3He Melting Pressure Measurements in High Magnetic Fields

Experimental methods and preliminary results of high-precision measurements of the ³ He melting curve in high magnetic fields have been described. The purpose of this work is twofold. One is to establish a reliable millikelvin temperature scale in high fields (T 10 mK, B 15 T), The other is to investigate a high field region of the magnetic phase diagram of solid ³ He by measuring the melting pressure down to temperatures below 1 mK. Besides the two superfluid transitions, the phase transition temperatures between the spin ordered solid and the paramagnetic solid, T _HFP , were determined at B = 12 and 10 T with good accuracy, which is an extension of previous measurements up to 8 T. The present T _HFP (B) line can not be scaled to that at a higher density with a single Grüneisen parameter, indicating a variation of density dependencies of the multiple-spin exchange interactions.     

Studies of 2D Cryocrystals by STM Techniques

Scanning tunneling microscopy (STM) studies of two-dimensional (2D) cryocrystals ⁴ He, Kr, Xe) physisorbed on graphite surfaces are presented. Individual helium atoms, usually thought to be invisible with STM, were recently observed on graphite surfaces at a density corresponding to the commensurate solid. Here we show that a local elastic deformation seems to be the principal mechanism responsible to render the atoms visible. Recent tight-binding calculations of the local density of states (LDOS) of graphite which predict the appearance of an energy gap support this picture. I-z curve measurements for the case of ⁴ He show a sharp drop (increase) of the tunneling current I at a certain tip-surface distance z during retraction (approach) of the tip. This drop (increase) may be associated with the tunneling of a single He adatom, opening new possibilities to study the quantum tunneling of atoms via STM.     

Development of systems reliability analysis code SECOM-2 for seismic PSA

An integrated code system SECOM-2, developed at the Japan Atomic Energy Research Institute (JAERI), has the following functions for systems reliability analysis in seismic probabilistic safety assessments (PSAs): (1) calculation of component failure probability, (2) extraction of minimal cut sets (MCSs) from a given fault tree (FT), (3) calculation of frequencies of accident sequences and core damage, (4) importance analysis with several measures with consideration of unique parameters of seismic PSAs, (5) sensitivity analysis, and (6) uncertainty analysis. This paper summarizes the special features of SECOM-2 to perform the analyses mentioned above. At JAERI, using an integrated FT which represents seismically induced core damage due to all initiating events as a system model to calculate core damage frequency of a nuclear power plant, SECOM-2 can calculate conditional point estimate probabilities of system failures, losses of safety functions, and core damage as a function of earthquake motions. The point estimate is computed by a method which gives an exact numerical solution using the Boolean arithmetic model method. As for consideration of correlation of component failure, which has been an important issue in seismic PSAs, a new technique based on direct FT quantification by a Monte Carlo simulation is being added to SECOM-2. Adding this technique, the core damage frequency can be calculated not only with the upper bound approximation based on MCSs but also with a near exact solution taking into account the correlation among all components. This paper also presents the preliminary results of a seismic PSA of a generic BWR plant in Japan performed at JAERI to demonstrate the functions of the SECOM-2 code.     

Oxidation of Fe (II) in sulfuric acid solutions with dissolved molecular oxygen

The oxidation of Fe(II) with dissolved molecular oxygen was studied in sulfuric acid solutions containing 0.2 mol · dm⁻³ FeSO₄ at temperatures ranging from 343 to 363 K. In solutions of sulfuric acid above 0.4 mol · dm⁻³, the oxidation of Fe (II) was found to proceed through two parallel paths. In one path the reaction rate was proportional to both [Fe⁻²⁺]² andp _{o 2} exhibiting an activation energy of 51.6 · kJ mol⁻¹. In another path the reaction rate was proportional to [Fe²⁺]², [SO ₄ ^{2 −}], andp _{o 2} with an activation energy of 144.6 kJ · mol⁻¹. A reaction mechanism in which the SO ₄ ^{2 −} ions play an important role was proposed for the oxidation of Fe(II). In dilute solutions of sulfuric acid below 0.4 mol · dm⁻³, the rate of the oxidation reaction was found to be proportional to both [Fe(II)]² andp _{o 2}, and was also affected by [H⁺] and [SO ₄ ^{2 −}]. The decrease in [H⁺] resulted in the increase of reaction rate. The discussion was further extended to the effect of Fe (III) on the oxidation reaction of Fe (II).     

