Aging of fuel-containing materials (fuel debris) in the Chornobyl (Chernobyl) Nuclear Power Plant and its implication for the decommissioning of the Fukushima Daiichi Nuclear Power Station

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Journal of Nuclear Materials 576 (2023) 154224

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Aging of fuel-containing materials (fuel debris) in the Chornobyl (Chernobyl) Nuclear Power Plant and its implication for the decommissioning of the Fukushima Daiichi Nuclear Power Station Toru Kitagaki a,∗, Viktor A. Krasnov b, Atsushi Ikeda-Ohno a,c,∗ a

Collaborative Laboratories for Advanced Decommissioning Science (CLADS), Japan Atomic Energy Agency (JAEA), 2-4 Shirakata, Tokai-mura, 319-1195 Ibaraki, Japan Institute for Safety Problems of Nuclear Power Plants (ISP-NPP), National Academy of Science of Ukraine, 36a Kirova, Chornobyl, Kyiv 07270, Ukraine c Advanced Science Research Center, Japan Atomic Energy Agency (JAEA), 2-4 Shirakata, Tokai-mura 319-1195 Ibaraki, Japan b

a r t i c l e

i n f o

Article history: Received 22 June 2022 Revised 28 December 2022 Accepted 28 December 2022 Available online 30 December 2022

a b s t r a c t Nuclear fuel debris, the debris formed after the severe accidents of nuclear power plants, is a complex material containing a wide range of elements, compounds, and radiation. This complexity renders all the stages of the treatment of nuclear fuel debris (including removal, storage, and (interim- and final) disposal) extremely difficult and troublesome in the technical context. The whole treatment of nuclear fuel debris is also an extremely long-term process for tens of thousands of years. During this extremely long period of time, the aging of nuclear fuel debris is an unavoidable but critical issue, as the aging could change the physical/chemical properties of the fuel debris and, consequently, could potentially affect the strategy and planning of the treatment of the fuel debris. This, needless to say, applies to the decommissioning of the damaged reactors of the Fukushima Daiichi Nuclear Power Station (1F). In order to estimate the potential effects of aging on the nuclear fuel debris remaining in the damaged 1F reactors, the severe accident at the Chornobyl (Chernobyl) Nuclear Power Plant Unit 4 (ChNPP-4) is a unique and valuable source of information on the aging of materials containing nuclear fuels. Given this background, this review aims at collecting and summarizing the knowledge and information about the aging of materials containing nuclear fuels (fuel-containing materials, FCM) formed as a result of the accident at ChNPP-4 in the light of the decommissioning of 1F. The aging on FCM at ChNPP-4 is classified according to its origin (physical, chemical, or biological) and surrounding conditions (temperature, humidity, and radiation). Based on this classification and the knowledge/information collected from ChNPP-4, the potential effects of aging on the nuclear fuel debris at 1F are discussed. © 2023 Elsevier B.V. All rights reserved.

1. Introduction In the severe accidents at the Fukushima Daiichi Nuclear Power Station (FDNPS, hereafter, 1F) in March 2011, high temperature reactions between nuclear fuels and structural materials (e.g. cladding tubes, control rods, or concrete) occurred due to the unexpected loss of coolant, eventually producing "(nuclear) fuel debris". The resultant fuel debris is expected to have remained in the reactor pressure vessels (RPVs) and/or primary containment vessels (PCVs) [1]. The fuel debris is a very complex mixture being composed of a variety of radionuclides including uranium (U) and plutonium (Pu) isotopes from nuclear fuels (UO2 and PuO2 ) and ∗

Corresponding authors. E-mail addresses: [email protected] (T. Kitagaki), [email protected] (A. Ikeda-Ohno).

https://doi.org/10.1016/j.jnucmat.2022.154224 0022-3115/© 2023 Elsevier B.V. All rights reserved.

fission products (e.g. Cs, Sr, etc.). This radiochemical complexity of fuel debris poses a significant technical difficulty in making the overall decommissioning strategy of the damaged 1F reactors, such as the assessment of radiation exposure and nuclear criticality, accountancy of nuclear materials, countermeasures to prevent/retard potential leakage of radioactive materials into the surrounding environment, etc. Additionally, the removal/dismantling of the fuel debris and the subsequent treatment/management including storage and potential disposal of the retrieved materials are expected to pose many practical difficulties and problems. As a matter of fact, the removal/dismantling of the fuel debris at 1F is presumed to require more than thirty years to accomplish [2]. During this long-lasting removal/dismantling process, the fuel debris remains in the RPVs/PCVs under extremely harsh conditions with highradiation, high-temperature, and high-humidity environments. After the removal/dismantling, the fuel debris has to be stored in in-

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Journal of Nuclear Materials 576 (2023) 154224

terim storage containers for further several tens of years to reduce the decay heat prior to the subsequent long-term storage. Given these facts, in order to perform the risk and safety assessment of the overall decommissioning process of the damaged 1F reactors in an appropriate and reliable manner, the potential effect of aging on the physical/chemical properties of the fuel debris should be evaluated comprehensively. To this end, reviewing the precedent studies reporting the aging of nuclear fuel debris in other damaged reactors is the only feasible approach at least for the present, as the characteristics of the fuel debris at 1F and the actual environment in the damaged reactors are mostly uncertain even to date due to the practical difficulty to investigate the actual status inside the severely damaged reactors of 1F. Even for the past severe accidents of nuclear reactors, the studies reporting the aging of fuel debris are very limited worldwide. In this context, a series of reports/publications on the aging of fuel containing materials (FCM) produced during the severe accident in the Chornobyl (Chernobyl) Nuclear Power Plant Unit 4 (ChNPP-4) in 1986 is an invaluable source of information to estimate the potential aging effect of fuel debris at 1F during the decommissioning process. The damaged ChNPP-4 reactor was initially encased in a shelter called "the sarcophagus", which is made of reinforced concrete, and it was further enclosed with a new safe shelter (safe confinement) in 2016. The "sarcophagus" was hastily built immediately after the accident as a temporary construction to prevent widespread dispersion of radioactivity. Due to the poor confinement capability of the initial "sarcophagus", the FCM remaining in ChNPP-4 were subject to long-term natural weathering including precipitation, seasonal variations in humidity and temperature, etc. This natural weathering (i.e. aging) of FCM eventually resulted in increasing the release of radioactive materials from the encased ChNPP-4 (i.e. the sarcophagus), which was primarily filled with radioactive dust stemming from the original nuclear fuels [3–6], into the surrounding environment as radioactively contaminated water and dust, triggering the decision to construct the second safe confinement. In addition to ChNPP-4, the severe accident at the Three Mile Island Nuclear Generating Station Unit 2 (TMI-2), USA, in 1979 is another potential source of information on fuel debris. However, the fuel debris generated at TMI-2 was removed from the reactor vessel by 1990 [7] and, consequently, suffered no weathering or aging. Hence, no study is reported for the "aging" of fuel debris of TMI-2. Given these facts, this study focuses on reviewing the aging of fuel debris in ChNPP-4 (i.e. FCM), which is the only source worldwide in terms of the aging of fuel debris. In this review, several different terms associated with (nuclear) fuel debris are used. At least in this review, these fuel-debrisrelated terms are defined as follows;

