Recombination Effect as Component of Residual Defect in Silicon Surface Barrier Detector

The thermal conductivity of graphite components used as in-core components in high-temperature gascooled reactors (HTGRs) is reduced by neutron irradiation during reactor operation. The reduction in thermal conductivity is expected to be reversed by thermal annealing when the irradiated graphite component is heated above its original irradiation temperature. In this study, to develop an evaluation model for the thermal annealing effect on the thermal conductivity of IG-110 graphite for the HTGRs, the thermal annealing effect evaluated quantitatively at irradiation temperatures of up to 1,200°C and neutron fluences of up to 1.5 dpa. Moreover, the thermal conductivity of IG-110 graphite was calculated by using a modified thermal resistance model considering the thermal annealing effect. The following results were obtained. (1) The thermal annealing effect on the thermal conductivity of IG-110 graphite could be evaluated quantitatively and a thermal annealing model was developed based on the experimental results at irradiation temperatures of up to 1,200°C and neutron fluences of up to 1.5 dpa. (2) The thermal conductivities of IG-110 graphite calculated by using the modified thermal resistance model considering the thermal annealing effect showed good agreement with experimental measurements. This study has shown that it is possible to evaluate the annealed thermal conductivity of IG-110 graphite by using the modified thermal resistance model at irradiation temperatures of 550–1150°C and irradiation fluences of up to 1.5 dpa.     

Neutron Irradiation Effect on Isotopically Tailored 11B4C

The present investigation was carried out to elucidate the effects of neutron irradiation on the dimensional change and thermal conductivity of isotopically tailored ¹¹B₄C. The specimens used in the present investigation are 99% ¹¹B-enriched ¹¹B₄C, 91% ¹⁰B-enriched ¹⁰B₄C, and β-SiC. ¹⁰B₄C was sampled from an irradiated material used as a neutron absorber in a “JOYO” MK-II control rod. The ¹¹B₄C and SiC specimens were irradiated in the experimental fast reactor “JOYO” to fluences of 1:94 × 10²⁶ n/m² (E > 0:1 MeV) at 530°C and 3:12 × 10²⁶ n/m² (E > 0:1 MeV) for ¹⁰B₄C, with a ¹⁰B burnup of 47:3 × 10²⁶ cap/m³. Measurements on changes in dimensions and thermal conductivity, as well as postirradiation annealing up to 1400°C, were carried out. The results of such measurements indicated that the changes in the dimensions and thermal conductivity of neutron-irradiated ¹¹B₄C were substantially smaller than those of ¹⁰B₄C and SiC. Postirradiation annealing measurements for ¹¹B₄C showed that the thermal conductivity was almost completely recovered at 1400°C. The changes in thermal conductivity by annealing were analyzed in terms ofphonon scattering theory. The onset of recovery of the thermal conductivity of ¹¹B₄C agreed well withirradiation temperature; however, the recovery of length did not coincide with irradiation temperature.     

Positron annihilation and nano-indentation analysis of irradiation effects on the microstructure and hardening of A508-3 steels used in Chinese HTGR

The irradiation and annealing behavior of Chinese A508-3 reactor pressure vessel (RPV) steel (0.04 wt% Cu) after 3 MeV Fe-ion irradiation ranging from 0.1 to 20 dpa at room temperature (called RTRPV) and high temperature (250?°C, called HTRPV) was studied by positron annihilation Doppler broadening (PADB) spectroscopy and nano-indentation hardness. PADB showed that the density of vacancy-type defects was higher for low-temperature irradiations. The higher hardness was found after high-temperature irradiation because of the formation of solute clusters during irradiation. Positron annihilation measurements revealed the interaction and clustering of vacancies with solute clusters which were introduced by Fe-ion irradiation. For both RTRPVs and HTRPVs, the positron defect parameter and positron diffusion length showed the recovery of the irradiation-induced defects. Total recovery was observed after annealing at 450 °C.     

