Evolution of Chernobyl Corium in Water: Formation of Secondary Uranyl Phases

Two crystalline phases, which are analogues of common secondary uranyl minerals, namely, becquerelite (Ca[(UO2)6O4 (OH)6]·8H2O) and phurcalite (Ca2[(UO2)3O2 (PO4)2]·7H2O) were identified on the surface of a Chernobyl corium-containing sample affected by hydrothermal alteration in distilled water at 150 °C for one year. Phases were characterized using Single-Crystal X-ray Diffraction Analysis (SCXRD) as well as optical and scanning electron microscopy. Features of the structural architecture of novel phases, which come from the specific chemical composition of the initial fragment of Chernobyl sample, are reported and discussed. Precise identification of these phases is important for modelling of severe nuclear accidents and their long-term consequences, including expected corium–water interaction processes at three damaged Units of the Nuclear Power Plant Fukushima Daiichi.


Introduction
A severe nuclear accident at the 4-th Unit of the Chernobyl Nuclear Power Plant (ChNPP) on 26 April 1986 was characterized with high-temperature interactions between U-oxide nuclear fuel, zircaloy cladding, and construction materials such as steel, serpentine and concrete [1]. Products of corium formation and solidification in the form of solid solutions "UO 2 -ZrO 2 " with different U/Zr ratio were identified in the matrices of socalled Chernobyl "lava" and "hot" particles [2,3]. In addition, corium products were discovered recently in the matrix of an unusual material which consisted of mainly molten and oxidized steel [4]. Such a material was formed during an initial very high-temperature (at least 2400-2600 • C) stage of the accident and it was injected into room 305/2 (right below the reactor core) where it rapidly solidified without interaction with silicate construction material (serpentine and concrete). According to a very cautious estimate, room 305/2 contains about 60 tons of the fuel [5].
It was found (for the first time in 1990) that matrices of Chernobyl "lava" interact with the environment. This process is accompanied with the formation of uranyl-phases such as UO 4 ·4H 2 O; UO 3 ·2H 2 O; UO 2 ·CO 3 ; Na 4 (UO 2 )(CO 3 ) 3 , etc. [6,7]. Moreover, the formation of uranyl phases, as assumed, could happen on the surface of some "lava" samples stored under laboratory conditions without humidity control [3,8].
The experimental study of the chemical alteration of Chernobyl corium and "lava" is very important in order to model behavior of these highly radioactive materials over long periods of time [9][10][11]. The information obtained can be applied to predict properties periods of time [9][10][11]. The information obtained can be applied to predict properties of molten fuel materials contacting water since 2011 at Units#1, 2 and 3 of the Fukushima Daiichi Nuclear Power Plant (F-1 NPP).
Herein, we report the results of precise phase identifications of two uranyl compounds, which were formed on the surface of the Chernobyl sample collected in room 305/2 of the Chernobyl "Shelter" [4] and used in previous experiments on hydrochemical alteration [10]. New-formed phases were characterized using several experimental techniques including Single-Crystal X-ray Diffraction Analysis (SCXRD) as well as optical and scanning electron microscopy. Features of the structural architecture of novel phases, which come from the specific chemical composition of the initial fragment of the Chernobyl sample, are reported and discussed.

Chernobyl Corium-Containing Sample
The Chernobyl corium-containing sample ( Figure 1) consisted of mainly Fe3O4 and inclusions of solid solutions "UO2-ZrO2" (i.e., corium solidification products) with a broad range of U/Zr ratio and was used for chemical alteration experiment in distilled water at 150 °C for one year. Details about chemical and phase composition of this sample have been reported before [4]. The main interest to study this particular type of Chernobyl highly radioactive sample is related to evaluating of physico-chemical durability of corium-steel interaction products over a long time in water under increased temperature. It is assumed that similar materials can be discovered in the near future at Units #1, 2 and 3 of F-1 NPP. Figure 1. One of the highly radioactive samples consisted of molten and oxidized steel and corium. It was collected by V.A. Zirlin and L.D. Nikolaeva in room 305/2 (right below former reactor core) of the Chernobyl "Shelter" in 1990: general view (a); and small broken fragments prepared for alteration test (b).

Hydrothermal Alteration Experiment
The 0.15-g fragment of the Chernobyl corium-containing sample and 10 ml of distilled water were placed in a steel autoclave equipped with a 25-ml Teflon liner. The experiment was carried out at a temperature of 150 °C and lasted about a year.
As the result of this hydrothermal experiment, a highly altered sample of corium-containing material was obtained, the surface of which was covered with yellowish crystals of various sizes and shapes ( Figure 2). According to the visual observation of secondary phases using an optical microscope under polarized and cross polarized light, Figure 1. One of the highly radioactive samples consisted of molten and oxidized steel and corium. It was collected by V.A. Zirlin and L.D. Nikolaeva in room 305/2 (right below former reactor core) of the Chernobyl "Shelter" in 1990: general view (a); and small broken fragments prepared for alteration test (b).

