Revisiting the method to obtain the mechanical properties of hydrided fuel cladding in the hoop direction

The method reported in the literature to calculate the stress–strain curve of nuclear fuel cladding from ring tensile test is revisited in this paper and a new alternative is presented. In the former method, two universal curves are introduced under the assumption of small strain. In this paper it is shown that these curves are not universal, but material-dependent if geometric nonlinearity is taken into account. The new method is valid beyond small strains, takes geometric nonlinearity into consideration and does not need universal curves. The stress–strain curves in the hoop direction are determined by combining numerical calculations with experimental results in a convergent loop. To this end, ring tensile tests were performed in unirradiated hydrogen-charged samples. The agreement among the simulations and the experimental results is excellent for the range of concentrations tested (up to 2000 wppm hydrogen). The calculated stress–strain curves show that the mechanical properties do not depend strongly on the hydrogen concentration, and that no noticeable strain hardening occurs. However, ductility decreases with the hydrogen concentration, especially beyond 500 wppm hydrogen. The fractographic results indicate that as-received samples fail in a ductile fashion, whereas quasicleavage is bserved in the hydrogen-charged samples

Axial strength of Zircaloy-4 samples with reduced thickness after a simulated loss of coolant accident

To investigate wall-thinning impact on axial load resistance of Zircaloy-4 cladding rods after a LOCA transient, axial tensile samples have been machined on as-received tubes with reduced thicknesses between 370 and 580 μm. After high temperature oxidation under steam at 1200 °C with measured ECR ranging from 10 to 18% and water quenching, machined samples were axially loaded until fracture. These tests were modeled using a fracture mechanics approach developed in a previous study. Fracture stresses are rather well predicted. However, the slightly lower fracture stress observed for wall-thinned samples is not anticipated by this modeling approach. The results from this study confirm that characterizing the axial load resistance using semi-integral tests including the creep and burst phases was the best option to obtain accurate axial strengths describing accurately the influence of wall-thinning at burst region

Influence of outer zirconia transient cracking and spalling on thermomechanical behaviour of high burnup fuel rod submitted to RIA

The French “Institut de Radioprotection et de Sîreté 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. © 2006 Taylor and Francis Group, LLC

Combined influence of pressurization and bendingof pre-hydrided unirradiated rod sectionsafter simulated transportation conditions

International audienceDuring the transportation of spent fuel assemblies, filling gas and fission gas release pressurize the fuel rods contained in the transportation cask. The rods are heated by their residual power, increasing the pressurization stresses and facilitating radial hydride precipitation during subsequent cooling. Radial hydrides have a detrimental influence on cladding ductility in some loading situations.In the present study, radial hydrides formed during simulated transportation are first evaluated, then their influence on pressurized fuel rod bending resistance is studied using a four point bend test device.The samples are pre-hydrogenated SRA Zircaloy-4 cladding tubes with alumina pellets inserted within the cladding tube. The samples are then tightened with Swagelocks and pressurized. The four point bend device is first used to precipitate radial hydrides using combined internal pressurization and bending imposed to the rod samples to simulate the fuel assembly transportation. The radial hydride precipitation heat treatment consists in heating the device at 350°C to dissolve hydrogen and cooling it slowly. After this, to simulate a lateral impact of fuel assemblies, the pressurized sample is loaded at room temperature under combined internal pressurization and four point bending. This type of loading induces a combination of axial and hoop stresses varying in the azimuthal direction between the two central rollers of the four point bending device. Internal pressurization and fuel pellets presence are shown to significantly strengthen the samples. Besides a rather negligible influence of radial hydrides on sample failure was observed under this loading configuration. After this mechanical test, the sample is cut and examined to check radial hydrides distribution at different azimuthal locations. These examination allows analyzing axial to hoop stress ratio influence on radial hydride precipitation but also to better understand the strengthening influence of fuel pellets and internal pressurization

Combined influence of pressurization and bendingof pre-hydrided unirradiated rod sectionsafter simulated transportation conditions

International audienceDuring the transportation of spent fuel assemblies, filling gas and fission gas release pressurize the fuel rods contained in the transportation cask. The rods are heated by their residual power, increasing the pressurization stresses and facilitating radial hydride precipitation during subsequent cooling. Radial hydrides have a detrimental influence on cladding ductility in some loading situations.In the present study, radial hydrides formed during simulated transportation are first evaluated, then their influence on pressurized fuel rod bending resistance is studied using a four point bend test device.The samples are pre-hydrogenated SRA Zircaloy-4 cladding tubes with alumina pellets inserted within the cladding tube. The samples are then tightened with Swagelocks and pressurized. The four point bend device is first used to precipitate radial hydrides using combined internal pressurization and bending imposed to the rod samples to simulate the fuel assembly transportation. The radial hydride precipitation heat treatment consists in heating the device at 350°C to dissolve hydrogen and cooling it slowly. After this, to simulate a lateral impact of fuel assemblies, the pressurized sample is loaded at room temperature under combined internal pressurization and four point bending. This type of loading induces a combination of axial and hoop stresses varying in the azimuthal direction between the two central rollers of the four point bending device. Internal pressurization and fuel pellets presence are shown to significantly strengthen the samples. Besides a rather negligible influence of radial hydrides on sample failure was observed under this loading configuration. After this mechanical test, the sample is cut and examined to check radial hydrides distribution at different azimuthal locations. These examination allows analyzing axial to hoop stress ratio influence on radial hydride precipitation but also to better understand the strengthening influence of fuel pellets and internal pressurization

