P.A. Hahn

20 K cryo-transfer tem observation of nascent displacement cascade damages in low temperature D-T neutron-irradiated metals at RTNS-II

Abstract TEM specimens of a variety of metals were irradiated below 16 K with D-T neutrons at RTNS-II and cryotransferred below 24 K to a JEM-200CX electron microscope. After the observation of nascent defects at temperatures below 24 K, the specimen temperature was raised in the electron microscope to examine changes in the defects. Defects observed at low temperature generally do not change much for heat treatment below stage III, but major structure changes are caused by 200 keV electron illumination. In electron-illuminated areas of gold, some defects disappear during annealing to 200 K and bubbles appear during 175 K observation after 200 K anneal, in areas where no defects were observed previously. There is a similarity of defect evolution in electron-illuminated areas in a number of metals. In nascent displacement cascades in most metals, both interstitials and vacancies form small aggregates of sub-microscope size; some observable interstitial loops are also formed.

Fusion neutron irradiation effects in commercial Nb3Sn superconductors

Abstract This is a preliminary report on a series of fusion neutron (14.8 MeV) irradiation studies on pure and alloyed multifilamentary Nb 3 Sn superconductors at the Livermore Rotating Target Neutron Source RTNS-II. One set of specimens (2 pure Nb 3 Sn and one alloyed — (Nb-1.2 wt% Ti) 3 Sn — conductor) was irradiated in-situ at 14 K in 3 cycles up to a maximum fluence of 1.3 × 10 22 n / m 2 (0.32 eV deposited damage energy) and T c and j c measured in magnetic fields up to 15.5 T. The critical parameters were also recorded after isochronal annealing cycles at 80, 180 and 300 K. Six identical sets including 3 additional wires (one pure liquid diffusion processed and 2 alloyed — (Nb-0.8 wt% Ti) 3 Sn and (Nb-2.0 wt% Ti) 3 Sn — conductors) were irradiated at room temperature at different positions from the source in order to accumulate approximately 2 ×, 1 ×, 1 2 , 1 4 , 1 8 and 1 16 of the maximum dose of the in-situ experiment. A comparison of the in-situ results with the room temperature irradiation data showed no significant differences in the behaviour of T c and j c as a function of neutron fluence and only small recovery effects were found after the isochronal anneals at the end of the in-situ experiment.

Simulation of fusion reactor conditions for superconducting magnet materials

We report on a comprehensive study of neutron irradiation induced changes of critical current densities in a variety of Nb-Ti and some Nb 3 Sn superconductors with Ti additions as well as of changes in normal state resistivity of the copper stabilizer at 8 T. The samples were irradiated at 5 K and subjected to room temperature annealing cycles after each irradiation step. The results on Nb-Ti show, in general, small degradations of j c , which do not exceed 20% over the lifetime fluence at the magnet location. Contrary to the low-field data, we find a rather uniform radiation response of metallurgically different NbTi materials at high fields. The results on (Nb, Ti) 3 Sn show the usual increase of critical current densities with neutron fluence, followed by a sharp decrease of j c . However, the peak of j c is shifted to much lower fluences in the alloyed samples as compared to pure Nb 3 Sn. Finally, results on the resistivity change of “magnet” copper are presented, which indicate that a considerable increase of resistivity (∼60% at 8 T) cannot be avoided under reasonable operating conditions of the magnet.

Neutron Irradiation of Superconductors and Damage Energy Scaling of Different Neutron Spectra

Three different neutron sources were used to irradiate identical sets of NbTi superconductors up to about half the lifetime dose of a superconducting magnet in a fusion reactor. Based on a careful source characterization of the TRIGA Mark-II reactor in Vienna, the spallation neutron source IPNS at Argonne and the 14 MeV neutron source RTNS-II at Livermore, the damage energy cross sections were calculated for four different types of NbTi alloys (42, 46.5, 49 and 54 wt% Ti). The experimental results on the variations of critical current densities Jc with neutron dose are found to scale within the experimental uncertainties with the appropriate damage energy cross sections. This first explicit proof of damage energy scaling for Jc-variations in superconductors is considered to be most valuable for the evaluation of radiation damage in superconductors under fusion reactor conditions.

Irradiation disordering and ordering of Cu3Au by fusion neutrons

Ordered and partially ordered Cu/sub 3/Au alloys (S = .30 - .99) have been irradiated at 4K and 300K with fusion neutrons at RTNS-II. The disordering rate was measured by monitoring electrical resistivity. The analysis of 4K irradiations and a comparison with fission reactor irradiations indicated that the disordering rate depended upon the long-range order parameter, S, dS/d phi t = -S(k/sub 1/-k/sub 2/S), where k/sub 1/ and k/sub 2/ are scaled with damage energy. The results of 300K irradiation indicated that reordering competed significantly with disordering in the partially ordered sample. Compared to the results of 4K irradiation, the net disordering rate at 300K was higher than that at 4K. This difference and the dependence of disordering rate on S is discussed in terms of the effects of disorder and thermal displacements on cascade cooling processes.

