Study of Creep and Hydride Re-Orientation Behaviour in ...contemporary-energy.net/v04n01a06-Hygreeva-Kiran-Namburi.pdf · Figure 6. Measurements of internal diameter [Di] and thickness

International Journal of Contemporary ENERGY, Vol. 4, No. 1 (2018) ISSN 2363-6440 ___________________________________________________________________________________________________________

___________________________________________________________________________________________________________ H. K. Namburi, P. Halodova, P. Bublikova, K. Jakub: “Study of Creep and Hydride Re-Orientation Behaviour in E110 Fuel …”, pp. 50–60 50

DOI: 10.14621/ce.20180106

Study of Creep and Hydride Re-Orientation Behaviour in E110 Fuel Cladding at Dry Storage Conditions

Hygreeva Kiran Namburi1*, Patricie Halodova1, Petra Bublikova1, Krejčí Jakub2

1)Centrum výzkumu Řež

250 68 Husinec-Rez, Czech Republic; [email protected] 2)UJP Praha

156 00 Praha, Czech Republic Abstract Zirconium based alloys are commonly used as material for fuel claddings in the light water reactors. Claddings act as first metallic barriers against loss of fission products during the nuclear power plant operation, intermittent storage or final dry storage. During the reactor operation, claddings are subjected to different stress levels at high temperatures as well as neutron radiation. This results in their creep and the integrity of claddings is always critical issue for the safe performance of the power plants and storage. In this work, E110 cladding creep behaviour at dry storage conditions having different hydrogen levels is studied. Test specimen was oxidized in an autoclave to have desired hydrogen content. Creep test was performed in a horizontal furnace with internal pressure by applying thermo-mechanical cycle. The failure of the fuel cladding occurred by extensive ballooning and wall thinning. Optical and transmission electron microscopy investigations were made on creep tested specimen to study the deformation process and identify the mode of deformation. The examination of samples from the deformed regions showed formation of dislocations, secondary precipitate particles and precipitation of hydrides on the grain boundaries. Creep deformation was associated with the grain boundary sliding.

1. Introduction Zirconium based fuel claddings serve as barrier between the pellets and the fuel rod environment, avoiding the release of fuel or fission products during service into the reactor core or cooling system or after service into storage containments. Zirconium and its alloys have a significant property, a very low neutron absorption cross section, 30 times less than iron. This is the main reason for the selection of zirconium and its alloys as core materials to obtain better neutron efficiency in thermal reactors. In addition, zirconium alloys also possess good corrosion resistance, mechanical strength and are relatively resistant to radiation damage [1-2]. During the reactor operation, claddings accumulates hydrogen due to oxidation and are subjected to degradation resulting from their exposure to high temperature, high pressure and irradiation. When the hydrogen content in zirconium alloys exceeds the terminal solid solubility limit, diffused hydrogen atoms react with the zirconium matrix and formation of zirconium hydride occurs [3]. The content of the absorbed hydrogen into the cladding tubes depends on many factors and is part of ongoing research work. In any case, the amount of hydrogen increases with the increase in the fuel burn-up, hence hydrogen content is an increasing quantity with respect to reactor cycles [4]. Possible degradation mechanisms of fuel claddings include [5]: creep [6], hydrogen embrittlement (reduction in ductility) [7] and Delayed Hydride Cracking [8, 9] etc.

Figure 1 shows the phase diagram of the binary Zr-H system. Different phases of zirconium hydrides are visible. The δ-zirconium hydride phase has a face centered cubic structure (ZrH1.66), γ-hydride phase has a body centered tetragonal structure (ZrH) and ε-hydride phase has a face centered tetragonal structure (ZrH2). δ- and ε- hydrides are the two stable hydride phases in the form of platelets, while the γ-hydrides are the metastable hydrides in the form of needles. ε- hydrides are observed at very high hydrogen contents and γ-hydrides are formed in quenched specimens [3].

