is connected with formation ofrelatively conservative (fixed) forms of radionuclides.
Remobilization is an opposite process toimmobilization.
According to deterministic kinetic model oftransformation [14], activity of the mobile form ofradionuclide (AM) is :k 1 - k3
AM = A0 {-----------------[(exp(-k1t) - exp(-k2 - k3)t]-k1 + k2 + k3
k3
+ ---------[1 - exp(-k2 - k3)t]}k2 + k3

4. Mobility of uranium isotopes in the soils
containing hot particles.
The radionuclide status in solid substances iscommonly studied by leaching procedure and researchof its distribution between different phases andchemical forms. Solutions that don’t dissolve matrix
matter are used. So admixtures elements residing out ofnodes of crystal lattice are leached. Nuclides ofdaughter radioactive elements are leached fromuranium and thorium minerals. Atoms resided inside acrystal or in the narrow capillaries migrate to the crystal
surface in the result of diffusion over vacancies,
internodes and capillaries. Desorption of the elementfrom the surface depends on the concentration of
displacing element in the solution and proceeds toattainment of equilibrium. Parent radionuclides moveinto solution only in the result of destruction of crystal
lattice, i.e. dissolution of the matrix matter.
Mobility of uranium isotopes and fission products
in soils contaminated by solid-phase accidental fallouts

where AM and A0 - activities of radionuclide mobile
and initial forms, kn - constants of rate of mobilization,

96

1,00

M obile form s

0,80
0,60
0,40
0,20
0,00
0

5

10

15

20

t,years-1-2-3-ourdata-lit.data-lim its

Fig. 1. Kinetic curves of 90Sr mobile forms accumulation in soddy-podzolic soils in the exclusion zone:
1 - k1 = 0.195, k2 = 0.02; 2 - k1 = 0.12, k2 = 0.02; 3 - k1 = 0.30, k2 = 0.02 year-1.
0,16

Mobile for

0,12

0,08

0,04

0,00
0

2

4

6

8

10

12

14

16

t, years
-1

-2

-3

-lit.data-our data-lim its

Fig. 2. Kinetic curves of 137Cs mobile forms accumulation in soddy-podzolic soils in the exclusionzone:
1 - k1 = 0.195, k2 = 1.7; 2 - k1 = 0.195, k2 = 1.0; 3 - k1 = 0.195, k2 = 2.7 year-1.
plutonium activity were carried out with Si (Au) detector within a 2% error. Content of uranium in thesoil was estimated by the mass-spectrometric isotopic
dilution method within a 0.8% error. Standard 235U
solution with 238U/235U isotope ratio equal to 0.1022
was used as an isotopic label. Limiting relative errors
for all the experimental measurements were estimatedwithin 95% confidence interval.
To check of the obtained 238U/235U ratios, a seriesof duplicate analyses were performed at the
mass-spectrometric laboratory by Prof. N.P.Tsherback,which confirmed validity of the previous results.
Extraction of uranium from the soils was carried out
in these experiments with concentrated hydrochloric
acid. After solid phase separation isolation of theuranium from the solution and its preparation towas evaluated by leaching method in series of works[1-3, 14, 15]. For these purposes sequential leaching of
nuclides from solid phase by water solutions of salts
and acids was performed.
Experiments on leaching of uranium isotopes fromthe top layer soils, which contained hot particles, werecarried out at the laboratory of transuranium elements
geochemistry of Department of Environmental
Radiogeochemistry of Institute of Geochemistry andPhysics of Minerals [14]. Soils were sampled in 1986
mainly at the industrial site of Chernobyl.
Determinations of atomic (mass) ratios such as
238 235
U/ U and 235U/236U were carried out by the
MI-1320 and MI-1201T mass-spectrometers with 0.2%
limiting
relative
error.
Alpha-spectrometricdeterminations of 234U/238U activity ratios as well as

