sustainability

Article Long-Term Aging of Chernobyl Fuel Debris: Corium and “Lava”

Bella Zubekhina 1,2,*, Boris Burakov 1,3 , Ekaterina Silanteva 1,3, Yuri Petrov 1, Vasiliy Yapaskurt 4 and Dmitry Danilovich 5

1 V.G. Khlopin Radium Institute, 28, 2-nd Murinskiy Ave., 194021 St. Petersburg, Russia; [email protected] (B.B.); [email protected] (E.S.); [email protected] (Y.P.) 2 Chemical Department, Lomonosov Moscow State University, 1, Leninskie Gory, 119991 Moscow, Russia 3 Ioffe Institute, 26, Politekhnicheskaya, 194021 St. Petersburg, Russia 4 Geological Department, Lomonosov Moscow State University, 1, Leninskie Gory, 119991 Moscow, Russia; [email protected] 5 Faculty of Chemistry of Substances and Materials, Saint-Petersburg State Institute of Technology (Technical University), 26, Moskovskii Ave., 190013 St. Petersburg, Russia; [email protected] * Correspondence: [email protected]

Abstract: Samples of Chernobyl fuel debris, including massive corium and “lava” were collected inside the Chernobyl “Sarcophagus” or “Shelter” in 1990, transported to Leningrad (St. Petersburg) and stored under laboratory conditions for many years. In 2011 aged samples were visually re- examined and it was confirmed that most of them remained intact, although some evidence of self-destruction and chemical alteration were clearly observed. Selected samples of corium and “lava” were affected by static leaching at temperatures of 25, 90 and 150 ◦C in distilled water. A 2 normalized Pu mass loss (NLPu) from corium samples after 140 days was noted to be 0.5 g/m at ◦ 2 ◦ 2 ◦


25 C and 1.1 g/m at 90 C. For “lava” samples NLPu was 2.2–2.3 g/m at 90 C for 140 days. The
 formation of secondary uranyl phases on the surface of corium and “lava” samples altered at 150 ◦C Citation: Zubekhina, B.; Burakov, B.; was confirmed. The results obtained are considered as an important basis for the simulation of fuel Silanteva, E.; Petrov, Y.; Yapaskurt, V.; debris aging at Fukushima Daiichi nuclear power plant (NPP). Danilovich, D. Long-Term Aging of Chernobyl Fuel Debris: Corium and Keywords: leaching; chemical alteration; Chernobyl “lava”, corium; fuel debris; Fukushima Daiichi “Lava”. Sustainability 2021, 13, 1073. NPP; plutonium https://doi.org/10.3390/su13031073

Academic Editor: Jorge García Ivars Received: 9 December 2020 1. Introduction Accepted: 18 January 2021 Published: 21 January 2021 The severe nuclear accident in the 4th Unit of the Chernobyl nuclear power plant (NPP)—presently in the territory of Ukraine—occurred on 26 April 1986 [1,2]. During

Publisher’s Note: MDPI stays neutral a planned test, unit 4 of the RBMK (High-power channel-type reactor) reactor was un- with regard to jurisdictional claims in intentionally drawn in a potentially unstable (with positive reactivity) condition, which published maps and institutional affil- was followed by a combination of unexpected events that caused an uncontrolled nuclear iations. chain reaction to occur within its extended core, resulting in a large amount of energy suddenly being released. In some local parts of the reactor core, very high temperatures were reached as a result of prompt supercriticality. This vaporized the superheated cooling water and ruptured the reactor core in a highly destructive steam explosion, followed by an open-air reactor core fire that released a considerable amount of airborne radioactive Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. contamination [3,4]. Chernobyl corium was formed before the explosion as a result of ≥ ◦ This article is an open access article the short time high-temperature ( 2400–2600 C) interaction between the nuclear fuel distributed under the terms and and the zirconium cladding in the local part of the reactor core [5–7]. Basically, solidified conditions of the Creative Commons corium consists of solid solutions “ZrO2-UO2”, but it may also include molten steel or steel Attribution (CC BY) license (https:// oxidation products. creativecommons.org/licenses/by/ After the explosion, an interaction between fuel fragments, solidified corium and ◦ 4.0/). silicate materials (concrete, serpentinite, sand) at an initial temperature of 1500–1600 C

