Characteristics of gas multiplication and position sensing of Ne- and Ar-based gases have been investigated and compared with each other for position sensitive proportional counters.The measurements were carried out in the high gas multiplication region, i.e. the transition region from the proportional to the self-quenching streamer (SQS) mode. As the quench gas, CH₄ and, in some cases, C₂H₅OH vapor were added with proper mixing proportions. In position sensing, it is advantageous to use gas mixtures which need lower applied anode voltages to obtain high gas multiplication for good S/N ratio. The experimental results suggest that Ne-based gases have advantages in lower anode voltages by about 250 V for 4.5 keV X-rays. It may be concluded that Ne-based gases are useful in the application in which the counter construction is mechanically severe.

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Starting from a static or time dependent multigroup transport equation, static or kinetic nodal equations whose unknowns are the fission sources in each reactor are rigorously derived for coupled reactors. Since these equations are exact independently of the strength of the coupling, they can also be used to the usual single reactor dividing the core into appropriate subregions. In this case, the coupling coefficients give the exact expressions of the transport kernel of the FLARE method by the transport equation.
In the case of a single reactor, the nodal kinetic equations are reduced to the point-reactor kinetic equations which are more reasonable than the usual ones, and a reactivity equation which gives the rigorous relation between the reactivity and the period is derived. Some comparisons with the usual method are made and discussed.

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In a PWR core, approximately 1/3 of the burned fuels are replaced by fresh fuels after about a year's operation. Reload cores are designed so as to minimize the radial power peaking factor under design requirements and some administrative constraints. The conventional way of generating an appropriate fuel loading pattern is a method of trial and error, which is laborious and requires much computer time. A computer aided system has therefore been developed to speed up and make more efficient the determination of fuel shuffling configurations in reload cores. The system consists of a rule-base system employing artificial intelligence techniques, a pin-wise diffusion code and a feedback program based on fuel assembly power distributions. It was successfully applied to cores with only burnable poison rods, with gadolinium-dispersed fuels and also to cores employing low leakage fuel management strategy.

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The oxygen potentials and lattice parameter of oxide fuels (UO₂ and UO₂-2 w/oGd₂O₃) irradiated in commercial BWRs have been measured by using a solid electrolyte galvanic cell technique and X-ray diffractometry. The fuel burn-up was 1330 GWd/t for the former measurements and 635 GWd/t for the latter. The oxygen potentials at 750°C for irradiated UO2 ranged from 420 kJ/mol at the fuel rim to -480 kJ/mol at the fuel center. The corresponding values for irradiated UO₂-2 w/oGd₂O₃ ranged from -460 to -540 kJ/mol. The lattice parameter near the fuel rim in the irradiated UO₂ increased with burn-up, and was distinctly larger than that at the fuel center. This lattice dilatation was mainly due to the accumulation of radiation defects, not to a decrease in O/M ratio, and was almost recovered after the annealing at 1, 600°C for 5h.

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Photochemical reactions of U (VI) ion with halogen and pseudohalogen anions, I^-, Br^-, Cl^-and NCS^-, were studied in aqueous phosphoric acid solutions under irradiation of nitrogen laser.The formation of U(IV) was observed in the reactions with I^-, Br^- and NCS^-, but not with Cl^-.The yield of U (IV) increased in the order Br^-<NCS^-<I^-. This order was the same as the quenching rate constants of the excited U (VI) ion with these anions, but the reverse of their standard redox potentials. These facts are consistent with the electron transfer mechanism between excited U (VI) and these anions. The rates of the formation of U (IV) in the presence of Br^- were measured by the spectrophotometric method. It was found that the rate equation was first order in both U (VI) and Br^-. The results were reasonably interpreted by a series of reaction processes involving U (V) and Br radical. The photo-oxidation of tris (1, 10-phenan-throline)-Fe ( II) ion by U (VI) was also studied in the same procedures. Based on the rate law, we proposed a one-electron transfer mechanism involving U (V) as an intermediate.

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The effects of operation mode and perforated-plate type on axial mixing of mixer-settler region in both dispersed and continuous phases were studied for a 5 cm I. D. pulsed perforated-plate extraction column of pulser feeder type. The axial mixing coefficient was simultaneously measured to both phases by using the "dynamic tracer co-injection method" proposed by the authors. The characteristics of both phases are observed obviously. Relatively to what it was in the continuous phase, the dispersed phase had short reaching time and long retardation time. The superficial axial mixing coefficient for dispersed phase becomes smaller than for the continuous phase. And experimental results showed that a backflow took place only in dispersed phase at a test section of 50 cm in axial length. The values of backflow ratio are observed from 0.5 to 1.8% of the dispersed net flow.