In a broad sense, the terms "FCM" and "fuel debris" are equivalent. In this publication, however, "FCM" is used to refer specifically to the fuel - containing materials generated in ChNPP-4, in order to differentiate the "fuel debris" of ChNPP-4 and other "fuel debris" including the fuel debris remaining at 1F. When comparing the FCM in ChNPP-4 and the fuel debris in 1F (1F-fuel debris), there is a significant difference in chemical composition stemming from the difference in the type of nuclear reactors. That is, ChNPP-4 is the light water-cooled and graphitemoderated reactor "RBMK-10 0 0 , while 1F reactors are boiling water reactors (BWR), resulting in the difference in components of a reactor core and structure materials. This indicates that the original components of FCM and 1F-fuel debris is expected to be different. Additionally, FCM and 1F-fuel debris have different histories of environmental exposure. That is, the FCM in ChNPP-4 had been exposed to humid air and constant precipitation under an atmospheric condition, while the 1F-debris was initially exposed to water including seawater containing some microorganisms and was subsequently kept cooled by water cleaned with reverse osmosis (RO) mainly under an inert N2 atmosphere [8,9]. Therefore, the aging effect is expected to be different between the FCM in ChNPP-4 and 1F-fuel debris. Nonetheless, we believe that reviewing the aging of FCM at ChNPP-4 is valuable for the assessment of potential aging effects on the fuel debris at 1F, as the environmental/storage conditions of 1F-fuel debris will surely vary and the conditions of 1F-fuel debris could become comparable to that of the FCM in ChNPP-4 during the long-lasting decommissioning process. Given the above facts, the objective of this review is to infer potential aging processes of 1F-fuel debris from a series of reports/publications on the aging of FCM at ChNPP-4, which helps consider possible countermeasures to prevent/retard the aging of fuel debris at 1F, and to comprehend future research needs for the decommissioning of 1F. This review primarily covers the bulk materials remaining in the sarcophagus, and does not deal with particulates which were released from the damaged ChNPP-4 reactor/sarcophagus into the surrounding environment. 2. Aging of FCM at ChNPP-4 The mechanism of material aging (weathering) is generally categorized into three [10]: physically (or mechanically), chemically, and biologically-based. The physical (or mechanical) aging is the disintegration and/or deterioration of materials without chemical alteration (change). The chemical aging is caused by chemical reactions without biological effects, while the biological aging stems from biological interactions involving animals, plants, microorganisms, etc. At the damaged ChNPP-4, the characteristics of FCM and their exposure (i.e. aging) conditions vary depending on the location in the sarcophagus, which is linked directly to a series of chronological events occurred in ChNPP-4 during the accident. Hereafter, typical characteristics of FCM and the relevant aging conditions are described according to the chronological events occurring during the severe accident at ChNPP-4 and circumstances after the construction of the sarcophagus.

• fuel-containing materials (FCM): any kinds of nuclear-fuelcontaining materials (i.e. containing U and/or Pu) that are produced as a result of the severe accident of ChNPP-4, • lave-like materials containing nuclear fuels (LMNF) : solidified products that contain FCM in a silicate matrix; one of the major silicate sources is serpentite which was the biological shield around the reactor core, • corium: molten materials that are formed during the core melting accident in a nuclear reactor core, as well as their solidified products; corium usually contains fuel elements i.e. U, some fission products, and structural materials (Zr, Fe, Ni, Cr), • molten core-concrete interaction (MCCI) products: reaction products between molten core materials (i.e. corium) and concrete (structure material of nuclear reactors), as well as their solidified products, and • (nuclear) fuel debris: any kind of nuclear-fuel-containing materials that are produced as a result of the severe accident of nuclear reactors.

2.1. Chronological description of events occurring during the severe accident at ChNPP-4 On 26 April 1986, a non-nuclear steam explosion occurred in the reactor core of ChNPP-4 and the reactor core was completely destroyed [11]. The formation and migration of FCM produced by this accident can be summarized as follows [11,12]. The core melting was already started prior to the explosion, and high-temperature interaction between UO2 (nuclear fuel) and zircaloy (zircalloy with 1%-Nb) (cladding material) occurred in the reactor core, the temperature of which reportedly rose above 2

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Fig. 1. Cross section of the destroyed Chornobyl Nuclear Power Plant Unit 4 covered with the sarcophagus. The graphics are modified from the original graphics in Ref. [13].