Sustainability 2021, 13, 1073. https://doi.org/10.3390/su13031073 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, 1073 2 of 9

occurred, causing the formation of a highly radioactive silicate melt—so-called Chernobyl “lava” or “lava-like fuel containing masses”—below the reactor shaft (mainly in room #305/2). Later, “lava” melt penetrated into different premises and solidified, forming brown and black colored streams. Only a small amount of massive corium survived in room #305/2, on the surface of steel pipes [6,7]. Samples of Chernobyl massive corium and “lava” were collected inside the “Shelter” in 1990 by scientists of the V.G. Khlopin Radium Institute (KRI) [5–7]. For many years these samples were stored at KRI under laboratory conditions at temperatures of around 20–25 ◦C in air without humidity control. In 2011, these samples were visually re-examined and it was confirmed that most samples remained intact. Only a couple of samples of black “lava” demonstrated mechanical self-destruction into small fragments. The formation of secondary uranyl phases were also observed in local areas of some corium and “lava” pieces [7]. Although no information has been reported so far about uranyl phases formed on the surface of Chernobyl “hot” particles, some evidence of higher leach rates of 90Sr have been published [8,9] for fuel particles in contaminated soils, comparing them to ones from the Chernobyl cooling pond, where U-oxide particles are less affected by oxygen. A study of Chernobyl sample aging is important not only for the optimal retrieval of highly radioactive materials located inside the Chernobyl “Shelter”, but also for the evaluation of fuel debris’ chemical behavior at Fukushima Daiichi NPP. Another important task is to conclude the role of self-irradiation on aging phenomena in corium and similar solids containing high amounts of radionuclides [10]. For this reason, experiments on the chemical alteration of Chernobyl corium and “lava” were recently carried out at KRI.

2. Materials and Methods The following Chernobyl samples were used for leaching and alteration experiments: (1) A total of 3 fragments of corium from room 305/2—for leaching experiments at 25 and 90 ◦C and for alteration experiments at 150 ◦C in distilled water. Chemically, Cher- nobyl massive corium is very inhomogeneous [11]. The plutonium (238,239,240Pu) con- tent in massive corium samples used for leaching tests varies from 1.4 to 5.6 MBq/g. (2) Two fragments of black “lava” and Two fragments of brown “lava”—for leaching at 90 ◦C and alteration at 150 ◦C in distilled water. Distribution of U and other alpha-emitters in matrices of Chernobyl “lava” (both black and brown) is relatively homogeneous [12]. Bulk content of plutonium (238,239,240Pu) in “lava” matrices is 1.5 and 3 MBq/g for black and brown “lava”, respectively [6,7,13,14]. Before the leaching experiments, all samples have been weighted and analyzed by non-destructive analytical methods (gamma-spectrometry, X-ray fluorescence analysis, optical microscopy). The weights of corium fragments used in this research varied from 0.15 to 0.18 g; the weights of “lava” fragments were 0.3 and 0.7 g for black and brown samples, respectively. Conditions of static leach tests, including the calculation of normalized mass loss, were the same as in our previous work [11]. Leaching experiments were performed at temperatures of 25 and 90 ◦C in distilled water. For accelerated chemical degradation, long- term alteration experiments were performed at a temperature of 150 ◦C in distilled water. For measurements of Pu in leachates, a Canberra 7401 alpha-spectrometer was used. For studying the corium phase composition various techniques were used includ- ing, XRD, Raman Spectroscopy and SEM-EDX methods. Two polished samples with carbon coating were prepared for SEM-EDX analysis (Figure1) using Tescan Vega 3 SBH equipped with an INCA X-Act EDS detector and a JEOL JSM-6480LV Scanning Elec- tron Microscope equipped with a W thermal emission cathode and an Oxford X-Maxn 50 detector (active area 50 mm2). The XPP correction was applied using Oxford Instru- ments INCA software. The accelerating voltage was 20 kV; electron beam current—10 nA, working distance—10 mm. Sustainability 2021, 13, x FOR PEER REVIEW 3 of 9

Microscope equipped with a W thermal emission cathode and an Oxford X-Maxn 50 de- tector (active area 50 mm2). The XPP correction was applied using Oxford Instruments INCA software. The accelerating voltage was 20 kV; electron beam current—10 nA, Sustainability working2021, 13, x FOR distance—10 PEER REVIEW mm. 3 of 9

Microscope equipped with a W thermal emission cathode and an Oxford X-Maxn 50 de- tector (active area 50 mm2). The XPP correction was applied using Oxford Instruments Sustainability 2021, 13, 1073 3 of 9 INCA software. The accelerating voltage was 20 kV; electron beam current—10 nA, working distance—10 mm.

