Fission Gas Bubble Formation Research Articles

The formation mechanisms of high burnup structure in UO2 fuel

The formation process of high burnup structure (HBS) in UO 2 fuel is accompanied by the subdivision of original grains and the formation of fission gas bubbles, which can accelerate the fission gas swelling and release, and correspondingly raises some safety concerns over the extension of operating life of the nuclear fuel in the reactor. The UO 2 fuel specimens were irradiated up to a maximum burnup of 120 GW·d/tU in High Flux Engineering Testing Reactor (HFETR) of Nuclear Power Institute of China (NPIC). High resolution scanning electron microscope (SEM) observations were carried out in the fuel specimens with different local burnups of 48–120 GW·d/tU. Our results show that there exist two restructuring mechanisms during the formation of the HBS in UO 2 fuel matrix: polygonization and recrystallization. Furthermore, a comparison with earlier experiments published at the CEA of France suggests that the deposition of fission fragments and recoil particles is the main mechanism responsible for the formation of small grains on the inner wall of large bubbles.

Microstructural Changes and Chemical Analysis of Fission Products in Irradiated Uranium-7 wt.% Molybdenum Metallic Fuel Using Atom Probe Tomography

Understanding the microstructural and phase changes occurring during irradiation and their impact on metallic fuel behavior is integral to research and development of nuclear fuel programs. This paper reports systematic analysis of as-fabricated and irradiated low-enriched U-Mo (uranium-molybdenum metal alloy) fuel using atom probe tomography (APT). This study is carried out on U-7 wt.% Mo fuel particles coated with a ZrN layer contained within an Al matrix during irradiation. The dispersion fuel plates from which the fuel samples were extracted are irradiated at Belgian Nuclear Research Centre (SCK CEN) with burn-up of 52% and 66% in the framework of the SELENIUM (Surface Engineering of Low ENrIched Uranium-Molybdenum) project. The APT studies on U-Mo particles from as-fabricated fuel plates enriched to 19.8% revealed predominantly γ-phase U-Mo, along with a network of the cell boundary decorated with α-U, γ’-U2Mo, and UC precipitates along the grain boundaries. The corresponding APT characterization of irradiated fuel samples showed formation of fission gas bubbles enriched with solid fission products. The intermediate burnup sample showed a uniform distribution of the typical bubble superlattice with a radius of 2 nm arranged in a regular lattice, while the high burnup sample showed a non-uniform distribution of bubbles in grain-refined regions. There was no evidence of remnant α-U, γ’-U2Mo, and UC phases in the irradiated U-7 wt.% Mo samples.

Comparison of fission gas swelling models for amorphous U3Si2 and crystalline UO2

Theoretical models are used in support of the I2S-LWR (Integral Inherently Safe LWR) project for a direct comparison of fuel swelling and fission gas bubble formation between U3Si2 and UO2 fuels. Swelling in uranium silicide is evaluated by analyzing the bubble coarsening process’ effects on the bubble population and size distribution. The uranium dioxide is examined with two models: one which calculates the swelling behavior with a fixed grain radius and a second which incorporates grain growth into the model. These two models use the diffusion of gas atoms to the grain boundaries to calculate the swelling. The different mechanisms controlling the swelling of the fuels are introduced including the “knee point” caused by the amorphous state for the U3Si2. The U3Si2 model shows that, for the burnup expected for the I2S reactor, swelling in U3Si2 will not reach the “knee point” with accelerated swelling. The results from the fixed grain UO2 model and U3Si2 model are used to compare the swelling of each fuel at the same fission rate.