2600 °C, to form the initial FCM. It is estimated that the explosion of the reactor core dispersed 3.5 vol% of the total nuclear fuel materials that were initially loaded in the reactor into the environment. Although the reliability of this estimated value is still under discussion, there is no doubt that the majority of the nuclear fuel materials remains in the reactor building even after the explosion. As a result of the explosion, the bottom part of the reactor core subsided into the floor below (room 305/2, see Figs. 1 & 3 [12,13]), eventually merging the reactor core and Room 305/2 to form a single space (Fig. 1 [13]). Consequently, FCM was initially accumulated at Room 305/2 (mainly at the southern part) immediately after the explosion. The accumulated FCM was kept heated at Room 305/2 due to the significant decay heat and possible graphite-originated fire. Under such a high temperature condition, the initial FCM interacted with construction materials such as steel, sand, and concrete at Room 305/2 for maximally ∼10 days after the explosion. During this stage, the lower plate is the main material in contact with FCM. The lower plate consisted of steel and serpentinite, the composition of which is SiO2 32–40% Fe2 O3 3–8.5%, CrO3 0.3–2.4%, MgO 43–45%, FeO 0.2–2.0%, Al2 O3 0.6–3.5%. Hence, the interaction between the initial high-temperature FCM and the lower plate resulted in the formation of silicate-based molten FCM or lava (LMNF). The resultant silicate-based molten materials were stratified into three major layers during ∼10 days after the explosion. The lightest top layer consisted of the admixture of U, Zr, Al, K, Ca, etc. in a silicate matrix and it formed black LMNF when solidified. The constituents of the middle layer were essentially comparable to the top layer and it was the source of brown LMNF when solidified. The difference in color between the top- and middle layers presumably stems from the content of UO2+ x in the silicate matrix, the detail of which is described later. The heaviest bottom layer was composed primarily of Fe-Cr-Ni, mainly forming metallic phases. After the stratification, the molten material spread further to the other areas of the reactor building by three different routes: large horizontal (LH), small vertical (SV), and large vertical (LV) streams (Fig. 3) [14]. The majority of the light top layer spread out as the LH stream. The LH stream from Room 305/2 first flowed into an adjacent Room 304/3 to fill the floor completely. The stream further poured into Corridors 301/5 and 301/6, and eventually flowed down to reach Room 217/2, where the famous black-ceramic FCM, "the Elephant’s Foot", was formed (Fig. 2). The resultant FCM associated

Fig. 2. Apperent change of “Elephant foot” (massive black LFCM found in Room 217/2) by aging.

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Table 1 Summary of location, source of molten flow, appearance, and aging condition of FCM in the sarcophagus of ChNPP-4.

Room No 305/2

Location Floor level from the basementa [m] +10.0

301/5 301/6 304/3 217/2 210/6 210/7 012/15 012/7 a

+9.0 +6.0

Source flow Original source of molten flow LH LH LH LH SV LV

+2.2 0.0

LV LV

Type of major FCM observed Black / brown LMNF

Condition of aging

Black LMNF Black LMNF Black LMNF Black LMNF Black LMNF Brown LMNF / metallic FCM / black LMNF Brown LMNF Brown LMNF

Note

Ambient conditions comparable to the outside environment Known as "Elephant foot" Accumulation of rainwater Accumulation of rainwater, freezing condition in winter, constant airflow

Slag and pumice Slag and pumice

The basement is "Level 0.0 0 0 in Fig. 1.

with these streams were basically black LMNF, indicating that the FCM originated from the lightest top layer. The lower (i.e. heavier) layer of the stratified molten FCM accumulated in Room 305/2 flowed down via a stream distribution corridor to form the LV stream and reach Room 210/7. The solidified FCM accumulated in Room 210/7 were found to be brown ceramic LMNF and metallic materials. This suggests that the LV stream and the resultant FCM found in Room 210/7 originate from the middle- and bottom layers of the original FCM accumulated in Room 305/2. The LV stream further poured down onto the floors of Rooms 012/15 and 012/7, where basin-bubblers filled with water were installed. The poured molten stream supposedly interacted with water at high temperature in Rooms 012/15 and 012/7, forming hard massive glass and brittle porous materials. These solidified FCM are often referred to as "slags" and "pumice", respectively. The two solidified FCM are both brownish, suggesting that these FCM originate from the middle layer of the original FCM in Room 305/2. Another vertical stream SV poured down to reach Room 210/6. The solidified FCM found in Room 210/6 were black, indicating that the SV stream and the resultant FCM originate from the lightest top layer of the original FCM in Room 305/2. The final distribution of FCM and their appearance are summarized in Table 1 and Fig. 3. Over 200 pieces of FCM samples collected from the damaged reactor building of ChNPP-4 were systematically analyzed by electron microscopy to understand the elemental composition of various FCM [14]. The results suggested that the difference in color between brown and black LMNF presumably stems from the inclusion of UO2+ x in the silicate matrix. In LMNF, uranium surely has solubility for a (molten) silicate glass matrix. Upon cooling, the dissolved uranium in the silicate matrix is precipitated as an independent phase (e.g. UO2+ x ) with the size of μm, which is often referred to as "inclusion", while other elements such as Fe are dissolved in the silicate matrix. As dark amber glasses do, the higher the additive (i.e. dissolved) content in a silicate matrix becomes, the darker the matrix becomes [15]. In the same manner, the dissolution of Fe and other soluble elements, that exhibit absorption particularly in the visible region, in a silicatebased molten material resulted in the formation of dark glass materials, i.e. black LMNF, as the solidified product. The components of the original molten FCM from the reactor core (i.e. UO2 and zircaloy) are hardly dissolved in the molten silicate matrix, and they are heavier than the silicate matrix. Consequently, these components were sedimented and condensed at the bottom of the silicate-based molten layer. The inclusion in LMNF were primarily confirmed to be UO2+ x . The presence of μm-order inclusion reduces the transparency of the silicate matrix, but the color of inclusion phase remains visible. As a result, the bottom layer of the silicate-based molten material formed a solidified LMNF containing

mainly UO2+ x inclusion in the silicate matrix, serving as a source of brown LMNF, the color of which originates from the inclusion of UO2+ x . The inclusion in LMNF were confirmed to be UO2+ x as a major phase with additional phases of (U,Zr)O2 and (Zr,U)SiO4 with 6–12 wt%-U, which was found to be a unique compound formed in ChNPP-4 and is named “Chernobylite”. The solid solution of (U,Zr)O2 (cubic) with a high Zr content (40∼50 atom%-Zr) is stable at high temperature, but the phase separates into (U,Zr)O2 (cubic) and (Zr,U)O2 (tetragonal or monoclinic) upon cooling [16]. The shape of the inclusion of UO2+x and (U,Zr)O2 phases depends on the molten state and the cooling conditions, e.g. round, a droplet shape in the order of μm, while (Zr,U)SiO4 is an octahedral or a prismatic pyramidal shape in the order from μm to hundreds of μm in size. The dissolution of other elements in the silicate matrix, such as Mg, Al, K, Ca, Zr, and U, was also confirmed [14], contributing to the coloring of the silicate matrix of LMNF. There is no significant difference in the chemical composition of silicate matrix itself between black and brown LMNF. 2.2. Aging conditions of FCM after the severe accident at ChNPP-4 The sarcophagus had undergone diurnal/seasonal environmental changes (e.g. temperature fluctuation, precipitation, sunbeam, etc.) until it was covered with the outer safe confinement in 2016. Due to the poor hermitical ability of the sarcophagus, rainwater constantly penetrated the reactor building and accumulated in the sarcophagus even after its construction. This means that FCM had been subject to natural weathering (aging) for approximately 30 years until the completion of the outer safe confinement. Shown in Fig. 4 are the seasonal variations in precipitation and ambient temperature at a meteorological station adjacent to the sarcophagus. Although the variation in temperature inside the sarcophagus is milder than that outside, hot and humid summer (up to 25 °C and 200 mm/month of precipitation) results in a high humidity environment inside the sarcophagus and accumulation of rainwater in the reactor building, while severe winter down to −10 °C causes the accumulated rainwater to freeze, which begins to melt with the arrival of spring. In general, the hot and humid environment is expected in summer across the whole reactor building, although the accumulation of rainwater is more significant on lower levels of the reactor building (e.g. Rooms 012/7 and 001/3, Fig. 5). This also means that the effect of the freezingmelting of accumulated rainwater is more serious in the lower levels of the reactor building. The accumulated rainwater contacting FCM was found to be slightly basic (pH = 8.6–9.6). This alkaline pH is presumed to result from the contact with basement concrete. The water contains Na+ , K+ , carbonate, and sulfate species as major ionic components [17]. Such basic rainwater was also found to dissolve troublesome 4