Figure 1. SEM-BSE microphotographs of two polished samples of Chernobyl massive corium.

Summarizing the EDX analysis results, we conclude that the main phases of Cher- nobyl corium are:

- Matrix consists of an iron oxide phase doped with Al и Cr (from 1 to 13 wt. % of Al Figure 1. SEM-BSE microphotographs of two polished samples of Chernobyl massive corium. and from 1 toFigure 15 wt. 1. SEM-BSE % of Cr). microphotographs This phase ofwas two identified polished samples by Raman of Chernobyl spectroscopy massive corium. and XRD analysis asSummarizing SummarizingFe3O4 (PDF the the EDX#011111). EDX analysis analysis Different results, results, we concludecontents we conclude that of the Cr main thatin phasesthe the ironmain of Chernobyl oxide phases of Cher- phase affectscoriumnobyl the different are:corium are: levels of contrast in the BSE pictures. - Matrix consists of an iron2 oxide2 phase doped with Al иCr (from 1 to 13 wt. % of Al - Minor phases- of Matrixsolid solutions consists of “UO an iron-ZrO oxide” (Figure phase doped2) with with the Al following и Cr (from composi- 1 to 13 wt. % of Al and from 1 to 15 wt. % of Cr). This phase was identified by Raman spectroscopy and tions: and from 1 to 15 wt. % of Cr). This phase was identified by Raman spectroscopy and (1) Zr-U-O withXRD 50–60 analysis wt. % as of Fe 3ZrO4 and(PDF 13–20 #011111). wt. Different % of U; contents of Cr in the iron oxide phase affectsXRD theanalysis different as levelsFe3O of4 (PDF contrast #011111). in the BSE Different pictures. contents of Cr in the iron oxide (2) mainly U-oxide phase with admixtures of Zr (1–2 wt. %) and Fe (1–3 wt. %); - Minorphase phases affects of the solid different solutions levels “UO2-ZrO of contrast2” (Figure in2) the with BSE the followingpictures. compositions: (3) mainly Zr-oxide- (1)Minor phaseZr-U-O phases with with of admixtures solid 50–60 solutions wt. % ofof Zr U“UO and(1.5–32 13–20-ZrO wt.2” wt. (Figure%) % and of U; 2)Fe with (3–10 the wt. following %). composi- In addition, an certain(2)tions: mainlyamount U-oxide of the phase Si-Fe-bearing with admixtures phase of and Zr (1–2 metallic wt. %) inclusions and Fe (1–3 wt.with %);

Fe, Ni and Cu were found.(3)(1) Zr-U-O mainly with Zr-oxide 50–60 phase wt. with % of admixtures Zr and 13–20 of U wt. (1.5–3 % of wt. U; %) and Fe (3–10 wt. %). (2) mainly U-oxide phase with admixtures of Zr (1–2 wt. %) and Fe (1–3 wt. %); (3) mainly Zr-oxide phase with admixtures of U (1.5–3 wt. %) and Fe (3–10 wt. %). In addition, an certain amount of the Si-Fe-bearing phase and metallic inclusions with Fe, Ni and Cu were found.