Effect of Neutron-Irradiation on Thermal Conductivity,Electric Resistivity and Thermal Expansion of Boron Carbide

Boron carbide possessing porosities of 91 and 77%T.D. were irradiated at about 300°C to 6.0x10¹⁸～ 3.3×10¹⁹ n/cm² in helium atmosphere. The thermal conductivity, the electric resistivity and the linear thermal expansion were determined. Upon irradiation to 3.3×10¹⁹ n/cm², cracking occurred in all of the 91%T.D. specimens and in two of the five 77%T.D. specimens. The thermal conductivity changed with irradiation more markedly in the case of the 77%T.D. specimen, which also increased its specific electric resistivity by a factor of 10⁴, while that of the 91%T.D. specimen increased by less than 10-fold. This significant difference in thermal and electric conductivity change by irradiation is attributed to the difference in microstructure. It was observed that the recovery of radiation damage began at about 300°C when the specimens were subjected to heating and the changes observed in respect of thermal conductivity, electric resistivity and linear thermal expansion.     

Observation and analysis of damage structure in Al and Al/Mg (N4) alloy after irradiation with 100 and 400 keV aluminium ions

A comparative TEM study has been made of ion irradiation damage structure in pure aluminium, commercial aluminium (grade 1100) and in a modified N4 (Al/2.95% Mg) alloy of the type used in the construction of the calandria of the Winfrith prototype SGHW Reactor. Atom displacements equivalent to many years neutron irradiation were simulated by bombardment with 100 and 400 keV Al⁺ ions to doses of up to 200 dpa at temperatures between 30 and 250°C. Dynamic observations of damage formation were made during irradiation with 100 keV ions in a linked heavy-ion accelerator/200 keV electron microscope, and further results were obtained by 400 keV Al⁺ ion bombardment in a Cockcroft-Walton accelerator. Dislocation structure and voids were seen in aluminium irradiated with 100 and 400 keV A1⁺ at temperatures between 30 and 250°C. Void swelling of 8.7% at 104 dpa was a maximum at 1̃50°C in type 1100 aluminium. No voids were found at temperatures μ 250°C. No voids were seen in the Al/Mg N4 alloy after bombardments up to 200 dpa with 100 keV A1⁺, and 150 dpa with 400 keV Al⁺ at temperatures between 50 and 170°C. The void-resistant property is consistent with observations in the USA of neutron-irradiated 5052 Al alloy which has a similar magnesium content to the modified N4 alloy. The 1100 alloy and N4 results have been analysed using the rate theory of swelling. The absence of voids in the N4 alloy indicates an effective vacancy annihilation mechanism, which possibly occurs at small precipitates formed during irradiation.     

Postirradiation Annealing of Boron Carbide Pellet Irradiated in Fast Breeder Reactor

This paper reports some irradiation effects and recovery behavior of neutron irradiated boron carbide pellets that were used as control rod elements in the Enrico Fermi Fast Breeder Reactor. Measurements were carried out on changes in lattice parameters, thermal expansion, helium release, elastic moduli and microstructure observations by annealing the irradiated pellets at elevated temperatures. The increase in unit cell volume of B₄C upon irradiation was found to be 0.22%. The recovery in lattice parameter began at around 500°C and completed at 1,000°C. It was found that the pellet showed a sharp increase in a dimensional change at about 700 to 800°C with a large amount of helium release, and the pellet which showed larger swelling released smaller amount of helium.     

The evolution of He+ irradiation-induced point defects and helium retention in nuclear graphite

The atomic-level study of point defect evolution in nuclear graphite is essential for a deep understanding of irradiation-induced property changes. The evolution of helium ion irradiation-induced point defects and helium retention in nuclear graphite ETU-10 and ETU-15 were studied by positron annihilation Doppler broadening (PADB) experiments and thermal desorption spectroscopy (TDS) measurements. The graphite samples were implanted with 10¹⁵, 10^16_, and 10¹⁷ cm^?2 of 200 keV He⁺ at operation temperatures below 373 K. Frenkel pairs were created during ion irradiation and they annihilated during annealing. Three stages of interstitial-monovacancy annihilation are suggested. At low temperatures, the initial annihilation would be refined only to the recombination of intimate metastable Frenkel pairs. When temperature increases, the annihilation would expand to a larger extent that isolate interstitials and vacancies annihilate with each other. In the case of high doses irradiation, vacancy clusters form at elevated temperatures. The retention and release of helium is tightly related to the evolution of the defects, especially the vacancies. The small over-pressured He-V clusters (He_nV) are thought to be the most possible form of helium retention under irradiation.     