Thermal properties of U–Mo alloys irradiated to moderate burnup and power

A variety of physical and thermal property measurements as a function of temperature and fission density were performed on irradiated U–Mo alloy monolithic fuel samples with a Zr diffusion barrier and clad in aluminum alloy 6061. The U–Mo alloy density, thermal diffusivity, and thermal conductivity are strongly influenced by increasing burnup, mainly as the result of irradiation induced recrystallization and fission gas bubble formation and coalescence. U–Mo chemistry, specifically Mo content, and specific heat capacity was not as sensitive to increasing burnup. Measurements indicated that thermal conductivity of the U–Mo alloy decreased approximately 30% for a fission density of 3.30×1021fissionscm−3 and approximately 45% for a fission density of 4.52×1021fissionscm−3 from unirradiated values at 200°C. An empirical thermal conductivity degradation model developed previously and summarized here agrees well with the experimental measurements.

Microstructurally Explicit Simulation of Intergranular Mass Transport in Oxide Nuclear Fuels

Transport of fission products (FPs) inside fuel pellets is an important mechanism that affects microstructure evolution as well as fuel performance. To study this phenomenon for low fuel burnups, when solid-state diffusion is likely to be the controlling mechanism that sets the stage for subsequent phenomena, e.g., fission gas bubble formation and linkage, we created a three-dimensional (3-D) finite element model based on the real microstructure of a depleted UO2 sample. The model couples grain bulk, grain boundary (GB), and triple junction (TJ) diffusion by using 3-D elements for grain bulks, two-dimensional elements for GBs, and one-dimensional elements for TJs. Grain boundary percolation theory is applied in one case study, and the result shows that the presence of high-diffusivity TJs reduces the effect of GB percolation. The model is also used with mass generation from grain bulks, and it is found that localized regions with a high concentration of FPs can form in the presence of a dominant GB percolation path. The work introduces an approach to model diffusion through GBs and TJs at a fair computational cost that can be applied to study the effects of microstructure on FP transport.

Recrystallization and fission-gas-bubble swelling of U–Mo fuel

At high burnup, U–Mo fuel exhibits some form of recrystallization, by which fuel grains are subdivided. The effect of grain subdivision is to effectively enhance fission gas bubble (FGB) swelling due to increased grain boundaries. Inter-granular FGB swelling, i.e., FGB formation and growth at the grain boundaries, is much larger than the intra-granular FGB swelling. Recrystallized fuel volume fractions of U–Mo fuels irradiated to fission densities reaching 5.7×1021f/cm3 were measured. Analytical expressions of recrystallization kinetics of U–Mo fuel during irradiation have been developed through the usage of the Avrami equation, a phenomenological equation which is also used to describe similar typical transformation reactions, such as new phase formation. In this work, we present a novel FGB swelling model of U–Mo fuel that is expressed in terms of Mo content, extent of cold work (fuel powder fabrication method), and fission density.

HIGH BURNUP CHANGES IN UO2FUELS IRRADIATED UP TO 83 GWD/T IN M5(R)CLADDINGS

Since the 90's, EDF and AREVA-NP have irradiated, up to very high burnups, lead assemblies housing <TEX>$M5^{(R)}$</TEX> cladded fuels. Post-irradiation examination of high burnup <TEX>$UO_2$</TEX> pellets show an increase in the fission-gas release rate, an increase in fuel swelling, and formation of fission-gas bubbles throughout the pellets. Xenon abundances were quantified, and phenomena leading to this bubble formation were identified. All examinations provided valuable data on the complex state of the fuel during irradiation. They show the good behavior of these fuels, exhibiting various microstructures at very high burnups, none of which is likely to lead to problems during irradiation.

Measurement of the specific heat of Zr–40 wt%U metallic fuel

The specific heat capacities of un-irradiated and irradiated metallic Zr–40 wt%U fuel have been measured between 50 °C and 1000 °C with a differential scanning calorimetry. The irradiated fuels have three different burnup levels of 0.38, 0.70 and 0.92 g-fission product (FP)/cm 3. The measured specific heat for the un-irradiated fuel is representative and consistent with the values estimated from the Neumann–Kopp rule. The irradiated fuels exhibited a complicated behavior of the heat capacities. The unique characteristics of the specific heat capacities can be explained by the recovery of radiation damage, the formation of fission gas bubbles and fission gas release, and a phase transition in the irradiated fuels. An examination of the microstructure revealed that multiple large bubbles were formed in the irradiated fuel during specific heat measurement. The measured specific heat is expected to enable us to estimate the stored energy in the metallic fuel during certain accident scenarios and to determine the thermal conductivity of zirconium–uranium metallic fuel.