FigureFigure 2. SEM-BSE 2. SEM-BSE microphotographs microphotographs of corium sample of corium with differentsample phaseswith different of (U,Zr)O phases2 solid solutions.of (U,Zr)O Content2 solid of U varies from 1.5 to 82 wt. %; Zr—from 1 to 67 wt. %. For “UO2-ZrO2” phase the U and Zr contents are comparable solutions. Content of U varies from 1.5 to 82 wt. %; Zr—from 1 to 67 wt. %. For “UO2-ZrO2” phase (50–60the wt. U % and of Zr Zr and contents 13–20 wt. are % of comparable U). (50–60 wt. % of Zr and 13–20 wt. % of U). Figure 2. SEM-BSE microphotographs of corium sample with different phases of (U,Zr)O2 solid solutions.In addition, Content an of certain U varies amount from of 1.5 the to Si-Fe-bearing82 wt. %; Zr—from phase and1 to metallic67 wt. %. inclusions For “UO2 with-ZrO2” phase The simplifiedFe,the Ni Uchemical andand CuZr contents were and found. radionucliare comparablede (50–60composition wt. % of Zrof and Chernobyl 13–20 wt. % corium of U). and “lava” are presented Thein Tables simplified 1 and chemical 2. and radionuclide composition of Chernobyl corium and “lava” are presented in Tables1 and2. The simplified chemical and radionuclide composition of Chernobyl corium and “lava” are presented in Tables 1 and 2.

Sustainability 2021, 13, 1073 4 of 9

Table 1. Bulk contents of the chemical elements (except oxygen) in matrices of Chernobyl “lava” and massive corium [7,13].

Element Content, wt. % Material U Zr Fe Na Mg Ca Si Al Black “lava” 4–5 2–6 0.3–6 2–10 1–5 3–13 19–36 3–8 Brown “lava” 7–10 5–6 1–2 4 4 5 31–33 4 Massive corium 1–40 0.2–20 40–95 No data

Table 2. Contents of main radionuclides in matrices of Chernobyl “lava” and massive corium [7,11].

Radionuclide Content, MBq/g (Recalculated for April 1986) Material 137Cs 154Eu 155Eu 244Cm 241Am 243Am 239,240Pu 238Pu Black “lava” 20–40 1–3 2 0.1 1 0.03 1 0.5 Brown “lava” 50–60 3–4 5 0.2–0.3 3 0.06 2 1 Massive corium 0.1–30 0.2–5 0.1–1 0.03–0.07 0.1–1 0.01–0.04 1–4 0.4–1.6

3. Results The normalized mass losses of Pu for Chernobyl black and brown “lava” are shown in Table3 and Figure3. Black and brown “lava” did not demonstrate essential differences ◦ 2 in Pu mass loss. The total NLPu (distilled water, 90 C, 140 days) was 2.2 and 2.3 g/m for black and brown “lava”, respectively. The normalized mass losses of Pu from corium matrix are shown in Table3 and Figure4.

Table 3. The normalized mass losses of Pu from different materials in distilled water.

Total Pu Normalized Mass Loss Sample Conditions of Leaching in Distilled Water, g/m2 25 ◦C for 7 days 0.3 [13] black “lava” 25 ◦C for 270 days 5.7 [13] 90 ◦C for 140 days 2.2 ± 0.2 25 ◦C for 8 days, 0.4 [15] 1 25 ◦C for 7 days 0.1 [13] brown “lava” 25 ◦C for 270 days 4.7 [13] 90 ◦C for 140 days 2.3 ± 0.2 25 ◦C for 140 days 0.5 ± 0.08 Corium 90 ◦C for 140 days 1.1 ± 0.12 ◦ −4 −3 PuO2 single crystals 90 C for 28 days from 2 × 10 to 9 × 10 [16] 239 ◦ −3 PuO2 ceramic pellet 25 C for 28 days 4.6 × 10 [16] LWR (Light-water reactor) spent 25 ◦C for about 150 days 3 × 10−3 [17] U-oxide fuel, (burnup 54.5 MWd/kgU) 1 performed in 1991.