97

Table 2. Uranium isotopic composition and content in soils in the near-field zone of the Chernobyl NPP[1].
238 235 *235 236 *234
No Soil sampling point
U/ U
U/ U
U/238U** 238U (µg/g) 235U,%
1.38
2.46
1
Site of the IV unit
39.68
16.08
3.06
0.796
2.18
2
North-western part of the cooling pond
44.82
26.12
2.75
1.173
1.92
3
Vicinity of Yanov railway bridge
51.21
14. 282.38
4Left bank of the Prypiat
53.3
8.16
2.00
0.745
1.84
5
Yanov railway station
52.6
13.05
3.96
0.910
1.87
6
Red forest
57.8
26.77
2.32
0.848
1.70
7
v. Yanov
62.8
35.72
2.57
0.764
1.57
8
v. Leliv
77.5
83.2
2.39
1.092
1.27
9
Paryshev channel
93.8
140. 3
3.22
0.632
1.06
10 v. Burakivka
102.4
47.26
2.18
0.826
0.97
11 v. Tolsty Lis
106.3
76.6
1.94
0.911
0.93
12 town of Prypiat
108.4
92.4
1.82
2.14
0.91
13 Chernobyl
112.9
103.7
1.86
0.948
0.88
1.013
0.79
14 v. Cherevach
126.3
194.8
1.69
*
Ratios between mass concentrations of uranium isotopes;**
Ratios between activities of uranium isotopes.
Table 3. Isotopic composition of uranium in leachate solutions before and after oxidation of uranium.
Solution without
Oxidized solution
Leaching
Soiloxidation
No
Sampling pointagent
238 235
235
238
235
235
238 235
U/ U
U, %
U/ U
U, %
U/ U 235U, %1 Cooling pond
8 Ì HNO3
44.82 2.18 %
28.42
3.4 %
11.74
7.85 %
2 Yanov railway station 2 Ì HNO3
1.86
%
40.61
2.4
%
12.17
7.6
%
52.6
3 v. Yanov
6 Ì HNO3
62.8
1.57 %
48.4
2.02 %
20.32
4.69 %
4 v. Leliv
7 Ì HCl
77.5
1.27 %
47.9
2.04 %
23.68
4.05 %
5 v. Leliv
2 Ì HNO3
77.5
1.27 %
51.1
1.92 %
31.22
3.1 %6 Paryshev channel
7 Ì HCl
93.8
1.05 %
15.64
6.0 %
2.744
27 %
7 Paryshev channel
1.04 %
61.4
1.6 %
18.93
5.0 %
8 Ì HNO3
95.4
acids with admixtures were used (Table 3). It was
found that the 238U/235U ratio for the first leachates
ranged from 15.6 to 61.4, whereas in the case of more
complete extraction of uranium from the same soils it
varied from 44.8 to 93.8. On these grounds the authorsof the work assumed that the reduction in the 238U/235U
isotopic ratio in solutions might be explained by thefact that the technogeneous fuel uranium (or at least
some part of it) differed from the natural one in its
higher ability to leaching. Dependence on the238
U/235U ratio value on leaching conditions testified
that the mobile uranium both of natural and fuel origin
is not completely transferred into a solution. In such acase a dynamic equilibrium should be establishedbetween the solid and liquid phases of the system.
Subject to the changes in the physico-chemicalleaching conditions such an equilibrium should be
disturbed resulting in changes in the uranium isotopic
composition of the leachate solution.
To estimate an effect of oxidation of the liquid
phase upon the displacement of uranium isotopic
composition oxidation of the aliquot part of theleachate solution was performed with persulphuric

mass-spectrometricanalysiswasperformed.
Experimental data on isotopic composition of uranium
and its content in liquid phase are presented in Table 2.
Obtained data on the uranium contents in soils did
not exceed the Clark values varying from 0.632 to 2.14µg/g. This testified at least to comparability oftechnogeneous uranium content in soils to the natural
one. The 238U/235U ratio in the uranium extracted fromthe soils varied from 44.82 to 126.3; 235U/236U ratio
varied at more greater extent. Shifts in the isotopic
composition relative to natural and even totechnogeneous uranium were pronounced: 238U/235U
isotopic ratios for some leachate-solutions were foundto be considerably lower than that of the initial fuel.
Besides, dependence of uranium isotopic
composition in soils upon their distance from thesource was observed: uranium enrichment by 235U
isotope increased with approaching to the IV unit, i.e.
with enrichment of soils by technogeneous (fuel)uranium.
Mobility of uranium isotopes in the solid phase was
evaluated by the method of their sequential leaching in
the variable physico-chemical conditions: different

