Study of Structure-Phase State of Oxide Films on E110 and E635 Alloys at Pre- and Post-Irradiation Stages

Transcription

1 A.A. BOCHVAR HIGH-TECHNOLOGY RESEARCH INSTITUTE OF INORGANIC MATERIALS (SC «VNIINM») 18TH INTERNATIONAL SYMPOSIUM ON ZIRCONIUM IN THE NUCLEAR INDUSTRY «ROSATOM» STATE ATOMIC ENERGY CORPORATION MAY 15-19, 2016 Study of Structure-Phase State of Oxide Films on E110 and E635 Alloys at Pre- and Post-Irradiation Stages A.Yu. Shevyakov 1, V.V. Novikov 1, V.A. Markelov 1, A.V. Obukhov 2, G.P. Kobylyansky 2 (1) A. A. Bochvar High-Technology Research Institute of Inorganic Materials (SC VNIINM ) (2) Research Institute of Atomic Reactors (SC SSC RIAR )

2 Introduction The requirement to ensure corrosion resistance of zirconium-based alloys, caused by the higher fuel burnup, increasing the duration of use cycles of the product, etc. leads to the need to develop of new and modernization of existing alloys. Determination of the effect of alloying and phase compositions on the corrosion process of determining the need to involve of structural studies of methods. To clarify the mechanisms of oxidation (corrosion) of requires knowledge of structural and phase transformations in the alloy and the oxide film. 2

3 Purpose of work The study of the structural-phase state of oxide films formed on the surface of the fuel claddings of E110 and E635 alloys after autoclave and reactor tests, and the identification of factors influencing the degradation of their structure and protective properties. Main tasks: 1) The development of highly sensitive methods for research of structural-phase state of oxide films after autoclave and reactor tests. 2) The research of structural-phase state of oxide films after autoclave test. 3) The determination of the influence of the phase composition on the protective properties of the oxide films formed after autoclave test. 4) The research of structural-phase state of oxide films after reactor irradiation. 5) The comparative analysis of the results after reactor irradiation with the results after autoclave test. 3

4 Methods of preparing specimens μm Mechanical grinding Electrolytic polishing Ion thinning 4

5 The parameters of the ion etching 0 to 30 specimen TEM-specimen Accelerating voltage applied to each of the two guns was set to 5 kv, and the amperage value was 6 ma. Angle of inclination of the specimen relatively to ion beams (grazing angle) was not more than 6, it is need for minimization of introduced radiation defects in the ion thinning. A thin TEM-specimen in any given section of the oxide film, parallel to the plane of the metal-oxide, can be obtained by changing of the modes (unilateral or bilateral thinning, accelerating voltage, current ion beam, bombardment angle, rotation or oscillation of the specimen, etc.). 5

6 Methods of preparing of the irradiated oxide films In the "hot cells" of SC SSC RIAR carried cutting disc of the cladding tube. Cutted discs are mechanically polished in the glove protective box and is subjected to ink jet electrolytic polishing, avoiding direct contact with the sample. To protect the inner chamber of system ion etching by of the spray of the active material was made special aluminum cup. 6

7 Oxide film before irradiation Tetragonal and monoclinic phases of the oxide film Nearby to the interface of "metaloxide" there is a significant proportion of the tetragonal phase ZrО 2 (lattice BCT). With increasing distance from the interface of "metal-oxide" the monoclinic phase of ZrO 2 becomes the main component. 7

8 Oxide film of the E110 alloy Columnar grains of ZrO 2 monoclinic phase longitudinal section 20 nm 100 nm Equiaxial grains of ZrO 2 tetragonal phase 50 nm Metal 20 nm longitudinal section 10 nm 8

9 Structural characteristics of the oxide film and the behavior of the precipitates of β-nb particles Crystalline β-nb particle Amorphous β-nb particle Nearby to the interface of "metal-oxide" there is a significant proportion of the tetragonal phase ZrО 2, which has ultrafine-grained structure with grain size 5 10 nm. With increasing distance from the interface of "metal-oxide occurs the transition from equiaxed grains of tetragonal ZrO 2 phase to columnar grains of monoclinic ZrO 2 phase with grain size nm. 9

10 The amorphization process of β-nb particles Crystalline β-nb particle in the oxide film The beginning of the amorphization process of β-nb particle in the oxide film Amorphous β-nb particle in the oxide film In initial stage of oxidation process all precipitates of β-nb particles are in the oxide film in the crystalline state. At distances of the order of nm from the interface of "metal-oxide" begins the amorphization process of the periphery part of precipitates. All β-nb precipitates are in an amorphous state upon reaching the oxide film thickness of about 1 μm. 10

11 Oxide film of the E635 alloy Oxide film in initial stage of its growth contains a significant proportion of the ZrO 2 tetragonal phase. This phase consists of fine, equiaxed grains with size of 5 10 nm. (HCP) mono ZrO 2 "metal oxide" tetra-zro tetra-zro mono-zro tetra ZrO 2 2 mono-zro 2 With increasing of oxidation time gradually occurs transition from equiaxial grains of ZrO 2 tetragonal phase to columnar grains of ZrO 2 monoclinic phase. The grain size of the monoclinic phase is nm. 11