Chemical alteration of Chernobyl “lava” has been observed inside the “Shelter” after several years after the accident. Some new-formed phases identified are UO3 × 2H2O (an analog of epiianthinite), UO4 × 4H2O (an analog of studtite), UO2CO3 (an analog of rutherfordine), Na4(UO2)(CO3)3 and sodium carbonates [7,18]. During the dry storage of Chernobyl samples (corium and “lava”) in laboratory conditions, chemical alteration and formation of secondary uranyl minerals (“yellow phases”) were clearly observed for most of them. Fluorescence of secondary phases in UV-light confirms that uranium in these phases partially oxidized from U (IV) to U (VI) (Figure5). Sustainability 2021, 13, x FOR PEER REVIEW 5 of 9

After 1 year of contact with distilled water at a temperature 150 °C, samples of “la- va” and corium were significantly altered. On the surface of Chernobyl corium, second- ary uranyl minerals of two forms were observed (Figure 6, right). One of them, with needle-shaped crystals, was not observed in alteration experiments at a lower tempera- ture (90 °C). For “lava”, sample chemical degradation was clearly observed (Figure 6, left), in contrast with the “fresh” sample (Figure 7b). Different alteration processes took place dependent on the phase: formation of “gel” on the surface of the sample, corrosion Sustainability 2021, 13, 1073 5 of 9 of metallic inclusions in the “lava” matrix and oxidation of uranium in U-bearing phases with the formation of secondary uranyl minerals (Figure 6, left).

Figure 3. The differential normalized mass loss of Pu for black and brown “lava” in distilled water at temperature 90 ◦C. Sustainability 2021, 13, x FOR PEER REVIEWFigure 3. The differential normalized mass loss of Pu for black and brown “lava” in distilled water6 of 9 NL(Pu) in logarithmicat temperature scale. 90 “Days” °C. NL(Pu) in figure in logarithmic does not refer scale. to duration “Days” in of figure leaching does experiment, not refer to but duration time from of the beginning of leachingleaching experiments. experiment, Expanded but uncertainty time from the for beginning all data is lessof leaching than 20% ex ofperiments. the value. Expanded uncertainty for all data is less than 20% of the value.

Figure 4. ◦ TheFigure differential 4. The differential normalized normalized mass loss mass of Pu loss for coriumof Pu for samples corium insamples distilled in waterdistilled at temperaturewater at 25 and 90 C. NL(Pu) in logarithmictemperature scale. 25 and “Days” 90 °C. in NL(Pu) figure doesin logarithmic not mean scal duratione. “Days” of leaching in figure experiment, does not mean but time duration from theof beginning of leachingleaching experiments. experiment, Expanded but uncertaintytime from the for beginning all data is of less leaching than 20% experiments. of the value. Expanded uncertainty for all data is less than 20% of the value.

Figure 5. Alteration of the surface of Chernobyl corium from room 217/2 with formation of secondary uranyl minerals in the light (left) and UV luminescence of alteration products (the same sample) (right). Sustainability 2021, 13, x FOR PEER REVIEW 6 of 9

Figure 4. The differential normalized mass loss of Pu for corium samples in distilled water at temperature 25 and 90 °C. NL(Pu) in logarithmic scale. “Days” in figure does not mean duration of Sustainability 2021, 13, 1073 6 of 9 leaching experiment, but time from the beginning of leaching experiments. Expanded uncertainty for all data is less than 20% of the value.

FigureFigure 5. 5.Alteration Alteration of the of surface the surface of Chernobyl of Chernobyl corium from co roomrium 217/2 from with room formation 217/2 ofwith secondary formation uranyl of minerals in thesecondary light (left )uranyl and UV minerals luminescence in the of alteration light (left products) and UV (the sameluminescence sample) (right of alteration). products (the same sample) (right). After 1 year of contact with distilled water at a temperature 150 ◦C, samples of “lava” and corium were significantly altered. On the surface of Chernobyl corium, secondary uranyl minerals of two forms were observed (Figure6, right). One of them, with needle- shaped crystals, was not observed in alteration experiments at a lower temperature (90 ◦C). For “lava”, sample chemical degradation was clearly observed (Figure6, left), in contrast with the “fresh” sample (Figure7b). Different alteration processes took place dependent on Sustainability 2021, 13, x FOR PEER REVIEW the phase: formation of “gel” on the surface of the sample, corrosion of metallic inclusions7 of 9 in the “lava” matrix and oxidation of uranium in U-bearing phases with the formation of secondary uranyl minerals (Figure6, left).

FigureFigure 6. 6.Formation Formation of secondary of secondary phases on phases the surface on ofthe Chernobyl surface brown of Chernobyl “lava” (left )brow and coriumn “lava” (right ()left after) 1and year co- ◦ riumcontact (right with distilled) after water1 year at 150contactC. with distilled water at 150 °C.