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Fig. 3. Distribution and appearance of FCM in ChNPP-4. The graphics are modified from the original graphics in Ref. [12].

Fig. 4. Seasonal changes in mean ambient temperature and precipitation at ChNPP-4 for the decade of 2006 to 2016 [17].

radionuclides (e.g. Sr, Pu, and Am isotopes) from FCM, posing the potential risk of migration of radioactive contaminants to the surroundings [18]. Due to the difference in atmospheric pressure and temperature, there is a constant air flow from the lower to the higher floors of the reactor building in the sarcophagus [18]. Such an air flow induces directly the erosion on the surface of the degraded FCM and also promotes the aging of FCM with moisture (see below).

cycle of thermal expansion/contraction potentially causes thermomechanical destruction on fragile types of FCM, such as pumicetype LMNF in Rooms 012/15 and 012/7 [19]. The aging of LMNF by thermo-mechanical cracking was also found to generate radioactive aerosols [20], which facilitates the spreading of radioactive contaminants not only in the sarcophagus but also into the surrounding environment.

2.3.2. Erosion Results of laboratory-based experiments by ISP-NPP using brown LMNF samples indicated that air flow in the sarcophagus can enhance the erosion of FCM [21]. The erosion rate was also estimated to be 1.9 μg/cm2 per year under the experimental conditions simulating the diurnal and seasonal effects inside the sarcophagus [21]. The effect of erosion is presumably en-

2.3. Type of aging 1 - Physical aging 2.3.1. Thermo-mechanical cracking Temperature fluctuation ranging from −10 to 25 ºC throughout the year (Fig. 4) could induce thermal expansion/contraction of FCM in the sarcophagus by the freeze-thawing of water. Such a 5

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Fig. 5. Migration and distribution of rainwater in the sarcophagus [116].

hanced on porous and cracked materials (e.g. pumice-type LMNF). This also means that the FCM fractured by thermo-mechanical cracking could be affected more significantly by erosion than the non-fractured ones. Hence, the combination of thermo-mechanical cracking and erosion could accelerate the physical aging of FCM, particularly on pumice-type LMNF accumulated in the lower levels of the sarcophagus.

Hence, the effect of self-irradiation on the FCM of ChNPP-4 is expected to be insignificant, although its potential cannot be excluded completely. In fact, as discussed in the next sub-Section 2.4, the irradiation under water induces radiolysis to produce chemically reactive species that cause chemical aging.

2.3.3. Self-irradiation Another type of physical aging on FCM could be caused by selfirradiation, particularly with α -particles. On vitrified glasses with high radioactivity, the effect of self-irradiation by α -particles was reported when the cumulative dose in a vitrified glass exceeded 1017 decays/cm3 , resulting in changes in volume [22,23]. Although the direct comparison between the homogeneously doped vitrified glass and the heterogeneous LMNF is not possible, the α -radiation in/from the LMNF of ChNPP-4 is expected to be far weaker and it requires ∼10 0 0 years to reach the level of 1017 decays/cm3 [24].

2.4.1. Reaction with moisture 1 - uranium phases As described in Section 2.2, the condition in the sarcophagus had been humid until the completion of the new safe shelter in 2016, and the accumulated rainwater still remains inside the sarcophagus even after the construction of the new safe shelter. This means that FCM in the sarcophagus are exposed to moisture (and in part soaked with water), potentially inducing water-related reactions. In particular, a high radiation level in the sarcophagus induces the radiolysis of water molecules in moisture and the accumulated water. This eventually produces radical species (e.g. OH∗ )

2.4. Type of aging 2 - Chemical aging

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and reactive substances (e.g. H2 O2 ) [25], triggering chemical reactions (i.e. chemical aging) on FCM. Water radiolysis is known to induce the oxidation of UO2 (U(IV)) to UO2 2+ (U(VI), known as "uranyl(VI)") [26]. The formed "uranyl(VI)" species is readily soluble in water and form aqueous complexes with existing anionic species, such as carbonates [27]. The evaporation of water from such aqueous uranyl(VI) solutions results in the formation of various uranium minerals, such as UO3 ·nH2 O (metaschoepite), UO4 ·4H2 O (studtite), UO2 CO3 (rutherfordine), and Na4 (UO2 )(CO3 )3 , all of which were found on the surface of aged black LMNF collected from Room 210/7 [28,29]. Additionally, needle-shape crystals containing Na+ , UO2 2+ , and sulfate species were observed on aerosols collected from Room 012/7 [19], which is presumed to be a product from water- and air-related reactions on the UO2+ x phase in FCM. Hence, the chemical aging with moisture/water results in the oxidation of UO2+ x in FCM to UO2 2+ , consequently depositing a series of uranyl(VI) minerals on the surface of FCM. 2.4.2. Reaction with moisture 2 - zirconium phases As described in Section 2.1, zirconium oxide (ZrO2 ) is another important component of FCM (particularly LMNF). At room temperature, the thermodynamically stable phase of ZrO2 is monoclinic, which changes to tetragonal and further to face-centered cubic with increasing temperature [30]. The dissolution of U in ZrO2 , however, forms stable (Zr,U)O2 solid solution phases with tetragonal and face-centered cubic structures even at room temperature [16]. In the presence of moisture, the phase transition from tetragonal- to monoclinic ZrO2 could occur in the temperature range between 30 and 300 °C, resulting in volume expansion [31]. The transition from tetragonal- to monoclinic ZrO2 could be also promoted under high radiation background [32]. The initial form of ZrO2 -related phase (i.e. Zr-rich (Zr,U)O2 ) in FCM is expected to be tetragonal, which could change to monoclinic with volume expansion under humid environment and high radiation background in the sarcophagus particularly for high Zr-content phases. If the inclusion of tetragonal-(Zr,U)O2 in a silicate glass matrix (i.e. LMNF) undergoes the phase transition to monoclinic, the resultant volume expansion causes cracks in the silicate matrix. In fact, cracks were confirmed around the tetragonal-(Zr,U)O2 inclusions in a brown LMNF collected from Room 210/7 [19]. Such cracks eventually affect the fragility (i.e. physical strength) of FCM. Hence, the combination of humid environment and high radiation background could alter both the chemical property of Zr-related inclusions and the physical property of overall FCM.

