Solid Transmutants Research Articles

Degradation of electrical resistivity of tungsten following shielded neutron irradiation

A major challenge for heat transfer in nuclear materials is to ensure thermal mobility after high amounts of neutron irradiation. Tungsten is widely selected as a heat transfer material in fusion reactors. In metals, thermal conductivity is dominated by electrons’ ability to transfer energy. Neutron irradiation generates point defects, clusters, and solid transmutation (e.g.rhenium and osmium in tungsten), which inhibit electron motion. The purpose of this work is to quantify the irradiation-induced change in electron mobility and deconvolute transmutation and microstructural effects on observed changes to electron mobility. Single and polycrystalline tungsten were fast neutron irradiated in the High Flux Isotope Reactor at Oak Ridge National Laboratory to doses between 0.2 and 0.7 displacements per atom (dpa) and temperatures from 500 °C to 1000 °C. Grain growth was observed in all samples. Microstructure and transmutation were quantified. The geometric orientation of samples with elongated grains has been shown to affect electrical resistivity. A mathematical model was developed and used to deconvolute solid-solution transmutation, grain, and temperature-dependent lattice effects on resistivity. At ∼0.4 dpa at ∼590 °C, the combined resistivity degradation due to voids, vacancies, interstitials, and dislocations is estimated to be greater than the contribution from solid solution Re transmutation, which is greater than the contribution from grain boundaries. At doses of ∼0.7 dpa at ∼750 °C, solid solution Re contributions are greater than all other effects combined. This work establishes a basis to predict the effects of irradiation temperature and transmutation on thermal properties of tungsten and highlights the importance of irradiation temperature.

Phase-field modeling of the clustering of transmutation element rhenium in irradiated tungsten

Incident neutrons in irradiated W can cause the formation of solid transmutation elements, of which Rhenium (Re) is the most abundant. We apply the phase-field method to investigate the clustering and growth of the Re-rich precipitate in irradiated W based on the spinodal decomposition mechanism, and their effects on the mechanical and thermal properties of W. Needle-like precipitates are reproduced and comparable to experimental observations. We then vary the irradiation dose and temperature to study their influences on the microstructure evolution of the Re-rich precipitate. Simulation results show that the average diameter, the number density, and the coverage rate of the precipitates significantly increase with the increase of the irradiation doses but slightly increase with the increase of the temperature. The effects of the Re-rich precipitate on the mechanical and thermal properties of W are also investigated. Results show that the Vickers hardness increase and the thermal conductivity degrade due to the formation of the Re-rich precipitate, especially at high irradiation doses. Conventional simulation methods can hardly handle the effect of the needle-like precipitates on the mechanical and thermal properties. In this work, we compare these two properties by using needle-like precipitates and circular precipitates. Our results clearly show that the needle-like precipitates give a better consistency with experimental results of the Vickers hardness increase, and reveal the anisotropic ability of the heat transfer in neutron-irradiated W. The current results can provide a systematic understanding of the Re clustering behavior from the microstructure evolution to its influences on the mechanical and thermal properties of W materials.

Quantitative APT analysis of solid transmutants in ferritic-martensitic steels after irradiation in SINQ targets: Experimental validation of neutronic calculations

The properties of materials applied in fission and fusion reactors and targets of spallation neutron sources can be greatly degraded by radiation-induced transmutants. A precise prediction of the production of transmutants is highly important for the safety assessment of these facilities. The prediction is normally made using neutronic calculations. Experimental validation of the calculation results is essentially necessary, but usually very challenging. In the present work, a quantitative analysis of transmutants in ferritic-martensitic (FM) steels F82H, Eurofer 97, and ODS Eurofer after irradiation at 9.6 and 19.6–21 dpa in the SINQ targets was performed using Atom Probe Tomography (APT). The results indicate that solid transmutants Ti, Sc, Ca, K, Cl, and S can be precisely measured and a gas element Ar could also be detected. The difference between the measured concentrations and that calculated with the MCNPX code is less than 30% for most of detectable nuclides of the transmutants. This work demonstrates that the neutronic calculations applied to STIP irradiation experiments are reliable.

Neutron irradiation tolerance of potassium-doped and rhenium-alloyed tungsten

Tungsten (W) based materials for fusion reactor applications have been developed in Japan for the last decade to improve thermo-mechanical properties and tolerance with respect to neutron-irradiation effects. Potassium (K) doping for dispersion strengthening and alloying by rhenium (Re) for solid solute softening and strengthening were applied to W fabricated by powder metallurgy. Thereby modified W materials, e.g. K-doped W and K-doped W-3%Re, demonstrated a higher recrystallization temperature threshold, an increase in strength and ductility, and a reduction in ductile-to-brittle transition temperature (DBTT) compared to pure W in the non-irradiated state. Embrittlement caused by displacement damage and its amplification due to solid transmutation products is expected to be an intrinsic issue of W based materials under neutron irradiation environment inherent to future fusion reactors. In this paper, the effect of K-doping and alloying by Re on the neutron-irradiation-induced embrittlement of W was investigated at neutron fluence causing a considerable amount of solid transmutation products.