Hydrothermal Alteration Experiment
The 0.15-g fragment of the Chernobyl corium-containing sample and 10 mL of distilled water were placed in a steel autoclave equipped with a 25-mL Teflon liner. The experiment was carried out at a temperature of 150 • C and lasted about a year.
As the result of this hydrothermal experiment, a highly altered sample of coriumcontaining material was obtained, the surface of which was covered with yellowish crystals of various sizes and shapes ( Figure 2). According to the visual observation of secondary phases using an optical microscope under polarized and cross polarized light, three types of morphologies were found: prismatic, lamellar and flattened needle-like crystals ( Figure 3). Pictures of the secondary phases were collected using a digital microscope, Keyence VHX-1000.

Chemical Composition
The chemical analyses were carried out with a Hitachi FlexSEM 1000 scanning electron microscope equipped with EDS Xplore Contact 30 detector and Oxford AZtecLive STD system of analysis. Analytical conditions were: accelerating voltage 20 kV and beam current 5 nA. Only Ca, Mn, P, Si, U and O were recorded in Phu; Ca, U and O-in Bqr.
Contents of other elements with atomic numbers higher than that of beryllium were below the detection limits. The following standards and X-ray lines were used: Ca-CaF 2 , K α ; Mn-Mn 2 SiO 4 , K α ; Si-SiO 2 , K α ; P-NdP 5

Single-Crystal X-ray Diffraction Studies
Single crystals of Bqr_1, Bqr_2 and Phu were selected under an optical microscope in polarized light, coated in oil-based cryoprotectant and mounted on a cryoloops. The diffraction data were collected using a Rigaku XtaLAB Synergy S X-ray diffractometer operated with a monochromated microfocus MoKα tube PhotonJet-S (λ = 0.71073 Å) at 50 kV and 1.0 mA and equipped with a CCD HyPix 6000HE hybrid photon-counting detector [15]. The frame width was 1.0 o in ω, and exposures ranged from 12 to 110 s for each frame. CrysAlisPro software [16] was used for the integration and correction of diffraction data for polarization, background and Lorentz effects, as well as for absorption correction. An empirical absorption correction based on spherical harmonics implemented in the SCALE3 ABSPACK algorithm was applied. The unit-cell parameters (Table 1) were refined using the least-squares techniques. The structures were solved by a dualspace algorithm and refined using SHELX programs [17,18], incorporated in the OLEX2 program package [19]. The final model included coordinates and anisotropic displacement parameters for all non-H atoms. H atoms were localized from different Fourier maps and were included in the refinement with bond lengths and isotropic displacement parameters restraints. The crystal structures of Bqr_1 and Bqr_2 were refined as two-component inversion twins with statistically equal contribution of components (0.54 (3) where n is the number of reflections and p is the number of refinement parameters.

