Effects of hydrogen content and temperature on fracture toughness of Zircaloy-4

The influence of hydrogen content and temperature on the fracture toughness of a Zircaloy-4 commercial alloy was studied in this work. Toughness was measured on CT specimens obtained from a rolled material. The analysis was performed in terms of J-integral resistance curves. The specimens were fatigue pre-cracked and hydrogen charged before testing them at different temperatures in the range of 293–473 K. A negative influence of the H content on material toughness was important even at very small concentrations, being partially restored when the test temperature increased. Except for some specimens with high H concentration tested at room temperature, the macroscopic fracture behaviour was ductile. The role of Zr-hydrides and Zr(Fe,Cr)₂ precipitates in the crack growth and the dependence with hydrogen content were analysed by observation of the fracture surfaces and determination of the Zr(Fe,Cr)₂ precipitates density on them.     

Evaluation of effect of hydrogen on toughness of Zircaloy-2 by instrumented drop weight impact testing

Hydride-assisted degradation in fracture toughness of Zircaloy-2 was evaluated by carrying out instrumented drop-weight tests on curved Charpy specimens fabricated from virgin pressure tube. Samples were charged to 60 ppm and 225 ppm hydrogen. Ductile-to-brittle-transition behaviour was exhibited by as-received and hydrided samples. The onset of ductile-to-brittle-transition was at about 130 °C for hydrided samples, irrespective of their hydrogen content. Dynamic fracture toughness (K_ID) was estimated based on linear elastic fracture mechanics (LEFM) approach. For fractures occurring after general yielding, the fracture toughness was derived based on equivalent energy criterion. Results are supplemented with fractography. This simple procedure of impact testing appears to be promising for monitoring service-induced degradation in fracture toughness of pressure tubes.     

The influence of hydride blisters on the fracture of Zircaloy-4

The fracture behavior under near plane-strain deformation conditions of Zircaloy-4 sheet containing solid hydride blisters of various depths has been examined at 25 and 300 °C. The study was based on material with either model ‘blisters’ having diameters of 2 and 3 mm or a continuous layer of hydride; in all cases, the substrate material contained discrete hydride precipitates. The fracture strains decrease rapidly with increasing hydride blister/layer depth to levels of about 100 μm deep, and then remain roughly constant. For a given blister depth, the material is significantly more ductile at 300 °C than at room temperature although measurable ductility is retained even at 25 °C and for large blister depths. The material is somewhat more ductile if the hydride is in the form of a blister than in the form of a continuous layer (rim). The hydride blisters/layers are brittle at all temperatures, and crack shortly after yielding of the ductile substrate. Consequently, both experimental evidence and analytical modeling indicate that fracture of the sheet is controlled by the crack growth resistance of the substrate at 25 °C. At elevated temperatures, the hydride particles within the substrate are quite ductile, inhibit crack growth, and failure eventually occurs due to a shear instability.     

Failure behavior of Zircaloy-4 cladding after oxidation and water quench

Simulated LOCA (loss of coolant accident) tests and subsequent mechanical tests on Zircaloy-4 cladding were carried out to evaluate the failure behavior of the cladding. Zircaloy-4 claddings were oxidized in a steam environment from 900 to 1250 °C for a given time period followed by a flooding of cool water to simulate LOCA tests. After the simulated LOCA test, the ductility of the oxidized cladding was evaluated by mechanical tests such as ring compression test and 3-point bend test. Evaluation of the absorbed contents such as hydrogen and oxygen were also carried out. The results showed that Zircaloy-4 cladding failed during thermal shock when the ECR (equivalent cladding reacted) value exceeded 20%. Lower boundary of brittle failure at thermal shock corresponds to 20% of ECR line calculated by the Baker-Just equation regardless of test temperature. On the other hand, boundary of ductile failure by the mechanical test did not followed after the ECR line. It rapidly decreased above 1000 °C to show that all Zircaloy-4 claddings behaved brittle fracture above 1150 °C when it oxidized at 300 s. Microstructural analysis revealed that boundary of ductile failure by the mechanical test fitted well when the absorbed oxygen content inside the prior-β layer was below 0.5 wt%.     

In situ study of the oxidation of Zircaloy-4 by ESEM

Zircaloy-4 has been oxidised in environmental scanning electron microscope (ESEM), at a temperature of 700 °C and the surface of the material has been observed during the oxidation, directly in the ESEM. The material has been subsequently analysed by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Several aspects of the oxidation were revealed in these experiments. Zircaloy-4 oxidises at a different rate depending on the crystallographic orientation of the grains at the surface. The iron and chromium in the secondary phase particles (SPP) diffuse to the surface and oxidise to produce a profile at the surface of the material. The in situ observations reveal no cracks at the surface during the oxidation. Subsequent examination of the surface (ex situ) by AFM reveals the presence of pores in the oxide. TEM examinations indicate the presence of weak internal planes in the oxide layer, inducing cracks parallel to the metal-oxide interface.     

