Protective Oxide Film on Alloy X750 Formed in Air at 973 K

An effective pre-oxidation method for Alloy X750 was developed to reduce general corrosion in an oxygenated aqueous environment such as in BWR core water. The optimum condition of preoxidation in air at elevated temperatures was found to be 5–20 h at 973 K by considering the allowance condition of heat treatment for age-hardening.

Some characteristics of the corroded oxide film have been clarified by surface analyses with XMA, SIMS, AES, XPS etc. The film was composed of double oxide layers, namely a highly crystallized NiFe₂O₄ outer layer and a high Cr₂O₃ content inner layer. The passive property of the film has been recognized to be due to the nature of the oxides whereby NiFe₂O₄ restricts the dissolution of metals because of its low solubility and Cr₂O₃ restricts the diffusion of metal ions because of its high binding energy and low diffusion coefficient.     

Determination of Cell Averaged Diffusion Constants Based on Transport/Diffusion Perturbation Theory

A unified definition for cell-averaged diffusion constants has been established on the basis of transport and/or diffusion perturbatlon theories. The diffusion constants are derived by equating each component of reactivities obtained by transport theory for a heterogeneous cell to a corresponding component of reactivities by diffusion theory for a homogenized cell. The derived diffusion constants are applied to infinite uniform lattices and heterogeneous lattices composed of different cells. In infinite lattices the present diffusion constants are compared to the conventional flux-weighted cross sections and Benoist's anisotropic diffusion coefficients. In heterogeneous lattices they are compared with Rowlands-Eaton cross sections and the extended Benoist's diffusion coefficient. The present formula is applied to a heterogeneous slab lattice. It reveals that the present method gives a more accurate result for k _eff compared to the case with the conventional diffusion coefficient defined by D= 1/3Σ_tr or the extended Benoist's diffusion coefficient together with the conventional flux-weighted cross sections.     

Effects of Hydrogen Peroxide on Intergranular Stress Corrosion Cracking of Stainless Steel in High Temperature Water, (V)

The difference in electrochemical corrosion potential of stainless steel exposed to high temperature pure water containing hydrogen peroxide (H₂O₂) and oxygen (O₂)is caused by differences in chemical form of oxide films. In order to identify differences in oxide film structures on stainless steel after exposure to H₂O₂ and O₂ environments, characteristics of the oxide films have been examined by multilateral surface analyses, e.g., X-ray diffraction (XRD), Rutherford back scattering spectroscopy (RBS), secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS). Preliminary characterization results of oxide films confirmed that the oxide film formed under the H2O2 environment consists mainly of hematite (α-Fe₂O₂), while that under the O₂ environment consists of magnetite (Fe₃O₄). Furthermore oxidation at the very surface of the film is much more enhanced under the H₂O₂ environment than that under the O₂ environment. It was speculated that metal hydroxide plays an important role in oxidation of stainless steel in the presence of H₂O₂. The difference in electric resistance of oxide film causes the difference in anodic polarization properties. It is recommended that several anodic polarization curves for specimens with differently oxidized films should be prepared to calculate ECP based on the Evans diagram.     

Hydrogen Concentration Dependence of Tritium Tracer Diffusion Coefficient in Alpha Phase of Niobium

In order to examine influences of coexistent hydrogen isotopes on diffusion behavior of tritium in niobium, tracer diffusion coefficients D_t of tritium in alpha phase of hydrogenated and deuterized niobium (α-NbH_xT_y and α-NbD_xT_y (x<0,8,y<<x)) have been measured at 473 K, 493 K and 553 K. The data on D_t show typical hydrogen concentration dependence: D_t of tritium for both α-NbH_xT_y, and α-NbD_xT_y, decreases with hydrogen concentration under all experimental conditions. The obtained concentration dependence of D_t of tritium differs from that of D_t of protium in α-NbH_x or of deuterium in α-NbH_xT_y. On the other hand, no appreciable differences in the concentration dependence of D_t of tritium between α-NbH_xT_y, and α-NbD_xT_y, are observed: there are no definite isotope effects due to the coexisting hydrogen isotopes. This result suggests that D_t of tritium for a tritiated niobium (α-NbT_x) is not very different from that for α-NbH_xT_y and α-NbD_xT_y. The chemical diffusion coefficient D* of tritium is also evaluated on the basis of the obtained D_t of tritium and of a literature value of a thermodynamic factor F for Nb-H and Nb-D systems.     

