Stress-corrosion cracking of Zircaloy-2 in methanol-iodine solutions

Annealed Zircaloy-2 cracks intergranularly when uniaxially stressed in boiling methanol-iodine solutions containing 0.002 to 20.0 g $\text{I}{}_2\text{1}$. The time to failure decreases with increasing iodine concentration. Stressing to 75% of the 0.2% proof-stress increases the dissolution rate by about 20 times. Both the corrosion rate and the time to failure under stress are independent of impressed electrochemical potential. The activation energy for dissolution, 8350 cals/mole for the unstressed condition, is reduced to 3360 cals/ mole under applied stress. This activation energy probably corresponds to the adsorption of I₂ on the Zircaloy surface. The mechanism of the stress-corrosion cracking seems to be one of stress—assisted dissolution controlled by the adsorption of I₂.     

Fracture mechanics analysis of iodine-induced crack growth in Zircaloy-4 tubing between 500 and 700°C

Low ductility failure of zircaloy tubing due to iodine-induced stress corrosion cracking (SCC) can occur up to about 700°C. The time-to-failure behavior of Zircaloy-4 cladding tubes containing iodine has been described by the elastic-plastic fracture mechanics model CEPFRAME for the temperature region 500 to 700°C. The model includes an empirically-determined computation method for the incubation period of crack formation, as a portion of the time-to-failure, as well as an elastic-plastic model for describing crack growth due to iodine-induced SCC. The total life time of the cladding tube is obtained by adding the crack initiation and crack propagation periods. The incubation period is a temperature-dependent function of both the depth of surface damage (both fabrication pits and machined notches) and the applied load, and is 40 to 90% of the time-to-failure. The elastic-plastic crack growth model is a modified version of the stress intensity K_I-concept of linear-elastic fracture mechanics. The extensions of this concept take into account a plastic strain zone ahead of the crack tip, which effectively increases the crack depth, and in addition, a dynamic correction factor for the crack geometry which is essentially a function of the effective crack depth. Unstable crack growth is predicted to occur when the residual cross section reaches plastic instability.Model results show good agreement with experimental data of tube burst tests at 500, 600, and 700°C. The crack growth velocity at all three temperatures is a power function of stress intensity ahead of the crack tip; the exponent is 4.9. The model can estimate time-to-failure of as-received cladding tubes containing iodine within a factor of 2. Application of the model to temperatures below 500°C is possible in principle. Due to the increasing scatter in experimental data, the structural transformation of the cladding by recrystallization, and the growing importance of creep strain, CEPFRAME has an upper temperature limit of approximately 650°C. The model is suitable for use in computer codes describing LWR fuel rod behavior during reactor transients and accidents.     

Deformation and Fracture Behavior of Zircaloy-2 Deformed at Constant Strain Rate in Iodine Environment, (I)

The relation between strain rate and iodine stress corrosion cracking (SCC) was studied on Zircaloy-2 subjected to uniaxial stress under constant extension rate in an iodine partial pressure of 4 Torr and at a temperature of 350°C. The specimens were machined from actual fuel cladding tube. In iodine environment, the tangentially directed specimens registered sharp decreases in stress beyond maximum point; this is attributed to crack initiation and propagation. Fracture ductility diminished with decreasing strain rate. Severe SCC was observed at strain rates below 2×10^?3min^?1. At high strain rates, the mechanism governing the rate of SCC appeared to be the time-dependent corrosion process. The axially directed specimens showed no signs of embrittlement due to gaseous iodine; this is attributed to the particular texture of the Zircaloy. Fracture surface observations indicated that transgranular cleavage fracture along the basal plane appeared to play a significant role in the stages of SCC initiation and propagation.     

Effects of ECC Water Injection Rate on Reflood Phase during LOCA

Some metal iodides such as of Fe, Al, Zr and Te are known to cause stress corrosion cracking (SCC) of Zircaloy just as iodine itself does. Therefore 15 metal iodides were selected as corrodants, and SCC tests were carried out using the internal gas pressurization method.

The results showed that: (1) only those metal iodides which react thermodynamically with Zr to produce ZrI₄ cause SCC of Zircaloy-2; (2) when SCC occurs, the reaction rate between the iodide and Zr seems to be a main factor in determining the SCC susceptibility; (3) gaseous ZrI₄ is the most corrosive agent; and (4) some species of metal iodides, such as PbI, cause SCC of Zircaloy-2 more easily than I₂ vapor.

