三种铀靶中裂变气体产物氪、氙的释放

选用溶液蒸干靶、粉状靶和块状靶等三种铀靶作为研究对象,初步研究了三种靶中气体裂变产物氪、氙的释放率。在西安脉冲堆辐照三种铀靶后,分别在常压循环和吸附真空两种释放模式下释放裂变气体产物氪、氙。采用间接法测定各气体产物的释放率。由固体裂变产物得到的总裂变数计算各气体裂变产物的总核数,由释放的气体裂变产物实测释放的产物总核数,以上两值之比表征释放率。结果表明:气体裂变产物释放率在三种铀靶中差异较大,在块状靶中最低,说明氪、氙的释放率与靶件的化学形态密切相关,氪、氙产物独立产额的大小也是决定释放率的重要因素。     

Effect of Grain Size on Microstructural Change and Damage Recovery in UO2 Fuels Irradiated to 23 GWd/t

The effect of grain size on microstructural change and damage recovery in U0₂ fuels was studied by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The as-irradiated lattice parameter of the standard fuel (grain size: 16/μm) was larger than that of the large-grained fuel (43 μm), indicating a larger number of fission-induced point defects in the lattice of the former fuel. This tendency was in contrast to previously reported results for low burnup fuels below 1 GWd/t. The lattice dilation in the present high burnup fuels was mainly due to the accumulation of vacancies. The lattice parameter of both fuels began to recover around an irradiation temperature of 450~650°C, and both had a complete recovery at 850°C. On annealing at high temperatures of 1,450~ 1,800°C, the bubble diameter in the standard fuel was larger than that in the large-grained fuel. This indicated that vacancy diffusion from the grain boundaries plays an important role during bubble coarsening at high temperatures.     

Effects of Annealing under High Static Pressure on Precipitation Behavior of Fission Gas Bubbles in Irradiated UO2

Annealing experiments were carried out on irradiated UO₂ in argon gas under high pressure (600 and 1,000 kg/cm²) as well as atmospheric, at temperatures of 1,400°–1,600°C. The effects of high external pressure on the behavior of fission gas bubbles in the irradiated UO₂ were studied by comparing replica electron micrographs of fractured surfaces of specimens annealed under different temperatures and pressures. The results indicate that high pressures such as above 600 kg/cm² can be effective in surpressing the growth of fission gas bubbles in both intergranular and intragranular zones, and in inhibiting the joining together of intergranular bubbles to form direct passages for fission gas release.     

Formation of Pellet-Cladding Bonding Layer in High Burnup BWR Fuels

Formation process of the pellet-cladding bonding layer was studied by EPMA, XRD, and SEM/TEM for the oxide layer on a cladding inner surface and the bonding layer in irradiated fuel rods. Specimens were prepared from fuels which had been irradiated to the pellet average burnups of 15, 27 and 42 GWd/t in BWRs. In the lower burnup specimens of 15 and 27GWd/t, no bonding layer was found, while the higher burnup specimens of 42 and previously reported 49 GWd/t had a typical bonding layer. A bonding layer which consisted of two regions was found in the latter fuels. One region of the inner surface of the cladding was made up mainly of ZrO₂. The structure of this ZrO₂ consisted of cubic phase, while no monoclinic crystals were found. The other region, near the pellet surface, had both a cubic solid solution of (U, Zr)O₂ and amorphous phase. Even in the lower burnup specimens having no bonding layer, cubic ZrO₂ phase was identified in the cladding inner oxide layer. The formation process of the bonding layer were discussed in connection with phase transformation by irradiation damage of fission products and conditions for contact of pellet and cladding.     

Thermal Recovery of Radiation Defects and Microstructural Change in Irradiated UO2 Fuels

Abstract

Thermal recovery of radiation defects and microstructural change in UO₂ fuels irradiated under LWR conditions (burnup: 25 and 44 GWd/t) have been studied after annealing at temperature range of 450-1,800°C by X-ray diffractometry and transmission electron microscopy (TEM). The lattice parameter of as-irradiated fuels increase with higher burnup, which was mainly due to the accumulation of fission induced point defects. The lattice parameter for both fuels began to recover around 450-650°C with one stage and was almost completely recovered by annealing at 850°C for 5 h. Based on the recovery of broadening of X-ray reflections and TEM observations, defect clusters of dislocations and small intragranular bubbles began to recover around 1.150–1,450°C. Complete recovery of the defect clusters, however, was not found even after annealing at 1,800°C for 5h. The effect of irradiation temperature on microstructural change of sub-grain structure in high burnup fuels was assessed from the experimental results.     

