Steady-state Swelling Rate Research Articles

Cavity Swelling of 15-15Ti Steel at High Doses by Ion Irradiation.

The titanium-stabilized austenitic stainless steel Fe-15Cr-15Ni, which shows enhanced resistance to irradiation swelling compared with more traditional 316Ti, has been selected as a core material for fast reactors. Data on the evolution of irradiation swelling in 15-15Ti steels at very high doses, which cannot be easily achieved by neutron irradiation, are still lacking. In this paper, the swelling behavior of the titanium-modified austenitic stainless steel 15-15Ti was investigated by pre-implantation of He at room temperature followed by Ni-ion irradiation at 580 °C to peak doses of 120, 240 and 400 dpa. Relatively small cavities were observed in the zone of helium implantation, while large cavities appeared in the region near the damage peak. A correction formula for the dpa curve was proposed and applied to samples with large swelling. It was found that the steady-state swelling rate of 15-15Ti remains at ~1%/dpa even at high doses. By comparing the swelling data of the helium-implanted and helium-free regions at same doses, 70 dpa and 122 dpa, the suppression of swelling by excessive helium can be deduced at such doses.

Analysis of swelling behaviour of ferritic/martensitic steels

ABSTRACTFerritic/martensitic steels show exceptional void-swelling resistance during neutron irradiation compared to conventional austenitic stainless steels, such as Types 316 and 304. Explanations for the difference have been proposed based on the different crystal structures and the different microstructures of the steels. In this paper, swelling behaviour of ferritic/martensitic steels HT9 and modified 9Cr–Mo are analysed to demonstrate how the complicated tempered martensite microstructure determines their excellent swelling resistance. The variation in steady-state swelling rate observed for different heats and different heat treatments is explained in terms of how chemical composition and heat treatment affect the microstructure. Microstructures of these steels are characteristically non-uniform, containing precipitate-rich and precipitate-deficient regions. Swelling resistance increases as number and size of precipitate-deficient regions are reduced by proper heat treatment.

Effects of dpa rate on swelling in neutron-irradiated Fe–Cr and Fe–Cr–Mo alloys

Data are presented on the void swelling of three model Fe–Cr ferritic alloys following irradiation in TEM packets in FFTF–MOTA over the range 373–600°C and a wide range of dpa rates. It is shown that raising the chromium level decreases the steady-state swelling rate at ∼420°C. Addition of Mo to the Fe–12Cr alloy does not change the swelling rate significantly but does lead to an apparent swelling of ∼3% that arises from the radiation-accelerated formation of Chi phase. Swelling tends to decrease with increasing irradiation temperature for all three alloys. It is shown that the sensitivity of swelling to dpa rate expresses itself not at the various packet positions in FFTF, each with their characteristic nominal dpa rates, but also in response to variations in dpa rate along the length of the packet containing the specimens. The latter introduces second-order uncertainties in determination of the dpa levels and dpa rates, but these are not sufficient to obscure the major conclusion concerning dpa rate and composition.

Prediction of swelling of 18Cr10NiTi austenitic steel over a wide range of displacement rates

The internal components of pressurized water reactors of Russian types WWER-440 and WWER-1000 are constructed of annealed 18Cr10NiTi steel, a close analog to AISI 321. Void swelling of the internals is a concern for plant life extension and predictive equations are required to assess the potential of swelling in critical components such as the baffle ring or reflection shield that surrounds the WWER core. The only previously available swelling data for this steel were derived at higher than PWR-relevant displacement rates in the BOR-60 fast reactor. The swelling equation previously developed from these data does not incorporate the effect of displacement rate on swelling. Using heavy-ion irradiation at very high dpa rates (10 � 2 and 10 � 3 dpa s � 1 ) and doses (5–100 dpa) and coupling the results to available neutron data a swelling equation has been developed that specifically incorporates the effect of dpa rate on void swelling. Experimental results allow description of the swelling peak, the incubation period and the steady-state swelling rate over a wide range of irradiation temperature. For the first time it appears possible to describe both ion and neutron data on this steel within the framework of a single empirical model. Swelling maps constructed from this model permit forecasting of the behavior of the steel in WWERs under the required irradiation conditions, not only at already attained exposure doses, but more importantly to higher dose levels that will be reached following plant life extension.