Figure 7. Chernobyl samples before leaching experiments: black “lava” from room 217/2 (a), brown “lava” from steam discharge corridor used for alteration experiment (b) and fragments of corium from room 305/2 (c).

4. Discussion As assumed, the mechanism of plutonium release into water is mainly related to the dissolution of the silicate glass-like “lava” matrices and not the dissolution of the chem- ically stable crystalline inclusions of zircon (Zr,U)SiO4 and solid solutions (Zr,U)O2-(U,Zr)O2. It is necessary to mention that the NLPu from black and brown “lava” during short time leaching tests performed in 1991 [15] and 25 years later [13], are com- parable. This indicates the relative chemical stability of “lava” matrices over long periods of time, although the effects of mechanical self-destruction of the black “lava” of the “Elephant foot” inside the “Shelter” and during storage under laboratory conditions were reported [7,11,19]. Plutonium release from the corium samples is less than one from “lava”. It is im- portant to note that in contrast to the glass-like Chernobyl “lava”, the geometric surface area of which is considered as very realistic, the corium surface area could not be evalu- ated properly using formal geometrical measurements of the samples because of its high porosity. Therefore, the NL of Pu from corium, calculated for the real surface area, is expected to be less than that from “lava”. At the same time, release of Pu from corium was much higher in comparison with spent U-oxide fuel, which is surprisingly compa- rable with pure PuO2 (Table 1). It is assumed that Pu in matrix of corium exists not only in stable forms such as an admixture in the crystalline structure of the solid solutions “ZrO2-UO2”, but also in chemically unstable species, such as nano-sized Pu-oxide parti- Sustainability 2021, 13, x FOR PEER REVIEW 7 of 9

FigureSustainability 6. Formation2021, 13, 1073 of secondary phases on the surface of Chernobyl brown “lava” (left) and co- 7 of 9 rium (right) after 1 year contact with distilled water at 150 °C.

