K. Farrell

Experimental effects of helium on cavity formation during irradiation—a review

Abstract Cavity (void) formation and swelling in non-fissile materials during neutron irradiation and charged particle bombardments are reviewed. Helium is the most important inert gas and is primarily active as a cavity nucleant. It also enhances formation of dislocation structure. Preimplantation of helium overstimulates cavity nucleation and gives a different temperature response of swelling than when helium is coimplanted during the damage process. Helium affects, and is affected by, radiation-induced phase instability. Many of these effects are explainable in terms of cavity nucleation on submicroscopic critical size gas bubbles, and on the influence of the neutral sink strength of such bubbles. Titanium and zirconium resist cavity formation when vacancy loops are present.

Void swelling and defect cluster formation in reactor-irradiated copper☆

Copper disks were irradiated at temperatures of 182 to 500 °C with moderated fission neutrons under carefully controlled conditions to a damage level of 1.1 to 1.3 dpa at a damage rate of 2× 10−7dpa/s. The lower temperature limit for void formation was found to lie between 182 and 220 ° C, with a maximum swelling of about 0.5% at 300 to 350 ° C and no swelling at 500 °C. At 182°C, vacancies produced stacking fault tetrahedra instead of voids.

Correlation of neutron and heavy-ion damage: I. The influence of dose rate and injected helium on swelling in pure nickel☆

Abstract Displacement damage structures in pure nickel at the 1 dpa level are compared for two widely disparate damage rates, 10 −7 dpa/s for neutron irradiations and 3 X 10 −3 dpa/s for self-ion bombardments over a range of temperatures spanning those for void formation. Peak swelling at about 0.7% is found at 400° and 600°C, respectively. At equivalent swelling temperatures, voids in the ion-bombarded material are larger and fewer than those from neutron irradiation, especially at temperatures above the peak swelling temperature. Additions of 20 appm He, matching that generated in the neutron irradiations, were made to the ion-bombarded nickel either prior to ion bombardment (preinjection) or during ion bombardment (simultaneous injection). This helium caused increased swelling at the upper and lower temperature extremes. Simultaneously implanted helium did not otherwise significantly affect microstructures, whereas preinjected helium increased the dislocation density and caused more but smaller voids over the full temperature range of swelling.

Simulation of first wall damage: effects of the method of gas implantation

Cavity formation in an austenitic alloy of similar composition to Type 316 stainless steel has been explored with regard to various methods of gas implantation. Irradiations were conducted at 900 K to doses of 1, 10, and 70 dpa with helium injection levels of 20 appm/dpa. Highest swelling (18%) was exhibited by the unimplanted reference material; a lesser amount by simultaneous helium injection (11%). Greatly reduced swelling due to profuse cavity nucleation was the results of the preinjection of 1400 appm He, either at room temperature (S = 1%) or at 900 K (4%). The dislocation density was not sensitive to helium injection technique. Simultaneous injection of 50 appm H/dpa, along with the He, may have caused a modest increase in the cavity and dislocation concentrations at higher doses. The observations are compared with a theory of void growth kinetics to estimate the relative influence of voids and dislocations as point defect sinks.

A TEM study of neutron-irradiated iron☆

Abstract The results of a transmission electron microscopy study of the defect structure in iron neutron-irradiated to low fluences (⩽ 1 dpa) at temperatures of 455–1013 K are presented. The dislocation microstructures coarsen with increasing irradiation temperature from decorated dislocations, through clusters of dislocation loops, to near-edge, interstitial dislocation loops with b = a 〈100〉, and network segments. Significant cavity formation occurred only at 548–723 K, with homogeneous distributions found only at 623 and 673 K. The maximum swelling of 0.07% occurred at 673 K. Large cavities had a truncated octahedral shape with {111} facets and {100} truncations. Damage halos were observed around boron-containing precipitates. The effects of interstitial impurities on microstructural development and the differences in the observed microstructures compared to those in refractory bcc metals are discussed.

The effects of irradiation temperature and preinjected gases on voids in aluminum

Abstract Voids in high purity aluminum irradiated to a fast (E>1 MeV) fluence of 4 × 1020 n/cm2 at 125 (0.43T m) and 150°C (0.45T m) are fewer in number but very much larger in size than those in material irradiated at 55°C (0.35T m). Additionally, at 125 and 150°C, the voids adopt a variety of shapes including plates, ribbons, cylinders and more equiaxed polyhedra, and are frequently associated with particles of transmutation-produced silicon. At the higher temperatures voids are larger near grain boundaries than in grain interiors. Injection of hydrogen or helium prior to irradiation causes an increase in the number of voids and a corresponding decrease in size in specimens irradiated at 150°C; 3 at. ppm He is more effective than either 3 or 9 at. ppm H. The gases do not appear to influence swelling. A commercial purity (99 per cent) aluminum subjected to the same irradiation treatments did not develop voids whether preinjected with gases or not; the visible radiation damage consisted solely of small lo...

