Rim Structure Formation Research Articles

Influence of Ni/Co binders and Mo2C on the microstructure evolution and mechanical properties of (Ti0.93W0.07)C–based cermets

In this research, solid–solution powder of (Ti0.93W0.07)C was synthesized by high–energy ball mill method followed by carbothermal reduction process. Subsequently, the acquired powder was blended with Ni/Co and Mo2C secondary carbide, and sintered under the optimized temperature (1510 °C) for 1 h to produce the modulated cermets. A typical core–rim structure formation with solid–solution phases was confirmed by backscattered electrons studies using a Field Emission electron scanning microscope. The hardness of the synthesized cermets was enhanced by increasing the specific amount of Mo2C. The acquired results demonstrate that the binder type has a prominent influence on the microstructure and hardness of the prepared cermets. The hardness of (Ti0.93W0.07)C–xMo2C–Ni cermet increased ~ 9%, when nickel was partially substituted by cobalt.

Diametral strain of fast reactor MOX fuel pins with austenitic stainless steel cladding irradiated to high burnup

The C3M irradiation test, which was conducted in the experimental fast reactor, “Joyo”, demonstrated that mixed oxide (MOX) fuel pins with austenitic steel cladding could attain a peak pellet burnup of about 130 GWd/t safely. The test fuel assembly consisted of 61 fuel pins, whose design specifications were similar to those of driver fuel pins of a prototype fast breeder reactor, “Monju”. The irradiated fuel pins exhibited diametral strain due to cladding void swelling and irradiation creep. The cladding irradiation creep strain were due to the pellet–cladding mechanical interaction (PCMI) as well as the internal gas pressure. From the fuel pin ceramographs and 137Cs gamma scanning, it was found that the PCMI was associated with the pellet swelling which was enhanced by the rim structure formation or by cesium uranate formation. The PCMI due to cesium uranate, which occurred near the top of the MOX fuel column, significantly affected cladding hoop stress and thermal creep, and the latter effect tended to increase the cumulative damage fraction (CDF) of the cladding though the CDF indicated that the cladding still had some margin to failure due to the creep damage.

Measurements of Crystal Lattice Strain and Crystallite Size in Irradiated UO2 Pellet by X-ray Diffractometry

Lattice parameters, crystallite sizes, and nonuniform strains of high-burnup UO2 fuel samples were measured using micro-X-ray diffractometry in order to investigate the effects of grain subdivision and strain distribution between crystallites on the microstructural changes, so-called rim structure formation, in UO2 pellets. The samples were prepared from fuel rods irradiated in commercial reactors, and pelletaveraged burnups of the samples were in the range from 41 to 61GWd/t. While the lattice parameters of the samples increased linearly in the burnup region up to approximately 70 GWd/t, the lattice parameters slightly decreased and tended to level off above 70GWd/t. The measured crystallite sizes were in the range from 100 to 200 nm and these were nearly the same as those of the “recrystalized grains” in the rim structure. The elastic strain energy densities, which were evaluated from the lattice parameters and nonuniform strains, tended to increase with burnup and show two plateaus at burnups of approximately 50 and 70GWd/t, respectively. The saturation of the measured strain energy density at approximately 50GWd/t can be attributed to the tangle of accumulated dislocations, and the increase in the strain energy density above 60GWd/t can be explained by the strain caused by crystallite rotation under the restraint conditions between crystallites.

