Irradiation In High Flux Isotope Reactor Research Articles

Microstructural development of tungsten and tungsten–rhenium alloys due to neutron irradiation in HFIR

The microstructural development of pure tungsten (W) and tungsten–rhenium (Re) alloys due to neutron irradiation in the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory, TN, USA, was investigated in this work. The irradiation conditions were ∼1 displacements per atom (dpa) at 500 and 800°C. After the neutron irradiation, microstructural observations were performed using a transmission electron microscope (TEM). Large amounts of precipitates identified as sigma- and chi-phases were observed in not only the W–Re alloys but also in the pure W after the neutron irradiation. The precipitates observed in the pure W were coarse and larger than those in the W–Re alloys. This was considered to be caused by the transmutation products of W and Re, namely, Re and osmium (Os), respectively, under irradiation in the HFIR with a higher contents of thermal neutron flux.

Tritium trapping in silicon carbide in contact with solid breeder under high flux isotope reactor irradiation

The trapping of tritium in silicon carbide (SiC) injected from ceramic breeding materials was examined via tritium measurements using imaging plate (IP) techniques. Monolithic SiC in contact with ternary lithium oxide (lithium titanate and lithium aluminate) as a ceramic breeder was irradiated in the High Flux Isotope Reactor (HFIR) in Oak Ridge, Tennessee, USA. The distribution of photo-stimulated luminescence (PSL) of tritium in SiC was successfully obtained, which separated the contribution of 14C β-rays to the PSL. The tritium incident from ceramic breeders was retained in the vicinity of the SiC surface even after irradiation at 1073K over the duration of ∼3000h, while trapping of tritium was not observed in the bulk region. The PSL intensity near the SiC surface in contact with lithium titanate was higher than that obtained with lithium aluminate. The amount of the incident tritium and/or the formation of a Li2SiO3 phase on SiC due to the reaction with lithium aluminate under irradiation likely were responsible for this observation.

Fracture toughness and Charpy impact properties of several RAFMS before and after irradiation in HFIR

As part of the development of candidate reduced-activation ferritic steels for fusion applications, several steels, namely F82H, 9Cr–2WVTa steels and F82H weld metal, are being investigated in the joint DOE–JAEA collaboration program. Within this program, three capsules containing a variety of specimen designs were irradiated at two design temperatures in the ORNL High Flux Isotope Reactor (HFIR). Two capsules, RB-11J and RB-12J, were irradiated in the HFIR removable beryllium positions with europium oxide (Eu 2O 3) thermal neutron shields in place. Specimens were irradiated up to 5 dpa. Capsule JP25 was irradiated in the HFIR target position to 20 dpa. The design temperatures were 300 °C and 500 °C. Precracked third-sized V-notch Charpy (3.3 × 3.3 × 25.4 mm) and 0.18 T DC(T) specimens were tested to determine transition and ductile shelf fracture toughness before and after irradiation. The master curve methodology was applied to evaluate the fracture toughness transition temperature, T 0. Irradiation induced shifts of T 0 and reductions of J Q were compared with Charpy V-notch impact properties. Fracture toughness and Charpy shifts were also compared to hardening results.

Mechanical properties of neutron-irradiated nickel-containing martensitic steels: I. Experimental study

Tensile and Charpy specimens of 9Cr–1MoVNb (modified 9Cr–1Mo) and 12Cr–1MoVW (Sandvik HT9) steels and these steels doped with 2% Ni were irradiated at 300 and 400 °C in the High Flux Isotope Reactor (HFIR) up to ≈12 dpa and at 393 °C in the Fast Flux Test Facility (FFTF) to ≈15 dpa. In HFIR, a mixed-spectrum reactor, ( n, α) reactions of thermal neutrons with 58Ni produce helium in the steels. Little helium is produced during irradiation in FFTF. After HFIR irradiation, the yield stress of all steels increased, with the largest increases occurring for nickel-doped steels. The ductile–brittle transition temperature (DBTT) increased up to two times and 1.7 times more in steels with 2% Ni than in those without the nickel addition after HFIR irradiation at 300 and 400 °C, respectively. Much smaller differences occurred between these steels after irradiation in FFTF. The DBTT increases for steels with 2% Ni after HFIR irradiation were 2–4 times greater than after FFTF irradiation. Results indicated there was hardening due to helium in addition to hardening by displacement damage and irradiation-induced precipitation.

The study of temperature variation during HFIR irradiation on vanadium

The effects of temperature variation on pure vanadium and vanadium model alloys under neutron irradiation were studied in high flux isotope reactor (HFIR). Pure vanadium and four vanadium model alloys (V–5Cr, V–5Ti, V–4Cr–4Ti, V–4Cr–4Ti–0.1Si) were irradiated under variable temperature conditions at 633/793 K for a total 8 of irradiation cycles. After the irradiation, the microstructure of the samples was compared to those of the same alloys, which were continuously irradiated at 793 K.