On mechanisms by which a soft neutron spectrum may induce accelerated embrittlement

Abstract Both low displacement rates and softened neutron spectrum favor survival of a higher fraction of point defects per displacement for producing micro-structural changes leading to hardening and embrittlement. Low displacement rate results in low bulk recombination rate. A high thermal to fast neutron flux ratio results in a large fraction of point defects produced in small cascades from (n,γ) and (n,α) reactions. Defects from such cascades generally avoid in-cascade recombination, while most of the defects created in large cascades produced by fast neutrons are lost to in-cascade recombination. Thus thermal neutrons produce more available defects per unit displacement dose. It is argued that the spectral effect may dominate the accelerated embrittlement observed in ferritic steels at the High Flux Isotope Reactor (HFIR) pressure vessel location. The rate effect is expected to be a secondary factor at temperatures as low as 50°C, where the HFIR data were obtained. Our analysis suggests generally that components subject to neutron environments with high thermal-to-fast ratios and irradiated at low temperatures may be subject to accelerated radiation effects.

An evaluation of low temperature radiation embrittlement mechanisms in ferritic alloys

Abstract Investigations underway at Oak Ridge National Laboratory (ORNL) into reasons for the accelerated embrittlement of surveillance specimens of ferritic steels irradiated at 50°C at the High Flux Isotope Reactor (HFIR) pressure vessel are described. Originally, the major suspects for the precocious embrittlement were a highly thermalized neutron spectrum, a low displacement rate, and the impurities boron and copper. Each of these possibilities has been eliminated. A dosimetry experiment made at one of the major surveillance sites shows that the spectrum at that site is not thermalized. A new model of matrix hardening due to point defect clusters indicates little effect of displacement rate at low irradiation temperature. Boron levels are measured at 1 wppm or less, which is inadequate for embrittlement. Copper and nickel impurities are shown to promote radiation strengthening at high doses but not at the low doses pertinent to the surveillance data. It is shown that a copper embrittlement scenario has other drawbacks, and it is argued that copper impurity is not responsible for the accelerated embrittlement of the HFIR surveillance specimens. The dosimetry experiment revealed unexpectedly high levels of reaction products in some of the fast flux monitors, which are found to be caused by an exceptionally high ratio of gamma ray flux to fast neutron flux at the pressure vessel. Gamma rays can also induce atomic displacements, leading to the suggestion that the accelerated embrittlement may be provoked by gamma irradiation.

Effects of gamma-induced displacements on HFIR pressure vessel materials

Abstract Accelerated radiation-induced embrittlement of the High Flux Isotope Reactor (HFIR) surveillance materials has been investigated since its discovery in 1986. Recent comprehensive dosimetry experiments revealed the presence of an intense gamma field at the HFIR surveillance locations near the pressure vessel. Gamma-induced reactions were found to dominate the response of several dosimeters and were crucial for the explanation of dosimetry results. This finding precipitated an assessment of the gamma-induced displacements-per-atom (dpa) rate, which was found to exceed the neutron-induced dpa rate at all locations analyzed. When the sum of neutron and gamma dpa is used for the interpretation of the HFIR surveillance results, the HFIR data are consistent with data from the Oak Ridge Research Reactor (ORR) and other test reactors. The accelerated embrittlement is therefore explained in terms of hitherto uncounted dpa induced by gamma rays.

A helium-induced shift in the temperature dependence of swelling☆

Abstract In an annealed austenitic alloy undergoing bombardment with 4 MeV Ni ions to doses between 1 and 70 dpa at 840, 900, 950, 1025, and 1100 K, the introduction of simultaneously-implanted helium at a rate of 20 appm He/dpa moves the swelling versus temperature curve up the temperature scale by 40 to 70 K. Co-implantation of hydrogen (deuterium) at a rate of 50 appm D/dpa simultaneously with the helium causes little or no additional systematic effects. The major change in microstructure caused by the gases is an enhancement of cavity nucleation by factors of 2 to 5 at 840 to 950 K, increasing to factors of thousands at 1100 K. Concurrently there is a reduction in the size of cavities and in swelling at all temperatures below 1025 K, and an increase in cavity size and in swelling at 1100 K, where the cavities are gas stabilized. At 1025 K the increase in nucleation of cavities outpaces the reduction in size and causes increased swelling. The primary effects of the gases are decided at low doses, below 1 dpa, where cavity nucleation is completed and where the conditions governing cavity growth are established; at higher doses swelling is determined by cavity growth, which is dependent on dose only. The gases cause copious formation of cavities on grain boundaries, boding ill for mechanical properties.