Shinji Ishimoto

Crystallography of zirconium hydrides in recrystallized Zircaloy-2 fuel cladding by electron backscatter diffraction

Precipitation morphology and habit planes of the δ-phase Zr hydrides, which were precipitated within the a-phase matrix grains and along the grain boundaries of recrystallized Zircaloy-2 cladding tube, have been examined by electron backscatter diffraction (EBSD). Radially-oriented hydrides, induced by residual tensile stress, precipitated in the outside region of the cladding, and circumferentially-oriented hydrides in the stress-free middle region of the cladding. The most common crystallographic relationship for both types of the hydrides precipitated at the inter- and intra-granular sites was identical at (0001)α // {111}δ, with {10&1macr;7}α // {111}δ, being the occasional exception only for the inter-granular radial hydrides. When tensile stress was loaded, the intra-granular hydrides tended to preferentially precipitate in the grains with circumferential basal pole textures. The inter-granular hydrides tended to preferentially precipitate on the grain faces opposite to tensile axis. The change of prioritization in the precipitation sites for the hydrides due to tensile stress could be explained in terms of the relaxation effect of constrained elastic energy on the terminal solid solubility of hydrogen at hydride precipitation.

Terminal Solid Solubility of Hydrogen in Unalloyed Zirconium by Differential Scanning Calorimetry

Zircaloy-2 cladding lined with pure Zr (Zr liner cladding) was developed as a candidate for pellet-clad interaction/ stress-corrosion cracking (PCI/SCC) failure resistant BWR cladding, and its good performance has been proved in many power ramp tests, up to burnups of about 50GWd/t. However, at higher burnups above 50–60GWd/t, another type of PCI failure, which apparently possesses an outsidein type cracking, has been recently reported. Though a hydride precipitation assisted-failure mechanism has been proposed for this type of fuel failure, there is no established theory. When considering the fuel performance, two phenomenological hydride behaviors seem to be important: (1) a dynamic relationship between hydride precipitation at crack tips and crack propagation in Zircaloy, and (2) a heterogeneous accumulation of hydrides in the Zr liner region adjacent to the Zr liner/Zircaloy-2 interface, which was observed in high burnup BWR claddings after base irradiation. The latter phenomenon may influence on hydride re-precipitation in the cladding outside region at power ramp conditions. In order to clarify both phenomena, basic understanding of dissolution and precipitation behavior of hydrides not only in Zircaloy-2 but also in Zr liner is essential. The terminal solid solubility during the dissolution of hydrides (TSSD) at heatup and during the precipitation of hydrides (TSSP) at cooldown have been the subjects of many reports for Zr and its alloys, using various measuring techniques. However, there is considerable scatter in the published data, especially in TSSP data. Moreover, there are no systematic data sets of TSSD and TSSP solvi for Zircaloy-2 and unalloyed Zr using the same technique. In our previous study, we have derived data sets of TSSD and TSSP for the current Zircaloy-2 and improved High Fe Zircaloy for BWRs, by using differential scanning calorimetry (DSC), and best fit equations for the two solvi were presented. In this subsequent study, the same DSC technique was applied to obtain data sets of TSSD and TDDP for unalloyed Zr. The present data for unalloyed Zr were compared to the previous data for the two types of Zircaloys. 1. Experimental (1) Materials In this study, two types of unalloyed Zr were subjected to terminal solid solubility (TSS) measurements. One was current Zr liner material, which was directly prepared from a tube shell of Zr liner cladding with 72.5mm outside and 41.5mm inside diameter. The shape of the Zr liner specimens was a 4mm square, about 0.6–0.7mm thick and weighing about 60–70mg. The other Zr specimens with almost the same shape as the Zr liner specimens were prepared from a Zr slab, supplied by National Bureau of Standards (NBS). Table 1 shows impurity concentrations of the two types of unalloyed Zr specimens, which were used for the present TSS measurements. Gaseous impurities of H, N and O in the Zr liner specimens were somewhat lower than those in the NBS Zr specimens. The oxygen concentrations were 320 and 850 ppm for the former and latter specimens. Test samples were hydrogenated from the as-received hydrogen levels (Zr liner: 9 ppm; NBS Zr: 27 ppm) by (1) a corrosion reaction in water vapor of 9.6MPa at 400 C, or (2) by a gaseous hydrogenation at 500 C in a He/2%H2 mixed gas. The obtained hydrogen concentration ranged from the asreceived levels to 281 ppm. The details of the hydrogenation procedure were described previously. (2) Differential Scanning Calorimetry The TSSD and TSSP temperatures of the specimens were measured using the differential scanning calorimetry (DSC) technique. The details of the DSC instrument (Netzsch DSC-404) were described previously. The DSC measurements were carried out in the same manner as the previous study for Zircaloy-2 and High Fe Zircaloy. Namely, carrier gas was purified Ar at the flow rate of 50 cm/min, and the heatup and cooldown rate of 10 C/min was adopted for the maximum temperature of 500 C with a 5min holding period there. The first run data were excluded, because of inhomogeneity of hydrogen distribution in the specimens. Then the data of the subsequent second and third runs were adopted for evaluating DSC peaks. The analysis of DSC peaks resulting from hydride dissolution during heatup and hydride precipitation during cooldown followed the previous procedure.

