No abstract available

[en] The thermal and mechanical behaviour of fuel rods is significantly influenced by the extent of their relocation and by compliance of the cracked pellets. Movement of the cracked pellet pieces towards the cladding results in softer pellets with crack voids which accomodate some fractions of the thermoelastic pellet deformation and make the pellet more compliant under the restraint of the cladding. It is difficult to model such a pellet compliance indepently of experimental observations because the cracked pellet behavior is uncertain by nature. Electrically heated simulation of pellet-cladding mechanical interaction (PCMI) facilitates much quicker and more flexible experimentation than actual in-pile tests. Testing apparatus consists of the simulated fuel rod with hollow UO₂ pellets and a tungsten rod in the center, and a diameter measuring device including three pairs of diameter sensors. Test parameter include the pellet-cladding gap and the cladding thickness. Results show that rods with a smaller gap have a larger increasing rate of cladding diameter. This suggests that a group of cracked pellet pieces induced by thermal stress has an apparent compliance which increases with pellet-cladding gap. Results also show more sensitivity to cladding thickness than those calculated assuming pellets having intrinsic stiffness. This also suggests the compliant nature of cracked pellets. Such a compliant nature can almost be described by reducing the elasticity of the pellet. A simple pellet compliance model was obtained by fitting calculations with measurements to describe a cracked pellet as a uniform axisymmetric body with apparent elasticity. (orig.)

[en] Out-of-pile corrosion tests were carried out, utilizing a circulating autoclave, for elucidating nodular corrosion of Zircaloy-2 in 10 MPa steam at 773 to 793 K. The oxide nodule growth process was observed to be initiated by micro formation on the thin black oxide layer which was subsequently raised by further oxidation through the crack. Experiments on effects of surface preparation and heat treatment on corrosion were made to investigate some factors which could affect the nodular corrosion. (author)

[en] In order to investigate the effects of crystallographic orientation on deformation and crack initiation in iodine-induced SCC of zirconium alloys, uniaxial tensile tests of zirconium and Zircaloy-2 plates were conducted in an iodine atmosphere. The crystallographic orientation of individual grains was determined by an etch-pit technique prior to testing. After testing, the etch-pit technique showed that prismatic slip was predominant in the plastic deformation and that cleavage cracks extended along basal planes. The plastic deformation of individual grains was significantly influenced by their crystallographic orientations, which varied from one grain to another. Accordingly, in homogeneous plastic deformation occurred between grains. The crack initiation took place preferentially at grain boundaries where differences of crystallographic orientations were large between adjacent grains. This indicated that crack initiations was caused by stress concentration due to strain incompatibility at those grain boundaries. (orig.)

[en] In order to investigate effects of the distribution of intermetallic particles on the SCC of zirconium alloys, SCC tests were carried out on non-irradiated Zircaloy-2 and zirconium which were heat-treated under various conditions. Relatively large particles were randomly dispersed in samples annealed at temperatures in the α-phase domain. Quenching from relatively low temperatures in the (α+β)-phase domain, such as 1148 K, produced the grain boundary segregation of very smal particles. Widmanstaetten structures were seldom formed by the quenching. The frequency of transgranular cracks increased with the volume fraction of the particles on grain boundaries, while that of intergranular cracks increased with the averged spacing of grain boundary particles. Not interangular cracks, but transgranular cracks propagated rapidly enough to cause short time brittle fracture, and SCC susceptibility increased with the volume fraction of grain boundary particles. (orig.)

[en] In order to investigate effects of intermetallic particles on SCC initiation of zirconium alloys, tensile tests were conducted in an iodine atmosphere using zirconium plates with different amounts of impurities, and Zircaloy-2 plates. SCC susceptibility of zirconium increased significantly with its iron content. Even small amounts of iron could form the intermetallic compound ZrFe₂ whose particle size and number increased iron content. In the case of Zircaloy-2, two different types of ternary compounds were detected, namely Zr(CrFe)₂ and Zr₂(NiFe). Metallographic examinations showed that the particles located at grain boundaries were important sites of SCC initiation in zirconium alloys. The initiation probability increased significantly with the amount of the particles, which supported the strong correlation between SCC susceptibility of zirconium and its iron content. (orig.)