Results
The mineral becquerelite was discovered a century ago [20], and its chemical composition and lattice parameters were then additionally reported [21,22]. The crystal structure of becquerelite was first reported by Piret-Meunier and Piret [12]. Later, the structural model of becquerelite was refined to better values of convergence factors [23,24] and spectroscopic studies have been performed [25][26][27]. Our SCXRD investigations confirm known structural models, and atom arrangements; naming from the latest model reported by Burns and Li [24] was taken as a starting set in the current work. It should be noted that all previous studies described a becquerelite unit cell in a non-conventional Pn2 1 a setting (Table 2). Structural models of Bqr_1 and Bqr_2 are reported in a standard setting, which corresponds to the mm2 point group.   Coordination polyhedra of U atoms share equatorial edges and vertices to form layers of [(UO 2 ) 6 O 4 (OH) 6 ] 2composition that are arranged parallel to (010) (Figure 4a). The layer of uranyl pentagonal bipyramids can be described in terms of anion-topology as formed by triangles and pentagons [34] with a . . . PDPD . . . stacking sequence of polygonal chains [35][36][37] and 5 4 3 1 cyclic symbol [38,39] (Figure 4b). All pentagons are occupied by Ur, while all triangles are empty. This type of polygon arrangement is attributed to the so-called protasite or α-U 3 O 8 anion-topology, which was also found in the structures of a number of minerals and synthetic compounds like protasite [23], billietite [23], compreignacite [40], masuyite [41], agrinierite [42], α-U 3 O 8 [43], Na 2 [(UO 2 ) 3 O 3 (OH) 2 ] [44], etc. In between the U-bearing layers, one crystallographically non-equivalent Ca 2+ cation and eight H 2 O molecules are arranged (Figure 4c). Ca-centered polyhedra are organized in 1D units that are stretched along the [001]. Four out of eight H 2 O molecules are arranged in the coordination sphere of Ca 2+ cations, and four molecules fill the gap between the chains of Ca-polyhedra and link with U-layers and Ca-chains only through the system of H-bonds (Figure 4d; Table 5). It should be noted that the system of H-bonds in the structure of Bqr, which was revealed after the assignment of H atoms sites, in general, corresponds to that proposed by Burns and Li [24]. However, several discrepancies can be found; for instance, OW24· · · O8 instead of OW24· · · OW27, or OW30· · · O10 instead of OW30· · · OW24 in Bqr and [24], respectively.     The mineral phurcalite was discovered by Deliens and Piret [13], who have reported on its orthorhombic symmetry, chemical composition and its lattice parameters. The structural model of phurcalite was reported the same year [14]. Later, the structure of phurcalite was refined several times for different specimens from various localities ( Table 2) [28][29][30]. The most recent study reports on the H-bonding system, which was determined by a combination of SCXRD and modern computational methods [31]. The structural model of phurcalite reported in [31] was taken as a starting set of atoms in the current work.
The crystal structure of Phu ( Figure 5) contains three crystallographically independent U 6+ cations. The U-O Ur bond lengths range from 1.798 (3) to 1.822 (3) Å (Table 6) There are two crystallographically non-equivalent P 5+ cations in the structure of Phu, tetrahedrally coordinated by four O atoms each with <P-O> = 1.535 and 1.546 Å for P1 and P2, respectively. It is of interest that P-centered tetrahedra has slightly different coordination environment (Figure 6a). [P1O4] 3oxyanion shares an equatorial O2···O6 edge with Ur3 hexagonal bipyramid, an equatorial O11 vertex with Ur3 cation, and a bridged O13 atom, which is a part of a common O13···H 2 O20 edge between Ca1 and Ca2 polyhedra. The [P2O4] 3oxyanion also shares an equatorial O8···O15 edge with Ur3 hexagonal bipyramid, O18 atom with Ca1 coordination polyhedron, and O9 atom, which is a part of O5···O9 edge common between Ca2 and U2 coordination polyhedra. Slight deficiency of bond valence sums (BVS) for the P2 site, along with a slight elongation of the <P2-O> bond length (compared to that for P1; Table 6), and the results of chemical analysis, all indicate the presence of less than 0.1 Si atoms per formula unit (p.f.u.) in the structure of Phu; this allows considering P2 site as (P 0.91 Si 0.09 ). Such a distribution most likely comes from the fact that the P1 site is more tightly bonded than the P2 site, which prevents a larger Si cation from occupying it. Similar crystal chemical restrictions for the larger Se 6+ cations incorporation in tighter S 6+ sites were observed in a course of phase formation studies in the mixed actinyl sulfate-selenate aqueous systems [45][46][47][48][49][50].           (Figure 5a), which are arranged parallel to (010). Anion-topology of the layer corresponds to the phosphuranylite type with 6 1 5 2 4 2 3 2 cyclic symbol [38,39], and can be described as formed by triangles, squares, pentagons and hexagons [34], where all hexagons and pentagons are occupied by Ur, all triangles are occupied by phosphate oxyanions (Figure 5b), and all squares stay vacant. This is one of the most common topological types of U-bearing 2D units. About 50 compounds of both natural and synthetic origin and various chemical compositions are known nowadays (e.g. [34,[51][52][53][54][55][56][57]). Layers are formed by the large number of chains of dimers of edge-shared uranyl pentagonal bipyramids that are further connected by edge-shared U-centered hexagonal bipyramids. Neighbor chains are shifted by the half period as they lengthen, so that hexagonal bipyramids are arranged in front of dimers of pentagonal bipyramids. In these places, the chains are linked into a layer through the phosphate tetrahedra, which share an edge with hexagonal bipyramid from one chain, and a vertex with pentagonal bipyramid from a neighbor chain.
There are two non-equivalent Ca 2+ sites, one Mn 2+ site and six H 2 O molecules arranged in between the uranyl phosphate layers (Figure 5c).

