Dpa Rate Research Articles

Displacement damage rate dependence of defect cluster formation in α-Fe during irradiation

Formation kinetics of defect clusters in pure iron during irradiation has been numerically investigated by reaction rate theory, with focusing on nucleation process of vacancy clusters (voids) and self-interstitial-atoms (SIA) clusters under a wide range of atomic displacement damage rate (dpa rate) and temperature conditions. In the rate theory model, the size dependence of thermal stability of a defect cluster is treated for a wide range of cluster size. The numerical analysis shows that the nucleation processes of voids and SIA-clusters are quite different from each other. As to the voids, the nucleation rate of voids depends much on temperature and dpa rate, and has the individual peak temperature for each dpa rate, during which the peak temperature increases with increasing dpa rate. This tendency for void nucleation is similar to that for void swelling observed in experiments. As to the SIA-clusters, the nucleation rate of SIA-clusters does not depend much on temperature and has no peak temperatures because of the relatively high thermal stability of an SIA-cluster, indicating that the conventional model (di-interstitial model) is applicable to describe the nucleation of SIA-clusters in a wide range of temperature.

A neutron point kinetic model for fusion relevant calculations

In the framework of research activities on fusion reactors a great effort is dedicated by the scientific community to the development of tritium breeding blankets. One of the main goals is to assess the neutronic behaviour of such devices to analyse their tritium breeding performance and to evaluate the required data for their thermal–mechanic and thermal–hydraulic design. Many papers have been published on this topic considering some stationary condition to calculate such important quantities as heating power, gas production and dpa rates, tritium breeding ratio, etc., but not much attention has been focussed to neutronic transport analyses in transient conditions. The present paper proposes a simple model based on the point kinetics approximation, which has been set up deriving an alternative formulation of the time-dependent neutron transport equation. This approach allows to define some physical characteristics that can be interpreted in a statistical way, making possible to calculate these quantities numerically by the Monte Carlo method. The adoption of the aforementioned numerical method has the great advantage that complex geometries (as the fusion reactor's blankets are) can be analysed with acceptable computational times. Some simple cases have been investigated to implement the theoretical model proposed with MCNP5 code and to show its potentiality. Then, applications to fusion reactor ITER blanket module and to the Helium Cooled Test Blanket Module, to be tested in ITER, have been taken into account in order to assess their neutronic time-dependent behaviour and the results obtained have been critically discussed.

Phase-field simulations of gas density within bubbles in metals under irradiation

Phase-field simulations are used to study the evolution of gas density within irradiation-induced bubbles in solids. In our simulations, which use copper as a model material, the dpa rate, gas production rate, and defect diffusivities are systematically varied to understand their effect on bubble nucleation rates, bubble densities, and the distribution of gas concentration within bubbles and in the solid regions. We find that gas densities within bubbles fluctuate drastically in the early nucleation stages, when growth rates are highest, but converge to steady-state values during the later coarsening stages. The steady-state gas densities within bubbles correspond with the ratio of total accumulated vacancy content divided by the total accumulated gas content, in agreement with a thermodynamic analysis concerning free-energy minimization.

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.

Swelling of pure vanadium and V-5Cr at ∼430 °C in response to variations in neutron flux-spectra in FFTF

When irradiated at ∼400–436°C in the FFTF fast reactor, pure vanadium swells more than does V-5Cr, a behavior opposite of that observed at 500–600°C. The tendency of V to swell more than V-5Cr increases as the dpa rate increases. The swelling of both metals appears to be exceptionally sensitive to the dpa rate, with the initial swelling rate increasing strongly as the dpa rate decreases. If this sensitivity is not factored into the analysis of swelling data it will appear that swelling is completely unpredictable. When this sensitivity to dpa rate is recognized along with the tendency of both alloys toward self-organization via void lattice formation, then swelling of V and V-5Cr in response to irradiation becomes better understandable.