Effect of pre-oxide on Zircaloy-4 high temperature steam oxidation

International audienceAn experimental study has been carried out on the high temperature oxidation behavior of pre-oxidized Zircaloy-4. Prior to steam oxidation, cladding tubes have been pre-oxidized in moistened oxygen environment at 425°C, up to pre-oxide layer thicknesses of 35 and 65 µm. All samples have then been subjected to isothermal high temperature steam oxidation, in the 900°C-1200°C temperature range, followed by water-quenching. Metallographic examinations and mean hydrogen content measurements have then been performed. For all tested temperatures, the weight gains measured for pre-oxidized samples are lower than those obtained for bare cladding, indicating that the pre-oxide has a protective effect against oxidation at high temperature. The results obtained on the O2+steam pre-oxidized samples are in good agreement with published data on autoclave-pre-oxidized samples.Comparison with literature data obtained on irradiated cladding shows that, at 1000°C and 1200°C, laboratory-grown unirradiated pre-oxidation layers have a stronger protective influence than a corrosion layer formed under irradiation. This difference in protectiveness may be explained by a more heterogeneous oxide microstructure and a higher intergranular porosity for in-reactor corrosion in comparison with laboratory pre-oxidation conditions

Development of a manufacturing process for a simulating fuel for the study of fine fragmentation in LOCA

Development of a manufacturing process for a simulating fuel for the study of fine fragmentation in LOCA Mehdi Belqat (IRSN) Fabrice Rossignol (IRCER), Tatiana Taurines (IRSN) , Jean Desquines (IRSN) Context of the study-fuel fragmentation Fuel pellets undergo significant changes during their life in the reactor. Their condition is also potentially degraded during an incident or accident

Fracture mechanics analysis of Zircaloy-4 tubular samples after laboratory simulated LOCA transient

International audienceThis paper investigates the room temperature cladding embrittlement of Stress Relieved Annealed (SRA) Zircaloy-4 fuel cladding samples subjected to laboratory steam oxidation tests simulating Loss of Coolant Accident (LOCA) transients at 1 200 °C followed by water quenching. These high temperature oxidized tubes are mechanically tested using axial tensile samples with machined gage sections. Formerly performed studies suggested that Linear Elastic Fracture Mechanics (LEFM) could provide a good understanding of the sample failure process, including crack nucleation close to the oxidized surfaces followed by crack instability and sample failure at higher applied loads. However, it was found that when the applied stress intensity was calculated using solutions for plate material given in the Tada and Paris Handbook, with the cracks in the plate corresponding to a crack formed only on the cladding's outer surface – produced by oxidation on this surface alone – versus two opposing cracks formed on the cladding's inner and outer surfaces – produced by oxidation on both of these surfaces – the critical stress intensity was different between these two cases. It is shown here that for both of the foregoing configurations the same critical crack intensity values are obtained when the applied LEFM stress intensity values are calculated using three dimensional finite element modeling of the axial tensile samples. The critical stress intensity factors determined by this more accurate method – and calculated as a function of degree of surface oxidation and hydrogen content in the prior-β phase – could, thus be considered to be a true materials parameter correlating well with the experimentally determined effect of hydrogen content on the failure strength of the samples. © 2018 Elsevier Lt

Embrittlement of pre-hydrided Zircaloy-4 by steam oxidation under simulated LOCA transients

International audienceDuring a Loss Of Coolant Accident (LOCA), the mechanical behavior of high temperature steam oxidized fuel rods is an important issue. In this study, as-received and pre-hydrided axial tensile samples were steam oxidized in a vertical furnace and water quenched in order to simulate a LOCA transient. The samples were then subjected to a mechanical test to determine the failure conditions. Two different rupture modes were evidenced; the first one associated to linear elastic fracture mechanics and the second one is associated to sample failure without applied load. The oxidized cladding fracture toughness was determined relying on intensive metallographic analysis. The sample failure conditions were then back predicted confirming that the main rupture parameters are well captured. © 2015 Elsevier B.V