Keywords: Zirconium alloys; Nuclear fuel claddings; Creep; Hydrides orientation; Deformation

Article history: Received: 07 April 2017 Revised: 22 January 2018 Accepted: 23 January 2018



Figure 1. (a) Phase diagram of binary Zr-H system with details of the (α Zr) phase region [3]

Table 1: Chemical composition of E110 cladding

After the usage of fuel assemblies for a period of time, they are firstly sent to the wet storage in water for heat decay and then sent later to final disposal, called dry storage. Initial temperatures at the dry storage are higher than wet storage as the specific heats of cooling media are lower than water. Under this situation at specific conditions (internal pressure, temperature, hydrogen content) creep [10], hydride re-orientation [11-12] and related effects at spent fuel dry storage [13-15] are the most likely degradation mechanisms that could cause failure in fuel claddings. Therefore the study on mechanical integrity of the fuel cladding tubes is very important.

Current study is focused on examination of microstructural changes during creep of hydrided E110 cladding at dry storage conditions (high temperature) and results of failure are presented. In the present work, irradiation effect is excluded to enable to investigate the stress induced hydride reorientation during the creep of cladding at the given test conditions in detail.

2. Experiment Specimen examined in this study was fabricated from E110 alloy which has tubular section, and is 90 mm long with outer diameter of ~ 9.1 mm and wall thickness of ~

686 µm. The chemical composition of E110 cladding tube is given in Table 1. In this work, specimen preparation and creep test was performed at UJP, while the microstructural examinations were carried out at CVR.

2.1. Creep test

Test specimen was oxidized in an autoclave at a pressure of 10.7 MPa and temperature of 425 °C to reach hydrogen content of 203 ppm. After the autoclave oxidation, end-plugs (made of same material) were welded to the test specimen on either side by electron beam welding. Test specimen was then drilled and filled with argon gas to have internal pressure of 9.6 MPa, as hoop stress in the current experiment.

Creep test was performed in a horizontal furnace with inert atmosphere at temperature of 450 °C and exposure time of 39 hours. The exposition was divided in 13 periods. After each period the sample was measured (length, diameter, volume and weight gain). Slow cooling at a rate of 1°C/min was used at end of the creep test. Figure 2 shows the image of the deformed and ballooned specimen after the creep test with internal pressure of 9.6 MPa. To determine the microstructural changes and to evaluate the re-oriented hydrides (radial hydrides) after creep test, 2 locations were chosen from the deformed specimen as shown in Figure 3 (i) maximum ballooning (corresponds to maximum stressed region) and (ii) un-ballooned region. Schematic sectioning of specimen for analysis by optical microscope and transmission electron microscope is shown in detail in Figures 4 and 5. Specimens were analysed in the circumferential and longitudinal direction of the tube axis. After sectioning and before microscopy analysis, internal diameter as well as thickness of the individual sections were measured.

2.2. Specimen sectioning and metallography

Before the microscopic examinations, the samples were embedded into the cold epoxy resin. First half section of the sample is embedded in the tubes Axial Direction (AD), as shown in Figure 5a and the second half section is embedded in Circumferential Direction (CD) as represented in Figure 5b. The steps of metallography process are described in Table 2. The samples for Optical Microscopy analysis were prepared by etching them in a solution of 100 ml H2O, 4.5 ml HNO3 and 0.5 -1 ml HF, to inspect the hydride distribution in the cladding.

Samples for Transmission Electron Microscopy (TEM) analysis were obtained by cutting along the axial direction of the pipe to 6 segments, as shown in fig. 5c. From each segment, electron transparent TEM foils were prepared in diameter of 3 mm with the final

Element Zr Nb Fe Oxide layer

Concentration (wt. %) 99 1

Impuritires 500 (ppm)

10 μm



thickness of 55 µm. The foils were further electro-polished by using Fishione twin-jet electropolisher

(Model 140) in an electrolyte (5% solution of HClO4 in C2H5OH) at - 60 °C with voltage ranged 28 to 35 V.

Figure 2. Appearance of fuel cladding

after creep test Figure 3. Schematic showing selected deformed locations for

microstructural examinations

Figure 4. (a) Schematic representation of the reference tube (RD- Radial Direction, AD- Axial Direction & CD-

Circumferential Direction); (b) longitudinal and transverse sectioning planes.