Fig. 6. Diversity of microorganisms inside of the sarcophagus.

and on aerosols [38], although its microbial diversity is confirmed to decrease with increasing radiation doses [36,39]. Some of these microorganisms have the potential to create local acidic/reducing conditions and to promote the corrosion and/or dissolution of materials [40–42], namely biological aging, which was also suggested by lab-based experiments by V. A. Krasnov et al. [18]. In combination with physical- and/or chemical aging, biological aging could accelerate the deterioration of FCM and/or change the process of aging. 3. Potential aging of fuel debris at 1F 3.1. Fuel debris at 1F

2.4.3. Crystallization of silicate glass matrix In principle, the crystallization of glass occurs above the glass transition temperature, which is more than 500 °C in the case of alkali silicate glasses [33]. Such high temperature condition was presumably realized during the core melting event of ChNPP-4, leaving crystalline silicate phases in LMNF [19]. Although it is unlikely, the possibility to have irradiation-induced crystallization of amorphous materials cannot be denied completely [34]. The crystallization of silicate glass phases eventually cracks the matrix of LMNF to release the inclusion of UO2+ x as nanoparticles [20]. Therefore, the irradiation-induced crystallization of glass matrix could be a matter of concern associated with the chemical aging on FCM.

3.1.1. Distribution and characteristics Since the occurrence of the severe accident at 1F, the actual situations of the damaged reactors have been continuously estimated by accident analysis, simulation, and a limited number of on-site investigations [1]. For instance, the initial OECD/NEA projects in support of 1F have been useful in analyzing possible interactions among materials with modeling (e.g. BSAF-1 & -2 projects [43]), and estimating the probable compositions of corium, release rates of FPs, etc. [44]. In addition, the PreADES project has been able to review in-pile data of irradiated fuel testing (e.g. Phébus project and TMI-2 examinations) [45]. At 1F, six reactor units had been operated until the accident in March 2011, while the first three units (Units 1–3) were severely damaged by the accident. For the reactor Unit 1, the majority of RPV components melted and flowed down to the concrete bottom floor of the PCV. The bottom part of the molten material flowed onto the PCV floor was then reacted with concrete (i.e. a floor material) to produce molten-core concrete interaction (MCCI) products. The material on the PCV floor was subsequently soaked in the seawater injected at the initial stage of the severe accident and afterward in the reverse osmosis (RO)-cleaned water for cooling. As a result, the fuel debris remaining in Unit 1 of

2.5. Type of aging 3 - Biological aging In nature, diverse microorganisms, such as bacteria, archaea, protozoans, and fungi, can subsist in almost any kind of environment. Even under the significantly high-radiation environment inside the sarcophagus, various microorganisms were confirmed to exist in the accumulated water (Fig. 6) [18,35,36], sediment [37], 7

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Journal of Nuclear Materials 576 (2023) 154224

1F is expected to be accumulated mostly at the bottom floor of the PCV, forming three layers: the top layer directly contacting water, the middle layer composed of solidified molten core materials, and the bottom layer consisting of (presumably porous) MCCI products. The thickness of the deposits were estimated to be 0.8 to 1 m inside the pedestal. The reactor core components of Unit 2 were also expected to melt and be accumulated mostly at the bottom of the RPV. The fuel debris remaining at the bottom of the RPV is therefore expected to be a complex mixture of metallic materials (fuel assembly and structure components) and oxides (fuel materials). Part of the molten materials generated in the RPV of Unit 2 flowed down to the bottom floor of the PCV, distributing the fuel debris in the PCV. The fuel debris distributed at the bottom floor of the PCV of Unit 2 is expected to partly undergo MCCI, although the amount of MCCI products in Unit 2 is presumably much lower than that in Unit 1. Due to the failure of cooling after the hydrogen explosion, the fuel debris remaining in the RPV of Unit 2 is expected not to be soaked in water, while the majority of the fuel debris accumulated at the bottom of the PCV has continuous contact with cooling water. In Unit 3, a significant amount of the reactor structure components is expected to remain in the RPV, while the molten fuel components mostly flowed down to the bottom floor of the PCV to be accumulated as a solidified molten core material. The bottom layer of the accumulated molten core material presumably underwent MCCI to a large extent. The fuel debris at the bottom of the PCV is expected to be immersed completely in cooling water, while the debris remaining in the RPV contacts only with humid air. In summary, the main components of the fuel debris remaining in Units 1 and 3 are expected to be oxides, while the debris in Unit 2 is expected to contain a significant amount of metallic materials.

