The authors have been investigating the fundamentals of thermo-hydraulic instabilities which may appear during the start-up in natural circulation BWRs in order to understand their driving mechanisms and to examine the methods preventing their occurrence with an aim of establishing a rational start-up procedure and reactor configuration. In this paper, based on our proposed driving mechanisms of geysering and natural circulation oscillation, the effects of system pressure on their occurrences and features are investigated experimentally. It is made clear that an increase in the system pressure suppresses their occurrences. From the results, a consideration is drawn to understand the differences in the start-ups between thermal natural circulation boilers using fossil fuel and the Dodewaard reactor.
Lastly, a rational start-up procedure to prevent their instabilities from occurring is recommended and a new idea of separator is proposed.

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Developed as a remedy-oriented system for systematically analyzing and evaluating humanrelated incidents occurring in nuclear power plants, the present system aims particularly at identifying causal factors and at deriving proposals for specific hierarchical countermeasures. Unlike conventional methods, which are based on checksheet format, and are thus devoid of logical methodology for conducting the analysis, and which thus lack the means of searching for underlying causal factors, and which do not record factual information on the sequence of events, our system incorporates innovative techniques such as; (a) a modified fault tree method for searching the underlying causal factors, (b) compilation of related events into sequential charts, (c) technique for devising proposed hierarchical redundant countermeasures, and (d) implementation procedures set out in practical manual form for easy familiarization and application.
Through several trial applications, our method has proved to permit identifying underlying causal factors even down to those associated with the software aspects of human action and state of mind, and with the mode of management, organization, operating rules and document forms, all of which are liable to be overlooked.
The results of analysis are stored in database to permit subsequent searching to serve as useful material for activities aimed at promoting plant operation safety.

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The propagation characteristics of frequency-swept laser pulse in near-resonant medium were studied by the numerical calculation and the experiment. The characteristics of the frequency-swept laser pulse has been largely modified in comparison with the frequency-fixed laser pulse. These modifications were demonstrated also experimentally for Sm vapor. The results agreed well with the numerically calculated.

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Specimens obtained from the PWR type UO₂ fuel with cladding at 39 MWd/kg U were provided for out-of-pile anneal. Obtained results are :
(1) A transient fission gas release (FGR) from the out-of-pile, instantaneous triangular shape temperature (peak 980°C) was <0.5%. On the contrary, a transient FGR from in-pile referential test was about 4%. The difference was attributed to the flat temperature profile across fuel for the former and the concave temperature profile for the latter. Significant microcrackings in UO₂ fuel periphery observed only in the in pile test might be the main cause of FGR enhancement.
(2) Onset temperature of FGR of the provided specimen obtained from the anneal was about 1, 340°C. The maximum FGR was about 37% at 2, 000°C. Diffusion coefficient D (m²•s^-1) was D=6.2 × 10^-3 exp(-440, 000/RT) at temperatures, 1, 473 ?? T <1, 873 K.
(3) FGR from holding at 1, 400°C/150 min was about 5 %, while FGR from cycling (30 cycles between 1, 400°C/5 min and 800°C/5 min) was 2.8%. No enhancement of FGR occurred by the cycling.

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The effect of grain size on microstructural change and damage recovery in UO₂ fuels was studied by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The as-irradiated lattice parameter of the standard fuel (grain size : 16 μm) was larger than that of the large-grained fuel (43 μm), indicating a larger number of fission-induced point defects in the lattice of the former fuel. This tendency was in contrast to previously reported results for low burnup fuels below 1 GWd/t. The lattice dilation in the present high burnup fuels was mainly due to the accumulation of vacancies. The lattice parameter of both fuels began to recover around an irradiation temperature of 450650°C, and both had a complete recovery at 850°C. On annealing at high temperatures of 1, 4501, 800°C, the bubble diameter in the standard fuel was larger than that in the large-grained fuel. This indicated that vacancy diffusion from the grain boundaries plays an important role during bubble coarsening at high temperatures.

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Experimental studies for the column with a feedback stream have been performed for a H-D-T system. A side stream was recycled to the column through an equilibrator as an internal feed stream. The stage model approximately predicted the composition distribution within the column, and was applicable for the simulation of the column having the feedback stream. The feedback stream decomposed HD at the equilibrator into H₂ and D₂ ; purity of H₂ and D₂ in products was increased by30%. The experimental results indicated that the location and flow rate of the side stream were key parameters ; the side stream must be withdrawn from the location where concentration of HD is larger than an equilibrium composition ; and its flow rate should be chosen so that the amount of HD decomposed is almost equal to that supplied to the column. The large feed flow rate of the column having the feedback stream, which was summation of external and internal feed streams, varied the HETP especially for the upper part of the column. In the design stage of the column, it is required to prepare several different points for the side stream so that its location can be changed with the variation of the HETP.