Measurements of Crystal Lattice Strain and Crystallite Size in Irradiated UO2 Pellet by X-ray Diffractometry

Lattice parameters, crystallite sizes, and nonuniform strains of high-burnup UO2 fuel samples were measured using micro-X-ray diffractometry in order to investigate the effects of grain subdivision and strain distribution between crystallites on the microstructural changes, so-called rim structure formation, in UO2 pellets. The samples were prepared from fuel rods irradiated in commercial reactors, and pelletaveraged burnups of the samples were in the range from 41 to 61GWd/t. While the lattice parameters of the samples increased linearly in the burnup region up to approximately 70 GWd/t, the lattice parameters slightly decreased and tended to level off above 70GWd/t. The measured crystallite sizes were in the range from 100 to 200 nm and these were nearly the same as those of the “recrystalized grains” in the rim structure. The elastic strain energy densities, which were evaluated from the lattice parameters and nonuniform strains, tended to increase with burnup and show two plateaus at burnups of approximately 50 and 70GWd/t, respectively. The saturation of the measured strain energy density at approximately 50GWd/t can be attributed to the tangle of accumulated dislocations, and the increase in the strain energy density above 60GWd/t can be explained by the strain caused by crystallite rotation under the restraint conditions between crystallites.

Fission Gas Release Behavior from High Burnup UO2 Fuels under Rapid Heating Conditions

Fission gas release (FGR) behavior under rapid heating conditions of high burnup UO2 fuels with developed rim structure has been examined using two different out-of-pile heating techniques with no restraint pressure. The burnups of the fuel specimens were 36–86 GWd/tU. The bare fuel specimens were heated up to 600-1,800°C at heating rates of 1.7 to 4,600°C/s. The FGR process strongly depended on fuel burnup (extent of rim structure formation) and heating conditions (heating rate, terminal temperature). At lower heating rates below about 10°C/s, growth and interlinkage of fission gas bubbles controlled FGR only at higher temperatures above the threshold temperatures of 1,250--1,450°C, depending on fuel burnup. At higher heating rates above at least 90°C/s, instantaneous FGRs, which originated from the occurrence of microcracks and fuel fragmentation induced by the overpressurization of rim bubbles, arose at higher temperatures above 700°C, only for the high burnup fuels of 74 and 86 GWd/tU with developed rim structure. Almost no FGR was found for non rim-structured fuels, even at the higher terminal temperatures of 1,500--1,800°C. From these and our previous out-of-pile heating results, a map of heating condition dependent FGR under unrestrained conditions is proposed as a function of heating rate and terminal temperature.

Fission Gas Release Behavior from High Burnup UO2 Fuels under Rapid Heating Conditions

Fission gas release (FGR) behavior under rapid heating conditions of high burnup UO2 fuels with developed rim structure has been examined using two different out-of-pile heating techniques with no restraint pressure. The burnups of the fuel specimens were 36–86 GWd/tU. The bare fuel specimens were heated up to 600-1,800°C at heating rates of 1.7 to 4,600°C/s. The FGR process strongly depended on fuel burnup (extent of rim structure formation) and heating conditions (heating rate, terminal temperature). At lower heating rates below about 10°C/s, growth and interlinkage of fission gas bubbles controlled FGR only at higher temperatures above the threshold temperatures of 1,250--1,450°C, depending on fuel burnup. At higher heating rates above at least 90°C/s, instantaneous FGRs, which originated from the occurrence of microcracks and fuel fragmentation induced by the overpressurization of rim bubbles, arose at higher temperatures above 700°C, only for the high burnup fuels of 74 and 86 GWd/tU with developed rim structure. Almost no FGR was found for non rim-structured fuels, even at the higher terminal temperatures of 1,500--1,800°C. From these and our previous out-of-pile heating results, a map of heating condition dependent FGR under unrestrained conditions is proposed as a function of heating rate and terminal temperature.