Analysis for hydrogen particle balance of plasma-wall system in the large helical device

The hydrogen particle balance of the plasma-wall system in the large helical device (LHD) is analyzed, using a zero dimensional model for plasma particles, neutrals in vessel and hydrogen inventory in wall. Based on the measurement of neutral gas pressure, plasma density and the pumping speed of the cryo-pumping system, it is found that the hydrogen retained in the wall desorbes with short and long time constant. The short term desorption is of order of 10²¹ atoms with a time constant of a few minutes, which is much smaller than the wall pumping for one shot, 10²² atoms. In a long time scale of about one experimental day, the wall absorbs significantly large amounts of hydrogen, up to 10²⁴ atoms. One of the possible reasons for the large wall pumping is a carbon deposition layer on the first wall surface. The effect of hydrogen retention on density control is also discussed.     

Spent fuel radionuclide source term model for assessing spent fuel performance in geological disposal. Part II: Matrix alteration model and global performance

In the framework of the research conducted on the long term evolution of spent nuclear fuel under geological disposal conditions, a source term model has been developed to evaluate the instantaneous release of radionuclides (RN) (instant release fraction, IRF) and the delayed release of the RN which are embedded within the matrix. This model takes into account most of the scientific results currently available except the effect of hydrogen and the current knowledge of the uncertainties. IRF was assessed by considering the evolution with time of the RN inventories located within the fuel microstructure to which no confinement properties can be allocated over the long term (gap, rim, grain boundaries). This allows for bounding values for the IRF as a function of time of canister breach and burnup. The matrix radiolytic dissolution was modeled by a simple kinetic model neglecting the recombination of radiolytic species and the influence of aqueous ligands. The oxidation of the UO₂ matrix was assumed not to be kinetically controlled. Spent fuel performance was therefore demonstrated to mainly depend on the reactive surface area.     

Trapping of radiolytic hydrogen by amorphous cobalt oxysulfide

Hydrogen production from the radiolysis of liquid and gaseous hydrocarbons was studied in the presence of several transition metal sulfides. Cobalt oxysulfide obtained by aqueous precipitation was the most efficient admixture to decrease radiolytic production of hydrogen by pure hydrocarbons or mixtures of saturated and unsaturated hydrocarbons. Cobalt oxysulfide was characterized by XRD, scanning and transmission microscopy, and IR spectroscopy. It seems to be amorphous compound with the impurities of lamellar Co(OH)₂ phase. The organic phases were analysed before and after irradiation by γ-rays or protons in order to elucidate the origin of the effect. It has been shown that the solid does not change the composition neither the amount of the organic radiolysis products, neither in liquid nor in gas phase experiments. Therefore the presence of solid does not influence any radiolysis processes in the organics. Amorphous cobalt oxysulfide acts essentially as a trap of hydrogen, being able to absorb considerable amounts of H₂ (up to 0.5 mol H₂/at. Co). The study of the solid-gas interaction showed that slow reaction of cobalt oxysulfide with hydrogen occurs at ambient conditions independently of the irradiation of the system.     

Cyclic stress-strain response of textured Zircaloy-4

In this paper, the cyclic stress-strain response of textured Zircaloy-4 is investigated at room temperature in an incremental step test using fully reversed tension-compression loading under strain control. The material exhibits an asymmetry of stress response in both rolling and transverse directions, and the corresponding cyclic stress-strain curves can be expressed by a power law relation. Furthermore, phenomenological friction and back stresses are derived from an analysis of hysteresis loop shapes using the Cottrell scheme. It has been shown that the magnitude of the phenomenological friction stress in compression is always higher than that in tension for either rolling or transverse direction. While the magnitude of the phenomenological back stress, being independent of the loading direction, increases much more rapidly in transverse direction than that in rolling direction with increasing the plastic strain amplitude, and the trend in both directions can be expressed by a logarithmic relation. A further discussion suggests that (i) the intergranular thermal stress in the material is responsible for the difference in the phenomenological friction stress between tension and compression, thus leading to the asymmetry of stress response; (ii) the increase of the saturated stress with the plastic strain amplitude stems from the back stress that is primarily a direct consequence of the plastic strain incompatibilities between grains; (iii) the different performance between rolling and transverse directions results from the texture effect.     