Consideration on Effective Diffusivity of Strontium in Granite

Abstract

Diffusive behavior of strontium and certain kinds of divalent cations in Inada granite were studied by a through-diffusion method. In order to examine the effect of sorption onto overall diffusive behavior, two kinds of solutions were used: 0.1M KCl solution and deionized water. The effective diffusion coefficient (D_e ) and rock capacity factor (a) were (2.0–3.6) xl0–¹³ m²/s and less than 0.022 in 0.1M KCl solution and (0.32–1.7) x 10 ^?13m²/s and 1.5–2.4 in deionized water respectively. The D_e , and α in deionized water were much larger than those in 0.1M KCl solution. These results are well explained by taking into account the diffusion of sorbed ion or the surface diffusion. In support of this mechanism, most D_e , values of Sr reported for various rocks are found proportional to the sorptivity ( _ρR_d )-In the case that the sorptivity is low, D_e of Sr depends on porosity like that of nonsorbed iodide. The effective diffusion coefficient of Sr in rocks was well explained by taking into account pore and surface diffusion and was expressed as D_e=2.1 xl0^?10 _? ^1.3+3.5xl0″^?12 _ρR_d . The effective diffusion coefficient of divalent cations in the granite was found proportional to their diffusion coefficients in bulk solution.     

Effect of Metallographic Factor on Hydrogen Pick-up Properties of Zircaloy-2

Zirconium alloy sheets were prepared with varying Fe, Cr and Ni systematically. The corrosion and hydrogen pickup property were estimated in steam at 673 K, in water at 633 K and in super critical water at 673 K. The effect of the SPP and the oxide film on the hydrogen pick-up was studied from the hydrogen pick-up route using D₂O and the microstructure of the oxide film and secondary phase particle (SPP) in the oxide film. The hydrogen pick-up ratio decreased with increase of Fe and decrease of Ni. It was affected by Fe/Ni ratio of the matrix. The hydrogen pickup was not related to SPP but was related to the oxide film when the oxide film was relatively thick. The tetragonal ZrO₂ is considered to act as a barrier for hydrogen pick-up.     

Numerical model for separation of H-D gas mixture in batch-type concentric-tube thermal diffusion columns

The modeling simulation for the separation of H-D gas mixture in batch-type concentric-tube thermal diffusion columns have been analyzed from the transport equation coupled with the application of mass balance. The most important assumption is that the concentrations of H₂, HD and D₂ are locally equilibrium at every points in the column as H₂ + D₂ ↔ 2HD. The concentration distribution equation was derived and the concentration difference between the bottom and top ends of the column could be estimated. The degree of separation and separation factor for recovery of deuterium from H-D gas mixture in the batch-type cryogenic-wall thermal diffusion column were estimated.     

Tritium diffusion in zircaloy-2 in the temperature range −78 to 204° C

Tritium diffusion measurements in Zircaloy-2 were carried out over the temperature range ?78 to 204 °C by direct measurement of tritium diffusion gradients. The ⁶Li (n, α)³H reaction was used to inject tritium into the specimens and to produce initial tritium concentration in the range 0.0065 ppm to 0.013 ppm ³H by weight. Two diffusion components were identified from the concentration profiles: a surface trapping region approximately 5 μm thick and a normal diffusion profile characteristics of bulk diffusion. Surface release measurements of tritium verified the existence of a surface trapping layer. The bulk diffusion component was consistent with classical diffusion solutions and was given by: D = 0.00021_?0.00018^+0.005 exp?(8500 ± 200 cal/RT) cm² · sec^?1.The surface trapping was attributed to oxide films formed on the Zircaloy-2 at room temperature. The apparent diffusion coefficients for the surface region were consistent with: D = 4.0_?3.3^+19.7 × 10^?14 exp?(7200 ± 1500 cal/RT) cm² · sec^?1 over the temperature range 25 to 411°C.     

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 (-).     

The rates of thermal decomposition of LiOH(s), LiOD(s) and LiOT(s)

Kinetic studies on the thermal decomposition reactions, 2LiOH(s) → Li₂O(s) + H₂O(g), 2LiOD(s) → Li₂O(s) + D₂O(g), and LiOH(s) + LiOT(s) → Li₂O(s) + HTO(g), have been carried out with mass spectrometric and radiometric methods over the temperature range 530–690 K. Those reaction rates were found to be of first order in the quantity of released water. The rate-constants, k, in s^?1, were: k_H2O = 1.8 × 10⁸ exp(29 500/RT), k_D2O = 1.7 × 10⁸ exp(29 000/RT), and k_HTO = 1.6 × 10⁷ exp(30 700/RT), respectively. The apparent activation energy obtained for the thermal decomposition of LiOH(s), E_a = 29.5 ± 1.1 kcal/mol, coincided with a literature value of the enthalpy of reaction, ΔH°₆₀₀^° = 29.8 ± 1.0 kcal/mol.     

