Yeon Soo Kim

A thermodynamic evaluation of the titanium–oxygen system from O/Ti=0 to 3/2

Abstract Evaluation of the partial thermodynamic functions of the Ti–O system (up to O/Ti=1.5) from 1000 to 2000 K, using a methodology previously developed for other M–O, M–H and M–N systems, is presented. Satisfactory pressure–composition–temperature relationships for each phase have been obtained even in regions where no data are available. The results are presented as oxygen isobars superimposed on the phase diagram. The Gibbs energy of oxygen dissolution in solid titanium is also calculated.

Steam oxidation of fuel in defective LWR rods

Abstract Oxidation of UO 2 by pure steam at pressures of 7 and 70 atm and 500°C and 600°C was measured in a thermogravimetric apparatus. The kinetics are linear, vary as the square root of the steam pressure, and are consistent with initial rates extrapolated from higher-temperature experiments in 1-atm steam. At temperatures characteristic of normal operation of defective fuel rods, the rate of hydrogen production by thermal oxidation of the fuel in steam is small compared with that due to cladding corrosion. The presence of H 2 in the steam has a much greater retarding influence on fuel oxidation than on cladding oxidation. Other potential sources of fuel chemical reactivity in steam, including reaction in cracks in the hot pellet interior and radiolysis of steam by recoiling fission fragments, do not result in significant fuel oxidation. During the incubation stage of fuel-rod degradation, the bulk of the evidence indicates that fuel oxidation is not a major source of the hydrogen in the fuel–cladding gap that eventually may cause secondary-hydriding failure of the rod.

High pressure hydriding of sponge-Zr in steam-hydrogen mixtures

Abstract Hydriding kinetics of thin sponge-Zr layers metallurgically bonded to a Zircaloy disk has been studied by thermogravimetry in the temperature range 350–400°C in 7 MPa hydrogen-steam mixtures. Some specimens were prefilmed with a thin oxide layer prior to exposure to the reactant gas; all were coated with a thin layer of gold to avoid premature reaction at edges. Two types of hydriding were observed in prefilmed specimens, viz., a slow hydrogen absorption process that precedes an accelerated (massive) hydriding. At 7 MPa total pressure, the critical ratio of H2/H2O above which massive hydriding occurs at 400°C is ∼ 200. The critical H2/H20 ratio is shifted to ∼2.5 × 103 at 350°C. The slow hydriding process occurs only when conditions for hydriding and oxidation are approximately equally favorable. Based on maximum weight gain, the specimen is completely converted to δ-ZrH2 by massive hydriding in ∼5 h at a hydriding rate of ∼10−6 mol H/cm2 s. Incubation times of 10–20 h prior to the onset of massive hydriding increases with prefilm oxide thickness in the range of 0–10 μm. By changing to a steam-enriched gas, massive hydriding that initially started in a steam-starved condition was arrested by re-formation of a protective oxide scale.

DART Analysis of Irradiation Behavior of U-Mo/Al Dispersion Fuels

Abstract DART (Dispersion Analysis Research Tool) is a computational code developed for integrated simulation of the irradiation behavior of aluminum dispersion fuels used in research reactors. The DART computational code uses a mechanistic fission gas behavior model and a set of up-to-date empirical correlations to simulate the fuel morphology change as a function of burnup. Integrating a thermal calculation subroutine enables fuel material properties to be updated at each time step. This paper describes the primary physical models that form the basis of the DART computational code. A baseline validation was performed through the modeling of several U-Mo/Al mini-plate tests (RERTR-6, 7, and 9) in the Advanced Test Reactor (ATR). A demonstration problem is also presented through the calculation of fuel plate swelling and constituent volume fractions in full-sized plates from the AFIP-1 test in ATR.

