M. Horiki

Microstructural evolution in low-dose neutron-irradiated Fe-16Ni-15Cr alloy

Abstract Reactor irradiation of the base alloy of modified austenitic stainless steel, a candidate alloy for fusion reactor application, was performed with improved temperature control. All three types of defects — vacancy clusters in the form of stacking-fault tetrahedra, interstitial-type dislocation loops and voids — are more pronounced in samples irradiated with conventional temperature control than those with improved control. They are attributed to the nucleation of those defects during the start-up of the reactor. From the irradiation of thin foils, the role of freely migrating interstitials was extracted. For bulk irradiation, the variations of the three types of defects mentioned above with irradiation temperature are thoroughly investigated. Reaction of the less freely migrating point defects in SUS than in pure metals is pointed out.

Microstructure evolution by neutron irradiation during cyclic temperature variation

Abstract Utilizing a technique to control the temperature which is not influenced by the operation mode of a reactor, an irradiation during which the temperature was alternatively changed several times between two temperatures ( T -cycle) has been performed. Some defect structures are understood as combinations of the defect processes at lower and higher temperatures, and some others are understood if the defect processes during the transient between the two temperatures are taken into consideration. However, the most remarkable characteristic of defect processes associated with the temperature variation is the reaction of point defect clusters induced by lower-temperature irradiation at the higher temperature. During lower-temperature irradiation, there is a greater accumulation of vacancy clusters as stacking fault tetrahedra in fcc metals than that of interstitial clusters as dislocation loops. Vacancies evaporated from the vacancy clusters at higher temperature can eliminate interstitial clusters completely, and the repetition of these processes leads to unexpectedly slow defect structure development by T -cycle irradiation.

Identification of the nature of small point defect clusters in neutron irradiated Fe–16Ni–15Cr by means of electron irradiation

Abstract The nature of very small point defect clusters in Fe–16Ni–15Cr irradiated with fission neutrons was identified from their behavior under electron irradiation with a high voltage electron microscope. In this analysis, the defect clusters which grew during electron irradiation were judged to be of interstitial (I)-type and those which shrank or disappeared to be of vacancy (V)-type. The fraction of the number of I- and V-type defect clusters in a specimen irradiated as a bulk at 353 K were found to be 7% and 93%, respectively. In a specimen irradiated at 623 K as a bulk, most of defect clusters were I-type with a very small fraction of V-type. In a thin foil specimen irradiated at 573 K, the fraction of I-type defect clusters increased from 20% near the thin specimen edge to 70% at a thicker part of 120 nm. These results were consistent with those in the previous judgement from the shape and contrast of transmission electron microscope image.

Invisible and visible point defect clusters in neutron irradiated iron

Abstract Microstructure evolution in a low-dose neutron-irradiated iron was examined with a transmission electron microscope (TEM). The characteristics of the temperature dependence of defect structures in irradiated iron were as follows, (a) rather lower number density of defect clusters at 473 K than at 573 K, (b) development of dislocation loops and voids at an intermediate temperature of 623 K and (c) formation of irregularly shaped dislocation loops at 673 K. Comparing the defect structures produced by the irradiation with a conventional temperature control and that with an improved temperature control, nucleation of defect clusters was suppressed under neutron irradiation at 473 K. From the defect structures introduced by irradiations in which the irradiation temperature was cyclically changed between two temperatures (T-cycle irradiation), the accumulation of a large number of invisible vacancy type defect clusters by irradiation at 473 K was strongly suggested.

Irradiation of Uranium Carbides in JMTR

Post-irradiation examinations were performed on three types of uranium carbide UC, UC2 and U2C3, that were irradiated with a relatively high thermal neutron flux (7×1013 n/cm2·s or 9×l012 fission/cm3·s for UC) and in a dose range between 2.7×l015 and 3.3×1018 fission/cm3 (i.e. 2.1×l016 and 2.4× 1019 nvt) in the JMTR (Japan Material Testing Reactor). On UC and UC2, trends similar to previous works were obtained in the irradiation effects. New preliminary results, however, were obtained on U2C3 for changes in the electrical resistivity and the lattice parameter which showed reduced values after attaining a maximum at 1017 fission/cm3. Successive annealing effects on the resistivity and the lattice parameter following the reactor irradiations were examined by pulse heating. Two major steps were observed, at around 400 and 600°C, in the recovery processes. A low temperature step, which was revealed previously at about 200°C in UC and UC2, was missing in this study, because of higher irradiation temperatures (...

Defect structures introduced in iron under varying temperature neutron irradiation

Abstract Defect structures in iron under varying temperature neutron irradiation and those under constant-temperature irradiation were examined by a transmission electron microscope (TEM). For irradiation at a low temperature (473 K), microvoids were detected by positron annihilation lifetime (PAL) measurement. For irradiation at a high temperature (673 K), with increasing the irradiation dose a few interstitial (I)-type dislocation loops grew and the total number density of the loops decreased. In a varying temperature irradiation between 473 and 673 K, after a shift to the high temperature the absorption of interstitials to I-type dislocation loops was suppressed by microvoids introduced at the low temperature and the loops shrank or disappeared by vacancies released from microvoids. As the cycle of varying temperature irradiation became short, the effect of the suppression of the defect development became small.

Fission fragment damage of magnetic uranium compounds

Effects of fission (fragment) damage on the magnetic properties were investigated for some uranium compounds with NaCl-type crystal structure, such as uranium monocarbide (UC, paramagnetic) monophosphide (UP, antiferro) and monosulfide (US, ferro). The induced changes in the magnetic properties due to the fission damage were much pronounced in the magnetically ordered state. A shift of the magnetic transition point (either the Neel (TN) and Curie (TC) temperature) was observed, together with the changes of the magnetic parameters. In some cases, a new magnetically ordered phase was revealed by the fission damage even at room temperature irradiation.

Quenched-in vacancies in single crystalline uranium monocarbide (UC)

Abstract Low-temperature electrical resistivity change due to quenched-in vacancies in single crystalline uranium monocarbide (UC) has been investigated. The formation energies for carbon and uranium vacancies are 1.1 ± 0.3 and 3.7 ± 0.4 eV, respectively. The isochronal annealing curves of quenched-in resistivity, obtained by resistivity measurements at room temperature, show two stages at about 600 and 1100 K, whereas those obtained by resistivity measurements at 77 K exhibit only the stage at 1100 K. The annealing stage at 1100 K is ascribed to the migration of uranium vacancies, of which the activation energy is evaluated as 2.1 ± 0.5 eV. It is found that the quenched-in resistivity depends on temperature; a quadratic temperature dependence is observed below 220 and 160 K for the quenched-in carbon and uranium vacancies, respectively.