Fission Gas Release in Irradiated UO2 Fuel at Burnup of 45 GWd/t during Simulated Reactivity Initiated Accident (RIA) Condition

Pulse irradiation simulating reactivity-initiated-accident (RIA) condition was conducted for test rod prepared from fuel irradiated in a commercial reactor and fission gas release behavior from fuel pellet during pulse irradiation was investigated. Optical microscopy (OM) and scanning electron microscopy (SEM) observations and electron probe micro analysis (EPMA) were carried out for the test rod as a part of destructive tests after the pulse irradiation. Fission gas release during pulse irradiation was evaluated by EPMA and puncture test. Xenon depression was observed in the fuel pellet after pulse irradiation at periphery and center region. It is considered that the xenon depression at pellet periphery was due to rim structure formation and fission gas was mainly released from the pellet center region during pulse irradiation. Comparing the fractional fission gas release during pulse irradiation with other results of out-of-pile annealing tests, it was concluded that most fission gas, which was accumulated at grain boundary during base irradiation, was released from the center region of test fuel pellet during pulse irradiation.

The formation of core–rim structures in Fe40Al/(TiC–TiN–WC) cermets produced by pressureless melt infiltration

Abstract Ti(CN)-based cermets were produced by pressureless melt infiltration of TiC, TiN, and WC mixed powders by Fe–40 at.% Al iron aluminide intermetallic (Fe40Al). Before infiltration, the powder compacts were pre-sintered at different temperatures for different times in order to form hard Ti(CN)-based skeletons with given values of porosity. A core–rim morphology of the solid particles evidenced under SEM characterizes the microstructure of cermets made from pre-sintered Ti(CN) skeletons infiltrated by Fe40Al. The mechanism of the core–rim structure formation was studied using XRD, SEM, TEM, and EDS. This core–rim structure is composed of an almost pure Ti(CN) core surrounded by a bi-layer rim of (Ti,W)(CN) solid solution. However, direct infiltration of as-pressed TiC, TiN, and WC powder mixtures by Fe40Al led to mostly Ti(CN) cermets without core–rim structures. The formation of a (Ti,W)(CN) phase during the pre-sintering step determines the existence of the core–rim microstructure. The Ti(CN) core and the (Ti,W)(CN) inner rim formed at the pre-sintering stage by solid state reaction, while the outer rim was formed during the infiltration stage by the dissolution of (Ti,W)(CN) material from the inner rim and precipitation as a solid solution of low W content.

Fission Gas Release in Irradiated UO2 Fuel at Burnup of 45 GWd/t during Simulated Reactivity Initiated Accident (RIA) Condition

Pulse irradiation simulating reactivity-initiated-accident (RIA) condition was conducted for test rod prepared from fuel irradiated in a commercial reactor and fission gas release behavior from fuel pellet during pulse irradiation was investigated. Optical microscopy (OM) and scanning electron microscopy (SEM) observations and electron probe micro analysis (EPMA) were carried out for the test rod as a part of destructive tests after the pulse irradiation. Fission gas release during pulse irradiation was evaluated by EPMA and puncture test. Xenon depression was observed in the fuel pellet after pulse irradiation at periphery and center region. It is considered that the xenon depression at pellet periphery was due to rim structure formation and fission gas was mainly released from the pellet center region during pulse irradiation. Comparing the fractional fission gas release during pulse irradiation with other results of out-of-pile annealing tests, it was concluded that most fission gas, which was accumulated at grain boundary during base irradiation, was released from the center region of test fuel pellet during pulse irradiation.

Fission Gas Release Behavior in High Burnup UO2 Fuels with Developed Rim Structure

The effect of rim structure formation and external restraint pressure on fission gas release at transient conditions has been examined by using an out-of-pile high pressure heating technique for high burnup UO2 fuels (60, 74 and 90 GWd/t), which had been irradiated in test reactors. The latter two fuels bore a developed rim structure. The maximum heating temperature was 1500ºC, and the external pressures were independently controlled in the range of 10-150 MPa. The present high burnup fuel data were compared with those of previously studied BWR fuels of 37 and 54 GWd/t with almost no rim structure. The fission gas release and bubble swelling due to the growth of grain boundary bubbles and coarsened rim bubbles were effectively suppressed by the strong restraint pressure of 150 MPa for all the fuels; however the fission gas release remarkably increased for the two high burnup fuels with the developed rim structure, even at the strong restraint conditions. From the stepwise de-pressurization tests at an isothermal condition of 1500ºC, the critical external pressure, below which a large burst release due to the rapid growth and interlinkage of the bubbles abruptly begins, was increased from a 40-60 MPa level for the middle burnup fuels to a high level of 120-140 MPa for the rim-structured high burnup fuels. The high potential for transient fission gas release and bubble swelling in the rim-structured fuels was attributed to highly over-pressurized fission gases in the rim bubbles.

