Irradiation creep due to elastodiffusion

Irradiation creep due to elastodiffusion, i.e. anisotropic diffusion induced by the application of a uniform external stress, has been studied. This is found to be a first order effect as compared to the usual SIPA which is a second order effect. The sink strength of dislocations is derived, as a function of the orientation of the dislocation line relative to the stress direction. The creep rate due to the resulting bias differential among dislocations is calculated and compared with that due to the usual SIPA. Using point-defect data generated by computer simulation, the creep rates due to elastodiffusion in iron and copper are found to be up to thirty times larger than those due to the usual SIPA.The effects of an anisotropic dislocation structure on the irradiation creep rate is also investigated. It is found that the anisotropy of the dislocation structure causes an anisotropy of the creep rate, which is largest in the direction where there is an excess of dislocation line directions.     

Irradiation creep in transient irradiation environments

Creep in a transient irradiation environment is examined theoretically. Pulsing the irradiation flux can enhance the predicted climb-glide creep rate. The enhancement is due to cyclic transients in the point defect fluxes which are shown to be a function of pulse duty cycle, pulse frequency and temperature. The pulsed mechanism is only effective if the climb barrier height is of the order of a nanometer or less. Furthermore, the enhanced creep rate is expected to have a weak stress dependence. The SIPA creep rate, on the other hand, is reduced with pulsing because pulsing reduces the time-averaged interstitial flux as compared with a continuous irradiation having the same time-averaged damage rate. A final consideration in the present analysis is that of discontinuously stepping the irradiation temperature to induce a transient-enhanced climb-glide creep rate.     

Irradiation creep in Zr single crystals

A single crystal of crystal bar Zr was irradiated, unstressed, at 570 K in a fast (> 1 MeV) neutron flux of $\text{5.5 × 10}{}^{16}\text{n/m}{}^2\text{-s}$. After a dose of $\text{6 × 10}{}^{23}\text{n/m}{}^2$ a tensile stress of 25 MPa was applied during a period of steady reactor power. The loading strain was an order of magnitude smaller than that observed when an identical, unirradiated, crystal was loaded to the same stress. There followed a period of primary creep during which the creep rate decreased to a value of $\text{5 × 10}{}^{\mathrm{?6}}\text{h}{}^{\mathrm{?1}}$ in the first 24 hours of the test. For the final 2000 hours of the test the specimen was observed to creep at a rate of $\text{1 × 10}{}^{\mathrm{?6}}\text{h}{}^{\mathrm{?1}}$ when the reactor was at full power. During shutdowns, the creep rate decreased with time. The results will be discussed and compared with predictions from current theories for the mechanism of irradiation enhanced creep in light of the micro-structures observed.     

Irradiation creep of stainless steel in bending

The development is described of a test to measure irradiation enhanced creep in bending of 20% cold-worked Type-316 stainless steel. The test will be irradiated in the experimental fast reactor EBR-II. The rationale used in design selection is described. The selected beam designs, the supportive tests in other stress states and the measurement techniques are described in detail.     

Irradiation induced creep of ceramic nuclear fuels

This contribution gives a review of the experimental results and accompanying theoretical considerations. The mechanisms considered for irradiation creep are: relaxation of elastic stresses by fission spikes, promotion of dislocation slide by thermal spikes, preferential, stress-orientated nucleation of dislocation loops and preferential growth of dislocation loops. A survey over the irradiation creep rates attributed to steady-state creep shows $\text{ε}{}_{\mathrm{irr}}\text{~ σ · F}$ for oxide fuel in the stress and fission rate ranges of $\text{σ = 10–50}\text{MN/m}{}^2$ and $\text{F = 3 × 10}{}^{12}\text{–1 × 10}{}^{14}\text{f/cm}{}^3\text{· s}$ at burnups < 3%. There seems to be a continuous increase of the irradiation creep rate of oxide fuels with increasing temperature. However, that increase cannot be directly interpreted through a thermally activated process. It seems that the irradiation creep rate will also depend on fuel porosity, on plutonium distribution in mechanically blended UO₂-PuO₂, but not substantially on the plutonium content per se. Some results were already given for carbide and nitride fuels, which show the irradiation creep rate to be lower by about a factor of 10 than for oxide fuel under comparable conditions. Primary irradiation creep has been observed up to $\text{(3–5) × 10}{}^{19}\text{f/cm}{}^3$ and could prevail up to $\text{1 × 10}{}^{20}\text{f/cm}{}^3$.     