The radiation analyses of ITER lower ports

The ITER Vacuum Vessel has upper, equatorial, and lower ports used for equipment installation, diagnostics, heating and current drive systems, cryo-vacuum pumping, and access inside the vessel for maintenance. At the level of the divertor, the nine lower ports for remote handling, cryo-vacuum pumping and diagnostic are inclined downwards and toroidally located each every 40°. The cryopump port has additionally a branch to allocate a second cryopump. The ports, as openings in the Vacuum Vessel, permit radiation streaming out of the vessel which affects the heating in the components in the outer regions of the machine inside and outside the ports. Safety concerns are also raised with respect to the dose after shutdown at the cryostat behind the ports: in such zones the radiation dose level must be kept below the regulatory limit to allow personnel access for maintenance purposes. Neutronic analyses have been required to qualify the ITER project related to the lower ports. A 3-D model was used to take into account full details of the ports and the lower machine surroundings. MCNP version 5 1.40 has been used with the FENDL 2.1 nuclear data library. The ITER 40° model distributed by the ITER Organization was developed in the lower part to include the relevant details. The results of a first analysis, focused on cryopump system only, were recently published. In this paper more complete data on the cryopump port and analysis for the remote handling port and the diagnostic rack are presented; the results of both analyses give a complete map of the radiation loads in the outer divertor ports. Nuclear heating, dpa, tritium production, and dose rates after shutdown are provided and the implications for the design are discussed.

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.

Precipitate evolution in low-nickel austenitic stainless steels during neutron irradiation at very low dose rates

Neutron-induced microstructural evolution in response to long term irradiation at very low dose rates was studied for a Russian low-nickel austenitic stainless steel designated Х18Н9 that is analogous to AISI 304. The irradiated samples were obtained from an out-of-core support column for the pressure vessel of the BN-600 fast reactor with doses ranging from 1.7 to 20.5 dpa generated at 3.8 × 10 −9 to 4.3 × 10 −8 dpa/s. The irradiation temperatures were in a very narrow range of 370–375 °С. Microstructural observation showed that in addition to voids and dislocations, an unexpectedly high density of small G-phase precipitates was formed that are not usually observed at higher dpa rates in this temperature range. A similar behavior was observed in a Western stainless steel, namely AISI 304 stainless steel, irradiated at similar temperatures and somewhat higher dpa rates in the EBR-II fast reactor, indicating that irradiation at low dpa rates for many years can lead to a different precipitate microstructure and therefore different associated changes in matrix composition than are generated at higher dpa rates. The contribution of such radiation-induced precipitation to changes in electrical resistivity was measured in the Х18Н9 specimens and was shown to cause significant deviation from predictions based only on void swelling.

Helium effects on microstructural evolution in tempered martensitic steels: In situ helium implanter studies in HFIR

Microstructural evolutions in tempered martensitic steels (TMS) under neutron-irradiation, at fusion relevant He/dpa ratios and dpa rates, were characterized using a novel in situ He-implanter technique. F82H-mod3 was irradiated at 500 °C in HFIR to a nominal 9 dpa and 190 or 380 appm He in both in the as-tempered (AT) and 20% cold-worked (CW) conditions. In all cases, a high number density of 1–2 nm He-bubbles were observed, along with fewer but larger ≈10 nm void-like faceted cavities. The He-bubbles form preferentially on dislocations and various interfaces. A slightly larger number of smaller He bubbles were observed in the CW condition. The lower He/dpa ratio produced slightly smaller and fewer He-bubbles. Comparisons of these observations to the results in nano-structured ferritic alloy (NFA) MA957 provide additional evidence that TMS may be susceptible to He-embrittlement as well as void swelling at fusion relevant He concentrations, while NFA are much more resistant to these degradation phenomena.

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.

Very high swelling and embrittlement observed in a Fe–18Cr–10Ni–Ti hexagonal fuel wrapper irradiated in the BOR-60 fast reactor

The highest void swelling level ever observed in an operating fast reactor component has been found after irradiation in BOR-60 with swelling in Kh18H10T (Fe–18Cr–10Ni–Ti) austenitic steel exceeding 50%. At such high swelling levels the steel has reached a terminal swelling rate of ∼1%/dpa after a transient that depends on both dpa rate and irradiation temperature. The transient duration at the higher irradiation temperatures is as small as 10–13 dpa depending on which face was examined. When irradiated in a fast reactor such as BOR-60 with a rather low inlet temperature, most of the swelling occurs above the core center-plane and produces a highly asymmetric swelling loop when plotted vs. dpa. Voids initially harden the alloy but as the swelling level becomes significant the elastic moduli of the alloy decreases strongly with swelling, leading to the consequence that the steel actually softens with increasing swelling. This softening occurs even as the elongation decreases as a result of void linkage during deformation. Finally, the elongation decreases to zero with further increases of swelling. This very brittle failure is known to arise from segregation of nickel to void surfaces which induces a martensitic instability leading to a zero tearing modulus and zero deformation.

