Neutron Irradiated Molybdenum Research Articles

Deuterium Retention in Helium and Neutron Irradiated Molybdenum

Understanding and managing D retention in plasma facing components is essential for tritium safety in fusion reactors. Neutron irradiated and virgin low carbon arc cast (LCAC) Mo, as well as Mo foil samples with and without He pre-irradiation, were used to investigate D retention. D and He retention were investigated simultaneously in thermal desorption spectroscopy using a high resolution residual gas analyzer. Results show a significant increase in D retention with He pre-irradiation. Vacancies and vacancy clusters are found to retain D in LCAC samples, but neutron irradiated Mo retains more D in vacancy clusters.

Formation of dislocation channels in neutron irradiated molybdenum

The formation of deformation-induced dislocation channels in neutron irradiated molybdenum and the loop-dislocation interactions were investigated using transmission electron microscopy. It is suggested that the irradiation-induced defect - microstructure is unstable with respect to the stress. Under the influence of the stress, loops start gliding on their glide cylinders in transition region of a cascade where vacancies and interstitials may coexist. Results indicate that loop-loop interactions have a significant role in the early stage of channel formation; dislocations are not seen either within or at the ends of the embryonic channels. There are two consequences of loop-loop interactions: the reduction in the loop density due to their mutual annihilation in localized regions and the increase in the loop sizes due to the coalescence of loops having the same nature. On further stressing, the segments of the larger loops could serve as the source of mobile dislocations. The yielding may occur when the stress level is high enough to activate the coalescence-produced glide sources. Furthermore, results on the attractive loop-dislocation interactions suggest that they may not have a significant role in eliminating defect clusters. On the other hand, the repulsive interactions may have a substantial role in altering the microstructure of the irradiated metals.

Mechanism of 99Mo adsorption and 99mTc elution from zirconium-based material in 99Mo/ 99mTc generator column using neutron-irradiated natural molybdenum

The 99Mo adsorption and 99mTc elution mechanism were investigated using SEM–EDS to analyze the elemental composition of the adsorbent surfaces. From the changes of element composition, it was found that molybdate ions were adsorbed into the adsorbent by ion exchange with Cl− ion in the material. It was also revealed that 99mTc can be eluted in high oxidation state since the 99mTc in lower oxidation state easily made coordination bonds with other elements in the adsorbent.

Void Shrinkage and Void Lattice Formation in Neutron-irradiated Molybdenum

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Study of the temperature evolution of defect agglomerates in neutron irradiated molybdenum single crystals

Small angle neutron scattering as a function of temperature, differential thermal analysis, electrical resistivity and transmission electron microscopy studies have been performed in low rate neutron irradiated single crystalline molybdenum, at room temperature, for checking the evolution of the defects agglomerates in the temperature interval between room temperature and 1200 K. The onset of vacancies mobility was found to happen in temperatures within the stage III of recovery. At around 550 K, the agglomerates of vacancies achieve the largest size, as determined from the Guinier approximation for spherical particles. In addition, the decrease of the vacancy concentration together with the dissolution of the agglomerates at temperatures higher than around 920 K was observed, which produce the release of internal stresses in the structure.

Low-temperature thermally-activated deformation and irradiation softening in neutron-irradiated molybdenum

The effect of neutron irradiation on low-temperature deformation of Mo in two heat treatments, i.e. annealed and stress-relieved, was investigated. Specimens were irradiated at reactor coolant temperature (∼80 °C) to doses ranging from 7.2 × 10 −5 to 0.28 dpa in the High Flux Isotope Reactor. Tensile tests were carried out between −50 and 100 °C at strain rates of 1 × 10 −5–1 × 10 −2 s −1. Thermal activation analysis based on tensile data was performed to understand the low-temperature deformation mechanism. Irradiation softening and reduced dependence on test temperature and strain rate of the yield stress was observed in the annealed Mo after low-dose neutron irradiation (<∼0.003 dpa). Higher dose neutron irradiation caused athermal hardening only. The stress-relieved Mo showed a weaker dependence on test temperature and strain rate of the yield stress than the annealed Mo, and the dependence of the yield stress of the stress-relieved Mo was nearly unchanged after irradiation. Comparison of the experimental values of activation parameters with the theoretical predictions of dislocation models indicates that the Fleischer model of interactions of dislocations with tetragonal strains gave a better description of the activation process than the double-kink model, which implies a scavenging effect. The reduced test temperature and strain rate dependence following irradiation may be explained by the decreased effective stress due to trapping of interstitial solute species by neutron-produced defects.

