An iteration method using the Pade approximation is described to accelerate the inner iteration of finite difference equations of two-dimensional diffusion equation. In the case of void problems where the system includes regions of very low density, the convergence rate of the usual iteration method becomes extremely slow, and it becomes often impossible to obtain a converged solution. It is found that the present method using the Pade approximation can give the converged solution for such void problems.

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Calculations on the time-dependent neutron spectra in an ice block are reported for the temperature range of 8616 K.
Three models for the lattice vibrational frequency distributions were applied to calculate scattering kernels. Calculated spectra were compared with measured spectra. It was found that the calculation with the Debye model agrees well with the experiment.
From the present analysis, it was suggested that the Debye temperature Θ_D of the low temperature ice can be expressed as Θ_D = 208 + 0.3T_O where T_O is the medium temperature.

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Assuming the entrance of a rectangular straight duct to be uniformly covered with a film of radiation source, and the radiations to be emitted into the duct at intensities proportional to cos2θ (θ : Angle of radiation incidence), the unscattered component of radiations along the duct axis is shown to be expressed by where φ0 is the total flux from the radiation source, while a, b are non-dimensional parameters representing the perspective angle of radiation source vertical and horizontal spreads. This consequently proves the fallacy of the expressions given by Schaeffer and by
Selph for the same assumptions, i.e.
and which are widely cited in textbooks, handbooks and reports.

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This paper describes an assessment result on conservatism of current safety analysis concerning reflood behavior during a LOCA in a PWR by using the experimental data with cylindrical core test facility (CCTF) performed at Japan Atomic Energy Research Institute (JAERI). WREM code is selected for a representative of current safety analyses.
The predicted peak clad temperature with the WREM code was higher than the data, and it was confirmed that the WREM code had the overall conservatism against CCTF data.
The WREM code predicted the reasonable core boundary conditions and it was found that the conservatism of the code came mainly from the calculations on the incore thermal hydraulics and clad temperature. In addition, it was found that the conservatism of the WREM code against the CCTF data could be attributed to the neglection of horizontal fluid mixing between subchannels, the neglection of the heat transfer enhancement due to the radial core power profile, and the usage of the heat transfer correlations conservative against CCTF data.

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Fuel rod failure behavior has been studied under a reactivity initiated accident condition in Nuclear Safety Research Reactor (NSRR), JAERI. In the studies, metallurgical observations showed that the incipient fuel rod failure mode was oxygen-induced embrittlement of the cladding independent of the test conditions such as fuel designs and cooling environments except for pressurized and waterlogged fuels. Development of the oxidation layers'and embrittlement of fi-Zry were quantitatively evaluated through the metallurgical examinations. A diffusion equation of oxygen was solved under a finite system with moving boundary conditions to obtain the oxygen concentration and evaluate the cladding embrittlement. The calculation showed that the wall thinning due to the cladding melt is needed for the complete embrittlement because the wall thinning enhances the oxygen concentration in the j3-Zry, which well explain the experimental results. Therefore the failure threshold energy is determined by the cladding melting temperature. The failure threshold derived from this study is expected to be applicable to predicting the fuel rod failure behavior in computer analyses and also useful to evaluate the failure threshold energy for the new types of fuel rod.

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Initial plastic deformation behavior of zirconium alloy fuel cladding was described quantitatively by the deformation system of single crystal of a-zirconium, and a model was proposed to simulate the yield behavior of polycrystalline material. Based on the model, effects of crystallographic texture and stress state on the plastic deformation of the cladding were evaluated. Conclusions obtained from this investigation are :
(1) The proposed model shows good agreement with the von Mises' yield criteria for a material with isotropic properties.
(2) Plastic anisotropy of the cladding decreases when neutron irradiation affects prism slip more strongly than the other deformation systems.
(3) Dominant deformation systems for axial tension or internal pressurization of the cladding are predicted to be prism slip or tensile twin, respectively, when the stress state of the cladding reaches the yield condition.

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Recombination coefficient kR for hydrogen on Ni was evaluated experimentally by measuring the permeation rate of ion-driven deuterium in the temperature range of 573 873 K, with impurities on the specimen surface controlled at nearly the same composition prior to deuterium implantation. Calculation showed that kR's of upstream and downstream-side surface had an apparent activation energy of negative values, but of different values, and that of upstream-side surface increased as much as two orders of magnitude due to the deuterium ion implantation.