Scanning electron microscope (SEM) examination and electron probe microanalysis (EPMA) on the fracture surface of failed specimens revealed that ZrI₄, formed as the reaction product between the metal iodides and Zr, might induce SCC of Zircaloy-2 rather than the iodides themselves.     

Plane strain ductility of zircaloy-4 tubes as dependent on structure and texture

Zircaloy-4 tubes were cold rolled 50 or 80% to a series of textures normal for regular tube production and finish annealed at 510 and 575°C. Two different types of ring expansion specimens giving local transverse deformation under a plane strain condition were developed. Plane strain tests were performed out-of-reactor at room temperature and 350°C, after which the plane strain ductility was measured as local transverse elongation. Both at room temperature and at 350°C tubes recrystallized at 575°C have higher plane strain ductility than tubes partially recrystallized at 510°C. The plane strain ductility is independent of the texture over the texture range of practical interest for regularly produced Zircaloy tubes. This seems to be a consequence of the fact that the stress ratio $\text{σ}{}_{\mathrm{\theta }}\text{: σ}{}_{\mathrm{z}}$ in plane strain loading varies with the texture in such a way that the degree of texture softening during deformation is the same for all canning tube textures.     

Thermally activated deformation of Zircaloy-4

Tensile tests were carried out on Zircaloy-4 over the temperature range 298–798 K. Yield stress values at the strain rates $\text{1.33 × 10}{}^{\mathrm{?4}}\text{s}{}^{\mathrm{?1}}$ and $\text{6.67 × 10}{}^{\mathrm{?4}}\text{s}{}^{\mathrm{?1}}$ were used to determine the activation parameters. A peak in activation volume ($\text{V}{}_{\mathrm{app}}\text{= 3100 b}{}^3$) was observed at about 690 K; outside this temperature range the activation volumes became almost independent of temperature ($\text{V}{}_{\mathrm{app}}\text{= 200?300 b}{}^3$). The peak in activation volume was explained in terms of a basic rate controlling mechanism and dynamic strain aging. This analysis indicated that the peak could be ascribed to the negative value of the strain rate sensitive solute strengthening term $\text{M}$ and that the mechanism based on the non-conservative motion of jogs appeared to be more favored as the basic rate controlling mechanism of Zircaloy-4 than an impurity mechanism     

Failure Propensities of Pressurized Zircaloy Tube Containing Iodine and Other Chemical Species

Abstract

Failure propensities of Zircaloy-4 cladding tube internally pressurized with Ar gas containing iodine and iodine plus each of other chemical species were examined at 360°C, to study the effect of corrosive fission products (FPs) on the integrity of spent nuclear fuel rods during dry storage, and also to assess the capability of preventing the spent fuel rod degradation.

The iodine stress corrosion cracking (SCC) of Zircaloy tube occurred in the long time/low stress exposure tests at stresses much lower than the conventional “threshold stress”, with considerably large strains at failure. The addition of cesium to iodine perfectly suppressed the SCC. It is inferred from these results that the degradation of spent fuel rods induced by corrosive fission products is unlikely during dry storage. Even if iodine alone should take effect, a proper strain limit could prevent spent fuel rods from incurring iodine induced effects because of considerably large strains necessary for iodine SCC of Zircaloy tube at low stresses.     

Threshold conditions for iodine-induced stress corrosion cracking of unirradiated zircaloy-4 tubing under internal pressurization

The results are presented of stress corrosion cracking (SCC tests in which nuclear power reactor grade zircaloy-4 tubing specimens were internally pressurized with a mixture of helium and iodine at ($\text{633 ± 5}$) K. Both as-received and artificially preflawed specimens were tested at an initial iodine availability of ~60 g/m² zircaloy surface. It is shown that the failure times in these tests correlate more reliably with hoop stress than with nominal stress intensity or failure strain, and that a threshold hoop stress of ~295 MPa exists for SCC failure within test times up to 605 ks. The origin of this threshold stress is discussed and it is concluded that the observed behavior is consistent with either a critical stress or a critical strain rate being required for the formation of iodine-induced stress corrosion cracks in unirradiated zircaloy tubing.     