Swift heavy ion and fission damage effects in UO2

Irradiations of uranium dioxide, UO2 with different swift heavy ions were performed using wide ranges of energies and fluences from 72 MeV to 2.7 GeV in the range 5×109–1017 ions/cm2. The ions were Zn, Mo, Cd, Sn, Xe, I, Pb, Au and U. The threshold energy loss for formation of visible tracks in UO2 could be determined to be in the range 22–29 keV/nm. Fission products of fission energy are below this threshold but nevertheless form thermal spikes in UO2. Observable tracks are only found at the surface. By using 127I-beams of 72 MeV energy the consequences of fission product impact, i.e., lattice parameter increase, fission gas bubble formation, resolution of fission gas from bubbles and fission-enhanced diffusion were all observed and measured. The swelling of UO2 was confirmed to be small and the technologically important process of polygonization (grain subdivision, also called rim-effect in operating UO2-fuel) could be simulated. The results obtained are important in understanding the operating behavior of UO2, today’s fuel in nuclear electricity-producing power stations.

Release of unstable fission products from defective fuel rods to the coolant of a PWR

An analysis method has been developed to predict the contents of unstable fission products in the coolant of a PWR containing defective fuel rods under steady-state operating conditions. It takes into account the relevant physical processes, such as fuel pellet oxidation, enhancement of fission gas diffusivity due to fuel oxidation, and migration of fission products from the fuel-to-clad gap to the coolant by a first-order rate process. With the shape of grains treated as tetrakaidecahedron, the transport of fission products from the fuel matrix to the grain boundaries and formation of fission gas bubbles on grain boundaries are described by White and Tucker's model. In addition, the fraction of fission gas bubbles on grain boundaries interlinked to open surfaces is determined with the use of the Monte Carlo technique. Transport of the fission products from the gap to the coolant is assumed to occur by a first-order rate process. Comparison with experimental data shows that the present method predicts reasonably well the R B ratios for defective fuel rods with intermediate linear heating rates and underpredicts for fuel rods with very high or low linear heating rates.

Fission Gas Bubbles in Uranium-Aluminide Fuels

Formation of fission gas bubbles heretofore has not been observed in uranium-aluminide fuels. Recent irradiations to record high burnups offered a possibility to determine the onset of fission gas bubble formation in this type of fuel. Present experimental evidence suggests that UAl/sub 2/, UAl/sub 3/, and UAl/sub 4/ do not form fission gas bubbles at fission densities of 7 x 10/sup 21//cm/sup 3/ of fuel (60% depletion of 93% enriched /sup 235/U), and that pure uranium aluminide is likely to remain free of fission gas bubbles to very high /sup 235/U burnup at any enrichment. However, fission gas bubbles were found in these experimental fuels for the first time, but they were without exception associated with uranium-oxide inclusions that were evidently formed during fuel fabrication.

On the formation of gas bubbles in fissile material during reactor irradiation

Abstract The formation of fission gas bubbles in solids has been considered in conditions where the re-solution of gas from bubbles is appreciable. In such conditions the Greenwood theory is shown to apply only at high temperatures and short irradiation times. At low temperatures precipitation is delayed at short irradiation times. For longer irradiation times at all temperatures, the irradiation-enhanced generation rate due to re-solution of gas from bubbles outweights the natural generation rate and the number of nuclei increases linearly with time until numbers are eventually limited by the agglomeration of bubbles by random motion. The number of bubbles predicted by this nucleation-agglomeration theory agrees with experimental observation in the case of uranium dioxide, but may not agree in the case of uranium, for which the re-solution parameter may not be high enough. The effect of the theoretical predictions of bubble number and size on swelling and gas migration is discussed for uranium dioxide.