Thermal Recovery of Radiation Defects and Microstructural Change in Irradiated UO2 Fuels

Abstract

Thermal recovery of radiation defects and microstructural change in UO₂ fuels irradiated under LWR conditions (burnup: 25 and 44 GWd/t) have been studied after annealing at temperature range of 450-1,800°C by X-ray diffractometry and transmission electron microscopy (TEM). The lattice parameter of as-irradiated fuels increase with higher burnup, which was mainly due to the accumulation of fission induced point defects. The lattice parameter for both fuels began to recover around 450-650°C with one stage and was almost completely recovered by annealing at 850°C for 5 h. Based on the recovery of broadening of X-ray reflections and TEM observations, defect clusters of dislocations and small intragranular bubbles began to recover around 1.150–1,450°C. Complete recovery of the defect clusters, however, was not found even after annealing at 1,800°C for 5h. The effect of irradiation temperature on microstructural change of sub-grain structure in high burnup fuels was assessed from the experimental results.     

Effect of helium preinjection and prior thermomechanical treatment on the microstructure of type 316 SS

Samples of 316 SS were preinjected with 15 appm helium either hot (650°C) or cold (room temperature) and irradiated with 3 MeV Ni⁺ ions to a dose level of 25 dpa at 625°C in order to test the validity of helium preinjection as a means of simulation of transmutant helium production. Results for preinjected and single-ion irradiated samples were compared to samples irradiated at 625°C to a 25 dpa dose level with 3 MeV Ni⁺ and simultaneously injected with helium at a rate of 15 appm He/dpa (dual-ion irradiated samples). Preinjected samples exhibited bimodal cavity size distributions. Preinjected samples of solution annealed or solution annealed and aged material showed lower swelling than dual-ion irradiated samples. However, He preinjected 20% cold worked samples showed greater swelling than dual-ion irradiated samples.     

Damage accumulation and annealing in 6H–SiC irradiated with Si+

Damage accumulation and annealing in 6H-silicon carbide (α-SiC) single crystals have been studied in situ using 2.0 MeV He⁺ RBS in a 〈0 0 0 1〉-axial channeling geometry (RBS/C). The damage was induced by 550 keV Si⁺ ion implantation (30° off normal) at a temperature of −110°C, and the damage recovery was investigated by subsequent isochronal annealing (20 min) over the temperature range from −110°C to 900°C. At ion fluences below 7.5 × 10¹³ Si⁺/cm² (0.04 dpa in the damage peak), only point defects appear to be created. Furthermore, the defects on the Si sublattice can be completely recovered by thermal annealing at room temperature (RT), and recovery of defects on the C sublattice is suggested. At higher fluences, amorphization occurs; however, partial damage recovery at RT is still observed, even at a fluence of 6.6 × 10¹⁴ Si⁺/cm² (0.35 dpa in the damage peak) where a buried amorphous layer is produced. At an ion fluence of 6.0 × 10¹⁵ Si⁺/cm² (−90°C), an amorphous layer is created from the surface to a depth of 0.6 μm. Because of recovery processes at the buried crystalline–amorphous interface, the apparent thickness of this amorphous layer decreases slightly (<10%) with increasing temperature over the range from −90°C to 600°C.     

Study of PRIMAVERA steel samples by positron annihilation spectroscopy technique II – Lifetime measurements

In the present article, a positron annihilation lifetime technique was used for the study of VVER-440/230 weld materials, manufactured in the frame of the international PRIMAVERA project on microstructural investigation of the irradiated WWER-440 reactor pressure vessel steel. The present results complement our previous report of positron angular correlation experiments and provide in-depth characterization of vacancy type defects behavior under irradiation and thermal treatment. The results give new insight into the previously published atom probe tomography and angular correlation of annihilation radiation studies. The measurements do not show any association of phosphorus or its segregation to the open volume defects investigated by positron annihilation spectroscopy. The embrittlement effects related to the phosphorus seem to be effectively annealed-out during 475 °C thermal treatment and the post annealing microstructure and mechanical properties of the material are consequently affected mostly by agglomerations of vacancy clusters coarsened during thermal treatment.     