Unlimited damage accumulation in metallic materials under cascade-damage conditions

Most experiments on neutron or heavy-ion cascade-produced irradiation of pure metals and metallic alloys demonstrate unlimited void growth as well as development of the dislocation structure. In contrast, the theory of radiation damage predicts saturation of void size at sufficiently high irradiation doses and, accordingly, termination of accumulation of interstitial-type defects. It is shown in the present paper that, under conditions of steady production of one-dimensionally (1-D) mobile clusters of self-interstitial atoms (SIAs) in displacement cascades, any one of the following three conditions can result in indefinite damage accumulation. First, if the fraction of SIAs generated in the clustered form is smaller than some finite value of the order of the dislocation bias factor. Second, if solute, impurity or transmuted atoms form atmospheres around voids and repel the SIA clusters. Third, if spatial correlations between voids and other defects, such as second-phase precipitates or dislocations, exist that provide shadowing of voids from the SIA clusters. The driving force for the development of such correlations is the same as for void lattice formation and is argued to be always present under cascade-damage conditions. It is emphasised that the mean-free path of 1-D migrating SIA clusters is typically at least an order of magnitude longer than the average distance between microstructural defects; hence, spatial correlations on the same scale should be taken into consideration. A way of developing a predictive theory is discussed. An interpretation of the steady-state swelling rate of ∼1%/displacement per atom (dpa) observed in austenitic steels is proposed.

Dependence of steady-state radiation swelling rate of l 0.1C–16Cr–15Ni–2Mo–2Mn–Ti–Si austenitic steel on dpa rate and irradiation temperature

A large number of swelling measurement data on the 0.1C–16Cr–15Ni–2Mo–2Mn–Ti–Si austenitic steel used as a fuel cladding at temperatures 640–870 К in the BN-600 fast reactor were analyzed. It was found that within irradiation temperatures 690–830 К a steady-state swelling dose rate was from 0.45%/dpa to 1.1%/dpa. By the statistical model of point defect migration for the 0.1C–16Cr–15Ni–2Mo–2Mn–Ti–S steel the dependence of the steady-state swelling rate on the irradiation temperature and displacement rate was calculated. The calculation data were consistent with the experimental data.

Effects of titanium additions to austenitic ternary alloys on microstructural evolution and void swelling

Ternary austenitic model alloys were modified with 0.25 wt.% titanium and irradiated in the Fast Flux Test Reactor (FFTF) at dose rates ranging over more than two orders in magnitude. While lowering of the dose rate strongly increases swelling by shortening the incubation dose, the steady-state swelling rate is not affected by dose rate. Although titanium addition strongly alters the void microstructure, swelling at about 420°C does not change with titanium additions, but the sensitivity to dose rate is preserved.

Theory of Frank loop nucleation at elevated temperatures

The nucleation of Frank loops from immobile primary interstitial clusters produced by irradiation in collision cascades is considered. Owing to the thermal instability of primary vacancy clusters at elevated temperatures, interstitial clusters receive, on the average, a net vacancy flux acting against their further growth. However, it has been shown previously that, under the combined action of the dislocation bias and continuous loop coalescence, sufficiently large interstitial loops manage to grow despite the net flux of vacancies. The nucleation of a growing interstitial loop, then, constitutes the growth, beyond a critical loop size, of an individual small cluster in an ensemble of clusters that are shrinking on the average. In this paper, the rate of interstitial loop nucleation based on this theory is developed analytically. We show that fluctuations in point-defect fluxes produced by the random (in time and space) generation of point defects in packets during cascade irradiation may significantly increase, that is by orders of magnitude, the probability of a small cluster to survive and grow. It is found that this flux is sufficient to explain the loop number densities experimentally observable in stainless steels at elevated temperatures, as well as to provide through the dislocation climb the steady-state swelling rate of about 1% (NRT)−1 (displacements per atom)−1.

Compositional and temperature dependence of void swelling in model Fe–Cr base alloys irradiated in the EBR-II fast reactor

A series of annealed and aged Fe–xCr, Fe–12Cr–yC and Fe–12Cr–0.1C–zMo model alloys were irradiated in EBR-II at eight temperatures between 400°C and 650°C and dose levels ranging from 35 to 131 dpa. Swelling-induced density changes observed in the binary alloys generally peaked at mid-chromium levels, with the chromium and temperature dependence expressed primarily in the duration of the transient regime. The steady-state swelling rate at the lower irradiation temperatures was much higher than previous estimates, reaching ∼0.2%/dpa and possibly still climbing at higher neutron exposures. The dependence of swelling on molybdenum and carbon was more complex, depending on whether the temperature was relatively low or high. At temperatures of 482°C and above, the effect of carbon additions was very pronounced with swelling of Fe–12Cr jumping dramatically from near zero at 0.002% C to 6–10% at 0.1% C. This indicates that the major determinant of the composition and temperature dependence probably lies in the duration of the nucleation-dominated transient regime of swelling and not primarily in the steady-state swelling rate as previously envisioned. This raises the possibility that significant swelling may occur earlier in fusion and spallation neutron spectra where high gas generation rates may assist void nucleation.

