The method of shielding calculations, and production and decay of nuclides being employed for estimate of radioactive inventory frequently assumes the Maxwellian distribution (MD) for thermal neutron spectrum and 1/E for epithermal neutron spectrum. Actually each spectrum deviates largely from the MD and 1/E spectrum respectively in reactor pressure vessel (RPV) and biological shielding concrete (BSC). These assumption then leads activation calculation for residual radioactivity to noticeable error. In this paper it is studied how the deviation of the neutron spectrum affects activation, effective thermal neutron cross section and effective epithermal neutron cross section. It has been proved that they change according to position in the RPV, BSC and other reactor components. Therefore, neutron energy group structure is required to be so fine as to define neutron spectrum accurately.

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Sodium void reactivity worth in two metallic fueled fast reactors and a MOX fueled one was measured at the Fast Critical Assembly (FCA). Using the measured reactivity worth, bias factors to be applied to the calculation were determined for the non-leakage term and the leakage term separately by a least squares method. Based on these bias factors, the calculation accuracy for each term was assessed.
For the non-leakage term, JENDL-3.2 showed good agreement with the experiments. No difference in the calculation accuracy was observed between the metallic fuel and the MOX fuel, though their neutron spectra were largely different. When the enriched uranium was contained in the void region together with plutonium, however, JENDL-3.2 overestimated the non-leakage term by about 6%. The effect of the plutonium isotope composition was also examined.
For the leakage term, it was found that the accuracy of the current calculation depended upon the void fraction in the cell. The directional diffusion coefficient based on the Benoist's theory was validated by transposing the plate direction. Moreover, the design concept of LMFBR with the sodium plenum, which aims at the reduction of the sodium void reactivity, was also examined.

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Evaluation of thermal and mechanical loads on the containment vessel due to combustion of flammable gases is essential for addressing the important issues of safety analyses of nuclear power reactors under hypothesized severe accident conditions. In order to predict flammable gas detonation which is the most severe type of combustion, a two-dimensional numerial analysis code COMA has been developed using the explicit finite volume method and non-staggered mesh scheme. A new numerically stable transport calculation method has been devised to facilitate treatment of a large pressure discontinuity at the wave front with the minimum numerical diffusion. The test calculation of detonation propagation in a square cavity surrounded by walls showed that the calculated propagation velocity agreed well with the theoretical velocity by Chapman and Jouguet, and the wave reflection at rigid walls and interaction of detonation waves could be analyzed. The method was then validated through a comparison study with the hydrogen detonation tests in a cylindrical tube performed by NUPEC and BNL. The calculated results agreed with observed overall effects of hydrogen concentration and initial temperature on detonation wave propagation velocities.

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Droplet behavior in the BWR dryer and separator was analyzed. The computer program HIJET-AFIMA was used to simulate the liquid droplet behavior in a dryer with four stage wave-type vanes. This program can describe movement of liquid droplets, as influenced by gas flow, and their deposition on the vane surface, considering drag force which is exerted on droplets. Close agreement between calculations and air-water tests was obtained for the wetness fraction at the exit of each vane stage. The calculated results under reactor conditions showed that larger droplets (30-600μm in diameter) were concentrated in the first and second stages due to their larger inertia force. Smaller droplets (less than 30μm in diameter) were able to pass through the flow channel without deposition on the vane surface. This was because the effect of the drag force was exerted more strongly on smaller droplets. As a result, wetness fraction at the exit was small and less than 0.1% when inlet wetness was 10%.
The droplet behavior analyses in a separator showed that the deposition coefficient with swirl was much higher than that without swirl. Since the ratio of inertia force to drag one was almost the same everywhere in the separator, the difference in inlet quality had only a small effect on droplet deposition.

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Previously manufactured oxide dispersion strengthened (ODS) ferritic steel cladding tubes had inferior internal creep rupture strength in the circumferential hoop direction. This unexpected feature of ODS cladding tubes was substantially ascribed to the needle-like grain structure aligned with the forming direction. In this study, the grain morphology was controlled by using the martensite transformation in ODS martensitic steels to produce an equi-axial grain structure. A major improvement in the strength anisotropy was successfully achieved. The most effective yttria addition was about 1mass% in improving the strength of the ODS martensitic steels. A simple addition of titanium was particularly effective in increasing the strength level of the ODS martensitic steels to that of ODS ferritic steels.

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In order to determine effects of hydrogen peroxide on stress corrosion cracking of structural materials in the primary cooling systems of boiling water reactors, a high temperature high pressure water loop with controlled hydrogen peroxide concentrations and lower possible oxygen concentrations has been fabricated. Test specimens are installed in a stainless steel autoclave which has poly tetra-fluoro-ethylene (PTFE) inner liner to prevent decomposition of hydrogen peroxide on the autoclave surfaces. Hydrogen peroxide is injected into the autoclave inlet through the injection line which also has PTFE inner liner. The concentration of hydrogen peroxide is measured at the autoclave outlet by sampling water via the PTFE-lined sampling line. More than 65% of the injected hydrogen peroxide remains at the autoclave outlet at elevated temperature (288°C). Electrochemical corrosion potential (ECP) of stainless steel is then measured in the autoclave while changing hydrogen peroxide and oxygen concentrations. From these measurements it is concluded that, at the same oxidant concentration: (1) ECP of stainless steel exposed to hydrogen peroxide is higher than that exposed to oxygen; (2) ECP is much affected by specimen surfaces; and (3) ECP shows a hysteresis pattern for on its concentration dependence. ECP of stainless steel with an oxidized surface formed under high hydrogen peroxide concentration is much higher than that with a mechanically polished surface and it is less affected by oxidant species and their concentrations.

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Our earlier reported work for densities of mixed aqueous solutions was extended for the prediction of density of mixed organic solutions containing tri-n-butyl phosphate, diluent and solvates of the electrolytes-UO₂(NO₃)₂ and HNO₃. Two empirical equations for density of mixed organic solutions have been reported. The first equation predicts density at 298.15K while the other equation predicts temperature dependency of density in the range of 283.15K to 333.15K. A novel feature of this model is consideration of TBP as a solute, rendering equations valid over the full range of TBP concentrations. Good agreement was observed between the published experimental data and predicted values from model reported in this work.

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The elementary chemical reaction of cesium which is a volatile fission product with steam (water) shown in the title are analyzed from the first principle. The activation energies for both reactions of the forward and the backward are calculated for the first time with a computer program Gamess, which is based on the non-empirical molecular orbital theory. The calculated values are expected to agree with those of experiments within an accuracy of several tens of kJ/mol. Using the values obtained above, the rate constants are also calculated in the framework of the transition state theory. The derived rate constants will allow us to predict the orders of magnitude of experimental values under high temperature conditions (1, 000K or higher). Thus these rate constants are available as input data for the kinetic (non-equilibrium state) study of cesium in severe light water reactor accidents.

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