A SANS investigation of the irradiation-enhanced α–α′ phases separation in 7–12 Cr martensitic steels

Five reduced activation (RA) and four conventional martensitic steels, with chromium contents ranging from 7 to 12 wt%, were investigated by small angle neutron scattering (SANS) under magnetic field after neutron irradiation (0.7–2.9 dpa between 250 and 400 °C). It was shown that when the Cr content of the b.c.c. ferritic matrix is larger than a critical threshold value (∼7.2 at.% at 325 °C), the ferrite separates under neutron irradiation into two isomorphous phases, Fe-rich (α) and Cr-rich (α^′). The kinetics of phase separation are much faster than under thermal aging. The quantity of precipitated α^′ phase increases with the Cr content, the irradiation dose, and as the irradiation temperature is reduced. The influence of Ta and W added to the RA steels seems negligible. Cold-work pre-treatment increases slightly the coarsening of irradiation-induced precipitates in the 9Cr–1Mo (EM10) steel. In the case of the low Cr content F82H steel irradiated 2.9 dpa at 325 °C, where α^′ phase does not form, a small irradiation-induced SANS intensity is detected, which is probably due to point defect clusters. The α^′ precipitates contribute significantly to the irradiation-induced hardening of 9–12 wt% Cr content steels.     

Separation of Isotope Mixture in a Free Molecular Jet

With the aim of finding the most desirable set of conditions for powder metallurgical preparation of graphite matrix fuels containing thorium carbide, the properties and structure of fuel samples prepared by compacting various mixtures of powdered material followed by heating were investigated.

It was found that of the resulting compacts those consisting of (1) purified and milled natural graphite powder, (2) metallic thorium powder and (3) furfuryl alcohol as binder showed the best properties after heating to 600°C. These compacts possessed 1.90–2.40 g/cc density, 3.5–10.0 kg/mm² Vickers hardness, a thermal expansion coefficient at 900°C of about 14×10^?6/°C in the direction parallel to, and 2.5times;10^?6/°C normal to pressing direction, and a thermal conductivity, in the same temperature range, of 0.02 (parallel) and 0.07cal/cm-sec°C (normal).

Since these favorable properties and structural features are not impaired by heating to about 2,000°C subsequent to treatment at 600°C, it can be said that a new, simple and effective set of conditions for fuel preparation has been found.     

Etude de la formation des cavités dans le cuivre irradie aux ions cuivre de 500 keV

We have studied the formation of voids in thin foils of copper irradiated by 500 keV Cu⁺ ions, as a function of temperature, irradiation intensity and the dissolved gas content of the samples. A theoretical examination of the phenomena gives a good account of the effects of temperature, the rate of defect creation and the foil thickness, but it fails to account for the fact that at 400° C voids are formed for a dose of 3 × 10^?4 dpa/s but not for 3 × 10^?3 dpa/s. This is tentatively explained in terms of the influence of the gas content. The effect of gas content on the formation of voids and dislocation loops has moreover been demonstrated by degassing thin foils by annealing in ultrahigh vacuum, or by implanting helium before irradiation.     

The Extraction of Curium and Americium by Tri-n-Octyl Phosphine Oxide

Magnesium chromate anhydrate, magnesium chromate monohydrate and sodium chromate quadrahydrate were irradiated in the Liquid Nitrogen Temperature Loop at the Japan Atomic Energy Research Institute to study the behavior of ⁵¹Cr recoil atoms. The initial retentions of recoil ⁵¹Cr were measured at ?55°, ?40° and ?30°C, and the thermal annealing behavior of recoil ⁵¹Cr was examined extensively in the temperature range from ?20° to 80 °C. The effects of difference in total irradiation dose and of the water of crystallization on the annealing behaviors of ⁵¹Cr recoil atoms are discussed.     