Irradiation creep under 60 MEV alpha irradiation

Accelerator-produced charged-particle beams have advantages over neutron irradiation for studying radiation effects in materials, the primary advantage being the ability to control precisely the experimental conditions and improve the accuracy in measuring effects of the irradiation. An apparatus has recently been built at ORNL to exploit this advantage in studying irradiation creep. These experiments employ a beam of 60 MeV alpha particles from the Oak Ridge Isochronous Cyclotron (ORIC). The experimental approach and capabilities of the apparatus are described. The damage cross section, including events associated with inelastic scattering and nuclear reactions, is estimated. The amount of helium that is introduced during the experiments through inelastic processes and through backscattering is reported. Based on the damage rate, the damage processes and the helium-to-dpa ratio, the degree to which fast reactor and fusion reactor conditions may be simulated is discussed. Recent experimental results on the irradiation creep of type 316 stainless steel are presented, and are compared to light ion results obtained elsewhere. These results include the stress and temperature dependence of the formation rate under irradiation. The results are discussed in relation to various irradiation creep mechanisms and to damage microstructure as it evolves during these experiments.     

Irradiation creep in deuteron bombarded stainless steel

A commercial and a high purity version of cold worked type 316 stainless steel was irradiated with 9 MeV deuterons at 300°C under tensile stresses between 100 and 350 MPa and the irradiation creep rate was measured. The results are qualitatively discussed in the light of present theoretical models.     

Irradiation creep in non-fissile metals and alloys

The principal methods used in measuring irradiation creep in non-fissile metals and alloys are described and the limitations of the techniques emphasised. The theoretical models of irradiation creep are surveyed and the experimental data on thermal and fast reactor core component materials, such as zirconium alloys and austenitic steels, are reviewed. In particular, the effects of compositional and metallurgical variables and irradiation parameters (temperature, dose and dose rate) on the magnitudes of the irradiation creep are assessed. Finally, the additional theoretical studies required to further the understanding of the phenomenon and the experimental work necessary for optimising the design and operation of thermal and fast reactors are summarised.     

Irradiation enhanced creep in LMFBR fuel element cladding

Rate changes observed in irradiation enhanced creep and swelling in stainless steel cladding are ascribed to the precipitation of carbide. Empirical equations modified according to precipitation kinetics are consistent with results from fuel element irradiation and in particular describe the “second peak” phenomenon.     

Irradiation creep and interrelation with swelling in austenitic stainless steels

The available experimental data on irradiation-induced creep in austenitic stainless steels are summarized and the existing theories reviewed. Attention is paid to the influence of material composition and pretreatments on irradiation creep. In particular the stress, flux, fluence and temperature dependencies are reported and possible correlations of irradiation creep with the microstructural evolution, the swelling behaviour and the precipitation kinetics of the materials are outlined. The consequences of stress effects connected with swelling for the irradiation-creep behaviour, especially the stress-dependence, are discussed.     

Irradiation creep transients in Ni-4 at.% Si

In the course of irradiation creep experiments on Ni-4 at.% Si alloy, two types of creep transients were observed on the termination of irradiation. The short term transient was completed within one minute while the long term transient persisted for nearly ten hours. A change in the temperature distribution was excluded from the possible causes, partly because the stress dependence of the observed transient strains was not linear, and partly because the strain increase expected from the temperature change was much smaller than the observed value. Transient behavior of point defects was examined in conjunction with the climb-glide mechanism and the steady-state irradiation creep data. Calculated creep transient due to excess vacancy flux to dislocations was in good agreement with the observed short term transient. The long term transient appears to be a result of dislocation microstructure change. The present results suggest an enhanced irradiation creep under cyclic irradiation conditions which will be encountered in the early generations of fusion reactors.     