Figure 7. Chernobyl samples before leaching experiments: black “lava” from room 217/2 (a), brown “lava” from steam Figuredischarge 7. Chernobyl corridor usedsamples for alteration before experiment leaching ( bexpe) andriments: fragments ofblack corium “lava” from room from 305/2 room (c). 217/2 (a), brown “lava” from steam discharge corridor used for alteration experiment (b) and fragments of corium from room 305/2 (c). 4. Discussion As assumed, the mechanism of plutonium release into water is mainly related to the 4. Discussion dissolution of the silicate glass-like “lava” matrices and not the dissolution of the chemically stable crystalline inclusions of zircon (Zr,U)SiO4 and solid solutions (Zr,U)O2-(U,Zr)O2. As assumed, the mechanismIt is necessary of to plutonium mention that release the NLPu intofrom water black and is brownmainly “lava” related during to short the time dissolution of the silicateleaching glass-like tests performed “lava” matrices in 1991 [15] and 25 not years the later dissolution [13], are comparable. of the Thischem- indicates the relative chemical stability of “lava” matrices over long periods of time, although the ically stable crystallineeffects inclusions of mechanical self-destructionof zircon of(Zr,U)SiO the black “lava”4 and of the “Elephantsolid solutions foot” inside the (Zr,U)O2-(U,Zr)O2. It is necessary“Shelter” and to during mention storage that under the laboratory NLPu from conditions black were and reported brown [7,11 “lava”,19]. during short time leaching testsPlutonium performed release from in the 1991 corium [15] samples and is25 less years than onelater from [13], “lava”. are It iscom- important to note that in contrast to the glass-like Chernobyl “lava”, the geometric surface area parable. This indicates theof whichrelative is considered chemical as stability very realistic, of “lava” the corium matrices surface areaover could long not periods be evaluated of time, although the effectsproperly of using mechanical formal geometrical self-destruction measurements ofof the the samples black because “lava” of its highof the porosity. “Elephant foot” inside Therefore,the “Shelter” the NL ofand Pu fromduring corium, storage calculated under for the reallaboratory surface area, conditions is expected to be less than that from “lava”. At the same time, release of Pu from corium was much higher were reported [7,11,19]. in comparison with spent U-oxide fuel, which is surprisingly comparable with pure PuO2 Plutonium release from(Table 1the). It iscorium assumed samples that Pu in is matrix less of than corium one exists from not only“lava”. in stable It is forms im- such portant to note that in contrastas an admixture to the in glass-like the crystalline Chernobyl structure of the“lava”, solid solutionsthe geometric “ZrO2-UO surface2”, but also in chemically unstable species, such as nano-sized Pu-oxide particles, formed as a result of area of which is considered as very realistic, the corium surface area could not be evalu- the high temperature melting in the system of “Zr cladding—UOx—steel—oxygen”. ated properly using formal geomIt is importantetrical to measurements note that at lower temperaturesof the samples (25 and because 90 ◦C), the of dissolution its high of the porosity. Therefore, the“lava” NL of matrix Pu wasfrom not cori accompaniedum, calculated with “gel” formation.for the real Such surface behavior ofarea, the “glassy”is matrix of “lava” is controversial to nuclear waste glasses [20]. The material study of the expected to be less thansecondary that from phases, “lava”. formed At as athe result same of chemical time, alterations,release of is aPu subject from of futurecorium research. was much higher in comparisonThe results with obtained spent require U-oxide further fuel, clarification which from is surprisingly the precise study compa- of the corium chemical and phase compositions using EMPA and XRD methods. However, it is possible rable with pure PuO2 (Table 1). It is assumed that Pu in matrix of corium exists not only to make the following conclusions: in stable forms such as an admixture in the crystalline structure of the solid solutions 1. Pu release into distilled water from matrices of black and brown Chernobyl “lava” is “ZrO2-UO2”, but also in chemicallycomparable. unstable The total normalizedspecies, such Pu mass as lossnano-sized at 90 ◦C is 2.2–2.3Pu-oxide g/m2 forparti- 140 days. 2. Pu release from matrix of Chernobyl corium is less than that from “lava”. The total normalized Pu mass loss for 140 days is 0.5 and 1.1 g/m2 at a temperature of 25 ◦C and 90 ◦C, respectively. 3. In comparison with the irradiated U-oxide LWR fuel, Chernobyl corium demonstrates, essentially, a lower level of chemical durability. Normalized Pu mass loss in distilled ◦ 239 water at 25 C for spent fuel and even PuO2 is two orders of magnitude less than Sustainability 2021, 13, 1073 8 of 9

that for corium. It is assumed that at least part of the uranium and plutonium in the matrix of Chernobyl corium exist in chemically unstable forms. 4. The chemical alteration of Chernobyl corium and “lava” causes the formation of new formed phases with essentially different chemical and mechanical levels of durability. This has consequential effects on the long-term behavior of fuel debris.