Effects of Void Migration Process on Fission-Gas Release

The release rate of fission-gas from U0₂ was continuously measured during irradiation in the Hitachi Training Reactor. The U0₂ specimen was heated electrically in in-core assemblies by tungsten heaters, either arranged axially transversing the specimen (producing radial temperature gradient) or cylindrically outside the specimen (uniform heating). In a case of the axially heated annular U0₂ pellets with radial temperature gradient, the rate of fission-gas release increased with time under constant temperature and flux. This experimental result was explained by considering as fission gas release mechanism a combination of diffusion and pore migration processes. On the other hand, when U0₂ pellets were heated isothermally up to 2,000°C, the fission-gas release rate could be explained in terms of diffusion process alone. No effect of equiaxial grain growth was observed on the rate of fission-gas release.     

Investigation of Effect of Gas Release on Coolability of Locally Blocked LMFBR Fuel Subassemblies

The objective of the present study is to evaluate the temperature rise due to gas release behind local blockage in LMFBR fuel subassemblies.

The experiments were conducted using six sets of electrically heated 37-, 61-and 91-pin bundles. The central or edge subchannels of each test section were blocked by a non-heat generating blockage of stainless steel plate. The effect of operational and geometrical conditions on the coolability of the blocked bundle were investigated from the test results. The examined factors are gas release rate, sodium flow rate, spacer type and blockage location.

The high coolant flow more than 2m/s in the velocity at the blocked plane, forced the released gas to be accumulated within the recirculation region resulting in marked temperature increase. The dependency of the temperature rise on the gas release rate was classified into two stages; (1) the temperature increases with the gas release rate, (2) after reaching a peak value, the temperature gradually decreases with the gas release rate. From these conclusions empirical correlations were derived to estimate the temperature rise under the condition of blockage with gas release. It was deduced that fission gas release in an LMFBR fuel subassembly with a local blockage has a potential to cause a limited pin-to-pin failure propagation in the recirculation region.     

In-Pile and Out-of-Pile Grain Growth Behavior of Sintered UO2 and (U,Gd)O2 Pellets

Grain growth behavior of UO₂ and (U, Gd)O₂ fuel pellets was investigated with the data from the out-of-pile isothermal heating experiments and the irradiation test at the Halden Boiling Water Reactor. The laboratory data gave best-fitted equations by employing the following fourth power rate equations :

UO₂ : D²-D⁴ ₀=3.79×10¹⁸ exp(-142,000/RT)t,

(U, Gd) : D²-D⁴ ₀=4.98×10¹⁷ exp(-140,000/RT)t,

where, D ₀ and D are initial and final three-dimensional diameters (μm), respectively, R the gas constant (=1.987 cal/mole/K), T the absolute temperature (K) and t the time (h) (gadolinia content: 3~10%, temperature range: 1,700~2,000°C).

The calculated grain diameter with the above equations revealed an overestimation on specimens which involved noticeable fission gas bubbles on their grain boundaries. It was demonstrated that the in-pile grain growth model, as was given in the following equation, which took account of the retarding effects of growth by precipitated intergranular bubbles could describe the grain growth of the irradiated samples :

where f: Grain boundary fractional coverage (-).     

Dependence on Strain Rate and Temperature Shown by Yield Stress of Uranium Dioxide

Abstract

Specimens of polycrystalline UO₂ were strained in compression at constant strain rate from 0.05 to 10.0/min and at temperatures ranging 900°–2,000°C. The experimental results revealed characteristic yield point, and the yield stress varied with strain rate and temperature. The present data, obtained at strain rates higher than currently adopted, showed greater sensitivity on strain rate and a higher activation energy for yield flow.

The data are discussed in their relation to the mechanism that controls the yield strength. The yield strength at high strain rate would appear to be controlled primarily by the peierls stress.