98ammonium in presence of Ag+ cations. When uranium
was leached from the soil by hydrochloric acid,evaporation of the solution aliquot, conversion of thesalt residual into NO3-form (removing of Cl- ion traces)
and its dissolution in 1 M HNO3 preceded to theoxidation operation.
Values of 238U/235U ratio in leachate solutions
before and after oxidation of uranium are presented inTable 3. In the case of oxidation of the solution
uranium in the leachate solution was more enrichedwith 235U isotope in comparison with unoxidized
solution. The minimal detected 238U/235U ratio reached2.7, that corresponded to ∼27% enrichment by 235U.
Experiments to estimate the contents of uranium
valency forms in the leachate solutions were also
carried out. U(VI)/U(IV) ratios were found to varyfrom 0.6 to 1.7 [2].
In the experiments on absorption of uranium fromhydrochloric acid solutions by the anion-exchange
resin AV-17, the authors of the work [2] have found theanionic form that is characterized to U(V1) to beenriched by 235U isotope relatively to U (1V) form.
In the other series of experiments, processes ofleaching of fragmented radionuclides as well as fueluranium from the contaminated soils from left-bankflood-plain of the Prypiat river were studied. Soils were
sampled in the right bank of Murovka channel on
20.03.91. By this moment solid-phase fallouts
underwent destruction to some extent. (see Part 3).
The levels of surface contamination by 90Sr in thesampling site were from 3.7 to 7.4 MBq/m2.
Radionuclide compositions of the studied samples arepresented in Table 4.
Gamma-spectrometric analysis and radiochemical
determination of 90Sr content were carried out from thesame sample. Then the samples were sequentiallytreated by water with natural hydrochemicalcomposition, 1M solution of ammonium acetate, andconcentrated acid solutions. 90Sr and uranium contents
and uranium isotopic composition were determined ineach of leachate-solutions. Solid to liquid phases ratio
was 1:1, the mixture was stirred for 24 hours, thenseparation of the phases on the Buchner funnel withmembrane filters of 0.2 µm in diameter was carried out.
Acid leaching was carried out in teflon dishes underlight heating and periodical stirring during 24 hours.
Stringent regime of acid leaching resulted indissolution of uranium matrix and in partial orcomplete breakdown of natural uranium-containing
minerals, but, because of low uranium concentrations,the applied procedure did not make available
determination of its content in a number ofleachate-solutions (Table 5). High 235U contents were
found in water, salt and acid leachate-solutions.
Insoluble residue from the sample 2 after mineral
breakdown by hydrofluoric acid was the mixture ofaccessories (mainly zircon-rutile) and graphite
particles. Presence of 90Sr pointed to the definite
influence of graphite fuel-containing particles on theuranium isotopic composition.
In the leachate-solution from the mineral
concentrate [15], sampled at the cooling-pond bank,mass contents of heavy nuclides were determined by
the mass-spectrometer“Plasma Quad” (“VG
Instruments”) with ionization of the material in
inductively connected plasma. The relative contents ofmasses from 233 to 242 were the next: 233 − 6.29; 234− 45.1; 235 − 2712.2; 236 −455.69; 237 − 5.54; 238 −237999; 239 − 112.61; 240 − 9.68; 241 − 9.11; 242 −8.38. Ratio of mass of 235 to 238 corresponded here to
the enrichment of uranium with 235U isotope up to
1.126%, that is close to “mean” composition of ejected

Table 4. Radionuclide composition of soil samples from left-bank flood-plain of the Prypiat on 20.04.91 [3].
Radionuclide content, kBq/kg
Sample
144
106
137
134
90
Ce
Ru
Cs
Cs
Sr
1
2
3

57.4
8.5
16.2

18.1
2.5
3.0

134
18.2
34.6

13.7
1.82
3.57

Table 5. Uranium isotopes in the leachate-solutions from the soils [3].
Sample 1
Sample 2
Agent
238
235
90
*
238
235
90
H2O
NH4Ac
6N HNO3
HNO3+ HCl
HNO3+ HF
6N HNO3