Author Contributions: Conceptualization and methodology, B.Z. and B.B.; investigation, all authors; analytical support, E.S., Y.P., V.Y., D.D.; writing—original draft preparation, B.Z., B.B.; writing— review and editing. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Russian Foundation for Basic Research (RFBR) grant #18-33-01182. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. The data are not publicly available because original data owns to authors. Acknowledgments: The authors are very grateful to V. Zirlin and L. Nikolaeva (Khlopin Radium Institute) for the treatment of the highly radioactive Chernobyl samples. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Information on the accident at the Chernobyl nuclear power plant and its consequences. Sov. At. Energy 1986, 61, 845–868. [CrossRef] 2. Rippon, S.; Blake, E.M.; Payne, J. Chernobyl: The Soviet Report. Nucl. News 1986, 29, 59–66. 3. International Atomic Energy Agency. INSAG-7: The Chernobyl Accident: Updating of INSAG-1; Safety Series No. 75-INSAG-7; IAEA: Vienna, Austria, 1992. 4. Poml, P.; Burakov, B. Study of a “hot” particle with a matrix of U-bearing metallic Zr: Clue to supercriticality during the Chernobyl nuclear accident. J. Nucl. Mater. 2017, 488, 314–318. [CrossRef] 5. Burakov, B.E.; Anderson, E.B.; Galkin, B.Y.; Pazukhin, E.M.; Shabalev, S.I. Study of Chernobyl “hot” particles and fuel containing masses: Implications for reconstruction the initial phase of the accident. Radiochim. Acta 1994, 65, 199–202. [CrossRef] 6. Burakov, B.E.; Anderson, E.B.; Shabalev, S.I.; Strykanova, E.E.; Ushakov, S.V.; Trotabas, M.; Blanc, J.-Y.; Winter, P.; Duco, J. The Behaviour of Nuclear Fuel in First Days of the Chernobyl Accident. MRS Online Proc. Libr. 1997, 465, 1297–1308. [CrossRef] 7. Burakov, B.E. Lava-like materials formed and solidified during Chernobyl accident. In Comprehensive Nuclear Materials, 2nd ed.; Konings, R., Stoller, R.E., Eds.; Elsevier: Amsterdam, The Netherlands, 2019. 8. Konoplev, A.V.; Bulgakov, A.A. Kinetics of 90Sr leaching from fuel particles in soils of the Chernobyl exclusion zone. At. Energy 1999, 86, 136–141. [CrossRef] 9. Bulgakov, A.; Konoplev, A.; Smith, J.; Laptev, G.; Voitsekhovich, O. Fuel particles in the Chernobyl cooling pond: Current state and prediction for remediation options. Environ. Radioact. 2009, 100, 329–332. [CrossRef] 10. Malkovsky, V.I.; Yudintsev, S.V.; Ojovan, M.I.; Petrov, V.A. The Influence of Radiation on Confinement Properties of Nuclear Waste Glasses. Sci. Technol. Nucl. Install. 2020, 3, 1–14. [CrossRef] 11. Zubekhina, B.Y.; Burakov, B.E.; Bogdanova, O.G.; Petrov, Y.Y. Leaching of 137Cs from Chernobyl fuel debris: Corium and “lava”. Radiochim. Acta 2019, 107, 1155–1161. [CrossRef] 12. Vlasova, I.; Shiryaev, A.; Ogorodnikov, B.; Burakov, B.; Dolgopolova, E.; Senin, R.; Averin, A.; Zubavichus, Y.; Kalmykov, S. Radioactivity distribution in fuel-containing materials (Chernobyl “lava”) and aerosols from the Chernobyl “Shelter”. Radiat. Meas. 2015, 83, 20–25. [CrossRef] 13. Zubekhina, B.Y.; Burakov, B.E. Leaching of actinides and other radionuclides from matrices of Chernobyl “lava” as analogues of vitrified HLW. J. Chem. Thermodyn. 2017, 114, 25–29. [CrossRef] 14. Nasirow, R.; Poeml, P. Gamma-Ray Spectrometry of Chernobyl Ceramic Samples; JRC Institute of Transuranium Elements: Karlsruhe, Germany, 2013. 15. Rogozin, Y.M.; Smirnova, E.A.; Savonenkov, V.G.; Krivokhatsky, A.S.; Avdeev, V.A.; Sagaidachenko, E.Y. Leaching of radionuclides from some newly formed products, extracted from reactor zone of 4th unit of Chernobyl NPP. Radiochemistry 1991, 33, 160–167. 16. Zubekhina, B.; Burakov, B. Plutonium leaching from polycrystalline and monocrystalline PuO2. Radiochim. Acta 2018, 106, 119–123. [CrossRef] 17. Katayama, Y.B.; Bradley, D.J. Long-Term Leaching of Irradiated Spent Fuel. In Scientific Basis for Nuclear Waste Management; Northrup, C.J.M., Ed.; Advances in Nuclear Science & Technology; Springer: Boston, MA, USA, 1980. 18. Burakov, B.E.; Anderson, E.B.; Strykanova, E.E. Secondary Uranium Minerals on the Surface of Chernobyl “Lava”. MRS Online Proc. Libr. 1997, 465, 1309–1311. [CrossRef] Sustainability 2021, 13, 1073 9 of 9

19. Borovoy, A.A. Inside and Outside “Sarcophagus”; CE IAE: Chernobyl, Ukraine, 1990. (In Russian) 20. Ojovan, M.I.; Pankov, A.S.; Lee, W.E. The ion exchange phase in corrosion of nuclear waste glasses. J. Nucl. Mater. 2006, 358, 57–68. [CrossRef]