The temperature dependence of nickel-ion damage in nickel

The temperature dependence of void and dislocation structures was studied in high-purity nickel irradiated with 2.8 MeV ⁵⁸Ni⁺ ions to a displacement density of 13 displacements per atom (dpa) at a displacement rate of $\text{7 × 10}{}^{\mathrm{?2}}\text{dpa/sec}$ over the temperature range 325 to 625°C. Dislocation loops, with no significant concentrations of voids, were observed in specimens irradiated at 475°C and below. Specimens irradiated between 525 and 725°C contained both voids and dislocations. The maximum swelling was measured as 1.2% at 625°C. Analysis of the data by theoretical models for void nucleation and growth indicated that the swelling in the present experiment was principally limited by void growth at low temperatures and by void nucleation at high temperatures. The data were also compared with previously reported neutron and nickel-ion irradiation results.     

Dose and Temperature Dependence of Void Swelling in Electron Irradiated Stainless Steel

In a high voltage electron microscope, solution treated Type 316 stainless steel was electron-irradiated at temperatures in the range of 370–630°C to a dose of about 30 dpa. The swelling (ΔV/V) induced by the irradiation beyond about 5 dpa is well described by an empirical equation, ΔV/V=A(dpa) ⁿ , under constant void and dislocation densities. With increasing irradiation temperature, the fluence exponent n increases and the pre-exponent term A decreases. At 550°C irradiation, the fluence exponent takes the value of 1.5 due to the diffusion-limited void growth. The value of n larger than 1.5 at higher temperature (>550°C) is attributable to the surface reaction-limited void growth. The smaller value of n for the low temperature (?500°C) irradiation appears to arise from the dislocation assisted vacancy diffusion. The peak swelling temperature of the specimen irradiated to 30 dpa is about 570°C, which shifts to a higher temperature with increase in electron dose.     

A study on recovery kinetics of radiation damage in graphite

Through the measurements of galvanomagnetic properties, an annealing study has been made of a natural graphite compact irradiated with fast neutrons to a total dose of $\text{2.3 × 10}{}^{18}\text{n/cm}{}^2$. The isothermal annealing has revealed that the recovery taking place near 200°C consists of two atomic processes having different values of activation energy, 0.9 eV in the first stage and 0.45 eV in the second. The physical significance of those energy values is discussed with reference to the past experimental work. Discussions are also given of the isochronal annealing conducted over a temperature range up to 1000°C. However, the definite solution is still prevented of the mechanism, because of a discrepancy lying between theory and experiment on the migration energy of interstitial carbon atoms.     

Hardening and Recovery of Neutron-Irradiated Pressure Vessel Steel (ASTM A533B)

Specimens of ASTM A533B steel were studied to gain information on the annealing process following irradiation, through measurements of internal friction and of hardness.

The specimens were quenched from 900°C and tempered at 650°C, then irradiated in the JMTR reactor at 65°–75°C to a neutron dose of 1.4–1.7×10²⁰ n/cm² (E _n >1MeV).

Peaks were observed on the internal friction curves from unirradiated specimens. These peaks disappeared upon irradiation, but reappeared with annealing treatment at 150°C.

Radiation-anneal hardening was observed at 250°C. The recovery of radiation hardening begins at a temperature between 250° and 350°C, but is not completed even at 550°C.     

Positron beam and Raman analysis of hydrogen plasma treated and annealed Cz-Si

Positron beam annihilation Doppler-broadening (PADB) and Raman studies were carried out on H-plasma treated and annealed p-type Cz-Si. Samples were treated by a 13.56 MHz H-plasma for 2 h at RT (∼30 °C) and 250 °C, respectively. Annealing was done in air between 100 and 600 °C for annealing times between 10 and 480 min. The Raman spectra of the RT treated samples show no (or only very weak) H₂-molecule signals, in contrast to the samples H-plasma treated at 250 °C. Raman intensity changes as a function of temperature are observed, which are attributed to the evolution of voids or platelets. The positron annihilation data correlate with these results, since annealing of the RT plasma treated samples reduces the Doppler-broadening S-values with increasing temperature and/or annealing time. Together with an increasing positron diffusion length this suggests that defects acting as positron trapping centers are (partially) annealed out. For the sample plasma treated at 250 °C the following depth dependent behavior of the Doppler parameters is found. Below 100 nm depth the Doppler parameters follow the same trend as those of the RT treated samples. In a zone between 100 and 200 nm depth the S parameter strongly increases after annealing at 600 °C. This is attributed to the formation of positronium (Ps), indicative for the presence of nano-cavities capable of trapping molecular hydrogen in this region.