The temperature dependence of the yield stress for neutron-irradiated molybdenum

Molybdenum was neutron-irradiated near 80 °C at doses of 7.2 × 10 −5, 7.2 × 10 −4, 7.2 × 10 −3, 0.072 and 0.28 dpa. Irradiated Mo was tensile tested over a temperature range of −50 to 100 °C at a strain rate of 1 × 10 −3 s −1. It was found that the yield stress of irradiated Mo decreased at lower temperatures and increased at higher temperatures, resulting in reduced temperature dependence of yielding at lower doses (<0.001 dpa); the yield stress was increased, and the temperature dependence of yielding was nearly unchanged upon further irradiation at higher doses (>0.001 dpa). The temperature dependence of the yield stress for unirradiated and irradiated Mo is consistent with the theoretical expression of the Fleischer model for interactions of dislocations with tetragonal strain fields.

Void nucleation at elevated temperatures under cascade-damage irradiation

showed reasonable agreement between theory and experiment. This application also predicts correctly the temporal development of large-scale spatial heterogeneous microstructure during the void nucleation stage. Comparison between calculated and experimental void nucleation rates in neutron-irradiated molybdenum at temperatures where vacancy emission from voids is negligible showed reasonable agreement as well. It was clearly demonstrated that the athermal shrinkage of relatively large voids experimentally observable in molybdenum at such temperatures may be easily explained in the framework of the present theory.

The gel generator: a viable alternative source of 99mTc for nuclear medicine

The majority of the world's supply of 99mTc is derived from generators in which fission produced 99Mo is adsorbed on to alumina. While such systems consistently produce 99mTc with excellent physico-chemical and biomedical properties, their manufacturing processes, commencing with the irradiation of 235U, involve numerous complications with the result that few organisations, worldwide, are willing to participate in such an endeavour, despite the size of the market (> $US 100,000,000 per annum). Encouraging results from basic research on an alternative generator system, based upon zirconium molybdate gel prepared from neutron irradiated molybdenum trioxide, was the stimulus for the decision to build a small pilot plant at Lucas Heights to manufacture the new generator under conditions akin to routine production. From this series of generators, a database of comprehensive performance characteristics has been constructed to show that, indeed, ( n, γ) 99Mo can be successfully exploited to provide an acceptable source of 99mTc for use in both nuclear medicine clinics and centralised radiopharmacies.

Influence of heat-treatment on tensile behavior of neutron irradiated molybdenum

Tensile specimens of stress-relieved and recrystallized molybdenum were irradiated at five temperatures between 646 and 1073 K in FFTF/MOTA cycle 11. The exposure levels were in the range of 6.8 to 34 dpa depending on the irradiation temperatures. After irradiation tensile tests and hardness test were carried out at room temperature. The total elongation of stress-relived specimens irradiated at 792, 873 and 1073 K ranged from 9 to 13%, and those of recrystallized specimens irradiated at these temperatures were almost 0%. The fracture modes of the stress-relieved specimens irradiated at 646–873 K were transgranular type and those of recrystallized specimens irradiated at 646–1073 K were intergranular type. Grain growth and intergranular fracture surface were observed in the stress-relieved specimen irradiated at 1073 K but it still has 9% total elongation. It is important that the stress-relived molybdenum had enough ductility at room temperature after neutron exposures up to relatively high fluences.

Saturation of swelling in neutron-irradiated molybdenum and its dependence on irradiation temperature and starting microstructural state

Molybdenum and its alloys are of interest for potential fusion application due to their high melting points and relatively good thermal conductivities. The development of void superlattices during neutron irradiation appears to limit void swelling in pure molybdenum to values below 4% over a wide range of irradiation temperatures. The saturation value of swelling and the void superlattice parameter each appear to be somewhat dependent on temperature, but independent of the thermomechanical starting state. Formation of technetium and ruthenium by transmutation may also play some minor role in the microstructural evolution, but do not appear to be involved in the limitation of swelling observed in this and other experiments.