Annealing experiments of low temperature 14 MeV neutron-irradiated ordered and disordered Cu3Au by TEM

Annealing experiments using a liquidHe cryotransfer TEM holder were performed for ordered and disordered Cu3Au irradiated to (5–9) × 1020n/m2 with 14 MeV neutrons below 20 K. Almost all TEM-observed defect clusters in a thicker part of disordered Cu3Au were of the interstitial-type, and a significant fraction of those in a thicker part of ordered Cu3Au were expected to be interstitial-type. The mean image diameter of defect clusters in ordered Cu3Au was larger than that in disordered Cu3Au. The possible reason is the lack of long-range focusing along the [110] Cu chains in disordered Cu3Au, which results in a higher cascade temperature and a higher probability of vacancy-interstitial recombination in disordered Cu3Au than in ordered Cu3Au. Because of stage-III annealing, TEM-visible interstitial-type defect clusters started to be annealed out at around 250 K, and superlattice reflections were observed in disordered Cu3Au after room temperature annealing.

Comparison of PKA energy spectra, gas-atom production and damage energy deposition in neutron irradiation at various facilities

Abstract By dividing neutron-energy spectrum into four energy groups, (I) 10 MeV, contributions to damage parameters (PKA spectrum, damage energy and gas-atom production) from each of the energy group were calculated for neutron irradiations at various facilities with the SPECTER code developed by Greenwood and Smither [1]. The normalized PKA spectra and the gas-atom productions were compared to examine differences in damage parameters. Such comparisons were carried out among (1) irradiations at various positions in different fission reactors (i.e. KUR, JOYO and FFTF-MOTA), and among (2) those at various fission reactors. Damage parameters were also calculated at STARFIRE fusion reactor and RTNS-II. A possible method to correlate damages at different fission reactors is discussed. It is suggested that damages in fusion reactor can be simulated by the superposition of irradiations with fission and D-T neutrons.

Fusion neutron irradiation induced ordering and defect production in Cu3Au at high temperatures

We irradiate three Cu/sub 3/Au alloys different degrees of initial long-range order at temperatures between 300K and 434K. The resistivity of samples is monitored during irradiation and related to the long-term order parameter by the Muto relation. The results show that the ordering rate, which is proportional to the concentration of freely migrating vacancies, increases at the beginning and then decreases later with fluence. The decrease is a result of the continuous production of sinks in the form of dislocation loops. The effect of sinks on vacancy annihilation in some cases causes a reversed temperature dependence of ordering rate. The free vacancy production rate and the rate of sink production are determined using an ordering kinetics theory. The results of the 14 MeV neutron irradiations are compared to those obtained in other neutron spectra and particle irradiations. The estimated free vacancy production rate is also compared to the primary defect production rate measured at 4.2K in disordered samples.

Radiation imposed limits on superconducting magnets: A data base for copper stabilizers

Two of eight differently prepared copper stabilizer samples, previously irradiated in the RTNS-II at LLNL, the IPNS-1 and the BSR at ORNL, have been irradiated to a fluence of 1.33 /times/ 10/sup 22/ n/m/sup 2/ at RTNS-II. During the course of the irradiation the samples were periodically removed (without warming) for measurements of the transverse magnetoresistance and returned for continued irradiation. This experiment extends the range of neutron-irradiation-induced resistivity by a factor of five over the previous experiments. A simple model is developed which reproduces the magnetoresistance results of all the experiments to an accuracy of 2.5%. 13 refs., 6 figs.

Fusion neutron irradiation of Ni-Si alloys at high temperature

Abstract Two Ni-4% Si alloys, with different cold work levels, have been irradiated with 14-MeV fusion neutrons at 623 K, and their Curie temperatures have been monitored during irradiation. The results are compared to those of an identical alloy irradiated by 2-MeV electrons. The results show that increasing dislocation density increases the Curie temperature change rate. At the same damage rate, the Curie temperature change rate for the alloy irradiated by 14-MeV fusion neutrons is only 6–7% of that for an identical alloy irradiated by 2-MeV electrons. It is well known that the migration of radiation induced defects contributes to segregation of silicon atoms at sinks in this alloy, causing the Curie temperature changes. The current results imply that the relative free defect production efficiency decreases from one for the electron irradiated sample to 6–7% for the fusion neutron irradiated sample.