Role de l'iode sur le developpement de la reaction entre le combustible et sa gaine in acier inoxydable (type 316) dans les reacteurs

During the operation of a reactor, the fuel $\text{(U, Pu}{}_{\mathrm{2-x}}\text{)}$ reacts with its cladding (stainless steel). The role of iodine, a fission product, in this reaction has been examined. Out-of-pile experiments have been done to study the compatibility of stainless steel with iodine, in open or closed systems, isothennally or in a temperature gradient, with or without the presence of fuel, using metallography, X-ray diffraction and X-ray microprobe analysis. Under isothermal conditions, iodine reacts directly with 316 stainless steel. Taking into account its composition and thermodynamie variables, iodides form in the following order:$\text{Mnl}{}_2\text{, Mnl}{}_2\text{+ Crl}{}_2\text{, Mnl}{}_2\text{+ Crl}{}_2\text{+ Fel}{}_2\text{, Mnl}{}_2\text{+ Crl}{}_2\text{+ Fel}{}_2\text{+ Nil}{}_2$In a thermal gradient, spectacular corrosion of the steel at low temperature is accompanied by substantial loss of chromium. In the hot zones, iron and chrome precipitate in equivalent amounts. The presence of $\text{(U, Pu)O}{}_{\mathrm{-x}}$in no way alters these observed phenomena. Thus the corrosive action of iodine on steel does not require the presence of oxygen. In irradiated fuel, free iodine (or iodine liberated by the decomposition of caesium iodide, a compound of fission products) can take part in the development of the reaction between fuel and cladding by demanding the latter of chromium, which reduces its resistance to oxidation by $\text{(U, Pu)O}{}_{\mathrm{2-x}}$.     

The iodine-induced strain rate sensitivity of Zircaloy fuel rod cladding

The strain rate sensitivities and failure times characteristic of iodine-induced stress corrosion cracking of Zircaloy fuel rod cladding are important because they can be related to certain power ramping rates which may increase fuel rod failure probabilities. The CCSCC model developed in this paper approximates these failure characteristics by simulating the transition from the slower (less observable) non-corrosive creep cracking (CC) regime to the faster (more observable) stress corrosion cracking (SCC) regime. Components of the CCSCC model include: ZrI₄ production by chemical reaction between Zircaloy and iodine; diffusion of the ZrI₄ to the crack tip; chemisorption, embrittlement, and damage accumulation at the crack tip; and crack initiation times and growth rates.Results indicate that the Zr-I₂ SCC process is dominated by competition between chemical reaction and material creep rate phenomena, rather than by stress thresholds or by the requirement that a complete monolayer of ZrI₄ form on the exposed Zircaloy surface at the crack tip. Failure times are dominated by the time required to initiate active crack growth. The SCC process is apparently not limited by diffusion kinetics on the time scale of laboratory experiments. Conflicting results were found concerning the chemical reaction rate at the crack tip.     

Chemical aspects of iodine-induced stress corrosion cracking failure of zircaloy-4 tubing above 50°C

The chemical environment associated with iodine-induced SCC failure of Zircaloy-4 tubing above 500°C has been characterized. At the critical iodine concentrations which result in SCC initiation and propagation, most of the iodine is present as condensed zirconium subiodides (I/Zr ? 0.4). Only a small part of the iodine remains in the gas phase as ZrI₄. The gaseous ZrI₄ is probably responsible for crack initiation and propagation. The critical ZrI₄ pressures for SCC failure have been estimated in zircaloy/iodine reaction experiments performed with unstressed zircaloy tube specimens. These pressures were confirmed in additional creep rupture tests conducted under controlled ZrI₄ partial pressure conditions. The estimated critical ZrI₄ pressure above which low-ductility SCC failure of the zircaloy tubing always occurs, independent of time-to-failure, varies between 0.005 bar at 550°C and 0.043 bar at 800°C. Below the critical values, however, a rather wide range of ZrI₄ pressures is associated with the onset of the SCC, especially at temperatures below 800°C. A comparison of the experimental results with available thermochemical data in the Zr-I system indicates that the main reaction involved during crack propagation is chemisorption of iodine-containing species on the fresh zircaloy surfaces created by metal straining at the crack tip.     

Effect of Absorbed Hydrogen on the Stress Corrosion Cracking (SCC) Susceptibility of Unirradiated Zircaloy Cladding

Effect of absorbed hydrogen on the stress corrosion cracking (SCC) susceptibility of unirradiated Zircaloy cladding was examined. The data obtained from literatures showed that the normalized ratios of SCC threshold stress (σ_th ) to 0.2% yield stress (σ_0.2.) claddings, from which the influence of σ_0.2 had been eliminated, increased with increasing hydrogen contents below 50ppm in unirradiated Zircaloy-2 and -4. For Zircaloy-4, the break point was observed in the relationship between normalized ratios of σ_th to σ_0.2 and hydrogen content in sample at hydrogen content of approximately 50ppm. Thermodynamic calculations were carried out for the reaction between iodine gas and zirconium containing hydrogen. The results suggested that the reactions hardly occurred at increased hydrogen content and zirconium reacted with iodine gas only below 100 ppm of hydrogen. Since these tendencies correspond to those of the normalized ratios of σ _th to σ_0.2 on the hydrogen content, it is considered that hydrogen affects the reactions between iodine gas and zirconium and reduces the SCC susceptibility of Zircaloy cladding.     

