1. 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 so-called 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
2O; UO
3·2H
2O; 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 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.
3. 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
1a setting (
Table 2). Structural models of
Bqr_1 and
Bqr_2 are reported in a standard setting, which corresponds to the
mm2 point group.
The crystal structure of
Bqr contains of six crystallographically independent U
6+ cations. Each U
6+ cation is strongly bonded to two O
2- atoms, forming almost linearly within 7° O
2-≡U
6+≡O
2- uranyl cations (
Ur) with U–O
Ur bond lengths ranging from 1.724 (16) to 1.854 (19) Å (
Table 3 and
Table 4). All six
Ur ions are equatorially coordinated by five O atoms, which results in the formation of pentagonal bipyramids (U–O
eq = 2.16 (2)–2.78 (3) Å). Besides, three out of five equatorial bonds are accounted for by O atoms of the hydroxyl groups. There is also one crystallographically unique Ca
2+ cation in the structure of
Bqr, which is coordinated by four O
Ur atoms and another four O atoms of H
2O molecules with Ca–O = 2.36 (2)–3.049 (18) Å to form square antiprism coordination polyhedron.
Coordination polyhedra of U atoms share equatorial edges and vertices to form layers of [(UO
2)
6O
4 (OH)
6]
2– composition 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
43
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
3O
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
3O
8 [
43], Na
2[(UO
2)
3O
3 (OH)
2] [
44], etc. In between the U-bearing layers, one crystallographically non-equivalent Ca
2+ cation and eight H
2O molecules are arranged (
Figure 4c). Ca-centered polyhedra are organized in 1D units that are stretched along the [001]. Four out of eight H
2O 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).
Ur1 and
Ur2 ions are equatorially coordinated by five O atoms, which results in the formation of pentagonal bipyramids (U–O
eq = 2.252 (3)–2.512 (3) Å). The
Ur3 ion is equatorially coordinated by six O atoms to form hexagonal bipyramid (U–O
eq = 2.221 (3)–2.790 (3) Å). 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]
3– oxyanion 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
2O20 edge between Ca1 and Ca2 polyhedra. The [P2O4]
3– oxyanion 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
0.91Si
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].
The crystal structure of
Phu is based on the uranyl phosphate layers of [(UO
2)
3O
2 (PO
4) (P
0.91Si
0.09O
4)] compositions (
Figure 5a), which are arranged parallel to (010). Anion-topology of the layer corresponds to the phosphuranylite type with 6
15
24
23
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
2O molecules arranged in between the uranyl phosphate layers (
Figure 5c). Ca1 site is surrounded by three H
2O molecules and two O
Ur atoms, and two O atoms are shared with two distinct phosphate groups with <Ca1–O> = 2.424 Å. Ca2 site is surrounded by four H
2O molecules, two O
Ur atoms, and two O atoms are shared with two distinct phosphate groups with <Ca1–O> = 2.498 Å. Ca1 and Ca2 coordination polyhedra share common O13···H
2O20 edge to form dimer. The Mn3 site occupies an inversion center, which is arranged between two neighbor dimers of Ca-centered polyhedra. This site represents a rather classical octahedron surrounded by four H
2O molecules (Mn3–H
2O = 2.207 (3)–2.266 (4) Å) in the equatorial plane and another two apical O
Ur atoms with slightly elongated bonds (Mn3–O
Ur12 = 2.387 (3) Å), which can fit any of divalent cations. In the case of
Phu crystal, an electron density peak of
c.a. 1.1
e/Å
3 was arranged in this site. Chemical analyses showed the presence of Mn in the examined samples, the amount of which corresponds to the site occupancy revealed in a course of SCXRD studies. Moreover, the presence of a cation at the Mn3 site results in a formation of the Ca1-Ca2-Mn3-Ca2-Ca1 pentamers via sharing common edges between Ca- and Mn-centered coordination polyhedra (
Figure 6b). Pentamers are stretched along c.a. [102] and [-102] and separated by an additional H
2O23 molecule, which links uranyl phosphate layers and pentamers only through H-bonds.
4. 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 Pna21 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 Mn2+ cations occurs due to the heterovalent isomorphism of Si4+ cations in the P5+ sites. Additional compensation, if needed, may come from the replacement of H2O16 and H2O19 molecules, which form an equatorial plane of Mn-centered octahedron and are included in the coordination sphere of Ca2 cations, by O2– anions or OH– groups. Thus, the formula of the studied Phu crystal according to the SCXRD and SEM data could be given as Ca2Mn0.03[(UO2)3O2 (PO4) (P0.94Si0.06O4)]·7H2O. It is of interest that, in previous studies of natural phurcalite crystals, no additional cation sites except for Ca1 and Ca2 have been found within the interlayer space. This example shows that a possible re-investigation of phurcalite mineral samples is needed to check if any additional cations that may occupy the Mn3 octahedral site.
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.
5. 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 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.