Yasuichi Sasaki

The reaction of nuclear graphites with hydrogen

Abstract The corrosion reactions of various nuclear graphites with hydrogen were studied in the temperature range 970–1090°C at hydrogen partial pressures between 7 and 48 torr in helium stream of total pressure of 1 atm. Methane was the only gaseous product found in this reaction. The rates of corrosion reaction were found to be 10−1 ~ 10−2 μg/cm2 hr on the basis of methane yield at 28 torr hydrogen pressure at 1025°C. The influence of the hydrogen partial pressure on the methane production rate is expressed as: R=K(P H 2 ) 3 2 . The apparent activation energy for the reaction was found to be 22 kcal/mol, independent of the impurity content of the graphite. No clear correlations were found between the production rate of methane and the total ash content in the graphite, its degree of crystallinity (determined by X-ray diffraction) and its degree of graphitization (infered from butanol immersion density).

Studies on the Uranium Dicarbide Phase

The lattice parameters of UC2 are examined as a function of carbon content and annealing: conditions on sintered carbides between UC1·5 and UC2·2 (7.0 to 10.0W/0 of carbon). The values of UC2 obtained under equilibrium state are around a 0=3.5253 A and c 0=5.9994A. The lattice parameter measurements were made also on UC2 obtained under non-equilibrium conditions by are-melting and carbothermic reaction of UO2. The lattice parameters were appreciably smaller than on the specimens obtained under equilibrium. With annealing, both the lattice parameter and the combined carbon content showed a tendency to increase.

Thermal expansion of some pre-stressed nuclear grade graphites

Abstract Thermal expansion coefficients (CTE) of some nuclear grade polycrystalline graphites were measured in the range from room temperature to 1173 K after pre-stressing to levels comparable with their fracture strength. The CTEs has been found to increase with residual strain independently of the samples as well as their manufacturing processes. Analytical discussions are given on the effects of both pre-stressing and/or thermal annealing. Phenomenological formulae appropriately expressing CTEs in heating and cooling processes have been derived.

Effect of high-temperature neutron irradiation on the crystallites of graphite materials

Abstract Changes in crystal lattice parameter, c-axis lattice strain and apparent crystallite height induced by neutron irradiation at 720–1350°C were investigated by means of X-ray diffraction for fourteen specimens which included five bands of nuclear graphite and two kinds of pyrolytic graphite. All irradiated materials show a marked increase in c-axis lattice strain, while changes in the lattice parameters are very small. The relation between lattice strain and layer spacing was analyzed for pre- and post-irradiated graphites, and it is inferred that, whereas the predominant constituent of strain in the unirradiated graphite is the statistical fluctuation in distribution of layer spacings, for the irradiated material it is the distortion of layer planes. On the basis of the relation between c-axis lattice strain and apparent crystallite height, the stacking height of graphite layer planes is estimated to be about 70 A for pre-irradiated graphite.

The change in the ratio of thermal expansion coefficients of some nuclear graphites at elevated temperatures

Abstract The coefficients of thermal expansion (CTE) of eleven isotropic and anisotropic nuclear graphites made of either gilsonite coke or needle coke were measured from room temperature to 900°C by using a dilatometer, and they were compared with the BAF (Bacon anisotropy factor) measured at room temperature and the CTEs of crystallites. The CTE for each sample increased and its anisotropic ratio decreased with increasing temperature, and the latter showed the same value of BAF at elevated temperatures. A difference between the CTE of a bulk sample and that of crystallites, and also the change in an accommodation factor, were observed at elevated temperatures.

Effects of high temperature neutron irradiation on dimension and thermal expansion coefficient of nuclear grade graphites.

This paper describes the experimental results and discussions on the changes in dimension, volume and thermal expansion coefficient of five nuclear grade graphites which were made from petroleum coke or gilsonite coke and irradiated in the JMTR at 670–1,260°C up to the maximum neutron fiuence 3.3×1021 n/cm2 (E>0.18 MeV). Dimensional or volume shrinkages for all samples were observed after irradiation and their changes depended on the kind of graphite studied. The changes in thermal expansion coefficient were also observed for three of five samples. The relations between the changes in dimension or volume and thermal expansion coefficient were obtained, and it was clarified that both the rates of irradiation shrinkage in dimension and volume of the sample having larger thermal expansion coefficient were smaller than for the sample having smaller thermal expansion coefficient.

Preparation of Plutonium Metal, (I):Preparation of Plutonium Metal by Lithium Reduction of Ammonium Plutonium(IV) Fluoride

A method of producing Pu metal by the reduction of sodium plutonium(IV) fluoride is reported. After the valency of Pu in acidic solution was adjusted to tetravalent state by addition of nitrite (e.g. NaNO2), sodium plutonium (IV) fluoride was precipitated by adding hydrofluoric acid in the presence of sodium ion in the solution. The double salt obtained by the wet method was dried under reduced pressure at 110°C, and then reduced with Ca metal in Ar atmosphere. Over 80% Pu yield was obtained by reducing 1 g batch of the double salt.