Water Network in Room-Temperature Ionic Liquid: <Emphasis Type="Italic">N,N</Emphasis>-Diethyl-<Emphasis Type="Italic">N</Emphasis>-Methyl-<Emphasis Type="Italic">N</Emphasis>-2-Methoxyethyl Ammonium Tetrafluoroborate

Crystal structures of room-temperature ionic liquid (RTIL)-H₂O mixtures are determined by the X-ray diffraction method. The RTIL is N,N-diethyl-N-methyl-N-2-methoxyethyl ammonium tetrafluoroborate, [DEME][BF₄]. At 0.9 mol pct H₂O, two kinds of superstructures occur simultaneously without a strain. Also, the volume of the unit cell is very small only at 0.9 mol pct additives. This relates to the composite domain structure, including a twin-related one, as an elastic anomaly. At other water concentrations, such an extraordinary behavior is not observable. By assuming a sublattice having an equivalent lattice constant, a water network at 1 mol pct H₂O is simulated using a Monte Carlo (MC) method. The network develops over the medium range in the simulation box.     

Tensile properties of directionally solidified Al-4 wt pct Cu alloys with columnar and equiaxed grains

The tensile properties of directionally solidified Al-4 wt pct Cu-0.15-0.2 wt pct Ti alloys with equiaxed grains were determined and compared with the properties of directionally solidified Al-4 wt pct Cu columnar structures. The tensile properties of the equiaxed structure were isotropic, but varied with the distance from the chill face. The mechanical properties of the equiaxed structure were generally between those of the longitudinal and transverse columnar structures. The 0.2 pct offset yield stress(σ _y, MPa) is represented as a function of the grain size,d (mm), the average concentration, C_o (wt pct), and the local concentration, C (wt pct), by σ_y = [(15.7 + 22.6 C_o) + (1.24 + 1.04 C_o)d ^-1/2] + [15.7 △C], where △C = C - C_o. The equiaxed structure exhibits inverse segregation similar to that in the columnar structure.     

Electrolytic reduction of U(VI) to U(IV) in acidic chloride and acidic sulfate solutions

In order to examine the applicability of the electrolytic reduction process of U(VI) (originally developed for the chloride system by PNC) to the sulfate solution system, a fundamental study was made. In this study, the concentrations of various chemical species in the catholytes were calculated at 298 K at various percentages of uranium reduction, taking the chloro-complex and sulfato-complex formation reactions of uranium into consideration. The polarization characteristics of the electrolytic reduction of uranyl chloride and uranyl sulfate were determined, using titanium and platinum cathodes, respectively, at 303 ± 1 K. In conjunction with this process, the electrical conductivity of the catholyte, the electrical resistivity of the cation exchange membrane, and the diffusion coefficient of uranyl sulfate were also determined.     

Activities of FexO in CaO + CaF2 + SiO2 + FexO quaternary slags by disposable electrochemical oxygen probes

The activities of Fe_xO in CaO + CaF₂ + SiO₂ + Fe_xO quaternary slags were measured by means of solid-oxide galvanic cell. The Fe_xO activities in the slags are influenced by CaF₂ as well as SiO₂. At constant Fe_xO mole fractions, e.g., , at low SiO₂ mole fractions i.e., , the substitution of CaF₂ for CaO has an effect of raising the Fe_xO activity. At higher SiO₂ mole fractions, e.g., , however, such an effect becomes insignificant.