Electrical conductivity of alumina after heavy irradiation in a high-flux fission reactor

Abstract The electrical conductivities of seven different alumina specimens were measured after they were irradiated in the High-Flux Isotope Reactor (HFIR), Oak Ridge National Laboratory. Results indicated a moderate increase in the electrical conductivity due to the irradiation in HFIR up to 2.8 displacements per atom at 450–500°C. A large offset current was observed in the irradiated specimens after they were subjected to an applied voltage, which indicated the existence of a large number of trapped electrical charges. An abrupt release of the trapped electrical charges may be a trigger to cause the previously reported electrical breakdown under irradiation (called radiation-induced electrical degradation).

Tensile behavior of F82H with and without spectral tailoring

The effects of neutron spectrum on tensile properties of the low-activation martensitic steel F82H (8Cr–2WVTa) was examined using a thermal neutron shield to tailor the neutron spectrum for steels irradiated in the high flux isotope reactor (HFIR). The yield stresses of spectrally tailored specimens irradiated in HFIR to 5 dpa at 300°C and 500°C are on trend lines obtained from unshielded irradiation in HFIR. No significant effect of the neutron spectrum on tensile properties could be detected.

Swelling of F82H irradiated at 673 K up to 51 dpa in HFIR

Reduced-activation ferritic/martensitic steel, F82H (8Cr–2W–0.2V–0.04Ta–0.1C), and variants doped with isotopically tailored boron were irradiated at 673 K up to 51 dpa in the high flux isotope reactor (HFIR). The concentrations of 10B in these alloys were 4, 62, and 325 appm during HFIR irradiation which resulted in the production of 4, 62 and 325 appm He, respectively. After irradiation, transmission electron microscopy (TEM) was carried out. The number density of cavities increased and the average diameter of cavities decreased with increasing amounts of 10B. The number density decreased and the average diameter increased with increasing displacement damage. Swelling increased as a function of displacement damage and He concentration.

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.

Embrittlement of CrMo steels after low fluence irradiation in HFIR

Subsize Charpy impact specimens of 9Cr1MoVNb (modified 9Cr1Mo) and 12Cr1MoVW (Sandvik HT9) steels and 12Cr1MoVW with 2% Ni (12Cr1MoVW2Ni) were irradiated in the High Flux Isotope Reactor (HFIR) at 300 and 400°C to damage levels up to 2.5 dpa. The objective was to study the effect of the simultaneous formation of displacement damage and transmutation helium on impact toughness. Displacement damage was produced by fast neutrons, and helium was formed by the reaction of 58Ni with thermal neutrons in the mixed-neutron spectrum of HFIR. Despite the low fluence relative to previous irradiations of these steels, significant increases in the ductile-brittle transition temperature (DBTT) occurred. The 12Cr1MoVW2Ni steel irradiated at 400°C had the largest increase in DBTT and displayed indications of intergranular fracture. A mechanism is proposed to explain how helium can affect the fracture behavior of this latter steel in the present tests, and how it affected all three steels in previous experiments, where the steels were irradiated to higher fluences.

Void swelling resistance of phosphorus-modified austenitic stainless steels during HFIR irradiation at 300–500°C to 57 dpa

The austenitic stainless steel prime candidate alloy (PCA) is a Ti-modified alloy (14Cr16Ni) developed for void swelling resistance at 400–600°C by the US Fusion Reactor Materials (FMR) program. During irradiation in the High Flux Isotope Reactor (HFIR) to 34 and 57 dpa (70–75 appm He/dpa) at 300–600°C, th e PCA (and the closely related D9 type alloy) has been shown to be highly susceptible to void swelling at 500°C. Generally such alloys show good void swelling resistance to higher doses during irradiation in fast-breeder reactors (FBRs, 0.5–1 appm He/dpa) in the 20–25% cold-worked (CW) condition. New multiply-stabilized, phosphorus-modified PCA alloys have been developed, with specific amounts and combinations of minor alloy elements added for improved MC formation and stability characteristics during irradiation. After HFIR irradiation at 300–500°C to 34–57 dpa, the multiply-stabilized, phosphorus-modified PCA and phosphorus-modified D9 alloy showed better swelling resistance (by density-change measurements) than similar alloys without phosphorus, particularly at 500°C. Microstructural studies after 34 dpa at 500°C showed that void swelling resistance in the multiply-stabilized, phosphorus-modified PCA alloys was directly related to the formation of ultrafine dispersions of MC precipitates during HFIR irradiation.