Effects of Rim Structure Formation on the Thermal Conductivity of UO2 Pellets

Thermal diffusivities of UO2 pellets irradiated in a test reactor were measured by using a laser flash method. The maximum burnups of the samples were about 85 GWd/t and some of the samples contained rim structures. Thermal diffusivities of irradiated pellets decreased compared with those of unirradiated and simulated soluble fission products-doped UO2 pellets. Hysteresis phenomena in the thermal diffusivity of irradiated fuel, which had been reported before, were not clearly observed in UO2 pellets in which the rim structures had formed. The thermal conductivities for irradiated UO2 pellets were evaluated from measured thermal diffusivities, specific heat capacities of unirradiated UO2 pellets and measured sample densities. The thermal conductivities of irradiated UO2 were compared to those of unirradiated UO2 pellets. The relative thermal conductivities were normalized to those of 96.5%TD (Theoretical Density). These normalized thermal conductivities of irradiated UO2 pellets in which significant rim structures had formed tended to be slightly higher than those of irradiated UO2 pellets in which no rim structure had formed. This may be caused by an effect of recrystallization in rim structures.

Transmission electron microscopy observation on irradiation-induced microstructural evolution in high burn-up UO 2 disk fuel

In order to identify the conditions of the rim structure formation as a function of burn-up and temperature, and to clarify the formation mechanism of this restructuring, UO 2 fuel disks were irradiated at four thermal conditions, between 400 and 1300 °C, and at four different burn-ups, between 36 and 96 MWd/kgU, without external mechanical constraint. The microstructural evolutions as a function of the irradiation parameters are observed by high resolution scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM observations reveal the transition from original to sub-divided grains of rim structure and make clear that the burn-up threshold is between 55 and 82 MWd/kgU. The temperature threshold of this restructuring could be 1100±100 °C. Moreover, polyhedral sub-divided-grains with size ranging between 0.5 and 2 μm, not only rounded grains in the size range 150–350 nm, are clearly observed. These configurations are explained by assuming that the grain sub-divisions occurred homogeneously within the original polyhedral grains, while the existence of rounded grains might be due to free surface effects. TEM observations of re-structured samples show that most of sub-grain boundaries are low angle and are heavily decorated by fission gas bubbles in the range 3.5–8 nm. In the non-restructured samples, dislocations and small precipitates are present, and many of the bubbles form “strings” along dislocation lines. In specimens irradiated at high temperature, many dislocations seem to be anchored by fission product precipitates. These results suggest that the formation mechanism of the restructuring is based on polygonization, and the precipitates could have some “pinning effect” on dislocations and defect clusters.

Post-irradiation examination of high burnup Mg doped UO 2 in comparison with undoped UO 2, Mg–Nb doped UO 2 and Ti doped UO 2

The pellets of UO 2, magnesium doped UO 2 (Mg–UO 2), magnesium and niobium doped UO 2 (Mg–Nb–UO 2) and titanium doped UO 2 (Ti–UO 2) were irradiated to burnups ranging from 19 to 94 GWd/tU at temperatures 550–930°C. The solubility of magnesium in UO 2 was low around 2 mol%. The addition of magnesium and titanium caused to form large grain sized pellet on sintering. The swelling of pellets during irradiation was unchanged by magnesium addition below 60 GWd/tU in agreement with the literature rate for UO 2. The thermal conductivity of unirradiated Mg–UO 2 was higher than that of undoped UO 2, which seemed to also hold for irradiated specimens. Pellet fracturing occurred by irradiation mainly by thermal stress. The undoped and metal doped UO 2 pellets in the 84–94 GWd/tU range at the irradiation temperatures of 560–640°C showed large bubbles and sub-divided grains of sub-micron size and the rim structure formation all over the surface. The xenon release from the pellets during irradiation increased with increasing burnup. In the fuels of close burnups, the xenon release increased rapidly with increasing temperature above about 600°C. At high burnups, the effect of metal addition seemed to recede unclear perhaps due to the formation of heavily damaged fuel matrix.