Neutron irradiation effects on the microstructural development of tungsten and tungsten alloys

Data on the microstructural development of tungsten (W) and tungsten rhenium (Re) alloys were obtained after neutron irradiation at 400–800 °C in the Japan Materials Testing Reactor (JMTR), the experimental fast test reactor Joyo, and the High Flux Isotope Reactor (HFIR) for irradiation damage levels in the range of 0.09–1.54 displacement per atom (dpa). Microstructural observations showed that a small amount of Re (3–5%) in W–Re alloys is effective in suppressing void formation. In W–Re alloys with Re concentrations greater than 10%, acicular precipitates are the primary structural defects. In the HFIR-irradiated specimen, in which a large amount of Re was expected to be produced by the nuclear transmutation of W to Re because of the reactor's high thermal neutron flux, voids were not observed even in pure W. The synergistic effects of displacement damage and solid transmutation elements on microstructural development are discussed, and the microstructural development of tungsten materials utilized in fusion reactors is predicted.

Effect of activation cross section uncertainties in the assessment of primary damage for MFE/IFE low-activation steels irradiated in IFMIF

The present study is mainly aimed to provide the primary damage (displacements per atom, generation of solid transmutants and gas production rates) of structural materials irradiated in the high and medium flux test modules of the International Fusion Materials Irradiation Facility (IFMIF). We have investigated if the change of the composition during the irradiation time has effect on the prediction of the atomic displacements. The effect of the activation cross section uncertainties in the assessment of both solid transmutants and hydrogen and helium production is also analyzed. The results are provided element-by-element, so that the primary damage of any material irradiated in such neutron environments can be easily assessed; in this paper, we have predicted the primary damage of the low activation steel Eurofer.

Precipitation of Solid Transmutation Elements in Irradiated Tungsten Alloys

Tungsten-based model alloys were fabricated to simulate compositional changes by neutron irradiation, performed in the JOYO fast test reactor. The irradiation damage range was 0.17–1.54 dpa and irradiation temperatures were 400, 500 and 750 � C. After irradiation, microstructural observations and electrical resistivity measurements were carried out. A number of precipitates were observed after 1.54 dpa irradiation. Rhenium and osmium were precipitated by irradiation, which suppressed the formation of dislocation loops and voids. Structures induced by irradiation were not observed so much after 0.17 dpa irradiation. Electrical resistivity measurements showed that the effects of osmium on the electrical resistivity, related to impurity solution content, were larger than that of rhenium. Measurements of electrical resistivity of ternary alloys showed that the precipitation behavior was similar to that in binary alloys. [doi:10.2320/matertrans.MAW200821]

Transmutation analysis of realistic low-activation steels for magnetic fusion reactors and IFMIF

A comprehensive transmutation study has been performed for steels considered in the selection of structural materials for magnetic and inertial fusion reactors for the international fusion materials irradiation facility (IFMIF) neutron irradiation environment, as well as in the international thermonuclear experimental reactor (ITER) and a fusion demonstration reactor (DEMO) for comparison purposes. An element-by-element transmutation approach is used in the study, addressing the generation of: (1) H and He and (2) solid transmutants. The IEAF-2001 activation library and the activation code ACAB were applied in the IFMIF transmutation analysis, after proving the applicability of ACAB for transmutation calculations in intermediate energy systems.

Effects of solid transmutation and helium on microstructural evolution in neutron-irradiated vanadium

Pure V and V alloys with B and Cr irradiated in the HFIR to 10 dpa at 400, 500 and 600 °C are examined by TEM. Nine percent of V is transmuted to Cr during irradiation to 10 dpa, and more than 99% of 10B is transmuted to He and Li during the early stage of irradiation. The Cr generation suppresses cavity nucleation near grain boundaries in pure V. However in V–4.9Cr which contains 13at.%Cr after irradiation, cavities concentrate near grain boundaries, and there are few cavities in the matrix. At 400 and 500 °C, the effect of He on the cavity formation is not clearly observed. At 600 °C, B addition enhancese the cavity nucleation in pure V and V–4.9Cr in the matrix. Growth of cavities is also enhanced in pure V which is converted to V–9.4Cr.