Comparison of swelling and irradiation creep behavior of fcc-austenitic and bcc-ferritic/martensitic alloys at high neutron exposure

It is well-known that ferritic and ferritic/martensitic steels develop much less swelling than austenitic steels during neutron or charged particle irradiation. The prevailing assumption is usually that the steady-state swelling rate of bcc steels is inherently much lower than that of fcc steels, at least a factor of ten or more. It is shown in this paper that this perception is incorrect, with bcc steels having steady-state swelling rates perhaps only a factor two to four lower. It is thought to be significant that the creep compliance of the two types of alloys also differs only by a factor of about two. The lower swelling observed in bcc steels relative to fcc steels is shown in this paper to be primarily a consequence of much longer transient regimes prior to the onset of steady-state swelling. Several other commonly held perceptions concerning the swelling of both bcc and fcc steels are examined in this paper and also shown to require revision. These involve the effect of cold-work on swelling, the extent of the temperature regime of swelling, and the possibility that swelling might saturate eventually. It also appears that the use of well-controlled in-reactor materials tests that employ active temperature control tend to yield significantly lower values of void swelling compared to that obtained under more representative conditions typical of actual reactor operation. The role of previously ignored differences in displacement rate to determine the duration of the transient regime of void swelling is also shown to require reevaluation, especially for bcc steels.

Factors which control the swelling of FeCrNi ternary austenitic alloys

In agreement with limited earlier studies, a comprehensive irradiation experiment conducted in both EBR-II and FFTF demonstrates that while cold-working usually decreases void swelling of ternary FeCrNi alloys at relatively low irradiation temperatures, it in general increases swelling at higher irradiation temperatures. Aging of cold-worked specimens to produce cellular dislocation networks tends to further increase swelling, especially at higher nickel levels. The swelling of ternary alloy at lower nickel levels also appears to be sensitive to details of the preirradiation annealing treatment. The differences in the details of reactor operating conditions also exert an influence on void nucleation and thereby on the duration of the transient regime of swelling. In the current irradiation series this leads to the swelling developed in EBR-II at ∼ 30 dpa being consistently larger than that in FFTF. All of these results confirm an earlier conclusion that the primary variability of void swelling of FeCrNi alloys lies in the incubation and transient regimes, rather than in the steady-state swelling rate regime. Under certain conditions, the transient regime can be made to approach 0 dpa.

Neutron-induced changes in density and electrical conductivity of copper alloys at 16 to 98 dpa and 430° C

An irradiation experiment on copper alloys in the Fast Flux Test Facility (FFTF) at 430° C has been completed. Four exposure levels were reached with the maximum at 98 dpa. Immersion density measurements of a wide variety of copper alloys show swelling values ranging from 1 to 64% at 98 dpa. Copper that is dispersion-strengthened with 0.25% Al 2O 3 (designated A125) appears to be the most swelling resistant and promising high conductivity alloy. The steady-state swelling rate of relatively simple copper alloys was found to be ~ 0.5%/dpa at 430° C. Both the swelling and conductivity changes of the pure copper reference alloy were found to be very reproducible.

Microstructural and microchemical comparisons of AISI 316 irradiated in HFIR and EBR-II

Two alternative interpretations of a unique but limited set of swelling data have been advanced in an attempt to predict the influence of helium on the swelling of 20% cold-worked AISI 316. In this paper it is shown that swelling data for annealed AISI 316 provide substantial insight concerning the role of helium on swelling. Additional insight has been obtained by a series of microstructural and microchemical examinations conducted on specimens of both annealed and cold-worked steel after irradiation in EBR-II and HFIR. These reactors differ greatly in neutron spectra and in AISI 316 generate quite different helium/dpa ratios and solid transmutant levels. At least in the range 500–750°C, the results of these studies show that helium's influence on swelling is manifested in the cavity density but not in the dislocation density or the major features of the microchemical evolution of the alloy matrix. The results also suggest that the influence of helium at high displacement levels is not strong on either the total swelling or the steady-state swelling rate.

Influence of neutron spectra on the radiation-induced evolution of AISI 316

A single heat of AISI 316 stainless steel has been irradiated in both the EBR-II and HFIR reactors. These reactors have significantly different spectra, leading to differences in displacements per neutron, helium/dpa ratio, and solid transmutant generation rates. The influence of reactor spectra can be observed in the transmutation-altered elemental compositions and the void densities. At least in the range of 500–650°C there does not appear to be any significant spectral-related differences in the dislocation density, phase development or total swelling. It appears that, in this temperature range, the steady-state swelling rate per displaced atom that develops in this steel will be the same in fast and thermal reactors, as well as in projected fusion devices.