U/ U

10 (9.1%)
1.4 (41.6%)
36 (2.7%)
45 (2.17%)

U, µg/g

Sr, %

U/ U

0.39
0.11

3.0
4.2
91.7
1.05

22 (4.3%)110(0.9%)

154
15.7
38.3

Sample 3U, µg/g

Sr, %*

238

U/235UU, µg/g

-

1.7
4.5
91.4

22 (4.3%)
3.3(23.2%)
30 (3.2%)

0.015

5.4 ⋅10--4

90

Sr, %*
1.9
3.3
84

2.4
9.9

* 90

Sr content is given as percentage of bulk activity in the soil

99

Table 6. Extent of radionuclides leaching from the graphite hot particles in the experiment withsubsequent treatment with water and mineral acids [3].
Time of exposition, hours
for separate
total
fraction
Water 1
1
1
Water 2
4
3
Water 37
3Water 413
6
Water 523
10
Water 640
17
Water 7
67
27
HNO3
**
**
HF
**
**
HCl
**
**
** Has not been controled

Total leaching, %Treatment

90

137

Cs

U

32.7
52.9
66.4
75.1
85.7
90.5
94.3
**
**
**

75.7
89.3
92.4
93.5
94.5
95.6
96.0
**
**
**

69.0
69.1
69.2
79.6
80.3
80.5
99.1
99.75
99.98

Sr

Table 7. Uranium isotopic composition in water leachate-solutions from the graphite hot particles [3].
Conditions of leaching
Isotope ratios
235
234 238238
236
Time, hours
Leaching agent
U, %
U/ U
U/235U
U/238U
-4
91.70
1
H2O
1.078
1.8 ⋅ 10-3
1.6 ⋅ 10
-4
3
H2O
1.088
90.91
2.3 ⋅ 10
1.8 ⋅ 10-3
6
H2O
1.107
89.29
1.6 ⋅ 10-4
1.9 ⋅ 10-3
-4
10
H2O
1.068
92.60
1.5 ⋅ 10
1.8 ⋅ 10-3
-4
17
H2O
1.099
89.96
1.4 ⋅ 10
1.9 ⋅ 10-3
27
H2O
1.088
90.91
2.5 ⋅ 10-4
1.6 ⋅ 10-3
-4
HNO3
1.010
97.96
1.5 ⋅ 10
1.6 ⋅ 10-3
-4
HF
1.906
51.47
2.6 ⋅ 10
1.7 ⋅ 10-3
-4
HCl
1.986
49.36
2.4 ⋅ 10
1.7 ⋅ 10-3
138.6
Natural uranium
0.721<2 ⋅ 10-6
6 ⋅ 10-5
235(*) Table was drawn up after [3], % U was calculated for the convenience of discussion.
fuel. In groundwater in the “Red Forest” near the
Chernobyl NPP, the presence of uranium with mass
ratios 238U/235U from 50 to 54 was fixed in 1987 [16].
It corresponds to 1.8-2.0% enrichment of the fuel.
Comparison of the data from the cited works makes
it evident that the intervals of variations of uranium
isotopic composition in water phase in its leachingfrom soils in natural conditions and in the laboratory
are actually identical.

Leaching of radionuclides from graphite particles
was carried out in the hermetically sealed vessels undercontinuous stirring. After set time expiration solid
phase was separated on the membrane filter of 0.2 µm
in diameter and then was treated by the water or acidsolution again to continue the process of leaching.
Sequential effect of mineral acids HF and HCl in
different conditions, including under high temperature(200 0Ñ) was carried out in the hermetically sealed
teflon cylinder.
Nuclide composition of leachate-solutions fromgraphite hot particles was determined bymass-spectrometer“PlasmaQuad”(“VG
Instruments”). Solution of the natural uranium has astrictly constant 238U/235U ratio equal to 137.88. A
series of 10 measurements showed the 238U/235U ratioin 138.6 ± 2.0 when the time of the single measurement
was 3 min. Uranium content and its isotopic
composition were determined in the experimental
leachate solutions from hot particles.
Experimental results are presented in the Tables 6
and 7. Experimental results testify the difference in therates of dissolution of strontium, cesium, and uranium
nuclides in the process of their leaching from graphite