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Presented are results of numerical simulations for radionuclide diffusion through fracture networks in geologic layers. Actual fracture networks are expressed as two-dimensional honeycomb percolation lattices. Random-walk simulations of diffusion on percolation lattices are made by the exact-enumeration method, and compared with those from Fickian diffusion with constant and decreasing diffusion coefficients. Mean-square displacement of a random-walker on percolation lattices increases more slowly with time than that for Fickian diffusion with the constant diffusion coefficient. Though the same relation of mean-square displacement vs.time as for the percolation lattices can be obtained for a continuum with decreasing diffusion coefficients, spatial distribution of probability densities of finding the random-walker on the percolation lattice differs from that on a continuum with the decreasing diffusion coefficient.The percolation model results in slow spreading near the origin and fast spreading in the outer region, whereas the decreasing-diffusion coefficient model shows the reverse because of smaller diffusion coefficient in the outer region. We could derive a general formula that can include both Fickian and anomalos diffusion in terms of fractal and fracton dimensionalities and the anomalous diffusion exponent.

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The ¹⁷O(n, α)¹⁴C cross section has been evaluated for incident neutron energies from 10^-5 eV to 20 MeV for accurate calculation on the ¹⁴C production in nuclear reactors. Evaluation was based on the single-level Breit-Wigner formula for the thermal to resonance energy regions. In the higher energy region, a multi-step evaporation model code PEGASUS was used, and the results were normalized using the ¹⁶O(n, α) cross section ratio of JENDL-3 evaluation to PEGASUS calculation. Results are given in tabular and graphical forms, and also as one-group cross sections using typical BWR, PWR and FBR spectra of ORIGEN-2.KEYWORDS: cross sections, evaluation, 17O(n, μ)14C reaction, 14C production, resonance parameter, Breit-Wigner formula, multi-step evaporation model, PEGASUS, one-group cross sections, ORIGEN-2

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To apply ionization chambers to fusion fuel gas processing systems, high concentration tritium gas was experimentally measured with small volume 0.16 and 21.6 cm³ ionization chambers. From plateau curves, the optimum electric field strength was obtained as 100200 V/cm. Detection efficiency was confirmed as dependent on the ionization ability of the filled gas, and moreover on its stopping power, because when the range of the β-rays was shortened, the probability of energy loss by collisions with the electrode and chamber wall increased. Loss of ions by recombination was prevented by using a small volume ionization chamber. For example the 0.16 cm³ ionization chamber gave measurements with linearity to above 40% tritium gas. After the tritium gas measurements, the concentration levels inside the chamber were estimated from their memory currents. Although more than 1/4, 000 of the maximum current was observed as a memory effect, the smaller ionization chamber gave a smaller memory effect.

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This report describes the results of reactivity initiated accident (RIA) experiments using UO2 fuel pellets containing 0.29 w/o Nb2O5 additive. In the present experiment, internal pressure of fuel rod, axial elongation and surface temperature of cladding were measured. From the result of post-irradiation examinations such as visual inspection, dimensional mesurement and ceramography, fuel failure threshold and failure mechanism were explored. The results obtained were :
(1) Failure threshold of the Nb₂O₅ doped fuel was equal to or greater than that of NSRR experimental data which were adopted in the Licensing Guideline for RIA, Japan.
(2) Failure mechanism of the Nb₂O₅ doped fuel was local ballooning followed by cladding rupture. No significant differences in the failure mechanism existed between the doped and undoped prepressurized fuels, the latter used in previous NSRR experiments.

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The effect of stress ratio, irradiation, oxidation, specimen volume and cumulative damage on fatigue behavior and fracture toughness were examined for HTTR (High Temperature Engineering Test Reactor) component materials, IG-110 and PG X graphites, and the following conclusions were derived :
(1) The fatigue strength of IG-11 graphite, which is non-purified IG-110 graphite, decrease with decreasing stress ratio and crack extension speed was enhanced mainly by the increment of cyclic peak load.
(2) The increment of fatigue strength of IG-110 graphite irradiated at the temperature of 575650°C and neutron fluence of up to 1.92 × 10²⁰3.20 × 10²⁰ n/cm² corresponds to that of the static strength of irradiated one.
(3) The fatigue strength of IG-110 graphite oxidized at 500°C in air can be inferred by the S-N curve analysis and the rule of degradation of static strength and crack extension speed of oxidized IG-110 graphite was enhanced as oxidation proceeds.
(4) Cumulative fatigue damage D_f of High-Low typed fatigue test for IG-110 graphite were smaller than the values estimated by the Miner's low, whereas the values of D_f of Low-High tests were larger than the values estimated by the Miner's low.

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