1. XI конференция по реакторному материаловедению, Россия,
Димитровград, мая 2019 г.
RESULTS OF POST-IRRADIATION EXAMINATIONS OF THE SPONGE-BASED E100 OPT ALLOY OPERATED AS FUEL CLADDING MATERIAL IN ALTERNATIVE TVSA-ALPHA UP TO ATTAINING A BURNUP OF 42 MW DAY/KGU TO ESTABLISH THE EXPERIMENTAL DATABANK IN SUPPORT OF RUSSIAN FUEL LICENSING FOR PWR PLANTS
© Yu.D. Goncharenko, S.G. Eremin, E.V. Chertopyatov, A.V. Obukhov, T.M. Bulanova,
G.M. Shishalova
(JSC “SSC RIAR”, Dimitrovgrad, Russia)
A.Yu. Shevyakov, S.A. Bekrenev, V.V. Novikov, and V.A. Markelov, 2019
(JSC “VNIINM”, Moscow, Russia)

2. Structural examinations of cladding
To fill the databank on irradiated cladding materials properties, including licensing of Russian fuel for Western design reactors, the study of structural characteristics of the irradiated fuel rod claddings and measurements of hydrogen content were added with the determination of cladding mechanical properties in the longitudinal direction.
Metallography
Microstructure of cladding after polishing:
at the outer surface (left)
at the inner surface (right)
On the outer and inner side of the cladding, along fuel rod entire length, there is an oxide film tightly adhered to the metal.
The oxide film thickness on the cladding outer increases uniformly from 5μm in the lower part of the fuel rod to 10–12μm in the upper part.
On the inner side of the cladding, the oxide film thickness is the same along the entire length of the fuel rod and is about 10μm .
Typical microstructure of cladding cross-section after etching

3. Structural examinations of cladding
Metallography
Cladding lower part
Cladding upper part
Hydrogen content
Oxide film thickness
The cladding microstructure contains point and extended hydrides.
Extended hydrides have a predominantly tangential orientation.
The maximum length of extended hydrides does not exceed 80μm along the entire length of a fuel rod.
Hydrides are evenly distributed over the cladding cross-section.
Mass fraction of hydrogen in the lower part of cladding is %, in the upper part %.
A change in the hydrogen concentration along the fuel rod length corresponds to a change in the oxide film thickness on the cladding outer surface.

4. Structural examinations of cladding
TEM
The cladding material is fully recrystallized and consists of equiaxed α-Zr grains of about 3 microns in size, within which particles of the secondary phases are evenly distributed.
Three types of secondary phase precipitates were found in the cladding microstructure :
- β-Nb phase particles ;
- Laves phase particles
Zr(Nb, Fe) 2;
- fine radiation-induced phase.
Typical cladding microstructure at magnifications
~15 000х and ~ х

5. Structural examinations of cladding
TEM
β-Nb particles
The main characteristic of alloy E110 secondary phase is the β-Nb phase (BCC lattice). Particles of this phase have a globular shape, the average size is about 60 nm. The concentration of this phase is approximately 1×1020m-3.
The content of niobium in β-Nb particles depends on their equivalent diameter.
In the microstructure of as-fabricated sponged alloy E110opt. before irradiation, β-Nb particles contain about 90 at. % Nb.
Content of Nb (at. %) в частицах β-Nb vs. equivalent diameter: - cladding upper part; cladding middle; cladding lower part.

6. Structural examinations of cladding
TEM
Laves phase particles
Laves phase particles Zr (Nb, Fe) 2 (intermetallic compound with HCP lattice) also have a globular shape.
The concentration of particles in the Laves phase does not exceed 7·1018m-3.
The Laves phase particle diameter is in the range from 100 nm to 300 nm.
The Laves phase particles contain approximately 50 at. % Zr, and Nb, and the amount of iron in the particles is close to the detection limit of energy dispersive spectrometer.
In as-fabricated sponged alloy E110opt. Before irradiation, the content of Fe in Laves phase particles achieves 30 at. %.
Laves phase particles among β-Nb particles

7. Irradiation-induced precipitates
Structural examinations of cladding
TEM
Irradiation-induced precipitates
Distribution of particles of fine irradiation-induced phase in size in the cladding middle
Zone free of fine irradiation-induced phase particles with a prismatic orientation of the foil and the current vector
g=[002]
The third type of secondary phase detected - irradiation-induced precipitates.
Lamellar irradiation-induced precipitates (IIP) are formed under irradiation in niobium-doped zirconium alloys.
IIP particles lie in the basal planes of the crystal lattice.
The average IIP size is about 5 nm.
The concentration of IIP in the fuel cladding material is approximately 2·1022m-3.