Rim structure formation of isothermally irradiated UO 2 fuel discs

UO 2 fuel discs, which had been irradiated at an isothermal condition of 550–630°C to 51, 86 and 90 GWd/t without any restraint pressure, have been subjected to detailed microstructure observations, elemental analyses and density measurements. Their data were compared with previously reported results of high-burnup Zircaloy-clad type fuel pellets, so as to clarify the effect of pellet–cladding interaction (PCI) restraint on the rim structure formation. The porous rim structure, accompanying huge bubbles and high porosity, was recognized for the high-burnup discs of 86 and 90 GWd/t, but not for the 51 GWd/t disc. From the good coincidence between porosity increase and density decrease for the high-burnup discs, it was concluded that the precipitation and growth of coarsened rim bubbles substantially caused fuel swelling. The wide variety of rim bubble sizes and porosities at a given local burnup, which was recognized in the present PCI-free discs and Zircaloy-clad type pellets reported in other literature, possibly resulted from the external PCI restraint effect. The pressure difference between bubble internal and external pressures, and vacancy diffusivity would rate-control the growth of rim bubbles.

Lattice parameter changes associated with the rim-structure formation in high burn-up UO 2 fuels by micro X-ray diffraction

Radial variations of the lattice parameter and peak width of two high burn-up UO 2-fuels (67 and 80 GWd/tM) were measured by a specially developed micro-X-ray diffraction technique, allowing spectra acquisition with 30 μm spatial resolution. The results showed a significant but constant peak broadening, and a lattice parameter that increased towards the pellet edge and decreased again within the rim-zone. This lattice contraction coincided with other property changes in the rim region, i.e., porosity increase, hardness decrease and Xe depletion. In terms of local burn-ups, the lattice contraction followed the rate of the matrix Xe depletion measured by EMPA, exceeding greatly the contraction rate due to dissolved fission products. The observed behaviour can be equally explained by a saturation of single interstitials with subsequent recombination with excess vacancies, as by the saturation and enlargement of dislocation loops. The concentration and sizes of defects involved and their possible relation to the rim structure formation are discussed.

Rim structure formation and high burnup fuel behavior of large-grained UO 2 fuels

Irradiation-induced fuel microstructural evolution of the sub-divided grain structure, or rim structure, of large-grained UO 2 pellets has been examined through detailed PIEs. Besides standard grain size pellets with a grain size range of 9–12 μm, two types of undoped and alumino-silicate doped large-grained pellets with a range of 37–63 μm were irradiated in the Halden heavy water reactor up to a cross-sectional pellet average burnup of 86 GWd/t. The effect of grain size on the rim structure formation was quantitatively evaluated in terms of the average Xe depression in the pellet outside region measured by EPMA, based on its lower sensitivity for Xe enclosed in the coarsened rim bubbles. The Xe depression in the high burnup pellets above 60 GWd/t was proportional to d −0.5– d −1.0 ( d: grain size), and the two types of large-grained pellets showed remarkable resistance to the rim structure formation. A high density of dislocations preferentially decorated the as-fabricated grain boundaries and the sub-divided grain structure was localized there. These observations were consistent with our proposed formation mechanism of rim structure, in which tangled dislocation networks are organized into the nuclei for recrystallized or sub-divided grains. In addition to higher resistance to the microstructure change, the large-grained pellets showed a smaller swelling rate at higher burnups and a lower fission gas release during base irradiation.