The effect of high temperature reactor primary circuit helium on the formation and propagation of surface cracks in alloy 800 H and inconel 617

The depths of surface cracks caused by the combined effects of corrosion and creep strain have been measured in specimens of Inconel 617 (Ni-22Cr-9Mo-12Co-1A1) and Alloy 800 H (Fe-32Ni-20Cr). Tests were carried out at 1073–1223 K in impure helium simulating the primary circuit coolant of a high temperature gas-cooled reactor, and in air. A characteristic crack depth $\text{a}{}_{90}$ was derived to represent the observed crack length distribution. In both test environments, an incubation strain was observed for the formation of surface cracks. Although the presence of surface cracks led to an increase in the creep rate, the magnitude of the change was similar for the two test atmospheres. Finally, a method has been developed to allow the depth of surface cracks, characterized by $\text{a}{}_{90}$, to be plotted with the stress rupture curves in order to indicate whether the corrosion effects need to be considered in the derivation of design stresses.     

Transition probabilities for the alkali isoelectronic sequences Li I,Na I,K I,Rb I,Cs I,Fr I

Dipole transition probabilities, oscillator strengths, lifetimes (mean lives), and branching ratios derived from a numerical Coulomb approximation are presented for experimentally identified (and some extrapolated) states n ≤ 12, l ≤ 4 for each of the following members of the alkali sequences (Z_net is the net charge of the corresponding ion): Li I

$\text{Z}{}_{\mathrm{<mtext>net</mtext>}}$

=1–15,17–24 Na I

$\text{Z}{}_{\mathrm{<mtext>net</mtext>}}$

=1–24 K L

$\text{Z}{}_{\mathrm{<mtext>net</mtext>}}$

=1–7 Rb I

$\text{Z}{}_{\mathrm{<mtext>net</mtext>}}$

=1–6 Cs I

$\text{Z}{}_{\mathrm{<mtext>net</mtext>}}$

=1–5 Fr I

$\text{Z}{}_{\mathrm{<mtext>net</mtext>}}$

=2,4 The results are presented in transition diagrams and in tables giving energy-level values and transition wavelengths as well. An appendix on hydrogen results for 5 ≤ n ≤ 12, 4 ≤ l ≤ 11 is included to represent the high-angular-momentum states of all members of the alkali isoelectronic sequences.     

The effect of ligaments on the reinitiation of fracture at the tip of a crack arrested during a hypothetical thermal shock event in a water-cooled reactor pressure vessel

During a hypothetical thermal shock event involving a water-cooled nuclear reactor pressure vessel, a crack can propagate deep into the reactor vessel thickness by a series of run-arrest-reinitiation events. Within the transition temperature regime, crack propagation and arrest in pressure vessel steels is associated with a combination of cleavage and dimpled rupture processes, the dimpled rupture regions being contained within ligaments that are normal to the crack plane and parallel to the direction of crack propagation. The present paper models the effect of ligaments on the reinitiation of fracture at the tip of an arrested crack, and the results of a theoretical analysis define the conditions under which ligaments might increase the reinitiation value above $\text{k}{}_{\mathrm{IC}}$, assuming that they fracture by a ductile rupture process. By comparing the predictions with experimental results for model vessels subject to thermal shock, it is shown that the ligaments, which are present at arrest, are unlikely to fail entirely by ductile rupture prior to the reinitiation of fracture at an arrested crack tip. Instead it is suggested that the ligaments fail by cleavage, whereupon they do not markedly affect the reinitiation $\text{K}$ value, which thus correlates with $\text{K}{}_{\mathrm{IC}}$.     