Radiation effects in neutron-irradiated molybdenum studied by gamma-resonance and positron methods

Abstract The effect of ionizing radiation on various physical and mechanical properties of Mo was predominantly investigated by conventional methods such as X-ray diffraction analysis, residual electrical resistance, mechanical testing, electron microscopy and others. However, for objective estimation of the material postirradiation state, information on atomic and subatomic levels is required. In this case it must be kept in mind that the results of macroscopic and microscopic methods may sometimes differ considerably one from another. For example, the study of the processes of annealing of electron-irradiated Mo defects using the electrical resistance and the positron lifetime measurement methods1,2 under almost the same conditions brings about the defect return stages varying in temperature by 50 to 60 K which is likely to be the result of the selectivity difference of these methods. Electrical resistance is known to vary under the effect of all types of defects, while the positron annihilation processes are affected mainly by vacancies. Consequently, methods comparable in terms of their selectivity and enabling us to derive mutually complementing data on the physical phenomena, the same or at least similar by their nature, must be chosen for joint application. In this work, to study the radiation damage of Mo, neutron-irradiated at 350 K by 1018-1020 cm-2 fluences, the gamma-resonance and positron spectroscopy methods are used.

Zirconium molybdate gel as a generator for technetium-99m—II. High activity generators

The characteristics of zirconium molybdate gels were described in Part I of this paper. Here it is shown that zirconium molybdate gel, prepared from neutron irradiated molybdenum oxide with 99Mo specific activities between 4.3 and 13.5 GBq g −1 is sufficiently stable for use as a matrix in preparation of high activity 99mTc generators. Elution efficiencies of generators containing from 2 to 6 g of gel ranged from 83 to 50% for 10 mL elutions. Generators containing 150 g of gel were eluted with efficiencies of 80–98%, with the activity peak in the first 50 mL. Radionuclidic impurities were considerably below the limits set by the British Pharmacopoeia. Although the pH of eluates was less than 4, subsequent treatment with a small zirconium oxide bed provided adjustment to within BP limits, and reduced 99Mo impurity levels tenfold to values near 10 −4%. No significant differences were found between the biodistributions in rats of radiopharmaceuticals prepared with pertechnetate from gel generators and those from other generators.

Irradiation strengthening of metals

The variation of the athermal component of the yield stress of neutron irradiated molybdenum and niobium alloys are shown. An analysis of the data obtained on the properties of numerous metals, steels, and alloys (30 designations) having different types of crystal lattices permits one to conclude that the linear relationship between the squares of the athermal components of the yield stresses of the irradiated and the original specimens is a general distinguishing feature of irradiation strengthening of metals and alloys. The authors show that the degree of stability of the initial structure can radically affect the nature of dependence of irradiation strengthening of the alloys on the athermal component of yield stress.

Void formation and mechanical property of high temperature neutron-irradiated molybdenum

Electron beam melted molybdenum (PM) and powder metallurgy molybdenum (PM) which were pre-heat treated at 1173 and 1573 K for 1 hr were neutron irradiated in JMTR at 1073–1448 K up to 1.8–4.5 × 10 24n/m 2 . Both of Mo were recrystallized on annealing at 1573 K, but recovery structures were still observed on annealing at 1173 K. By irradiation of recrystallized Mo, no grain size changes were observed, while dislocation structures changed in the 1173 K annealed Mo. On the other hand, voids were observed in an the irradiated Mo. For-PM Mo, the void sizes were smaller and higher number density than those for EB-Mo. At higher temperature irradiation, heterogeneous void distribution was recognized near grain boundary. Yield stress and elongation for EB-Mo annealed at 1173 K were lower and larger, respectively than for unirradiated ones. On the contrary, for EB-Mo annealed at 1573 K the higher and lower elongation were obtained in comparing with EB-Mo annealed at 1173 K and with increasing irradiation temperature the elongation recovered. For PM-Mo annealed at 1173 K the yield stress were lower than for unirradiated one and elongation of 47 % at 1073 K irradiation rapidly reduced by irradiation at higher temperatures. Also, no elongation was obtained at all irradiation temperatures for PM-Mo annealed at 1573 K.