Allowing for the effects of residual stresses on creep in channel tubes

Conclusions The method enables one to calculate the creep strain in reactor structures with allowance for the relaxation of the residual stresses distributed over the wall thickness in the presence of radiation and temperature fields. The result for RBMK channel tubes is that the residual stresses have only a minor effect on the strain in prolonged operation. However, in short-time tests, this factor may appreciably distort the results and may even alter the sign of the strain. The internal stresses may also make themselves felt when one measures the strain due to radiation growth in zirconium-alloy specimens.Translated from Atomnaya Énergiya, Vol. 58, No. 1, pp. 13–17, January, 1985.     

Irradiation creep due to climb-induced glide in an anisotropic dislocation structure

Irradiation creep due to the climb-induced glide mechanism in an anisotropic dislocation structure is investigated, at the grain level, using a rate theory approach. The dislocation climb is considered to be affected by a net flow of either interstitials or vacancies to it, as a consequence of the combined effects of the intrinsic and stress-induced (SIPA) biases. In general, strong anisotropy of the creep rate is predicted from the theory, when the effects of the intrinsic and stress-induced biases are comparable. The theory is applied to zirconium. The results show that the creep rate, in general, depends strongly on the orientation of the basal projection of the stress direction in addition to its orientation relative to the $\text{c-}\text{axis}$ for this creep mechanism.     

Irradiation devices for the study of creep and swelling in ceramic fuels

In fuel element design for advanced nuclear reactors perfect knowledge of fuel behaviour under irradiation plays a decisive role, above all for long service lives and high burnups. Therefore, the development of fast breeder fuel elements within the framework of the Karlsruhe Fast Breeder Project included various irradiation rigs which allow continuous measurement during irradiation of fuel specimen creep and swelling. A survey is presented of some of these irradiation rigs.     

Irradiation creep of previously irradiated 304 stainless steel in a fast flux environment

Results of a fast flux neutron irradiation experiment designed to investigate the effects of high levels of prior irradiation (to 10²³ n/cm², E > 0.1 MeV) on the irradiation creep of type 304 stainless steel at 700 K (800°F) are reported. The steady state creep coefficient is found to increase by a factor of 7 as the specimen fluence increases from 3 to 10 × 10²²n/cm², (E > 0.1 MeV). A non-linear dependence of strain on stress is exhibited. The results of this study are compared with previously reported stress relaxation data and with predictions of a swelling enhanced irradiation creep model.     

Irradiation creep of oxide dispersion strengthened (ODS) steels for advanced nuclear applications

Ferritic oxide dispersion strengthened steels with different microstructure were in-beam creep tested in a temperature range from 300 to 500 °C. Irradiation was by He-ions. Elongation was determined as a function of stress and irradiation damage rate. Damage was investigated by transmission electron microscopy. A thorough analysis of the loops developing during irradiation creep did not show any dependence of orientation or size on the direction of the applied stress. At 400 °C radiation induced segregation was found (most probably an iron aluminide) which had no effect on irradiation creep. No pronounced influence of microstructure or dispersoid size on the irradiation creep behavior was detected. Irradiation creep compliance of PM2000 with dispersoids of about 30 nm diameter were found to differ little from material with dispersoids of only 2-3 nm diameter. This is in contrast to thermal creep where dislocation-obstacle interactions are extremely important. An assessment of the technical relevance of irradiation creep in advanced nuclear systems is presented.     

Irradiation induced creep experiments in the BR2 reactor using the resonant cavity method

A 16Cr16NiNb stainless steel (steel 1.4981) will be used as wrapper material for SNR 300. Therefore, some in-pile creep tests have been performed with this material in the temperature range 420–700°C. The main objective of this programme was to see, whether the creep rates of steel 1.4981 followed at low fluences ($\text{? 2.5 × 10}{}^{22}\text{n/cm}{}^2$, $\text{E > 0.1}\text{MeV}$) the same rules as for other austenitic stainless steels. The experiments were performed in the BR2 reactor at Mol/Belgium, using creep rigs which were developed and manufactured by CEN Grenoble. The creep strains were measured by the resonant cavity method. The paper describes the main characteristics of the creep capsules, and reports on the performance of those types of rigs. Finally, the experimental results are presented and discussed.     

The effect of internal pressure on the creep relaxation of a thin, straight pipe

The effect of constant internal pressure on the relaxation of bending moment on a long, thin, circular cross section cylindrical shell is determined. The inadequacy of several simple approximate analyses, previously proven useful in other creep applications, is established.