A New Non-destructive Technique for Hydrogen Level Assessment in Zirconium Alloys using EMAR Method

In order to study the applicability of EMAR (electromagnetic acoustic resonance) method to non-destructive hydrogen level assessment in fuel spacer bands at pool side, an ultrasonic transmitter and receiver together with an EMAT (electromagnetic transducer) were used. Unirradiated Zircaloy-2 thin plates were hydrogen charged for the measurements. An irradiated fuel cladding tube was also used to examine the detection sensitivity of the resonance spectrum of the irradiated material. The following results were obtained. Acoustic anisotropy Δf, defined by using two resonance frequencies for shear waves with different polarization, was adopted as a parameter to express the ultrasonic resonance property. A hydrogen concentration dependence of Δf was observed in the range up to 1,200 ppm. Specimen thickness and oxide thickness were found to have negligible effect, on Δf, and liftoff of the sensor up to 1mm did not affect the Δf value. The acoustic anisotropy proposed in this paper was not sensitive to any of specimen dimension, surface condition, or sensor liftoff.

Effects of Gadolinium Doping on Electrical Properties of UO2 Grain Boundaries.

Abstract Complex impedance measurements were made on Gd-doped UO2 to study Gd doping effects on electrical properties of the matrix and grain boundary. The Gd contents ranged from 0.5 to 10wt% temperature from 295 to 1,273 K; and frequency from 5 to 4×107Hz. The matrix conductivity increased almost linearly with Gd content, indicating that the number density of electron holes also increased almost linearly with increasing Gd. The grain boundary capacitance of the Gd-doped UO2 was larger than that of UO2 by about 3 orders of magnitude. The grain boundary conductivity of the Gd-doped UO2 was also larger than that of UO2, but decreased rapidly with increasing Gd content, between 0.5 to 10 wt%. The migration energy of electron holes across a grain boundary was larger than that in the matrix and seemed to increase with Gd content. It was presumed from these results that Gd ions segregated to the grain boundaries to form a potential barrier for the migration of electron holes. The barrier thickness was estimate...

Effect of Hydrogen Water Chemistry on Zircaloy-2 Fuel Cladding and Structure Material Performance

The effects of hydrogen addition to the feedwater on the corrosion and hydrogen uptake performance of Zircaloy-2 fuel cladding tubes, a water rod tube and spacer materials irradiated for four cycles in a BWR were evaluated. The uniform oxide behaviors of the cladding tubes, water rod and spacer materials were not affected by hydrogen water chemistry (HWC) condition. The hydrogen uptake and pickup fractions of the water rod and spacer materials were similar to those of water rods and spacer materials under normal water chemistry (NWC) conditions. As for the fuel rods, in spite of comparably heavy crud deposition, their hydrogen uptake and pickup fractions were clearly lower than the values under NWC conditions. Overall, the results indicated that HWC had no adverse effects on fuel performance.