amount of Pu presumably as (U,Pu,Zr)O2 or similar solid solution [62,63]. When the molten core materials flowed down to the bottom floors of the PCVs, the high-temperature molten materials melted concrete containing SiO2 , Ca(OH)2 , CaCO3 , Al2 O3 , Fe2 O3 , etc. to form MCCI products. Additionally, MCCI decomposes concrete to produce oxidizing gasses (e.g. CO2 and steam) to oxidize metallic Zr, and the resultant heat from the oxidation further accelerates MCCI [64]. Such Zr oxide phases (i.e. ZrO2 and (Zr,U)O2 ) can react with SiO2 (the major component of concrete) to form (Zr,U)SiO4 [14,50]. MCCI can also oxidize other metallic phases in the molten materials, precipitating the resultant oxide phases at the bottom of the MCCI region [51]. Hence, the MCCI products produced at 1F are likely to consist of the main matrix of impure SiO2 containing primarily (U,Zr)O2 , (Zr,U)O2 , and (Zr,U)SiO4 , and metallic phases (e.g. Fe) accumulated at the bottom. 3.2. Actual conditions at 1F 3.2.1. Presence of water One specific event that determines the characteristics of 1Ffuel debris is the immediate injection of seawater into the RPVs for urgent cooling after the loss of coolant. Seawater had been injected into the reactor cores for approximately two weeks after the accident, and a water treatment system to remove multiple radionuclides (Advanced Liquid Processing System: ALPS) was subsequently installed [65]. The system can remove Sr and Cs (major problematic radionuclides) based on ionic exchange, as well as other ions with reverse osmosis and desalination [65]. The seawater circulated by the installed water circulation system was continuously diluted with RO water, which eventually leaked into the PCVs. As a result, fairly radionuclide-free desalinated water was accumulated at the bottoms of the PCVs. In fact, results of water analysis indicated that the water accumulated in the PCVs of Units 1–3 had neutral pH (around 7) and the concentrations of major ions (e.g. Na, Mg, Si, Ca, Mn, Fe, Zn, and Sr) were below detection limit (< 5 ppm) [66]. Hence, the current characteristics of water in the PCVs are expected to be comparable to the low-middle grade deionized water. In order to avoid possible corrosion of RPV materials by the injected and circulated water, hydrazine, a strong reducing reagent, was added to the water at the initial stage of injection/circulation [67]. The addition of hydrazine reduces the concentration of dissolved oxygen and microorganisms in the water. It could, however, also affect the redox chemistry of 1F-fuel debris components, which is a major factor that determines the aging behavior of the fuel debris. A recent video clip reporting the actual condition inside the PCV of Unit 3 [68] reveals that water flow is rather low and static inside the PCV. The temperature of water accumulated at the bottoms of the PCVs is presumed to be comparable to the surrounding atmospheric temperature and follow the seasonal change of atmospheric temperature ranging from 10 to 40 °C (Fig. 7) [69], indicating that the actual water condition inside the PCVs of 1F is expected to be fairly mild and less aggressive to the remaining fuel debris.

3.1.2. Main components Sampling and analysis of the actual fuel debris at 1F is still ongoing and, therefore, the amounts, composition, and characteristics of 1F-fuel debris are mostly not certain even at this time. However, we can infer the composition and characteristics of the debris based on the results from laboratory-based experiments under the conditions simulating the actual environment at 1F [46–55]. The RPVs of 1F are composed mainly of fuel assemblies (UO2 , zircaloy, and stainless steel (SS)), control rods (SS and B4 C), and construction materials (zircaloy and SS). A high temperature environment (minimally 2500 °C∼ at the reactor cores) is presumed at the initial stage of the accident (i.e. loss of coolant) [56,57]. The main cause of such a high temperature environment was a combination of decay heat from the fuel materials and significantly exothermic reaction between zircaloy (cladding material) and water (steam) at high temperature [56–58]. The significant heat generated in the reactor cores can further diffuse inside the PCVs, resulting in high temperature conditions in the whole PCVs. The interaction between zircaloy and steam at high temperature is also known to oxidize metallic Zr to α -Zr(O) and ZrO2 , which possibly interact further with UO2 (fuel material) to form (U,Zr)O2 and metallic U (α -U) to a minor extent [48]. The (U,Zr)O2 phase is divided into (U,Zr)O2 and (Zr,U)O2 upon cooling. Additionally, when a high temperature interaction occurs among zircaloy, SS, and B4 C, the final solidified products contain Fe2 Zr- and NiZr2 -based intermetallic compounds and ZrB2 [48,59,60]. Hence, the major phase of oxide-based fuel debris remaining at 1F is expected to contain (U,Zr)O2 /(Zr,U)O2 , while the metallic debris is composed mainly of Fe- and Zr-based intermetallic alloys containing SS components (i.e. Ni and Cr) as well as borides. Among the damaged three reactors Units 1–3, Unit 3 had been operating with mixed oxide (MOX) fuels consisting of UO2 and PuO2 [61]. For this reason, the fuel debris remaining in Unit 3 is presumed to contain a significant

3.2.2. Atmospheric condition In order to prevent the damaged reactor cores from hydrogen explosion, the PCVs of Units 1–3 have been (and are still being) purged with nitrogen gas [67]. Thanks to this purging with nitrogen, the atmospheric condition inside the PCVs is basically kept under positive pressure and inert [69], although possible penetration of air into the RPVs/PCVs through cracks cannot be excluded completely. Due to the presence of accumulated water, the humidity inside the PCVs is expected to be close to saturation [68,70]. 8

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Journal of Nuclear Materials 576 (2023) 154224

Table 2 Summary of conditions in the damaged reactors of 1F and the major components of the fuel debris. Number of reactor unit 1 Presence of moisture/water