12 Behavior of precipitates of the Laves phase particles in the oxide film With increasing distance from the interface of "metal-oxide" occurs a transition of the particles of the Laves phase from the crystalline to the amorphous state. Сrystalline state Сrystalline state Amorphous state Amorphous state Amorphous state Сrystalline state Near to the interface of metal-oxide all the particles of the Laves phase Zr(Nb,Fe) 2 are in a crystalline state and has HCP lattice. Amorphization process of the precipitates begins at nm from the interface of "metal-oxide", at a distance of nm all the particles of the Laves phase transform into an amorphous state. 12

13 Changes in the composition of the Laves phase particles during amorphization Crystalline L-phase Reduction of iron and niobium content in the particles of the Laves phase and their oxygen enrichment occurs in the transition from crystalline to the amorphous state during oxidation. Over time the precipitates begin to dissolve in the oxide. Dissolution of boundaries Amorphous L-phase dissolution 13

14 Effect of yield of iron and niobium from Laves phase particles on porosity of the oxide film 1 μm In early stages of the oxide film growth concentration of micropores and microcracks is negligible. Increasing of oxidation time leads to yield of iron and niobium atoms from Laves phase particles into the oxide. О О Metal Concentration of micropores in the oxide film is growing with increasing of distance from the interface of "metaloxide". Microanalysis shows the enrichment of areas of the oxide film (contains pores) by iron and niobium, in later stages of its growth. 3,5 μm 14

15 Composition of defect areas in the later stages of oxidation 6 μm 12 μm By approaching the outer surface of the oxide film increases the concentration of micropores, but the composition of the local areas containing micropores is changing. At later stages of oxidation niobium, localized in the defective areas of the oxide film, distributes uniformly throughout oxide, but the iron remains in the areas, which contain micropores. 15

16 Irradiated oxide films The first electron-microscopic examination of the structural-phase state of the central parts of the irradiated oxide films of E110 alloy (thickness of about 10 μm) and E635 alloy (thickness of about 80 μm) was carried out in Russia. The oxide film on the fuel cladding of E110 alloy 20 nm 20 nm 10 nm The main structural component of the oxide film is a monoclinic modification of ZrО 2, which is confirmed by microdiffraction studies. In addition to monoclinic component of oxide film, was found a slight amount of the tetragonal ZrO 2 phase, in the form of individual grains. 16

17 The oxide film on the fuel cladding of E110 alloy 20 nm 20 nm Electron microscopic study of irradiated oxide film formed on the fuel rod cladding of E110 alloy, did not reveal the presence of particles of β-nb in their composition. It is shown that the oxide film is sufficiently dense, however there are areas containing in its composition a small amount of defects such as micropores. The presence of a small amount of defects in the oxide film after operation of the fuel rod cladding in reactor conditions indicates its good protective properties and high resistance to pore formation. 17

18 The oxide film on the fuel cladding of E635 alloy 20 nm 20 nm 20 nm Monoclinic modification of oxide film with individual grains of the tetragonal phase. Electron microscopic study of irradiated oxide film formed on the fuel rod cladding of E635 alloy, did not reveal the presence of Laves phase particles in their composition. It is shown that the oxide film contains a significant amount of defects such as micropores and microcracks, but their concentration corresponds to the concentration of defects in oxide films formed on the cladding of the E635 alloy after long period (~ 600 days) autoclave tests. 18

19 Comparison of microstructural characteristics after autoclave and reactor tests E635 alloy Irradiation in the VVER-1000 Autoclave tests Apparent differences in the microstructure of the oxide films and the corresponding concentration of micropores, after autoclave and reactor tests (apart from the lack of precipitates in the oxides under irradiation), is not observed until the thickness of oxide films ~ 40 μm formed during operation in the reactor. Thus, accelerated of corrosion of fuel rod claddings of E635 alloy under reactor irradiation does not occur due to the development of microporosity in oxide films. 19

20 Conclusions 1. The method for electron-microscopic study of samples of oxide films obtained on the fuel claddings from E110 and E635 alloys after autoclave testing and operation in the reactor was developed. 2. Studies have shown: - presence of two phase components ZrO 2 (tetragonal and monoclinic) was found in the oxide films of alloys E110 and E635. ; - with the increase of oxidation time all precipitates of second phases transform into an amorphous state in oxide films during autoclave testing; - the redistribution of the alloying elements (iron and niobium) between precipitates of the Laves phase particles and oxide film leads to formation of micropores, which in turn accelerate oxidation of E635 alloy products; - the oxide film formed on the surface of the fuel rod cladding of E110 and E635 alloys in the operation in the VVER-1000 reactor does not contain in the composition of the second phase precipitates; - the acceleration of corrosion of the fuel rod cladding of E635 alloy under reactor irradiation does not occur due to the development of microporosity in oxide films. 20

21 Thank you for your attention! 21