Transient failure of zircaloy cladding (original) (raw)
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Engineering Failure Analysis, 2010
The anisotropic plastic behavior and the fracture of as-received and hydrided cold-worked stress relieved Zircaloy-4 cladding tubes are investigated under thermal-mechanical loading conditions representative of pellet-clad mechanical interaction during reactivity initiated accidents in pressurized water reactors. In order to study the combined effects of temperature, hydrogen content, loading direction and stress state, axial tensile, hoop tensile, expansion due to compression and hoop plane strain tensile tests are performed at room temperature, 350 • C and 480 • C on the material containing various hydrogen contents up to 1200 wt. ppm (hydrides are circumferential and homogeneously distributed). These tests are combined with digital image correlation and metallographic and fractographic observations at different scales. The flow stress of the material decreases with increasing temperature. The material is either strenghtened or softened by hydrogen depending on temperature and hydrogen content. Plastic anisotropy depends on temperature but not on hydrogen content. The ductility of the material decreases with increasing hydrogen content at room temperature due to damage nucleation by hydride cracking. The plastic strain that leads to hydride fracture at room temperature decreases with increasing hydrogen content. The influence of stress triaxiality on hydride cracking is negligible in the studied range. The influence of hydrogen on material ductility is negligible at 350 • C and 480 • C since hydrides do not crack at these temperatures. The ductility of the material increases with increasing temperature. The evolution of material ductility is associated with a change in both the macroscopic fracture mode of the specimens and the microscopic failure mechanisms.
Behavior of zirconium fuel cladding under fast pressurization rates
Nuclear Engineering and Design, 2008
Rapid pressurization test was carried out to evaluate the mechanical behavior of the zirconium cladding under a fast strain rate as well as a biaxial stress state for simulating an out-of-pile reactivity initiated accident (RIA) behavior. Influence of temperature, hydrogen content and alloying elements have been addressed in the conducted mechanical tests. The results showed that pressurization rates of 5.4 GPa/s at room temperature and 3.1 GPa/s at 350 • C were achieved. The corresponding time to failure was similar to expected power transient duration during a RIA. Maximum hoop stress of Zircaloy-4 at room temperature and 350 • C increased, respectively by 24.3 and 16.8% when compared to the conventional burst test results. Failure mode switched from a ductile ballooning to a brittle failure which leads to an axial split of the cladding when the hydrogen was added at a nominal value of 600 ppm. When the test temperature increased, its effect was diminished. Addition of an alloying element influenced the mechanical property differently. Niobium acted beneficially against hydrogen embrittlement in that it increased the ductility of the metal matrix.
Quench Behaviour of the Zircaloy Fuel Rod Cladding Tubes
A series of separate-effects tests has been carried out at Forschungszentrum Karlsruhe on Zry-4 and Zr1%Nb fuel rod cladding to study the enhanced oxidation which can occur during quenching. In these tests single rod specimens were heated by induction to high temperatures and then rapidly cooled down by injection of a cold steam. The principal investigated parameters are the extent of the pre-oxidation (100 – 300 µm) and the surface temperature at the onset of cooldown (1100 –1600 °C). In particular the main objectives of the programme are to provide an extensive experimental database for the development of the detailed mechanistic quench models and to investigate the physico-chemical behaviour of the overheated fuel elements under different flooding conditions.
Proceedings of 18th …, 2005
Several high-burnup PWR-fuel-claddings associated to high corrosion levels have been subjected to RIA transients in the CABRI facility. The zirconia layer surrounding the Zircaloy-4 fuel rods exhibited a complex behavior during the RIA transients. Regularly spaced incipient cracks are initiated in the zirconia layer and stop at the Zircaloy interface beneath the oxide. These cracks are mainly initiated in the direction normal to the maximum principal stress. The crack density in post-test metallographies appears to be tightly linked to the maximum applied strain. This cracking process is sometimes followed by oxide spalling. Oxide spalling has a major impact on thermal-mechanical fuel rod behavior during such transients, thus the understanding and modeling of spallation is an issue that deserves attention. The present paper provides a quantitative analysis of the related data generated within the scope of the CABRI REP-Na tests, in the last decade, and more recently in the first tests of the CABRI Water loop program. A link with several other mechanical testing programs such as tensile tests, creep tests is also established.
Failure of hydrided zircaloy-4 fuel cladding tubes under RIA loading conditions
The anisotropic viscoplastic behavior and the fracture of cold-worked stress relieved Zircaloy-4 cladding tubes is investigated under reactivity initiated accidents loading conditions. The combined effects of temperature (from 25°C up to 480°C), hydrogen content (from 0 up to 1200 ppm) and stress/stain state (from uniaxial tension up to plane strain tension) are analyzed. A strengthening effect of hydride precipitates and a softening effect of dissolved hydrogen are observed. Ductility of the material increases with increasing temperature and decreases with increasing hydrogen content at room temperature. The embrittlement effect of hydrides is substantially reduced when increasing temperature. A slight effect of stress/strain triaxiality is evidenced. The evolution of material ductility is associated with a change in the macroscopic fracture aspect and the failure mechanisms. A model is proposed to describe the anisotropic viscoplastic behavior of the material.