Immediately after accident

Current

Atmosphere

Main components of fuel debris

2

3

RPV

High temperature vapor

PCV

Seawater

RPV

Wet

PCV

Near-neutral water

Immediately after accident

RPV PCV

N2 + H2

Current

RPV PCV

N2

RPV

Oxide

Metal and oxide

Oxide

PCV

Oxide and MCCI

Metal

Oxide and MCCI

3.3.1. Physical aging According to the description in Section 2.3, physical aging can be subdivided into thermo-mechanical aging, erosion by air, and self-irradiation. Seasonal variation in temperature at 1F (Fig. 7) would cause cracks between different phases with different thermal expansion coefficients [71], which would eventually make the whole fuel debris fragile. The thermal expansion coefficients of some representative phases expected in 1F-fuel debris are listed in Table 3. The thermal expansion coefficients of UO2 and ZrO2 (monoclinic), the representatives of (U,Zr)O2 and (Zr,U)O2 , respectively, are comparable to one another, indicating that the effect of thermo-mechanical aging would be less significant in the oxidebased fuel debris. On the other hand, the difference in thermal expansion coefficient is rather large among metallic phases (Fe, Zr, Fe2 Zr, NiZr2 , and ZrB2 ), suggesting that the variation in temperature could crack the metal-based fuel debris. Furthermore, there is the largest difference in thermal expansion coefficient between amorphous SiO2 and ZrSiO4 ((Zr,U)SiO4 ), the two main phases expected in MCCI products in 1F-fuel debris (Table 3). This means that the effect of thermo-mechanical aging would be the most significant in the MCCI-based fuel debris among the three major components of 1F-fuel debris. As a matter of fact, such an effect of thermo-mechanical aging was reported also for LMNF at ChNPP-4 [19,20]. Air flow could possibly induce erosion particularly on porous and cracked fuel debris. It is expected that the effect of erosion has been insignificant at 1F until now, as the damaged PCVs at 1F have been kept confined without forced exhaust and, therefore, the atmospheric condition inside the PCVs has been kept static. However, the condition is expected to change from static to dynamic when the operation of fuel debris removal, which involves drastic fluctuation of pressure inside the PCVs, commences. This means that erosion could be an issue in the future decommissioning works at 1F, particularly during the removal/dismantling of fuel debris. In order to estimate the degree of erosion progression that induces cracks on materials, one useful indicator is the fracture toughness, a measure of resistance to crack propagation of material [72]. The values of fracture toughness for some representative phases expected in 1F-fuel debris are summarized in Table 4. Alloys, such as zircaloy-2 and stainless steel, possess high fracture toughness, while other phases (oxides, intermetallic compounds, and borides) are rather weak against cracking. Hence, the effect of erosion could be much more significant on the non-metallic phases than the alloy phases (i.e. metallic phases). As mentioned in Section 2.3.3, the effect of self-irradiation on nuclear fuel debris is caused mainly by α -radiation. Among the three major components of oxide, metallic, and MCCI phases in

Fig. 7. Variation in temperature at the lower PCVs of 1F [69].

As mentioned in 3.2.1, the current atmospheric temperature inside the PCVs follows the seasonal change of the surrounding environment (Fig. 7). Hence, the current atmospheric condition inside the PCVs is also mild and less aggressive to the remaining fuel debris. However, when a large-scale operation of the fuel debris removal commences, the pressure inside the PCVs will be switched to negative in order to avoid the dispersion of radioactive aerosols, that are produced during the dismantling of fuel debris, into the surrounding environment. This will eventually allow oxygen to penetrate into the PCVs, changing the atmospheric condition inside the PCVs which would affect the long-term aging behavior of 1F-fuel debris. The conditions at 1F, together with the main components of 1F-fuel debris, are summarized in Table 2.

3.3. Potential aging at 1F As described in Section 3.1, the fuel debris at 1F is expected to be a mixture of oxide- and metallic compounds, and MCCI products, while the debris at ChNPP-4 (= FCM) is predominantly composed of silicate-based lava-like materials (= LMNF). The characteristics of the MCCI component in 1F-fuel debris are expected to be comparable to those of LMNF at ChNPP-4 and, therefore, we can infer the potential aging behavior of the MCCI-based fuel debris at 1F based on the knowledge and information on LMNF at ChNPP-4. The aging of oxide- and metal-based fuel debris at 1F could be also deduced from the information/knowledge about the inclusion and precipitate components of LMNF at ChNPP-4, in addition to the results from lab-based studies performed thus far. 9

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Journal of Nuclear Materials 576 (2023) 154224

Table 3 Thermal expansion coefficients of representative phases expected in the fuel debris at 1F.

Type of fuel debris Oxide

Metal

MCCI product

Phase UO2 Monocrinic-ZrO2 Zr Fe SUS316L Fe2 Zr NiZr2 ZrB2 SiO2 (fused) ZrSiO4

Thermal expansion coefficient [10−6 /K] 9.4 8.8 5.7 11.8 15.9 4.76 9.1–9.5 5.2 0.49 a-axis : 3.2–4.4 c-axis : 5.4–7.6

Table 4 Fracture toughness of representative phases expected in the fuel debris at 1F. YSZ: Yttria stabilized zirconia. Type of fuel debris Oxide

Metal

MCCI product

Phase UO2 cubic-(U,Zr)O2 YSZ Zircaloy-2 Stainless steel Fe2 Zr ZrB2 SiO2 (fused) ZrSiO4

Fracture toughness [MPa·m1/2 ] 0.6 1–3 2–4 50–60 100–500 2.5 2–6 0.8 1–4

Temperature [K] 293 293 293 293 273–373 – – 293 293 –

Reference [102] [102] [103] [103] [104] [105] [106] [102] [102] [107]

is several microns in steels [80], the formation of He-gas bubbles in the metal-based fuel debris could occur only within a micronorder region and, therefore, the effect of α -ray irradiation on the metallic 1F-fuel debris is considered to be insignificant. As already mentioned in Section 2.3, the effect of α -ray irradiation on MCCIbased phases is expected to be also insignificant. Hence, the α -ray irradiation on 1F-fuel debris would be concern potentially for the oxide-based phases, if at all.

Reference [108] [52] [109] [110] [111] [112] [113] [114] [115]

3.3.2. Chemical aging Moisture/water is considered to be a major factor that causes chemical aging on the fuel debris at 1F. Given the actual conditions described in Section 3.2, the PCVs of 1F are expected to be wetter than the RPVs, meaning that the chemical aging is likely to be more significant in the PCVs than in the RPVs. The major components of the fuel debris are expected to be oxide and metallic phases in the RPVs, while MCCI-based debris is expected to exist additionally in the PCVs of Units 1 and 3, as summarized in Table 2. As described in Section 2.4.1, the reaction between moisture/water and UO2 , one major phase of the oxide-based fuel debris in 1F, could result in the oxidation of uranium and dissolution as UO2 2+ (i.e. uranyl(VI)) under high radiation conditions, while ZrO2 , the other major phase of the oxide-based fuel debris, is expected to remain intact due to its high chemical stability. In the RPVs of 1F, the presence of moisture/water is expected to be limited and, therefore, the UO2 phase in the fuel debris is unlikely to be oxidized or dissolved at least under the current condition of the RPVs. A small amount of moisture/water in the RPVs could probably oxidize UO2 slightly to UO2+ x [81]. Although the solubility of UO2+ x is slightly higher than that of UO2 , which is hardly soluble [26], the oxidation/dissolution of UO2 (and related) phase(s) is likely to be insignificant in the RPVs of 1F. On the other hand, the amount of moisture/water is much larger in the PCVs than in the RPVs. This means that the oxidation /dissolution of UO2 by moisture/water should be more significant in the PCVs than in the RPVs. However, during the migration of the initially-formed hightemperature melt from the RPVs into the PCVs, the majority of the UO2 -based fuel debris in the PCVs is expected to have undergone high-temperature reactions with the structure materials containing zirconium [56], eventually forming (U,Zr)O2 [58]. The chemical stability of (U,Zr)O2 increases with increasing the Zr content [82], and the dissolution of (U,Zr)O2 is reported to be (much) slower than UO2 even in an aqueous carbonate (NaHCO3 ) solution containing H2 O2 [83]. Given all these facts, the oxidation/dissolution of UO2 related phases (i.e. UO2 , UO2+ x , and (U,Zr)O2 ) is considered to be insignificant both in the RPVs and the PCVs of 1F. As described above, ZrO2 , the other major phase of the oxide-based fuel debris at 1F, is likely to remain intact. When high-temperature UO2 Zr (Zr-U-O) melt, which was initially formed as a result of high-