High pressure oxidation of sponge-Zr in steam/hydrogen mixtures

Abstract A thermogravimetric apparatus for operation in 1 and 70 atm steam-hydrogen or steam-helium mixtures was used to investigate the oxidation kinetics of sponge-Zr containing 215 ppm Fe. Weight-gain rates, reflecting both oxygen and hydrogen uptake, were measured in the temperature range 350–400°C. The specimens consisted of thin sponge-Zr layers metallurgically bonded to a Zircaloy disk. The edges of the disk specimens were coated with a thin layer of pure gold to avoid the deleterious effect of corners. Following each experiment, the specimens were examined metallographically to reveal the morphology of the oxide and/or hydride formed. Two types of oxide, one black and uniform and the other white and nodular, were observed on sponge-Zr surfaces oxidized in steam environments at 70 atm. The oxidation rate when white-nodular oxide formed was a factor of two higher than that of black-uniform oxide at 400°C for steam contents above 1 mol%. The oxidation rate was independent of total pressure, the carrier gas (H 2 or He) and steam content above ∼1 mol%. The oxidation kinetics of sponge-Zr follows a linear law for maximum reaction times up to ∼ 6 days. The oxidation rate in steam-hydrogen mixtures at 70 atm total pressure decreases when the steam content approaches the steam-starved region (∼ 0.5 mol% steam at 400°C and ∼ 0.02 mol% steam at 350°C). Lower steam concentrations cause massive hydriding of the specimens. Even at steam concentrations above the critical value, direct hydrogen absorption from the gas was manifest by hydrogen pickup fractions greater than unity.

Chemical processes in defective LWR fuel rods

Abstract The results of several experimental studies aimed at improving understanding of the chemical processes that cause severe degradation of defective light-water reactor fuel cladding are reported. The competition between oxidation and hydriding of zirconium and zircaloy exposed to steam-hydrogen mixtures at 70 bar and 350–400°C was studied by thermogravimetry. A critical H 2 /H 2 O ratio of the gas separates regimes of these two types of reaction. For sponge-Zr, the critical ratios at 350 and 400°C are ≈ 5000 and ≈ 200, respectively. Radiolysis of steam by alpha particles was studied mass spectrometrically. The yield of the hydrogen radiolysis product in saturated steam at 290°C was found to be ≈ 8 molecules per 100 eV of deposited energy. An in-reactor experiment demonstrated that fission-fragment-irradiated steam can oxidize UO 2 to UO 2+ x .

High-pressure hydriding of Zircaloy

Abstract The hydriding characteristics of Zircaloy-2(Zry), sponge zirconium (as a liner on Zry plate), and crystal-bar zirconium exposed to pure H2 at 0.1 MPa or 7 MPa and 400°C were determined in a thermogravimetric apparatus. The morphology of the hydrided specimens was also examined by optical microscopy. For all specimen types, the rate of hydriding in 7 MPa H 2 was two orders of magnitude greater than in 0.1 MPa H 2 . For Zry, uniform bulk hydriding was revealed by hydride precipitates at room temperature and on one occasion, a sunburst hydride. In addition, all specimen types exhibited a hydride surface layer. In a duplex Zry/sponge-Zr specimen, Zry is more heavily hydrided than the sponge Zr layer.

Original ArticleAnalysis on the post-irradiation examination of the HANARO miniplate-1 irradiation test for kijang research reactor

The construction project of the Kijang research reactor (KJRR), which is the second research reactor in Korea, has been launched. The KJRR was designed to use, for the first time, U–Mo fuel. Plate-type U–7 wt.% Mo/Al–5 wt.% Si, referred to as U–7Mo/Al–5Si, dispersion fuel with a uranium loading of 8.0xa0gU/cm3, was selected to achieve higher fuel efficiency and performance than are possible when using U3Si2/Al dispersion fuel. To qualify the U–Mo fuel in terms of plate geometry, the first miniplates [HANARO Miniplate (HAMP-1)], containing U–7Mo/Al–5Si dispersion fuel (8 gU/cm3), were fabricated at the Korea Atomic Energy Research Institute and recently irradiated at HANARO. The PIE results of the HAMP-1 irradiation test were analyzed in depth in order to verify the safe in-pile performance of the U–7Mo/Al–5Si dispersion fuel under the KJRR irradiation conditions. Nondestructive analyses included visual inspection, gamma spectrometric mapping, and two-dimensional measurements of the plate thickness and oxide thickness. Destructive PIE work was also carried out, focusing on characterization of the microstructural behavior using optical microscopy and scanning electron microscopy. Electron probe microanalysis was also used to measure the elemental concentrations in the interaction layer formed between the U–Mo kernels and the matrix. A blistering threshold test and a bending test were performed on the irradiated HAMP-1 miniplates that were saved from the destructive tests. Swelling evaluation of the U–Mo fuel was also conducted using two methods: plate thickness measurement and meat thickness measurement.