Figure 5. Sectioned sample embedding in an epoxy resin (a) in the axial / longitudinal direction, (b) in transverse

direction (c) TEM specimen sectioning in to segments in Axial – Circumferential plane (left), preparation of TEM foils from each segment (right)



Figure 6. Measurements of internal diameter [Di] and thickness [t] of selected deformed sections 1-maximum, 2- intermediate & 3-unballooned region (in mm)

3. Results and discussion

3.1. General observation In Figure 2, the visual appearance of the cladding tube test specimen after creep test is shown. The deformation and ballooning of the cladding tube is clearly visible. There are no visible indications of opening or wide split, cracks and/or cavities on the outer surface of the test specimen. Figure 6, depicts the measurement of internal diameter and thickness of deformed regions as well as the diametric deformation in the maximum and un-ballooned regions.

3.2. Metallographic observations

3.2.1 Hydride morphology and distribution

Transverse (cross-sectional) and longitudinal metallographic images from the ballooned and un-ballooned regions are shown in Figures 7 and 8. A uniform layer of oxide thickness in all three cases is evident, which was formed during autoclave oxidation process. There is no presence of micro cracks in the radial direction of the tube but the oxide layer appeared to be damaged. The ballooned region surface of the crept specimen shows a region which appears uneven and contains depression marks, as in Figure 7 (a-b).

As seen in the Figures 7-8, optical examination of the maximum ballooned (section 1) and un-ballooned regions (section 3) revealed the presence of hydride platelets aligned in parallel to the loading direction i.e. hoop stress direction. These are called circumferential

hydrides formed on the circumferential-longitudinal planes and are in the form of long chains. In both regions, the average length of macro hydrides in the longitudinal direction was 180 ± 12 µm and in the circumferential direction was 200 ± 8 µm. Grains and grain boundaries are however not discernible optically, and thus, we cannot obtain any detailed information regarding location of the hydrides with respect to the grains and grain boundaries.

During the creep test at the applied temperature i.e. 450 oC, hydrogen is in dissolved state (applied temperature is above the hydride dissolution temperature). This is evident from the binary phase diagram of Zr-H system [3], as was shown in Figure 1. During the cooling phase of the test with decrease in temperature, hydrogen diffused to the local precipitation planes, up on reaching the local solubility limit, starts to form hydride precipitates in circumferential-longitudinal plane of the tube [6]. Figure 9, shows the 3D re-construction of the observed hydrides on the circumferential-longitudinal plane. In the Figure 9, on the right side is shown the habit planes of the circumferential hydrides in hexagonal zirconium lattice [6]. No Radial hydrides are observed. This indicates that the applied stress of 9.6 MPa was not sufficient enough to precipitate radial hydrides and that the stress induced hydride re-orientation mechanism wasn´t involved.

3.3. TEM observations

TEM microstructural studies were performed on the crept specimen to determine the local microstructural features at the deformed and un-deformed region. TEM



thin foils were prepared from 2 regions of the cladding tube in the longitudinal direction as in Figure 5c. Figure 10 shows microstructure encountered at the 2 regions, in the bright as well as dark field TEM imaging mode. The observed areas have mixture of polyhedral equiaxed grains, which contain a large number of precipitates of secondary phases within the grains and along their borders numerous dislocations are also present. The grain boundaries are straight or curved and often

occurring sub-grains and twinning. Electron diffraction determined the grain structure as in Figure. 11, which is formed by phase α -Zr of hexagonal lattice with the lattice parameters a = b = 3.136 Å, c = 5.039 Å, α = β = 90°, γ = 120°, the grain size ranges ~ 3-5 μm as seen in Figure 10. The grains are mostly equiaxed, as shown in Figure 11. No obvious grain elongation was observed in the longitudinal/axial direction.