1F-fuel debris, the oxide phase is expected to be most affected by α -radiation, resulting in the expansion of crystal lattice and the formation of nano-sized bubbles accumulating He gas in the matrix or at the grain boundary. The formation of He-gas bubbles further gives rise to internal pressure in a substance, producing micron-sized cracks [73]. The α -particles and recoil atoms generated by α -decay can produce the displacement of atoms in crystalline materials, the repetition of which eventually causes the above-mentioned effects (i.e. the expansion of crystal lattice, increase in the density and stress levels of defects, and the generation of nano-size He gas bubbles) [74]. The degree of atom displacement by α -radiation is often evaluated by the average number of displacement per atom (dpa) [75,76]. The α -radiation with 0.1 dpa causes a certain percentage of lattice expansion in spent nuclear fuels, while the generation of nano-sized He-gas bubbles occurs when dpa exceeds 1 [73]. Assuming that spent nuclear fuels with 40 GWd/tHM burnup are stored under normal atmospheric conditions, approximately 10 0 0 years are required to reach 1 dpa [73]. It is estimated that the spent nuclear fuels loaded in the 1F reactors had 22–26 GW d/tHM burnup immediately before the accident and the Pu content in the MOX fuels loaded in Unit 3 was 3.9%, which reaches ∼0.1 dpa 100 years after the accident [77]. This suggests that both the expansion of crystal lattice and the formation of nano-sized He-gas bubbles in the oxide-based fuel debris (i.e. the phases containing UO2 and PuO2 ) are anticipated to be insignificant and unlikely to occur during the initial stage of decommissioning of 1F, including the removal and dismantling of fuel debris. The α -ray irradiation to the metal-based phases of 1F-fuel debris could arise from the inclusion of α -nuclides in the metallic phases and/or the external irradiation from other fuel debris containing α -nuclides. It was reported that MeV-order α -ray radiation, covering 4–6 MeV radiation relevant to the α -radiation by U, Pu, and TRU nuclides, induces the formation of He-gas bubbles in stainless steel (i.e. the major component of metal-based fuel debris at 1F) [78,79]. However, considering the range of an α -particle 10

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Journal of Nuclear Materials 576 (2023) 154224

temperature reactions between UO2 (nuclear fuels) and metallic Zr (structure materials) in the RPVs of 1F, is fully oxidized and quenched, the melt is primarily solidified as cubic-(U,Zr)O2 , and then separated into cubic-(U,Zr)O2 and tetra-(Zr,U)O2 during cooling. The proportion of each phase depends on the initial U/Zr ratio and the cooling rate. Solid solution of tetra-(Zr,U)O2 is stable under normal atmospheric conditions [16] and it is presumed to be hardly soluble. Hence, the dissolution of ZrO2 -related phases (i.e. (mono-)ZrO2 and tetra-(Zr,U)O2 ) and cubic-(U,Zr)O2 with the Zr content more than ∼25% [83] considered to be insignificant at 1F. As described in Section 3.1.2, the metal-based fuel debris in 1F is presumed to be composed primarily of iron and zirconium. When in contact with moisture/water in the presence of oxygen, iron is known to corrode to form hydrated iron(III) oxides and Fe(III) oxyhydroxide (i.e. rust) on the surface [84]. As a matter of fact, such rust-like reddish debris was observed in quantity during the recent underwater monitoring inside the PCVs of Units 2 and 3 of 1F [85]. This suggests that the metallic fuel debris of 1F contains a large amount of iron-rich oxide phases, which are already corroded under the current water-rich conditions of the PCVs. If present, the iron-rich metallic fuel debris can be corroded also in the RPVs, although the corrosion should be much slower than that in the PCVs due to the lower content of moisture/water in the RPVs than in the PCVs. The rust layer formed on the surface of iron-rich metallic phases is stable and therefore acts as a protective layer to retard further atmospheric corrosion [86]. Given the current steady condition without significant fluctuation of airand water flows in the PCVs, the corrosion of the iron-rich metallic fuel debris in 1F is anticipated to be retarded by the formation of rust layers on the surface. However, when the operation of the fuel debris removal commences, the atmospheric conditions (e.g. pressure and airflows) inside the RPVs/PCVs will be fluctuating, probably promoting the removal of the protective rust layers from the surface. This leads to the emergence of new metallic layers on the surface and, consequently, the corrosion of the iron-rich fuel debris progresses. The structural steels in the reactors are high alloy steels with the Ni and Cr content of typically 5–20% in total, which is sufficient to form passive layers and retard the steel corrosion. However, there is the possibility that the iron-rich metallic phases of 1F-fuel debris in contact with high alloy steels could form an electrochemical environment. Such an electrochemical environment (i.e. anodic iron-rich metallic phases) would promote the corrosion of these metallic phases. A further danger with (passivating) high alloy steels could be the stress corrosion cracking (SCC) failure, particularly when chloride ions are present in the water. SCC and/or environmentally induced mechanical failures of damaged structures is particularly a longer-term issue for ∼20 years. Metallic zirconium (or zircaloy), another major metallic phase expected in the fuel debris of 1F, resists corrosion by forming a thin (ca. 1– 3 μm) passive oxide layer of ZrO2 on the surface even at room temperature [86], and the passive oxide layer becomes thicker (typically