Temperature and fission rate effects on the rim structure formation in a UO 2 fuel with a burnup of 7.9% FIMA

A BWR design UO 2 fuel irradiated to a burnup of 7.9% FIMA was selected for a careful calculational and experimental analysis because the rod experienced an unusual power history: it had two high power periods at 1.7% FIMA and between 4 and 5% FIMA causing increased fuel temperatures and thus increased gas release and damage recovery. As a consequence, two parameters generally considered to be important for grain subdivision (rim structure formation) were locally different from normal fuel, i.e. fission gas inventory and extent of radiation damage. Histories of temperature, fission rate and fission gas release were calculated at different radial positions. Microstructure observations (TEM, SEM) revealed the typical high burnup grain subdivision process (polygonization) which extended to a maximum of 1.65 mm ( r/ r 0 = 0.73) from the pellet surface inwards. For this radial position, the calculations yielded a local temperature of 1200°C and predicted that more than half of the fission gas was released during the second high power period for this radial position. The results give thus information on the importance of the fission gas inventory for the burnup threshold of restructuring.

Towards the mathematical model of rim structure formation

The high burnup LWR UO 2 fuels show a notable micro-structural change around the pellet outer zone which is called the rim structure. It is observed at temperatures as low as 400°C so that fission track and cascade mixing could be the key mechanism. SEM observation revealed that the structure primarily appears on free surfaces of UO 2, indicating that strong sink for point defects may play a big role. And as generic observations, increase of lattice parameter indicates extensive amounts of vacancies are stored in high burnup fuel, which may induce the restructuring interacting with dislocations of high density at high burnup. Considering these observations a model of reaction-diffusion process of defects with irradiation induced transport is proposed. The equations are investigated numerically. The model indicates that an instability starts when the dislocation network starts intensive interaction with vacancy flux which is modified by interstitial diffusion between spatial segments. It appeared to be similar to the Turing type instability which indicates that the rim structure formation is one kind of the self-organizing processes of open reaction-diffusion systems.

Polygonization and high burnup structure in nuclear fuels

The rim effect, i.e., the formation of the high burnup structure typical for the outer zones of LWR UO 2 fuel pellets at extended burnup, is presently being studied in many laboratories. It is caused by a subdivision of the original as-sintered grains of the UO 2 pellets into 10 4 to 10 5 new small subgrains. The aim of the activities presently ongoing in different laboratories is to define the conditions (burnup, temperature, pressure, type of fuel, grain size, etc.) for the formation of the rim structure and to understand the mechanism of this subgrain division, or polygonization. The aim of the present paper is to discuss the existing knowledge on such polygonization processes in other materials, to confront this knowledge with the observations on UO 2 fuel and to discuss the attempts to model the rim structure formation.

96/04981 Confluence of calculational and experimental information for determination of power distribution and burnup

A BWR design UO2 fuel irradiated to a burnup of 7.9% FIMA was selected for a careful calculational and experimental analysis because the rod experienced an unusual power history: it had two high power periods at 1.7% FIMA and between 4 and 5% FIMA causing increased fuel temperatures and thus increased gas release and damage recovery. As a consequence, two parameters generally considered to be important for grain subdivision (rim structure formation) were locally different from normal fuel, i.e. fission gas inventory and extent of radiation damage. Histories of temperature, fission rate and fission gas release were calculated at different radial positions. Microstructure observations (TEM, SEM) revealed the typical high burnup grain subdivision process (polygonization) which extended to a maximum of 1.65 mm (r/r0 = 0.73) from the pellet surface inwards. For this radial position, the calculations yielded a local temperature of 1200°C and predicted that more than half of the fission gas was released during the second high power period for this radial position. The results give thus information on the importance of the fission gas inventory for the burnup threshold of restructuring.