5. Leaching of nuclides from dispersed fuel ofgraphite composition.
Ability of uranium and fission products to leachingfrom hot particles ofpredominantly graphite
composition sampled in the premises of the“Sarcophagus” was studied in the work [3].
Radionuclide composition of sampled particles varied
noticeably. 90Sr content in some particles varied from37 to 814 kBq/g, that of 137Cs - from 2 to 12 MBq/g.
Radioactivity of a number of particles was caused
exclusively by nuclides of 134Cs and 137Ñs. These data
testified predominantly condensation genesis of theparticles.

100uranium-containing compounds:- natural minerals and soil compounds, containing
uranium with composition of about 0.7 % 235U;- technogeneous fuel-containing particles withuranium of about 1.1% 235U composition;- technogeneous fuel-containing particles containing
uranium of fresh fuel charged immediately before
the accident and enriched up to 2% of 235U;
statistical expected contribution of that particles isless than 1% of technogeneous uranium.
Relationship between technogeneous and naturaluranium in the studied soils increases with approachingnear the source (Chernobyl NPP Industrial site ) up to
their comparable contents [1, 15]. 235U contents could
not exceed in the most contaminated samples 1.1%
(238U/235U ratio was about 89.5). Part of 235U inslightly contaminated soils should be close to 0.7%.
According to work [1] 238U/235U ratio in the waterphase after extraction of uranium from contaminatedsoils varied from 126 to 40 (Table 2). The mostsignificant enrichment by 235U isotope (up to 2.4%)
was reached in the soil sampled directly near the IV
unit. Differences between expected isotopes ratios andthose obtained experimentally exceeded substantiallythe error of mass-spectrometric analysis. Finding of
high-enriched uranium presents the presumption thatisotopic heterogeneity of nuclear fuel in the IV unit
existed before the accident.
Isotope fractionation in leaching of uranium
observed in the work [1], confirmed by the work
[15]and probably by the other one [6], showed that
correspondence of uranium isotopic composition in thesolution to that of the soil could be reached provided
that the analyzed element was completely dissolved.
But hot particles are rather resistant to action of acids.
Complete dissolution of hot particles was observed
only after roasting of the samples at 650 0Ñ withparticles and their independence (Fig. 3). It is
explained by authors by the distinctions of formation of
uranium-containing particles: recrystallization of
uranium compounds following the “release” offragment radionuclides took place under conditions ofeffects of high temperatures and interaction between
construction materials and air oxygen.
Isotopic composition of uranium in sequential
water leachate-solutions from graphite particles was
uniform varying in ranges of 1.07-1.11% enrichment,
which was rather close to calculated values for the bulk
mass of the fuel. Only insignificant part of it, i.e. themost resistant to dissolution, corresponded to 2%enrichment.
Authors of the work [3] distinguish two groups ofuranium-containing phases in graphite particles by
their resistance to action of leaching agents:- depleted fuel with 235U content about 1%; itconstitutes the main mass (∼99%) of uranium that iscontained in the particles and dissolves readily in thewater and nitric acid;- uranium-containing phase with 238U/235U isotopes
ratio corresponded to that of the “fresh” 2% reactor
fuel; its content is quite insignificant, and its
dissolution requires high temperature and pressure.
The both phases contain 236U, formed in result of
nuclear processes.

6. Hypothesis for fractionation of uranium
nuclides in its leaching from fuel-containing
soils.
Heterogeneous composition of uranium-containing
compounds should be taken into account in an attempt
to explain the results of determination of uranium
isotopic composition in the soils contaminated bysolid-phase radioactive fallouts. The contaminatedsoils may be expected to contain the next100