Effect of irradiation in HFIR on tensile properties of Cr-Mo steels

Tensile specimens of tempered martensitic 9Cr-1MoVNb and 12Cr-1MoVW steels with and without additions of 2% Ni were irradiated in the High Flux Isotope Reactor (HFIR) at 400 and 600°C to displacement-damage levels of up to 72 dpa. Nickel was added to the steels to produce transmutation helium, and up to 700 appm He was produced in the steels with 2% Ni. Irradiation at 400°C caused an increase in strength and a decrease in ductility, while irradiation at 600°C caused a decrease in strength and an increase in ductility. Results at 400°C were similar to data from previous tests on steel irradiated to ~ 11 dpa (up to 110 appm He), where it was concluded that helium caused a strength increase in addition to the hardening caused by displacement damage alone. The present results indicate that the helium-enhanced strengthening at 400°C saturates with increasing helium concentration, similar to the saturation (at a lower strength level) with displacement-damage dose found in the absence of helium.

Impact behavior of 9Cr-1MoVNb and 12Cr-1MoVW steels irradiated in HFIR

Charpy impact specimens of 9Cr-1MoVNb and 12Cr-1MoVW steels were irradiated in the High Flux Isotope Reactor (HFIR) at 300 and 400°C to displacement-damage levels as high as 42 dpa. For both steels, irradiation caused large increases in the ductile-brittle transition temperature (DBTT) and a decrease in the upper-shelf energy (USE). The increase in DBTT and the decrease in USE were greater at 400°C than at 300°C. At 400°C, irradiation to ~ 40 dpa produced shifts in DBTT of 204 and 242°C for the 9Cr-1MoVNb and 12Cr-1MoVW steels, respectively. These are the largest shifts ever observed for these steels and are attributed to the high helium concentration generated during irradiation in HFIR.

On the saturation of the DBTT shift of irradiated 12Cr-1MoVW with increasing fluence

Irradiation of ferritic steels often results in an increase in the ductile-to-brittle transition temperature (DBTT), sometimes well in excess of 100°C, and also a substantial decrease in the upper shelf energy (USE). Two irradiation experiments were conducted to determine the fluence dependence of this degradation in impact properties. Identical subsize Charpy specimens of 12Cr-lMoVW steel were irradiated in the University of Buffalo Reactor (UBR) at 300°C to a fluence of 0.86 × 10 24 n/m 2 ( E > 0.1 MeV ) and in the High Flux Isotope Reactor (HFIR) at 300 and 400°C to a fluence of ∼1 × 10 26 n/m 2 ( E > 0.11 MeV ). Impact testing indicated that the DBTT was increased by irradiation at 300°C by 36 (UBR) and 164°C (HFIR). Corresponding decreases in the USE were 13 and 33%, respectively. Irradiation in HFIR at 400°C led to an increase in the DBTT of 217°C and a 50% drop in the USE. Tests on aged but unirradiated control materials indicated aging alone has an insignificant effect on the impact properties. The results show that the major portion of the irradiation embrittlement at 300°C occurs for fluences beyond 1 × 10 24 n/m 2 . Comparison of the results with data in the literature shows that the greatest effect on the impact properties of 12Cr-lMoVW steels results after irradiation in the temperature range of 300 to 400°C. Finally, a possible detrimental influence of small amounts of helium is noted and discussed.

Hfir irradiation facilities improvements — the HIFI project

The High-Flux Isotope Reactor (HFIR) has outstanding neutronics characteristics for materials irradiation, but some relatively minor aspects of its mechanical design severely limit its usefulness for that purpose. In particular, though the flux trap region in the center of the annular fuel elements has a very high neutron flux, it has no provision for instrumentation access to irradiation capsules. The irradiation positions in the beryllium reflector outside the fuel elements also have a high flux; however, although instrumented, they are too small and too few to replace the facilities of a materials testing reactor. To address these drawbacks the HFIR Irradiation Facilities Improvement (HIFI) Project, now under way and scheduled for completion in mid-1987, consists of modifications to the reactor vessel cover, internal structure, and reflector. Instrumentation access will be provided to the flux trap region, and the number of materials irradiation positions in the removable beryllium will be increased from four to eight, each with almost twice the available experimental space of the present ones.

Effect of low-temperature irradiation with (n,α) helium production on tensile properties of 12 Cr-1MoVW-type steels

Tensile specimens from small heats of ferritic (martensitic) steel based on 12 Cr-1 MoVW have been Irradiated in the High Flux Isotope Reactor (HFIR) at ∼50°C to displacement damage levels of up to 9.5 dpa. Two heats of 12 Cr-1 MoVW steel were irradiated, along with similar alloys to which nickel had been added. Transmutation of the 58Ni by thermal neutrons during irradiation in HFIR leads to helium production rates similar to those anticipated in a fusion reactor. Displacement damage is produced simultaneously by the fast neutron flux in the reactor. After irradiation, the specimens were tested at room temperature and 300°C. Yield strength and ultimate tensile strength of irradiated samples displayed considerable hardening over the unirradiated condition. The increased strength was accompanied by a decrease in ductility. Indications are that the hardening resulted only from the displacement damage and was not affected by the transmutation helium formed during irradiation.