Diffusivity and permeability of hydrogen in neutron irradiated molybdenum and platinum

Hydrogen permeability and diffusivity were measured in neutron irradiated (3.0 × 10 15 or 1.2 × 10 17 n/ cm 2) metals of pure molybdenum of bcc structure and platinum of fcc. Although hydrogen permeability in molybdenum was decreased by 10% to 20% by the irradiation, in platinum it was increased by about 10%. X-ray diffraction analysis of the irradiated metals was also performed to study the relation between hydrogen behaviour in the metals and its neutron irradiated structures.

The influence of purity, cold work and alloying elements on the anneal hardening in neutron-irradiated molybdenum

The influence of metallurgical factors on the post-irradiation anneal hardening in molybdenum was studied. Molybdenum with different purities, cold-worked molybdenum, and several binary molybdenum alloys were irradiated to a fluence of 9 × 10 22 n/ m 2 ( E n ⩾ 1 MeV) at about 423 K in JMTR. Hardness, positron lifetime measurements, and microstructure observations after isochronal anneals revealed two stages of post-irradiation anneal hardening in the temperature ranges 473–773 K and 873–1173 K. The hardening in the lower temperature range was observed in both an arc-melted molybdenum polycrystal (low purity material) and an electron-beam zone-melted molybdenum single crystal (high purity material), while the hardening in the higher temperature range was observed only in the single crystal which was attributed to the formation of micro-voids. These results are interpreted to show that micro-voids can grow to a critical size only in high purity materials and, therefore, contributing: to the hardening. Alloying with substitutional elements or cold work of molybdenum have some influence on the hardening in both temperature ranges.

Vacancy clusters in neutron irradiated molybdenum

Abstract A single crystal of molybdenum, irradiated at 100°C by fast neutrons to a fluence of 6.5 × 1019 neutrons/cm2 was examined by the positron annihilation lifetime technique. The results of the experiment imply that this molydbenum specimen contains: (a) small defects such as vacancies and divacancies, and loops; (b) larger vacancy clusters of average size about twelve vacancies; and surprisingly (c) relatively few vacancy clusters of intermediate size.

Defect recovery behavior of neutron irradiated molybdenum and tungsten of two purity levels

Abstract The recovery behavior of radiation-induced defects during post-irradiation annealing was studied on molybdenum and tungsten specimens of two different purity levels. An electrical resistivity measurement technique at liquid nitrogen boiling temperature (∼77°K) was used. Irradiation of both materials was conducted in Oak Ridge Research reactor at reactor ambient temperature (∼70°C). The accumulative neutron fluence received was 7.3E+19 neutrons cm−2 (En>l MeV) and 5.1E+20 neutrons cm−2 (thermal). It was found that the number of recovery stages appeared to be independent of either the material or the impurity content. The stages are then believed to be due to the recovery of intrinsic defects and the recovery mechanisms are most likely the same for molybdenum and tungsten on the homologous temperature scale.

Deformation behavior of neutron irradiated molybdenum in tensile and hardness tests

Both tensile and hardness tests were performed on molybdenum irradiated at reactor ambient temperature to a fast neutron fluence of 1.1 × 10 20 n cm −2 and comparisons of the deformation behavior in both tests are made as a function of post-irradiation annealing temperatures. Irradiation hardening was observed for all specimens and embrittlement was exhibited by those irradiated specimens annealed in the temperature range of 343 to 973 K. Very coarse slip lines, believed to be made by dislocation channeling, were observed in the plastically deformed region of the specimens which failed with a brittle fracture. It is shown that dislocation channeling could be one of the main factors leading to the irradiation embrittlement as evidenced by SEM and TEM observation. In addition, it is suggested that deformation behavior of irradiated molybdenum in tensile tests could be determined from hardness test results on the same material.