Influence of silicon on swelling and microstructure in Russian austenitic stainless steel EI-847 irradiated to high neutron doses

Void swelling and microstructural development of niobium-stabilized EI-847 austenitic stainless steel with a range of silicon levels were investigated by destructive examination of fuel pin cladding irradiated in three fast reactors located in either Russia or Kazakhstan. The tendency of void swelling to be progressively reduced by increasing silicon concentration appears to be a very general phenomenon in this steel, whether observed in simple, single-variable experiments on well-defined materials or when observed in multivariable, time-dependent irradiations conducted on commercially produced steels over a wide range of irradiation temperatures, neutron spectra and dpa rates. The role of silicon on microstructural development is expressed both in the solid solution via its influence on dislocation and void microstructure and via its influence on formation of radiation-induced phases that in turn alter the matrix composition. Surprisingly, increases in silicon level in this study do not accelerate the formation of silicon-rich G-phase, but act to increase the formation of Nb (C,N) precipitates. Such precipitates are known to be associated with delayed void swelling.

Neutron-induced swelling and embrittlement of pure iron and pure nickel irradiated in the BN-350 and BOR-60 fast reactors

Pure iron and nickel were irradiated in the range of 2–15 × 10 −7 dpa/s at 345–650 °C to very high neutron exposures in two fast reactors, BOR-60 and BN-350, to study void swelling and changes in mechanical properties of these two metals. Both nickel and iron swell in this temperature range with the maximum swelling rate at ∼500 °C in nickel, but possibly at ⩽350 °C for iron. It also appears that the swelling rate in nickel and possibly in iron may be dependent on the dpa rate, increasing with decreasing dpa rate. The evolution of mechanical properties of the two metals is quite different. The differences reflect the fact that b.c.c. iron is subject to a low-temperature embrittlement arising from a shift in ductile–brittle transition temperature, while f.c.c. nickel is not. Nickel, however, exhibits high temperature embrittlement, thought to arise from the collection of transmutant helium gas at the grain boundaries. Iron is not strongly affected by transmutation since it generates much less helium during equivalent irradiation.

Direct Conversion of High Energy Protons to Electricity Using a Solid-State Pin Junction Diode

Using a single junction PIN (p-type, intrinsic, n-type) diode, made of silicon, and doped with boron and phosphorus, high energy protons have been converted to electricity, through ionization from electronic stopping in the silicon, at an efficiency of 0.2%. A simulation of 3.02 MeV D-D protons has been performed, using a 3 MeV linear accelerator. Proton fluxes of ∼3x1010 protons·cm-2s-1 were incident on a PIN diode with 0.7 cm2 of surface area facing the incident protons. Losses in efficiency as a function of proton fluence are compared with dpa (displacements per atom) rates calculated using the Monte Carlo ion transport code TRIM (Transport and Ranges of Ions in Matter).

Void swelling of AISI 321 analog stainless steel irradiated at low dpa rates in the BN-350 reactor

In several recently published studies conducted on a Soviet analog of AISI 321 stainless steel irradiated in either fast reactors or light water reactors, it was shown that the void swelling phenomenon extended to temperatures as low as ∼300 °C or less, when produced by neutron irradiation at dpa rates in the range 10 −7–10 −8 dpa/s. Other studies yielded similar results for AISI 316 and the Russian analog of AISI 316. In the current study a blanket assembly duct from BN-350, constructed from the Soviet analog of AISI 321, also exhibits swelling at dpa rates on the order of 10 −8 dpa/s, with voids seen as low as 281 °C and only 0.65 dpa. It appears that low-temperature swelling occurs at low dpa rates in 300 series stainless steels in general, and also occurs during irradiations conducted in either fast or in mixed spectrum reactors as shown in other studies.