Discussion
Analogues of becquerelite discussed within this paper do not significantly differ in chemical composition and crystal structure from the previously studied natural samples. However, we report the crystal structure of becquerelite in the standard Pna2 1 setting for the first time, along with all H atom site assignments, which allow us to demonstrate the branchy H-bonding system. Investigation of phurcalite analogs have demonstrated differences in the structural architecture of known natural and obtained synthetic phases. Thus, the new octahedral site between the uranyl phosphate layers occupied by Mn atoms was found. It can be assumed that incorporation of a cation into the Mn3 site and the formation of pentamers result in a stronger linkage of uranyl phosphate layers into 3D structure. Compensation of an additional positive charge that comes with the incorporation of Mn 2+ cations occurs due to the heterovalent isomorphism of Si 4+ cations in the P 5+ sites. Additional compensation, if needed, may come from the replacement of H 2 O16 and H 2 O19 molecules, which form an equatorial plane of Mn-centered octahedron and are included in the coordination sphere of Ca 2 cations, by O 2anions or OHgroups. Thus, the formula of the studied Phu crystal according to the SCXRD and SEM data could be given as Ca 2  The Chernobyl corium-containing sample used in this research is a product of high temperature co-melting of U-oxide fuel, zircaloy cladding, steel, serpentine and concrete [4]. As a result, it has a unique and complex chemical and phase compositions. It can explain the composition of uranyl phases formed during the alteration experiment. Uranium comes from the relicts of overheated nuclear fuel (UO x ) and corium inclusions (U-Zr-O with high U/Zr ratio), which is easy to oxidize to the 6+ state in aqueous solutions. Calcium and Si come from the concrete. Phosphorus and Mn, most likely, come from construction steel of 10HSND grade (10XCHД in Russian), used in the low basic reactor plate "OR" ("OP" in Russian). This steel contains 0.5-0.8 wt.% Mn and up to 0.035 wt.% P [58].
During optical microscopy studies of the alteration products, several intergrowth of lamina and needle crystals were found (Figure 7a,b). SCXRD experiments showed that these are intergrowths of Bqr and Phu, which can be described as follows: rotation of Phu unit cell relative to the Bqr by 142.83 • around the c.a. [−0.25 0 1] axis, which corresponds to the approximate coincidence of the [001] direction in the structure of Bqr with the [−1 −11] direction in the structure of Phu (Figure 7b,c). In these directions both structures have similar arrangement of Ca polyhedra and U bipyramids neighbor to them. Hence, one can assume that intergrowing relates exactly to these structural fragments. To our knowledge, this is the first reported evidence of becquerelite and phurcalite intergrowth.  (Figure 7b,c). In these directions both structures have similar arrangement of Ca polyhedra and U bipyramids neighbor to them. Hence, one can assume that intergrowing relates exactly to these structural fragments. To our knowledge, this is the first reported evidence of becquerelite and phurcalite intergrowth.

Conclusions
Two analogues of common secondary uranyl minerals, becquerelite and phurcalite, formed on the surface of a Chernobyl corium-containing sample affected by hydrothermal alteration were identified and studied in detail. The results obtained are proposed to be included into a database for modelling of long-term behavior of corium-steel interaction products forming as a consequence of severe nuclear accidents.
The fact that, during hydrothermal experiment, only crystals with dense polymerization of uranyl polyhedra (i.e., that share common edges) were obtained, confirms our recently made assumption [56,57,59,60] that such structures should not crystalize at ambient temperature and an additional energy source is needed to obtain phases with dense architecture, while uranyl minerals and compounds with sparse structural units (i.e., that share only common vertices) can crystallize from aqueous solutions at ambient conditions.
The results of reported studies are important not only for predicting corium aging in anticipation of decommissioning, but also for evaluating the stability of corium, spent

Conclusions
Two analogues of common secondary uranyl minerals, becquerelite and phurcalite, formed on the surface of a Chernobyl corium-containing sample affected by hydrothermal alteration were identified and studied in detail. The results obtained are proposed to be included into a database for modelling of long-term behavior of corium-steel interaction products forming as a consequence of severe nuclear accidents.
The fact that, during hydrothermal experiment, only crystals with dense polymerization of uranyl polyhedra (i.e., that share common edges) were obtained, confirms our recently made assumption [56,57,59,60] that such structures should not crystalize at ambi-ent temperature and an additional energy source is needed to obtain phases with dense architecture, while uranyl minerals and compounds with sparse structural units (i.e., that share only common vertices) can crystallize from aqueous solutions at ambient conditions.
The results of reported studies are important not only for predicting corium aging in anticipation of decommissioning, but also for evaluating the stability of corium, spent fuel, and cemented U-bearing wastes under temporary storage and final repository conditions [61][62][63].
The chemical stability of the corium should be modelled taking into account potential formation of secondary uranyl phases and their further chemical and physical alteration. Short-term leach tests do not provide enough time for the growth of secondary mineral-like phases. Therefore, such an important process is usually not taken into account in the models [64][65][66][67][68][69], although uranyl phases are obviously less stable than U oxide.
It is known from the model experiments that analogues of becquerelite are formed during the aging of spent fuel [70]. Thus, one can assume that the initial chemical forms of uranium are less important in most cases for the formation of these phases than particular oxidizing conditions and properties of the environment [71][72][73][74][75][76][77]. Corium, which possibly formed at F-1 NPP may differ chemically from Chernobyl corium [4,10,78], but the products of its alteration in water would be similar.