Dissolution and precipitation behavior of hydrides in Zircaloy-2 and high Fe Zircaloy

In order to elucidate the terminal solid solubility during the dissolution of hydrides at heatup (TSSD) and during the precipitation of hydrides at cooldown (TSSP) for hydrogenated Zircaloy-2 and high Fe Zircaloy, differential scanning calorimetry (DSC) measurements have been carried out in the temperature range of 50-600 °C. The hydrogen concentrations in the two kinds of alloys ranged from 40 to 542 ppm. There was no difference in either TSSD or TSSP solvi between Zircaloy-2 and high Fe Zircaloy, and best-fit equations were derived for the two curves. In the present TSSP data, two different activation energies, separating into high and low temperature ranges at 260 °C, were obtained. Based on the widths of the DSC peak obtained during cooldown, the average precipitation (nucleation plus growth) rates of zirconium hydrides from super-saturated state were assessed. The activation energy of the precipitation rate was approximately equivalent to reported values of hydrogen diffusion coefficients of Zr and Zircaloys.     

Simulation study of carbon impurity dynamics on tungsten surfaces exposed to hydrogen ions

The depth profile of C impurity deposited on a W target exposed to H⁺ and C⁺ impurities at a concentration of C: 0.8% has been calculated in terms of segregation, diffusion and chemical erosion. For the segregation, the Gibbsian model has been used. For the diffusion, a concentration dependent diffusion model (C in WC and/or C) has been utilized. For the chemical erosion, the chemical erosion yield much lower than that for the H-C system has been applied. The calculated depth profiles at 653 K and 913 K are in good agreement with the XPS data. The agreement indicates that there is a significant contribution of segregation, which shifts the maximum C concentration to the top surface in the depth profiles. On the other hand, there are little contributions from diffusion and chemical erosion, which are related closely to formation of WC in the target.     

Heat capacity of hydrogenated Zircaloy-2 and high Fe Zircaloy

Heat capacities (C_p) of non-hydrogenated and hydrogenated Zircaloy-2 and high Fe Zircaloy were measured in the temperature range from 350 to 873 K, using a differential scanning calorimeter. The hydrogen concentrations in the two types of alloys ranged from 26 to 1004 ppm. The C_p values of the as-received alloys with 26-29 ppm hydrogen were in good agreement with literature data for low hydrogen Zircaloys. From this finding and observation of almost the same enthalpy changes for hydride dissolution for both alloys, it was concluded that there was no difference in C_p values between the two types of hydrogenated Zircaloys. The dissolution enthalpy of hydrides calculated from C_p data was 41.0 kJ/g-atom H. For Zircaloy-2 samples with higher hydrogen concentrations than 700 ppm, the phase transition from α+δ to α+β was observed at the eutectoid temperature of 824-827 K. Two types of models describing an additional heat capacity due to the hydride dissolution were presented based on the present C_p data and previously derived terminal solid solubility of hydrogen.     

Analysis of water radiolysis in relation to stress corrosion cracking of stainless steel at high temperatures - Effect of water radiolysis on limiting current densities of anodic and cathodic reactions under irradiation

Electrochemical corrosion potential (ECP) is an important measure for environmental factor in relation to stress corrosion cracking (SCC) of metal materials. In the case of SCC for in-core materials in nuclear reactors, radiolysis of coolant water decisively controls ECP of metal materials under irradiation. In the previous models for ECP evaluation of stainless steel, radiolysis of reactor water in bulk was considered to calculate the bulk concentrations of the radiolysis products. In this work, the radiolysis not only in bulk but also in the diffusion layer at the interface between stainless steel and bulk water was taken into account in the evaluation of ECP. The calculation results shows that the radiolysis in the diffusion layer give significant effects on the limiting current densities of the redox reactions of the radiolysis products, H₂O₂ and H₂, depending on dose rate, flow rate and water chemistry, and leads to the significant increase in the ECP values in some cases, especially in hydrogen water chemistry conditions.     

Characterization of hydrogen concentration in Zircaloy-4 using ultrasonic techniques

The relationship between hydrogen concentration precipitated as hydride particles and ultrasonic parameters, such as velocity and attenuation, was examined in Zircaloy-4 samples for potential applications in the Non-Destructive Test Field. Different amounts of hydrogen (up to 517 ppm) were introduced in the samples by gaseous charging. Ultrasonic attenuation measurements were performed with compressive waves at frequencies of 10 and 30 MHz, and propagation velocity measurements were performed at 10 MHz. Ultrasonic velocity showed an approximately linear increase with hydrogen concentration and it could be used as an assessment parameter when the hydrogen level is high enough. Attenuation versus hydrogen concentration has been fitted by a logarithmic equation at 10 MHz. At 30 MHz a fluctuating behavior of the attenuation prevented measurement of the hydrogen concentration.     