Influence of carbon addition on neutron-induced void swelling of Fe–15Cr–16Ni–0.25Ti model alloy

Addition of 0.05 wt% C to a model Fe–15Cr–16Ni–0.25Ti quaternary model alloy leads to a reduction in neutron-induced swelling at 430 °C. The transient regime of swelling is prolonged by carbon addition, most strongly at lower dpa rates. Contrary to the swelling behavior observed in carbon-free Fe–15Cr–16Ni and Fe–15Cr–16Ni–0.25Ti model alloys irradiated in the same experiment, Fe–15Cr–16Ti–0.25Ti–0.05C does not exhibit a strong dependence of swelling on dpa rate. It appears that carbon’s role, while not yet well-defined, operates via a solute-based or TiC complex mechanism rather than by a precipitate-based mechanism. A model is proposed whereby carbon stabilizes loop microstructures against unfaulting, where unfaulting is known to be a prerequisite to formation of the glissile dislocation network needed to establish a high swelling rate. This stabilization is proposed to counteract the tendency of loop unfaulting to occur more strongly at low dpa rates.

The synergistic influence of temperature and displacement rate on microstructural evolution of ion-irradiated Fe–15Cr–16Ni model austenitic alloy

An experimental investigation of microstructural evolution has been conducted on Fe–15Cr–16Ni irradiated with 4.0 MeV nickel ions. Irradiations proceeded to dose levels ranging from ∼0.2 to ∼17 dpa at temperatures of 300, 400, 500 and 600 °C at dpa rates of 1 × 10 −4, 4 × 10 −4 and 1 × 10 −3 dpa/s. The swelling was found to monotonically increase with decreases in dpa rate at every irradiation condition studied. The earliest and most sensitive component of microstructure to both temperature and especially dpa rate was found to be the Frank loops. The second most sensitive component was found to be the void microstructure, which co-evolves with the loop and dislocation microstructure.

The transport and fate of helium in martensitic steels at fusion relevant He/dpa ratios and dpa rates

Understanding, modeling and managing the effects of He and displacement damage on microstructural evolution and property changes are primary objectives of fusion materials research. We recently implemented an approach for producing controlled He-to-dpa ratios under neutron irradiation using a novel α-implantation technique. Thin 1–4 μm NiAl coatings were deposited on Eurofer-97 TEM discs to produce a uniform He deposition zone of 6–8 μm. The test matrix is aimed at characterizing the transport, fate and consequences of He and He-to-dpa ratio variation on alloys with a wide range of starting microstructure. We explore the effect He-to-dpa ratio and temperature on the microstructure of conventionally processed Eurofer-97. Bubbles were found at all irradiation temperatures, with estimated maximum diameters of ∼12, 6.9 and 1.4 nm at 500 °C (∼9 dpa and 372 appm He), 400 °C (∼3.9 dpa and 82 appm He) and 300 °C (∼3.9 dpa and 89 appm He), respectively.

The transport and fate of helium in nanostructured ferritic alloys at fusion relevant He/dpa ratios and dpa rates

A novel in situ 59Ni(n,α) reaction He-implanter technique is described, that can be used to characterize the effect of the He/dpa ratio on microstructural evolution and changes in the flow properties of various materials at fusion relevant conditions. Irradiations in the high flux isotope reactor resulted in α-implantation from 1 to 5 μm thick NiAl coatings on TEM discs producing a uniform He concentration of 5–50 appm He/dpa to a depth of 5–8 μm. Initial post-irradiation TEM observations of MA957 irradiated to 9 dpa and 380 appm He at 500 °C provide strong evidence for the existence of a high density of small ⩽1 nm size bubbles trapped on nano-scale YTiO clusters. In contrast, similar irradiations of Eurofer 97, described in a companion paper, produce a lower number density of larger cavities, which may be a mixture of bubbles and voids.

Microstructure and mechanical properties of austenitic stainless steel 12X18H9T after neutron irradiation in the pressure vessel of BR-10 fast reactor at very low dose rates

Results are presented for void swelling, microstructure and mechanical properties of Russian 12X18H9T (0.12C–18Cr–9Ni–Ti) austenitic stainless steel irradiated as a pressure vessel structural material of the BR-10 fast reactor at ∼350 °C to only 0.64 dpa, produced by many years of exposure at the very low displacement rate of only 1.9 × 10 −9 dpa/s. In agreement with a number of other recent studies it appears that lower dpa rates have a pronounced effect on the microstructure and resultant mechanical properties. In general, lower dpa rates lead to the onset of swelling at much lower doses compared to comparable irradiations conducted at higher dpa rates.