Figure 7. Maximum ballooned region (a-d) Hydrides oriented in the Radial-Circumferential orthographic plane;

and (e-h) Hydrides oriented in the Radial-Axial orthographic plane

(b)

(c) (d)

(e) (f)

(g) (h)

(a)



Figure 8. Un-ballooned region (a-d) Hydrides oriented in the Radial-Circumferential orthographic plane;and (e-f) Hydrides oriented in the Radial-Axial orthographic plane

Figure 9. 3D reconstruction of hydrides orientation in the creep tested hydrided E110 cladding and schematic of hydride formation on hydrides habit plane [6]

(a) (b)

(c) (d)

(e) (f)



Maximum ballooned . Un-ballooned

Figure 10. Grain Morphology α-Zr phase in bright field (BF) with the presence of secondary phase particles

Maximum ballooned

Un-ballooned

Figure 11. Grain Morphology α-Zr phase in bright field (BF) with the presence of secondary phase particles and dislocations



Secondary precipitate particles (SPP) within the grains are analysed in TEM in low and high magnification. SPP’s in the grains are of different size, shape and distribution within and along the grain boundaries as in Figure 12-13. The density of precipitates ρ ~5.32e19 m-3 are determined as the total number of particles (n = 463) to the sample volume. The mean particle size is 62 ± 3.4 nm. Secondary Laves phase Zr(Nb,Fe)2 and β -Nb particles were identified by electron diffraction. Further particles were identified as hydrides with typical morphology (Figure 15) of a length of 1-1.5 microns.

Hydrides were observed inside the matrix grains of α - Zr and growing along the grain boundaries.

At the deformed/ballooned region more dislocations were formed in the grain interiors and as well the dislocation density was increased in comparison to un-deformed/un-ballooned region, as evident in Figures 10-12 and 15. There is no noticeable difference in the secondary precipitate particle distribution and grain size in both the regions. Ballooning/deformation and increase in the diameter with reduced wall thickness of the cladding is associated with the grain boundary

Figure 12. The distribution of precipitates inside grains having dislocations, BF (left, maximum ballooned region), and inside and on the grain boundaries, BF (right, un-ballooned region)

Figure 13. Secondary particles Zr(Nb,Fe)2 inside the grain α-Zr BF (left) and the corresponding diffractogram where the [1 1 -2 3] zone matrix is nearly parallel

to the [1 1 -2 3] phase zone Zr(Nb,Fe)2 (right, ballooned region).



Figure 14. Secondary phase particles of Zr (Nb, Fe)2 inside grains in BF (left) and DF (centre) and the respective diffraction used for DF (right, un-ballooned region)

Figure 15. Micro hydrides in cladding sections of maximum (top) and un-ballooned (bottom) regions



sliding and increase in the dislocation density in the grains, microstructure comparison from Figure 11, ballooned and un-ballooned regions. This was also evident and reported in other investigations [17-22].

4. Conclusions Microstructural examination of hydrided E110 cladding sample from the ballooned regions after the creep test, has been carried out. The main results of the characterisation are as follows:

1. The hydrided cladding deformed due to creep at high temperature i.e. 450 oC displayed ballooning at location with increase in the diameter and zirconium hydrides precipitated in the direction parallel to applied pressure/ hoop stress, i.e. circumferential direction of the tube.

2. In the ballooned regions, no presence of radial hydrides i.e. precipitated in the radial direction of tube was observed. The applied stress i.e. 9.6 MPa could not cause formation of radial hydrides at given temperature. No stress induced hydride re-orientation was observed.

3. This concludes that stress level is below the critical level for formation of radial hydrides, which can cause fracture of the tube/ split.

4. From the results of TEM characterisation, increase in dislocations were observed in the grain interiors and as well their density was higher in ballooned region compared to the un-ballooned/un-deformed region.

Acknowledgements The presented work was financially supported by the Ministry of Education, Youth and Sport Czech Republic Project LQ1603 (Research for SUSEN). This work has been realized within the SUSEN Project (established in the framework of the European Regional Development Fund (ERDF) in project CZ.1.05/2.1.00/03.0108).

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Study of Creep and Hydride Re-Orientation Behaviour in ...contemporary-energy.net/v04n01a06-Hygreeva-Kiran-Namburi.pdf · Figure 6. Measurements of internal diameter [Di] and thickness