[en] In order to investigate effects of crystallographic orientation on deformation and crack initiation in iodine induced SCC of zirconium alloys, uniaxial tensile tests of zirconium and zircaloy-2 plates were conducted in an iodine atmosphere. Crystallographic orientation of individual grains was determined by an etch-pit technique prior to testing. After testing the etch-pit technique showed that prismatic slip was predominant in the plastic deformation and that cleavage cracks extended along basal planes. The plastic deformation of individual grains was significantly influenced by their crystallographic orientations, which varied from one grain to another. Accordingly, inhomogeneous plastic deformation occurred between grains. The crack initiation took place preferentially at grain boundaries where differences of crystallographic orientations were large between adjacent grains. This indicated that crack initiation was caused by stress concentration due to strain incompatibility at those grain boundaries. (author)

[en] In order to investigate the effects of crystallographic orientation on deformation and crack initiation in iodine-induced stress corrosion cracking (SCC) of zirconium alloys, uniaxial tensile tests of zirconium and Zircaloy-2 plates were conducted in an iodine atmosphere. Ater testing, the etch-pit technique showed that prismatic slip was predominant in the plastic deformation and that cleavage cracks extended along basal planes. The plastic deformation of individual grains was significantly influenced by their crystallographic orientations. Accordingly, inhomogeneous plastic deformation occurred between grains. The crack initiation took place preferentially at grain boundaries where differences of crystallographic orientations were large between adjacent grains. This indicated that crack initiation was caused by stress concentration due to strain incompatibility at those grain boundaries. (author)

[en] Low-cycle fatigue tests have been performed on Zircaloy-2 by a reversed-bending method in inert and iodine atmospheres at 623 K. Fatigue lives in both atmospheres followed the Coffin-Manson law. Cracks propagated by a transgranular shear mode, and the propagation was the process controlling fatigue life in an inert atmosphere. Fatigue life in a high concentration of iodine vapour was significantly shorter than in an inert atmosphere. The iodine vapour caused brittle fracture of a specimen mainly by the transgranular cleavage mode, and an accelerated crack propagation rate. The minimum vapour pressure of iodine to cause brittle fracture under fatigue conditions was about 27 Pa. (orig.)

[en] Detailed PIE has been carried out on three fuel assemblies irradiated in BWRs to 30 - 35 GWd/t. Our data base for fission gas release (FGR), has been extended to 39 GWd/t in rod burnup and to 43 GWd/t in pellet burnup. The FGR rate data measured on the three assemblies showed large scattering from 0% to 25%, as the previously reported data of less than 30 GWd/t. These scattering data can be related with the maximum powers they experienced beyond 10 GWd/t. The FGR rate seems to depend mainly on maximum powers, however, only a little on burnups. Micro gamma scanning and EPMA results revealed that local FGR rate varied radially in the pellet for a high FGR rate rod, i.e. almost 0% at the outer region and almost 100% at the center region. There existed a narrow transient band between them and local FGR rate showed a rapid change there. Ceramography and SEM observations showed that the local FGR rate variations related closely to pellet micro structural changes. At the center region, many large pores were found on the grain boundaries, connecting to each other and providing tunnels for gas release. The tunnel formation may do a key role to increase the local FGR rate. No remarkable changes were found at the outer region, except a thin pellet outer surface layer (pellet rim). The micro structure of the transient band indicated that the process of pore growth and tunnel formation related with the local FGR rate increase. The tunnels seem to control local FGR rate since their occurrence. Their effect, especially historical effect on FGR, should be considered in fuel behaviour analysis codes. The pellet rim showed to have very different structure. The original structure disappeared and very fine pores appeared. The observed rim structure looked still retaining most of fission gas. However, the numerical density of the pores is so high that pore connections and tunnel formations may occur and induce additional FGR. However, further investigations are required to evaluate the rim effect on FGR at high burnup. (author). 7 refs, 6 figs, 2 tabs