L each in g , %90
80
S r-9070

Ñ s-137
60

U

50
40
30
0

10

20

30

40

50

60

70

t, h

Fig. 3. Kinetics of leaching of 137Cs and 90Sr radionuclides, and fuel uranium from the graphite hotparticles [3].
101

independent experiments on determination of isotopic
compositions of the valent forms of uranium [2], whichshowed enrichment of U(VI) form by 235U isotope.
Different mobility of 235U and 238U isotopes causedby their unequivalent energetic position in the solid
phase should be assumed to explain satisfactorily thefractionation of uranium isotopes: nuclides with thelower energy of bond in the solid phase passpredominantly into liquid phase.
It is still unclear which of the uranium-containing
fractions in soil has unequivalent energetic bonds of235
U and 238U isotopes, and how that difference could
be generated.
Since fractionation of uranium isotopes in leachingfrom natural minerals usually does not take place, so
observed isotope separation in the experiments is due
to presence of technogeneous uranium of dispersed
fuel (solid-phase fuel-containing fallouts, formed inresult of the IV unit accident). Comparison of theresults of leaching of uranium from graphite particles(Table 7) and that from the soils (Tables 3 and 5) shows
the difference in the isotope enrichment of uranium.
Fuel-containing fragments of graphite particles formed
through condensation of evaporated fuel did notexhibited practically the isotope fractionation inleaching. Rebuilding of the structure took place inrecrystallization of uranium. Defects of the lattice thatwas characterized by the difference in the mobilities ofuranium isotopes were eliminated, even if they existed
in the initial substrate. Thus, only the particles formedby mechanical fragmentation of the fuel can containuranium with different mobility of isotopes.
Two kinds of the particles, therefore, can be
distinguished by the uranium isotopes mobilities in thedispersed fuel formed as the result of the Chernobyl
accident. They are the particles formed by mechanicalfragmentation of tablets and the particles formed inrecrystallization of the fuel.
It enables to assume the presence of at least two
beings of uranium in irradiated dispersed fuel:
1) Uranium of irradiated fuel in products offragmentation of the fuel - particle of indefinite

subsequent treatment with nitric acid at heating or with
mixture of chlorate and hydrofluoric acids [2]. In thecited work [1] completion of uranium extraction into
liquid phase was not qualified, so statement of uranium
isotopic composition in the solid phase (in soil)presented in Table 2 was not quite correct. However,this circumstance does not influence upon basic resultsof experiments on leaching of uranium fromcontaminated soils that testify the possibility offractionation of uranium isotopes 235U and 238U
between solid and liquid phases.
Factors of separation may be used for numericalrepresentation of fractionation effects. If 235U and 238U
contents in the mixture of uranium isotopes are
expressed in mole parts, the relative isotope contents inthe solid and liquid phases would be presented asfollows [16]:R= x /(1 − x) ,where: x is the mole part of 235U isotope, and (1 −x) is the mole part of 238U isotope.
Coefficient of the single separation is given by theequation:

α = R*/ R ,where the index * is attributed to the liquid phase.
Factors of separation calculated on the data of Table
3 are presented in the Table 8. Presence of any otheruranium isotopes which contents did not exceed 0.3%
by mass, was not taken into account in calculation of
the mole parts of 235U and 238U isotopes.
Factors of separation of uranium isotopes inleaching of uranium from different soils by the acids
are in the range 1.3 - 1.6; in leaching by the same but
oxidated solutions the factors vary in the range 2.5 - 5.
Result of uranium leaching from the soil sampled on
the bank of Paryshev channel (Table 8, No. 6) was the
exception. Fractionation of isotopes in oxidation with
7M HCl in this case exceeded by an order of magnitude
the effects of the rest experiments. Authenticity of themore high separation of isotopes in leaching byoxidated solutions was confirmed by the results ofTable 8. Factors of separation of uranium isotopes between solid phase and solution in its leachingfrom the soil.
No