Uptake of deuterium in partially D-depleted residual codeposits left on DIII-D tiles after laboratory thermo-oxidation experiments

Recent evidence has shown that tokamak carbon-based codeposits may become partially or fully depleted of hydrogen through thermo-oxidation, as the hydrogen content of the codeposits is removed more rapidly than the carbon content. In this study we examine the ability of such partially-depleted residual DIII-D divertor codeposits to uptake deuterium upon subsequent exposure to deuterium gas or deuterium plasmas. The partially D-depleted specimens used here were obtained from a previous study where DIII-D codeposits were oxidized for 2 h at 623 K (350 °C) and 267 Pa (2 Torr) O₂ [J.W. Davis et al., Thermo-oxidation of DIII-D codeposits on open surfaces and in simulated tile gaps, J. Nucl. Mater. 415 (2011) S789–S792]. In the present study some of these specimens, having undergone prior oxidation, were exposed to D₂ glow discharge plasmas or D₂ gas at 20 kPa (150 Torr) at 300 or 523 K. In the case of plasma exposure, no uptake of D was observed, while an increase in D content was seen following D₂ gas exposures. When the gas exposure took place at 300 K, heating the specimens in vacuum to 623 K for 15 min led to the release of all of the increased D content. For the gas exposure at 523 K, the increase in D content was found to require longer (8 h) vacuum baking to remove. However, in a reference codeposit specimen (from a closeby location on the tile), which had not been previously oxidized, there was a similar increase in D content following D₂ exposure at 523 K, but it could not be released even following 8 h vacuum baking at 623 K.     

Effects of water-to-cement ratio and temperature on diffusion of water in hardened cement pastes

Apparent diffusion coefficients (D_a) of water and activation energies (E_a) of diffusion in hardened cement pastes (HCPs) were determined as a function of water-to-cement (w/c) ratio (0.36–0.60) and temperature (293–323 K) using HTO and H₂¹⁸O as tracers. The values of D_a and E_a ranged from 1.1×10^?11 to 1.7×10^?10 m² s^?1 and from 21.5 to 31.3 kJ mol^?1, respectively. No significant difference between the D_a values of HTO and H₂¹⁸O suggests that water predominantly diffuses as H₂O molecule and dissociation of water is not significant even at high pH range in HCP. The values of E_a at low w/c ratio were higher than in bulk liquid water, suggesting a contribution of a different water regime, such as supercooled bulk water. Two simple models consisting of capillary and gel pores were considered to estimate the volume ratio of gel pores to total pores by optimizing the model to fit with the experimental data. The result suggests that HCP has a pore network mostly consisting of capillary pores with some very narrow pores plugged with hydrates, where HTO must diffuse through gel pores. This view of the HCP pore network was made available through analysis of E_a values.     

Derivation of Finite Medium Age-Diffusion Kernels in Source-Sink Theory

Based on a simple adsorption theory, a mathematical model was proposed to predict axial iodine profiles of the column of silica gel impregnated with silver nitrate (hereinafter referred to as Ag-S) in an off-gas treatment system for spent fuel dissolution. The unknown parameters of the model: the effective diffusion coefficient D_ea and the Langmuir coefficient K were determined by curve fitting of iodine profile experimentally obtained. At 423 K, D_ea and K were found to be 5.60×10^-7 m².s^-1 and 1.0×10⁵ m³.kg^-1, respectively. Using the parameter values, the model could well predict the iodine profiles obtained at 423 K in the previous works under different experimental conditions. Furthermore, the effect of silver contents on the iodine profiles was reasonably predicted. It was suggested that the proposed model is simple and would be useful to predict the iodine profiles in Ag-S adsorbent columns.     

Kinetics of initial stage of sintering of UO2 and UO2 with Nd2O3 addition

The kinetics of initial stage sintering of UO₂ powder were reinvestigated, using Ar-10% H₂ atmosphere. The effect of the addition of neodynium oxide was studied. The results revealed that surface and grain boundary diffusion mechanisms act simultaneously. The values of activation energies were found to be $\text{48.48 ± 3.51 kcal/mole}$ in the temperature range 870–942°C and $\text{89.88 ± 9.87 kcal/mole}$ in the temperature range of 942–1030°C for UO₂, and $\text{115.61 ± 7.77 kcal/mole}$ in the temperature range 1030–1150°C for UO₂ + Nd₂O₃. An important decrease in the calculated diffusion coefficient occurs by the addition of Nd₂O₃.     

Etude sur la Combination des Réservoirs et des Colonnes de Diffusion Thermique

The heat siphon has long been in general use for the purpose of enriching the isotopes present in a reservoir into a thermal diffusion column. K. Clusius and H.H. Bühler developed an alternative method, the gas oscillator, which they utilized as a gas connecting method.