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Density changes of simulated radioactive waste glasses, silica glass and Pyrex glass irradiated by an electron accelerator were measured by a "sink-float" technique. The density changes of the waste and silica glasses were less than 0.05%, irradiated at 2.0 MeV up to the fluence of 1, 7 × 10¹⁷e/cm², while were remarkably smaller than that of Pyrex glass of 0.18% shrinkage, Precision of the measurements in the density changes of the waste glass was lower than that of Pyrex glass possibly because of the inhomogeneity of the waste glass.

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The linear sweep voltammetric study of Ni in molten LiF-NaF-KF eutectic was carried out at temperatures between 470 and 540°C by using an indicator electrode made of Pt bead. The observed reduction wave for Ni(II) was considered due to the reversible two-electron reduction, Ni(II)→ Ni(0). Nickel (II) can be determined in molten LiF-NaF-KF eutectic with the detection limit of 4 × 10^-4 mol/dm³. The diffusion coefficient of Ni(II) was found to be 5 × 10^-6 cm² Is at 500°C and its activation energy, (5.86 ± 1.67) × 10^-4 J/mol.

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A hundred milligram of UO₂ was irradiated in JRR-4 (burn up 0.017%) and dissolved in boiling 3 M-HNO₃ (20 ml) to study the behavior of fission-product iodine (¹³¹I, 1 ng). During the dissolution, 8090% of the iodine volatilized together with ¹³³ Xe. Zeolitel3X (4g) trapped completely the iodine in the dissolver off-gas. Blowing a NO₂ flow through the solution was effective for expulsion of the remaining iodine and finally 9095% of the initial iodine was removed from the solution with the aid of addition of KIO₃. The iodine species in the solution includes I₂, I^-, IO₃^- (IO₄^-), organic iodides and other unknown species. The expulsion operations caused the unknown species to increase in relative abundance. This tendency was also noticed for iodine of higher concentration in 3 M-HNO₃.

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A code - PER8MIGR - based on finite element method, and incorporating an 8-node isoparametric elements has been developed for solving in three-dimensional treatment the groundwater flow and advection diffusion equations involving with the decay chain of radioactive substances. Galerkin's method is applied to discretize the permeability and advection diffusion equations, and linear matrix equations are solved by band matrix method.
The Code is verified for applicability by agreement seen in benchmark tests for groundwater flow in which comparisons are made of the results obtained using the code with those of one- and three-dimensional analyses and with HYDROCOIN Level-1 Case-1 solution, and also in tests for nuclide, migration with the results obtained using other codes - AT123D, SWIFT, F.E.M. code of LBL and MIG3D in INTRACOIN test. The present work should permit accurate estimation of the flow and diffusion behavior to be expected of high- and low-level radioactive wastes in geological formations of complex geometry.

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In this paper the method of balancing centrifuge rotors for initial three modes based on practical experience is presented though, depending on the size of the centrifuge, more than one flexural modes have to be encountered. The object is to provide useful and practical information, as technical information on balancing of centrifuge rotors is hardly available because most of the work is shrouded in the clouds of the so-called secrecy.
The rotor is balanced in three stages. Firstly, individual components and sub-assemblies are balanced in traditionally low speed rigid rotor balancing machines. Secondly, the complete rotor is balanced by using modal balancing. Thirdly, the rotor is run in a high speed testing facility and rotor balance quality is checked at the operating speed. During the second and the third stages the rotor is run in its own bearings under vacuum and the data is acquired with the aid of a computer.

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A coarse-mesh 3-D (X-Y-Z, Hexagonal-Z) discrete ordinates transport calculation method has been developed. This method employs an weighted diamond difference approximation, the weight in which is a function of neutron direction and scalar flux, and can be easily incorpolated into conventional discrete ordinates transport codes. Results obtained in four-group S₄P₀ calculations on simple fast reactors indicate that, though the computer time of this method has increased by 3050% compared with that of the conventional finite difference method using the same mesh, the errors of k_eff and the power distribution are reduced remarkably.

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