Calculation and measurement of helium generation and solid transmutants in Cu–Zn–Ni alloys

A method recently proposed by Garner and Greenwood [1]would allow the separation of the simultaneous and possibly synergistic effects of solid and gaseous transmutation in Cu–Zn–Ni alloys. Copper transmutes primarily to Ni and Zn, leading to an accelerated production of helium from the ingrowth of zinc in mixed-spectrum reactors. Adding Zn to Cu alloys eliminates the time required for this ingrowth and leads to accelerated helium rates which are comparable to the well-known effect in Ni. Using a suitable Cu–Zn–Ni alloy matrix in a comparative irradiation of thermal neutron shielded and unshielded specimens, the separate influences of the solid and gaseous transmutants should be distinguishable. To assess the potential of this proposal, it is necessary to show that we can accurately predict the isotopic and elemental evolution of these alloys. In the current study, several different Cu–Zn–Ni alloys, irradiated in the HFIR JP-23 experiment, were characterized by measuring of the helium content as well as the solid transmutants. The measured values are in good agreement with calculations based on neutron dosimetry, therefore allowing the successful design of the transmutation experiment proposed by Garner and Greenwood.

The influence of neutron spectrum and irradiation history on microstructural evolution in fusion structural materials

The impact of solid transmutant generation and temperature transients during irradiation are new research subjects for application of fission neutron irradiation data to fusion materials development. Fundamental research for these issues is in progress, and includes such approaches as alloying the transmutant elements before irradiation and varying temperature irradiation with charged particles. In addition, novel uses of fission reactors are under way using neutron spectral changes to control solid transmutation, and highly controlled periodically-variable temperature irradiation. The understanding of microstructural processes in such complex and the variable irradiation conditions is growing.

Materials design and related R&D issues for the force-free helical reactor (FFHR)

Materials issues related to the Force-Free Helical Reactor (FFHR) design are presented. In FFHR, which uses the Flibe liquid breeder from the main reason of safety, if there is no need to replace in-vessel materials in the lifetime of 30 years, the reactor can be operated with not only the high safety margin but also a high availability of the plant, resulting in reducing not only the cost of electricity (COE) but also the total amount of radiative wastes. Nuclear properties of induced radioactivity, solid transmutation products, and decay heat at the 14 MeV neutron fluence of 45 MWa/m 2 as well as materials compatibility with Flibe are investigated for JLF-1, V-alloy, SiC and high Z materials. In conclusion, control of metal impurities, transmutation of W and V, acceptable decay heat, and reaction kinetics with Flibe are pointed out as R &D issues.

Effects of solid transmutants and helium in copper studied by mixed-spectrum neutron irradiation

Microstructures of pure Cu and Cu–Ni–Zn alloys irradiated in High Flux Isotope Reactor (HFIR) at 573 K to 9.2 dpa and 673 K to 10.4 dpa have been observed with TEM. Transmutant Ni and Zn of ∼3 wt% were produced during the irradiation. The effect of Ni and Zn production during irradiation was interpreted based on the knowledge obtained from Fast Flux Test Facility (FFTF) irradiated Cu and Cu–Ni–Zn alloys. The effect of He produced from Ni in Cu–5Ni during irradiation in HFIR was consistent with that produced from 10B in Cu–5Ni– 10B during irradiation in FFTF.

The effect of neutron spectrum on the mechanical and physical properties of pure copper and copper alloys

The electrical resistivity and tensile properties of copper and oxide dispersion strengthened (DS) copper alloys have been measured before and after fission neutron irradiation to damage levels of 0.5 to 5 displacements per atom (dpa) at ∼ 100 to 400°C. Some of the specimens were irradiated inside a 1.5 mm Cd shroud in order to reduce the thermal neutron flux. The electrical resistivity data could be separated into two components, a solid transmutation component Δρ tr which was proportional to thermal neutron fluence and a radiation defect component Δρ rd which was independent of displacement dose. The saturation value for Δρ rd was ∼ 1.2 nΩ m for pure copper and ∼ 1.6 nΩ m for the DS copper alloys irradiated at 100°C in positions with a fast-to-thermal neutron flux ratio of 5. Considerable radiation hardening was observed in all specimens at irradiation temperatures below 200°C. The yield strength was relatively insensitive to neutron spectrum in specimens strengthened by dispersoids or cold-working.

Transmutation of Mo, Re, W, Hf, and V in various irradiation test facilities and STARFIRE

With the exception of V and Cu alloys, the formation of solid transmutants has not been thought to play a large role in the response of metals to irradiation. For a number of refractory elements, however, it appears that very large levels of solid transmutation can occur both in fission and fusion spectra. The phase stability and other properties of alloys can be strong functions of the changing composition. The spectral sensitivity of transmutation also poses some difficulty in correlating data from various neutron spectra. Transmutation of Mo, Re, W, Hf and V have been calculated for the Fast Flux Test Facility (FFTF), the High Flux Isotope Reactor (HFIR), and the STARFIRE fusion reactor. Results show that Re and W undergo sizeable transmutation in these spectra, and may limit the use of these elements in fusion reactor materials. Vanadium and hafnium have high transmutation rates in HFIR and may require spectral tailoring.