We analyzed both methods mathematically, resulting in the conclusion that the flow rate |σ _R | (g/sec) of the heal siphon and of the gas oscillator should have a value ranging from 3H to 5H, and that the period of the gas oscillator 2T (sec) has little influence on the enrichment process, which permits us to choose a value convenient for operation.     

Effect of Argon Ion Sputtering of Surface on Hydrogen Permeation through Vanadium, (II) Effect of Sputtering of Both Side Surfaces

As part of studies on plasma-wall interactions of fusion reactors, the effect of Ar ion sputtering of both the upstream side and the downstream side surfaces of a V coupon upon its H₂ permeability has been studied. The H₂ permeability measuring apparatus has been modified to install another ion gun for the use in sputtering the downstream side specimen surface. The H₂ permeation rates were measured at 673K by changing the H₂ pressure and sputtering mode. The downstream side sputtering has been observed to enhance the H₂ permeability, though its degree was smaller than that due to the upstream side sputtering. The both side sputtering resulted in the largest H₂ permeability value ever obtained for V. The rate of H₂ permeation through V seemed to be determined by the surface processes including the downstream side rather than the bulk diffusion even after the both side sputtering. Based on AES measurements, the enhancement of H₂ permeability caused by sputtering was attributed to the diminishing of the surface O impurity.     

Gas Chromatographic Measurements of Hydrogen Isotopes Mixture by Using Catalytic Oxidation Type Detector

A new type of gas chromatograph for quantitative measurements of hydrogen isotopes mixture has been developed through the adoption of catalytic oxidation type detector. Experiments using He as the carrier gas were successfully performed for the separation of D₂-HD and H₂-HD-D₂ mixtures. The minimum detection limits were 0.4–0.9 μl-STP for these components. The linear calibration curves were obtained in the range of 10 μl-STP–20 ml-STP. The sensitivities for H₂, HD and D₂ were 843, 774 and 654 (μl-STP)^?1, respectively. The maximum time needed for the analysis was about 16 min.     

Effect of Changes in Feed Rate and Cut on Circulating Flow within Thermal Diffusion Column for Isotope Separation

A two-dimensional axisymmetric convectional flow field of an isotope separating thermal diffusion column with continuous feed and draw-offs is analyzed through the Newton iterative numerical solution of the equations of change without the Boussinesq approximation. Computations are performed for Ar gas within an inner hot radius of 0.2 mm and an outer cold radius of 5 mm, between which the temperature differs by 300 K. The rate of feed F, supplied into the middle point of the column, is varied from 1 to 5 cm³/s, while the cut θ, or the ratio of the upper drawing-off to feeding rates, from 0.1 to 0.9. Comparison of flow vectors, temperature and density profiles among various sets of (F, θ) makes it clear that the flow fields agree with one another for the same value of the total transport; that is, θF_pc (pc: feed gas density) in the upper section and -(1-θ)pc in the lower section of the column. Observations on the mass flow vectors near the feed slit lead to the finding that the multiplicative effect of separation cannot be expected in the column with the inner radius r_c of 5 mm at the feed rate F exceeding 2 cm³/s(i.e F/ΔTr_c ⁴ )>0.1 [1/(s·K·cm)]), because the feed flow interrupts the whole-length circulation of the natural convection.     

Permeability of hydrogen and deuterium of Hastelloy XR

Permeation of hydrogen isotope through a high-temperature alloy used as heat exchanger and steam reformer pipes is an important problem in the hydrogen production system connected to be a high-temperature engineering test reactor (HTTR). An experiment of hydrogen (H₂) and deuterium (D₂) permeation was performed to obtain permeability of H₂ and D₂ of Hastelloy XR, which is adopted as heat transfer pipe of an intermediate heat exchanger of the HTTR. Permeability of H₂ and D₂ of Hastelloy XR were obtained as follows. The activation energy E₀ and pre-exponential factor F₀ of the permeability of H₂ were E₀=67.2±1.2 kJ mol⁻¹ and F₀=(1.0±0.2)×10⁻⁸ m³(STP) m⁻¹ s⁻¹ Pa^−0.5, respectively, in the pipe temperature ranging from 843 K (570 °C) to 1093 K (820 °C). E₀ and F₀ of the permeability of D₂ were respectively E₀=76.6±0.5 kJ mol⁻¹ and F₀=(2.5±0.3)×10⁻⁸ m³(STP) m⁻¹ s⁻¹ Pa^−0.5 in the pipe temperature ranging from 943 K (670 °C) to 1093 K (820 °C).