1999
A series of separate-effects tests has been carried out at Forschungszentrum Karlsruhe on Zry-4 and Zr1%Nb fuel rod cladding to study the enhanced oxidation which can occur during quenching. In these tests single rod specimens were heated by induction to high temperatures and then rapidly cooled down by injection of a cold steam. The principal investigated parameters are the extent of the pre-oxidation (100 -300 µm) and the surface temperature at the onset of cooldown (1100 -1600 °C). In particular the main objectives of the programme are to provide an extensive experimental database for the development of the detailed mechanistic quench models and to investigate the physico-chemical behaviour of the overheated fuel elements under different flooding conditions.
Rupture of spent fuel Zircaloy cladding in dry storage due to delayed hydride cracking
Nuclear Engineering and Design, 2008
Delayed hydride cracking in the Zircaloy alloy has been considered as a possible degradation mechanism of spent nuclear fuel cladding in interim dry storage. Some recent in-core fuel failures indicated that a long axial crack developed in the cladding was a secondary failure by delayed hydride cracking. The aim of this study is to define the effects of hydride reorientation on the failure of Zircaloy cladding. Different hydride orientations, the amount of zirconium hydride and various cracking types, all have been considered for their effects on the crack growth and stability of the cladding, and have been thoroughly discussed in this paper. A finite element computer code, ANSYS, has been used in conjunction with the strain energy density theory. In summary, the crack propagation will be aggravated if the hydride orientation is shifted from the circumferential to the radial direction. For a larger crack length, the zirconium hydride plays an important role in affecting the crack growth because the strain energy density factor increases as the hydride approaches the crack tip. Furthermore, when thermal effects are considered, a compressive stress exists at the inner side of the cladding, while a tensile stress is found at the outer side of cladding, thus resulting in crack propagation from the outer side to the inner side of the cladding. These findings are in accordance with other experimental results in related literature.
Journal of Nuclear Science and Technology, 2006
The French ''Institut de Radioprotection et de Sûreté Nucléaire'' (IRSN) conducted the REP-Na tests in the CABRI reactor within the framework of its research program on nuclear fuel safety. These tests were devoted to the study of Reactivity Initiated Accident (RIA). Cracking and spalling of the fuel rod zirconia layer were observed after several REP-Na tests. Sometimes, an ovalisation of the rod after the RIA transient is also observed. Metallographic examinations showed that the outer and inner zirconia cracks are regularly spaced and that the crack density is linked to the clad plastic hoop strain. An analogy with brittle thin film layering a ductile substrate submitted to a tensile test is made and helps to understand this specific phenomenon. A numerical simulation evaluates the thermo-mechanical behaviour of the rod, including the zirconia influence during a RIA. This work make it possible both to identify the spalling process and to clarify the preferential spalling along the less corroded azimuths for several tests. The influence of the transient spalling on the boiling crisis occurrence in PWR condition is finally addressed.
Numerical modeling of fuel rod transient response under out of pile test conditions
Progress in Nuclear Energy, 2019
As per revised Emergency core cooling system (ECCS) acceptance criteria, a precise prediction of fuel rod behavior is essential for realistic safety analysis of nuclear reactor. In this context, a one-dimensional code name 'TRAFR' (Transient Response Analysis of Fuel Rod) is developed to simulate the thermo-mechanical behavior of Zircaloy-4 cladding under transient conditions. The transient simulations for inert and oxidizing atmosphere were performed under out of pile test conditions and the predicted burst strains were in good agreement with the experiments conducted in past. Under inert atmosphere, the cladding rupture was delayed and burst strain was higher in all the phases due to the absence of oxidation kinetics. In the oxidizing atmosphere, the burst strain was considerably small at high temperature in mix phase (α+β) and β-phase due to increment in cladding strength and reduction in ductility. For the same internal pressure and clad surface boundary conditions, the temperature of failure was higher for oxidizing atmosphere due to heat generation by exothermic reactions at the surface of the cladding. The clad surface temperature rise rate decreased before the burst with increment in gap width between pellet and cladding owing to decrement in gap conductance. The code under-predicted the burst strain in the mix phase (α+β) and β-phase by 'Baker-Just' model. The reason for such deviation was abrupt parabolic oxide growth prediction with higher exothermic heat generation and subsequent faster reduction in cladding thickness which made 'Baker-Just' model approach more conservative than 'Cathcart-Pawel' model.
The effect of strain biaxiality on the fracture of zirconium alloy fuel cladding
Journal of Nuclear Materials, 2021
During a Reactivity Initiated Accident (RIA), nuclear fuel cladding experiences a multiaxial loading state in which the Pellet-Cladding Mechanical Interaction (PCMI) produces a strain biaxiality ratio (εzz/εθθ) of between 0 and 1. This study examines the effect of strain state and loading path on the fracture of Zircaloy-4 fuel cladding. A new mechanical test has been developed in order to apply several levels of strain biaxiality. Digital Image Correlation (DIC) is used to measure the strain field, to identify the onset of cladding failure, and to obtain the corresponding critical loading state. Results show that the strain biaxiality has a significant effect on the hoop strain at failure, and the smallest fracture strain is obtained for nearly plane strain conditions. A model is proposed for the accumulation of damage due to plastic deformation, based on the Lode parameter of the plastic strain rate tensor. The model is used to predict the failure hoop strain as a function of the strain biaxiality ratio (εzz/εθθ), and good agreement is found between the experimentally measured and predicted failure strains.