Distribution of transmutant nickel formed in fast neutron irradiated copper

Copper and some of its alloys have been proposed to serve as high heat flux components in future fusion energy devices [l]. Unfortunately both the thermal and electrical conductivities of copper-base alloys will decline during neutron irradiation as a result of void swelling and solid transmutants [2]. The two most important transmutants are nickel and zinc, with nickel having the largest influence on conductivity degradation on a per atom basis. Recent calculation of the transmutations formed in pure copper during irradiation in the Fast Flux Test Facility (FFJ’F) predicted that N 0.41 at% nickel and N 0.40 at% zinc would be produced in pure copper after full power operation for 300 effective full power days [3]. These calculations also showed that the generation rate of nickel in typical fusion spectra would be roughly twice as large and therefore the conductivity loss would be even greater for a given level of atomic displacement. The formation and distribution of transmutants in irradiated copper alloys is currently being studied. Using copper-nickel alloys, it has been shown that during electron irradiation, nickel exerts significant influence on the evolution of dislocation microstructure and on void swelling [4-61. Radiation-induced segregation of nickel at defect sinks such as voids and grain boundaries was also reported [5,6]. Zinc was shown not to segregate, but to migrate away from such sinks, however [6]. From these studies it is clear that the properties of copper will change during neutron irradiation as nickel and zinc accumulate and then interact with point defect fluxes near microstructural sinks. The objective of the present study is to verify experimentally the transmutation calculations and to investigate whether transmutant nickel also behaves in the same manner as observed in electron irradiations.

Neutron damage and activation of the first wall of inertial confinement fusion reactors: Recycling and waste disposal

Even though general conclusions cannot be derived for all the protection schemes in inertial confinement fusion (ICF) reactors, the feasibility of the ferritic alloy HT-9 as the main component of the first structural wall (FSW) in ICF facilities using thin-film Li17Pb83 liquid protection, flowing through porous tubes (INPORT), can be demonstrated as a solution in terms of radiation damage. Swelling and shift in the ductile-brittle transition temperature (DBTT) can be analyzed using the results of experimental fast-fission reactors, which are demonstrated to be good experimental tools in that ICF range. The good performance of HT-9 is remarkable. The generation of new solid transmutants and the depletion of initial constituents need also be considered. Further, a reduced-activation HT-9 (niobium-free) has been studied using recycling and shallow land burial (SLB) criteria. The recycling using that HT-9 is shown to be not feasible, as is SLB waste disposal. The unexpected critical role of some short-lived isotopes is remarkable, and more research on their nuclear data must be performed.

Recent Progress in Fusion Reactor Materials Studies: Focus on Transmutation and Radioactivation Aspects

A review is presented of recent progress attained in the understanding of the influence of transmutation on the development of fusion-relevant property change data. It is shown that early experiments on helium effects on void swelling, irradiation creep and tensile properties of austenitic stainless steels were often misleading, and the influence of helium is much smaller than originally expected. Similar definitive conclusions concerning the role of helium in ferritic steels, copper alloys, and vanadium alloys cannot'be drawn at this time. It is also shown, however, that the formation of solid transmutants can be very important in a number of alloy systems, especially for some of those proposed as reduced radioactivation candidates

Production of transmutants and displacement effect in the EURAC spallation neutron source for testing of fusion materials

Two aspects have been criticized when using spallation reactions for fusion material testing: different final damage structures produced by high-energy recoils, and adequacy of the production of gaseous and solid transmutants. Using an original procedure to compute the displacement efficiency it is concluded that no new damage structures can be expected from the high-energy recoils at low temperature. An interaction displacement efficiency representing the interference of subcascades is defined, and it has been found independent of the recoil energy. Computed atomic displacement cascades for recoils of 200 keV and 1 MeV support this conclusion. The rate of He and H production is well compared with fusion, and the contribution from high-energy reactions is remarked. Solid transmutant generation from high-energy spallation neutrons and fusion are compared. A small contribution in the production of important elements is found, the proton flux effect being less significant than that of the neutron flux.

New developments in fusion materials research using surrogate neutron spectra

The use of surrogate irradiation facilities to conduct fusion-relevant materials research requires that fission-fusion correlations be developed to account for the differences between the surrogate and fusion spectra. It is shown that with the exception of 59Ni isotopic doping of nickel-containing alloys and irradiation in a temperature-controlled environment it is almost impossible to study the effects of helium without introducing other important variables such as displacement rate, temperature history and solid transmutants. When helium effects are studied in the absence of differences in these variables, helium's impact on macroscopic properties is shown to sometimes be second order in magnitude. Differences in displacement rate and temperature history can completely dominate experiments directed toward the study of PKA recoil spectra and helium/dpa ratio.