8. Structural examinations of cladding
TEM
Reactor irradiation led to the formation of radiation defects in the form of dislocation loops and <c> dislocations. The average size of the dislocation loops is in the range from 13nm to 17nm. Radiation <с> - dislocations are evenly distributed in the grain body regardless the location of the β-Nb phase particles. независимо от расположения частиц фазы β-Nb.
In addition to the above phases and radiation defects, single hydrides up to 5µm long were found .
Hydrides in the upper (left) and middle (right) part of the cladding

9. Mechanical tests
The short-term mechanical properties of the irradiated fuel claddings in the longitudinal direction were determined on segmental samples.
Segmental samples were cut off from cladding fragments after removal of the fuel core at the EDM with a graphite electrode.
Segmental sample cut off from the cladding
Mechanical tests were carried out in argon in the temperature range from room temperature to 600°C on a universal testing machine located in the hot cell.
Segmental samples tested at room temperature and at 380°С, and 600°С

10. Mechanical tests
All test diagrams of longitudinal samples, as well as transverse ring samples, correspond to the plastic nature of fracture. It was determined that the plastic and strength properties of the fuel cladding in the longitudinal direction do not change significantly along the fuel rod length at a room temperature and at 380°C., Yield strength is in the range of 540 to 590 Mpa at a room temperature. At a test temperature of 380°C, it is in the range from 370 to 420 MPa.
In the transverse direction at room temperature, the yield strength is in the same range from 540 to 580 MPa. At a test temperature of 380 ° C, the range of yield strength change longwise a fuel rod in the transverse direction is slightly lower MPa.
Changes in the cladding mechanical properties in the longitudinal ( ) and transverse ( ) directions longwise fuel rods at a room temperature (left) and test temperature 380°С (right)

11. Mechanical tests
Changes in the cladding mechanical properties in the longitudinal ( ) and transverse ( ) directions longwise fuel rods at a room temperature (left) and test temperature 380°С (right)
Yield strength decreases slightly with increasing altitude coordinates both in the transverse and longitudinal directions.
At all altitude coordinates, the yield strength in the longitudinal direction exceeds the yield strength in the transverse direction, both at a room temperature test and at test temperature 380 ° C.
The uniform elongation in the transverse direction is in the range from 1 % to1,5 % (Тtest = 380°C) and from 1,9 % to2,2 % (room T).

12. Mechanical tests
Cladding mechanical properties of sponged alloy E110 opt. spent in the TVSA-ALPHA determined in the longitudinal direction at a room temperature and f 380°C should be supplemented with changes in the cladding strength and plastic properties in the temperature range up to 600° C. .
Changes in the cladding mechanical properties in the longitudinal direction
vs. test T.
yield point and yield strength
uniform and total elongation

13. Conclusions
Based on PIEs of cladding made of sponged E110opt alloy from TVSA-ALPHA spent for 3 campaigns up to 42 MWday/kgU, the following can be concluded:
On the outer and inner side of the cladding longwise the fuel rod, there is an oxide film tightly adhered to the metal. The oxide film thickness on the cladding outer side increases uniformly from 5 microns at a level of 350 mm from the lower grid upper plane, to microns at a level of 2950 mm. On the cladding inner side, the oxide film thickness is the same longwise the fuel rod and is about 10μm.
Point and extended hydrides were found in the cladding microstructure longwise the fuel rod . Extended hydrides have a predominantly tangential orientation. The maximum length of extended hydrides is in the range of 80–90 μm. Hydrides are evenly distributed over the cladding cross-section.
Hydrogen mass fraction in the cladding does not exceed 0,004 % wt.
The cladding material is fully recrystallized and consists of equiaxed α-Zr grains of about 3 microns in size, within which β-Nb phase particles , Laves phase Zr (Nb, Fe) 2 particles, and a fine-dispersed irradiation-induced phase particles are evenly distributed. The chemical composition of the secondary phase particles differs from the composition of these phases prior to irradiation, and also varies with the secondary phase particles size .

14. Conclusions
Mechanical properties of the cladding material in the longitudinal direction are at a good level both at 380 ° C and at a room temperature.
Both the strength and plastic properties of the cladding material change insignificantly longwise fuel rod, both at 20°C and 380°C.
The uniform elongation ranges from1 % to1,5 % (Тtest = 380°C) and from1,9 % to2,2 % (Тtest = 20°C).
The yield point ranges from 370 to 420 MPa (Тtest = 380°C) and from 540 to 590 MPa (Тtest = 20°C).
When the test temperature changes in the range from room temperature to 600C, the uniform elongation changes slightly at test temperatures up to 400C and are in the range from 1.5% to 2%, and as the test temperature rises, it increase to 3.5 % at 600C.
The yield point measured during testing of these samples, varies from 600MPa, at a room temperature, to 400 MPa, at °C. With a further increase in the test temperature to 600°C, the yield point ​​quickly decrease s to 100MPa.
The cladding strength properties in the longitudinal direction are insignificant but exceed the strength properties in the transverse direction, both at a room temperature and at 380°C longwise the fuel rod

15. XI конференция по реакторному материаловедению, Россия,
Димитровград, мая 2019 г.
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