The standard molar enthalpy of formation of CdMoO4

The molar enthalpies of solution of CdMoO₄(s), CdO(s), Na₂ MoO₄(s) and NaF(s) in (10 mol HF(aq) + 4.41 mol H₂O₂(aq)) dm⁻³ have been measured using an isoperibol type calorimeter. From these results and other auxiliary data, the standard molar enthalpy of formation of CdMoO₄(s) has been calculated to be Δ_fH°(298.15 K) = −(1034.3 ± 5.7) kJ mol⁻¹. This value of enthalpy of formation of CdMoO₄(s) agrees well with the estimated enthalpy of formation of this compound. There is no other report on the thermodynamic property measurements on this compound.     

Steam oxidation of Zr-1.5Nb-0.4Sn-0.2Fe-0.1Cr and Zircaloy-4 at 900-1200 °C

The steam oxidation characteristics for the Zr-1.5Nb-0.4Sn-0.2Fe-0.1Cr (HANA-4) and Zircaloy-4 claddings were elucidated at LOCA temperatures of 900-1200 °C by using a modified thermo-gravimetric analyzer. After the oxidation tests, the oxidation behaviors, oxidation rates, surface appearances, and microstructures of the as-received, as-oxidized, and burn-up simulated claddings were evaluated in this study. The high-temperature oxidation resistance of the as-received HANA-4 cladding was superior to that of the Zircaloy-4. The superior oxidation resistance of the HANA-4 cladding could be attributed to the higher Nb and the lower Sn within its cladding. The pre-oxidized layer formed at the low temperatures below 500 °C could retard the oxidation rate at the high temperatures above 900 °C. And the soundness of the pre-oxidized layer formed at a lower temperature could influence the oxidation kinetics and the rate constants during a steam oxidation at LOCA temperatures from 900 to 1200 °C.     

Radionuclide release from high burnup spent fuel during corrosion in salt brine in the presence of hydrogen overpressure

In the case of a contact between groundwater and Fe-based spent fuel disposal containers in a repository large amounts of hydrogen will be produced by the corrosion of iron, which may result in significant hydrogen pressures. To quantify to what extent the hydrogen overpressure may counteract radiolysis enhanced matrix dissolution, related experimental work has been performed. High burnup spent fuel was corroded in 5.6 mol (kg H₂O)⁻¹ NaCl solution applying H₂ overpressures (experimental set 1) <0.17 bar by radiolysis, (experimental set 2) 2.8 bar by Fe corrosion, (experimental set 3) 3.2 bar by external H₂ gas. In the absence of Fe (experimental set 3) the UO₂ matrix dissolution rate decreased by a factor of about 10. In this test the concentrations of U, Np, Tc in solution were found to be decreasing by at least two orders of magnitude, and ranging within the same level as in the presence of Fe powder (experimental set 2). However, Pu and Am concentrations (experimental set 3) were less affected, due to the high sorption capacity for these radioelements onto Fe corrosion products.     

Development of refractory armored silicon carbide by infrared transient liquid phase processing

Tungsten (W) and molybdenum (Mo) were coated on silicon carbide (SiC) for use as a refractory armor using a high power plasma arc lamp at powers up to 23.5 MW/m² in an argon flow environment. Both tungsten powder and molybdenum powder melted and formed coating layers on silicon carbide within a few seconds. The effect of substrate pre-treatment (vapor deposition of titanium (Ti) and tungsten, and annealing) and sample heating conditions on microstructure of the coating and coating/substrate interface were investigated. The microstructure was observed by scanning electron microscopy (SEM) and optical microscopy (OM). The mechanical properties of the coated materials were evaluated by four-point flexural tests. A strong tungsten coating was successfully applied to the silicon carbide substrate. Tungsten vapor deposition and pre-heating at 5.2 MW/m² made for a refractory layer containing no cracks propagating into the silicon carbide substrate. The tungsten coating was formed without the thick reaction layer. For this study, small tungsten carbide grains were observed adjacent to the interface in all conditions. In addition, relatively large, widely scattered tungsten carbide grains and a eutectic structure of tungsten and silicon were observed through the thickness in the coatings formed at lower powers and longer heating times. The strength of the silicon carbide substrate was somewhat decreased as a result of the processing. Vapor deposition of tungsten prior to powder coating helped prevent this degradation. In contrast, molybdenum coating was more challenging than tungsten coating due to the larger coefficient of thermal expansion (CTE) mismatch as compared to tungsten and silicon carbide. From this work it is concluded that refractory armoring of silicon carbide by Infrared Transient Liquid Phase Processing is possible. The tungsten armored silicon carbide samples proved uniform, strong, and capable of withstanding thermal fatigue testing.     

Comparison of binding energy to the sorption enthalpy of radionuclides with trap materials used in LMFBRs

In liquid metal fast breeder reactors (LMFBR), traps are provided in the primary coolant circuit to reduce the contamination due to the deposition of long lived γ-emitting nuclides. The binding energies of the radionuclides with iron and nickel were estimated using Pauling’s electronegativity. The results are comparable to the sorption enthalpies derived from the experimental isotherms.