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Static corrosion tests of a powdered simulated waste glass were performed in deionized water with and without bentonite at 90°C for periods of up to 130 days. The glass irradiated with thermal neutron for activating Cs in the glass was used as a glass specimen in order to determine the sorption of Cs on bentonite. In the corrosion tests without bentonite, it was observed that normalized elemental mass loss (NL) values for soluble elements (B, Li, Na and Mo) were larger than those for Si by a factor of three and continued to increase after saturation of Si. In the corrosion tests in the presence of bentonite, it was observed that the glass corrosion was enhanced, and a large amount of Cs was sorbed on bentonite.
The experimental results were analyzed by use of some corrosion models. The analysis showed that diffusion of the soluble elements is expected to be a dominant process for the glass corrosion, as well as the dissolution/precipitation reactions. In addition, it is expected that the glass corrosion in the presence of bentonite is largely affected by both ion-exchange equilibrium of the aqueous phase with Na-montmorillonite and precipitation of sepiolite from dissolution of dolomite.

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Interphase transfer kinetics of neodymium and nitric acid was studied using a single drop column with recycling organic phase via an external mixing vessel in H₂O-HNO₃/NaNO₃-Nd(NO₃)₃-tri-n-butylphosphate system. Experimental data have been analyzed by two new concepts for driving forces for transport : synthesized linear and logarithmic forms. The former is defined as geometrical-mean driving force, and the latter is the logarithm of the product of reciprocals of concentration ratios x/x^e and y/y^e against equilibrium states in each phase, i.e. In {(x^e•y^e)/ (x•y)}.
By applying thermodynamic logarithmic form of driving force along reaction coordinate, the net transfer fluxes of neodymium and nitric acid have been represented by chemical affinity under high ionic strengths over a wide range of solvent loading as flux=flux°f(1-exp(-A/RT)).

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The numerical models of WSPEEIDI (Worldwide version of System for Prediction of Environmental Emergency Dose Information) were revised. The new models are formulated with a terrain following vertical coordinates with the compressibility of the atmosphere being considered. The models have been integrated into a computer code system together with system control software, worldwide geographic database, meteorological data pre-processor and graphics. The performance of the code system has been evaluated against the Chernobyl case with reliable source terms, well established meteorological input and a comprehensive monitoring database, which were provided in ATMES (Atmospheric Transport Model Evaluation Study) coordinated by IAEA, WMO and CEC.

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In block-type HTGRs, control rod insertion depths into cores had to be maintained as small as possible at full power operations, to avoid a fuel temperature rise. Thus, specifications (poison atom density (N_BP) and radius (r)) of rod-type burnable poisons (BPs) had to be optimized so that the effective multiplication factor (keff) would be constant at a minimum value throughout a planned burnup period.
However, the optimization had been a time-consuming work until now since survey calculations had to be done for most possible combinations of N_BP and r. To solve this problem, I have found a optimization method consisting of two steps. In the first step, approximation formulas describing a time-dependent relation among effective absorption cross sections (Σa_BP), N_BP and r are used to select promising combinations of N_BP and r beforehand. In the second step, the best combination of N_BP and r is determined by a comparison between Σa_BP of each promising combination and expected one. The number of survey calculations was reduced to about 1/10 by the optimization method. The change in keff for 600 burnup days was reduced to 2%Δk by the method. Hence, it was made possible to operate reactors practically without inserting the control rods into cores.

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For powdered fuel processed in the nuclear fuel facilities, flooding is often thought to be the severest condition regarding the nuclear criticality safety evaluation. The reactivity of such a heterogeneous system as powdered fuel in water should be almost equal to that of the homogeneous one, when the fuel particle size is very small. The neutron multiplication factor was calculated for an infinite cubic array of slightly enriched UO₂ sphere particles immersed in water with various enrichments, water to fuel ratios and fuel particle sizes. The calculations were performed with a computer code module based on the collision probability method to solve the ultra-fine energy group equations of neutrons. The change in the neutron multiplication factor from the homogeneous system is dominated first by the change in the resonance escape probability and second by the change in the thermal utilization factor ; these changes and therefore their sum, depend almost completely on the mean uranium concentration (or water to fuel volume ratio) and rarely on uranium enrichment up to 10 wt% for a fuel particle size of 1 mm. The dependence determines the fuel particle size regarded as homogeneous in proportion to the negligible relative error of the neutron multiplication factors.

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