Thermochemistry of the alkali metal and alkaline earth-actinide complex oxides

After a brief discussion of the various techniques used for the preparation of actinide complex oxides, the present status of the thermochemistry of these compounds is reviewed. Perovskiterelated compounds with general formulae $\text{M}{}^{\mathrm{II}}{}_3\text{An}{}^{\mathrm{VI}}\text{O}{}_6$, $\text{M}{}^{\mathrm{II}}{}_2\text{M'}{}^{\mathrm{II}}\text{An}{}^{\mathrm{VI}}\text{O}{}_6$, $\text{M}{}^{\mathrm{I}}\text{An}{}^{\mathrm{V}}\text{O}{}_3$ and $\text{M}{}^{\mathrm{II}}\text{An}{}^{\mathrm{IV}}\text{O}{}_3$, as well as compounds of the type $\text{M}{}^{\mathrm{II}}\text{An}{}^{\mathrm{VI}}\text{O}{}_4$, $\text{M}{}^{\mathrm{I}}{}_2\text{An}{}^{\mathrm{VI}}\text{O}{}_4$, $\text{M}{}^{\mathrm{I}}{}_2\text{An}{}^{\mathrm{VI}}{}_2\text{O}{}_7$ and $\text{M}{}^{\mathrm{I}}{}_4\text{An}{}^{\mathrm{VI}}\text{O}{}_5$, are especially considered as, in the case of those species, thermodynamic data are available for compounds of several actinides and/or several alkali and alkaline earth metals.The stabilities of the complex oxides are discussed with respect to the parent binary oxides and to the aqueous ions; trends as a function of the size of the alkali or the alkaline earth cation are presented. Suggestions for synthesis of some analogous compounds with heavier actinides are also discussed.     

Emissivity of zirconium alloys in air in the temperature range 100–400°c

A technique is described for measuring the emissivity of materials in air. This technique was used to determine both the normal total emissivity ($\text{?}{}_{\mathrm{nt}}$) and the total hemispherical emissivity ($\text{?}{}_{\mathrm{ht}}$) of both unoxidized and oxidized samples of Zircaloy-2, Zr/2.5wt.% Nb and other zirconium alloys in the temperature range 100–400°C. Temperature had a negligible effect on the values obtained. However, $\text{?}{}_{\mathrm{nt}}$ values increased from $\text{0.158 ± 0.025}$ for the range of unoxidized zirconium alloys examined to $\text{0.6 ± 0.08}$ for the alloys when they were oxidized to have a $\text{2.0 × 10}{}^{\mathrm{?6}}\text{m}$ thick oxide.     

Plastic flow of ordered Zr3Al

Room-temperature tensile experiments established that ordered Zr₃Al of the Ll₂ type obeys the relationship $\text{σ}{}_{\mathrm{?}}\text{= σ}{}_{\mathrm{0,?}}\text{+ kd}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}$ where σ_? is the flow stress at a given strain ?, σ_0,? is a strain-dependent frictional stress, $\text{d}$ is the average grain diameter, and $\text{k}$ is a strain-independent constant of magnitude $\text{(2.8 ± 0.3) kg/mm}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$. Zr₃Al flows by fine, planar slip, and is susceptible to intergranular cracking.     

Matrix elements of the relativistic electron-transition operators

The formulas, which enable us to calculate the electric and magnetic multipole transition probabilities in relativistic approximation under various gauge conditions of the electromagnetic potential, are presented. The numerical values of the coefficients of the one-electron reduced matrix elements of the relativistic operators of the electric and magnetic dipole transitions between the configurations

$\text{K}{}_0\text{n}{}_2\text{l}{}_2\text{j}{}_2\text{α}{}_0\text{J}{}_0\text{j}{}_2\text{J?K}{}_0\text{n}{}_1\text{l}{}_1\text{j}{}_1\text{α}{}^{\mathrm{\prime }}{}_0\text{J}{}^{\mathrm{\prime }}{}_0\text{j}{}_1\text{J}{}^{\mathrm{\prime }}$

, where K₀ represents any electronic configuration, having the quantum number of the total angular momentum 0 ≤ J₀ ≤ 8 (the step is $\text{1}\text{2}$), and $\text{1}\text{2}\text{≤ j}{}_2$, $\text{j}{}_1\text{≤}\text{7}\text{2}$, are given.     

Sintering characterization of UO2 powders

The influence of a number of powder characteristics on the sintering behaviour of UO₂ has been evaluated. A good correlation is found between the green and sintered densities ($\text{d}{}_{\mathrm{g}}$ and $\text{d}{}_{\mathrm{s}}$) and the specific surface area as determined by Knudsen flow permeametry. This correlation, however, does not reveal the differences in behaviour which are borne out by the sintering index defined as: $\text{I}{}_{\mathrm{s}}\text{= (d}{}_{\mathrm{s}}\text{? d}{}_{\mathrm{g}}\text{)/(100 ? d}{}_{\mathrm{g}}\text{)}$. These differences are explained in terms of the absence or presence of open porosity within the powders, which can be measured by means of mercury porosimetry. Thermal treatments have a more pronounced effect on the sinterability of powders with open porosity than on those without.