Sampling site

Leaching
agent

R 235Uin soil

1
2
3
4
5
6
7

Cooling pond
st. Yanovv. Yanovv. Lelivv. Leliv
Paryshev channel
Paryshev channel

8 Ì HNO3
2 Ì HNO3
6 Ì HNO3
7 Ì HCl2 Ì HNO3
7 Ì HCl
8 Ì HNO3

0.0223
0.019
0.016
0.013
0.013
0.01060.0105

102

Solution
without oxidation
R*1
α1
0.035
1.58
0.02461.29
0.0206
1.29
0.021
1.61
0.019
1.51
0.064
6.0
0.016
1.55

after oxidation
R*2
α2
0.085
3.82
0.082
4.32
0.049
3.09
0.042
3.27
0.032
2.48
0.36
34.2
0.053
5.04

of Chernobyl NPP. - Kiev, Naukova dumka, 1992. 144 p. ( in Russian).
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E. V. ,
OlkhovickYu.À.,
Koromyslichenko T. I. et. al.
Plumbago hot
particles from Sarcophagus of Chernobyl NPP //Dokl. AN Ukraine. - 1993. - ¹ 11. - p. 170-174. ( inRussian).
4. Chernobyl Disaster. -Ed. by V.Barjakhtar. - Kiev.
-Naukova dumka.-1996.-575 p. (in Ukrainian).
5. Gerasimov A.S., Zarizchkaj T.S., Rudik A.P.
Reference book of formation of the nuclides in the
nuclear reactors. Energoatomizdat, M. , 1989. — 575
p. ( in Russian).
6. Kuzmina I.Å., Toêàrevsêij V.V. Destruction of
nuclear fuel after disaster in Reactor No 4 ofChernobyl NPP // Obninsk, 1996. - FEI-231.
7. Begichev S.N., Borovoj A.A., Burlakov V.V. et al
Fuel of reactor of 4 unit of Chernobyl NPP. -Ì.,1990ã., -21p.- -(preprint Atomic energy Institute;5268/3). ( in Russian).
8. Belyaev S.T., Borovoy A.A., Demin V.N. et al: The
Chernobyl Source Term. Radiation protection-53.
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Released during Three Major Nuclear Accidents,
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Luxembourg, 1-5 October 1990, v. 1, Report EUR
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Energy “The Accident at the Chernobyl NPP and its
Consequences” IAEA Post Accident Review
Meeting, Vienna, 25-29 August 1986.
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Chernobyl, Ukraine, March, 1994. OECO
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habitus. It may be partially oxidated. Uranium
isotopic composition is close to 1-2% enrichment.
2) Uranium of irradiated fuel in form of spherical or
fiber-shaped particles that undergone therecrystallization in result of evaporation andcondensation. Recrystallization fuel uranium as a partof graphite particle may be in oxide form or probably
metallic uranium. Isotopic composition should beclose to 1% enrichment, and insignificant part of thisuranium has isotopic composition close to 2%enrichment.
Fractionation of uranium isotopes between liquidand solid phases is not observed in exposure of thesecond type fuel particles to water solutions. When thefuel particles of the first type are exposed to water
solutions, uranium in liquid phase may be enriched by235
U isotope relative to the uranium of the solid phase
exposed to leaching.
What is the reason of different mobility of 235U and238
U isotopes in dispersed fuel? Two hypotheses areworth an attention.
The first one supposes heterogeneous compositionof nuclear fuel of RBMK reactor consisting of fractions
with different enrichment by 235U, uranium isotopes ofthe more enriched fraction having higher mobility due
to features of preparation technology.
Another hypothesis is based on the assumption that
selective loosening of bonds of separate 235U atoms
takes place in the solid body under action of slow
neutrons, to which 235U nuclei have relatively large
values of capture cross sections. Energy of the nuclei inexcited state is probably sufficient to loosening of thechemical bonds of 235U in the solid phase for a longtime. In this case the probability of the process of
excitation of 235U nuclei due to reversible absorption ofneutrons without fission is yet uncertain. It is also not
clear whether the excitation of 235U nuclei takes place
in a standard mode of operation of RBMK reactor, or it
is somehow connected with the emergency state of the
IV unit of the Chernobyl NPP.
Acknowledgments: the authors are grateful toprof. V.Barjakhtar for helpful discussion of the natureof fractionation of the uranium isotopes, toYu.Olkhovick and Yu.Slupizcky for giving some
unpublished data and to L.Kononenko for help inediting the paper. Authors also thank Dr.T Imanaka
and O.Nasvit for their attention to this work.

BIBLIOGRAPHY
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composition of uranium in soils of near zone ofChernobyl NPP // Docl. AN SSSR. - 1990. - ¹ 4. - p.
885-888 ( in Russian).
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