A numerical calculation technique for the two-dimensional two-fluid model has been developed. The control volume formulation and non-staggered mesh scheme are employed in order to confirm that solutions satisfy the conservation equations of the two-fluid model. Numerical instability due to the non-staggered mesh is overcome by considering additional flows induced from a local pressure gradient on control volume boundaries. The Godunov method and SIMPLE method are used to estimate the additional flow for compressible and incompressible two-phase flows, respectively. These methods are modified to take the density difference between the phases and the added mass effect into account.
The present method has been applied to analyses of two-dimensional two-phase flow discharge from a pipe and phase separation due to gravity in a horizontal circular pipe in order to investigate the capability of dealing with fundamental phenomena of two-dimensional two-phase flow. The present technique enables a small scale computer program to simulate two-dimensional two-phase flow and it should be a valuable tool for fundamental study of two-phase flow dynamics.

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A new method was developed to predict critical powers for a wide variety of BWR fuel bundle designs. This method couples subchannel analysis with a liquid film flow model, instead of taking the conventional way which couples subchannel analysis with critical heat flux correlations. Flow and quality distributions in a bundle are estimated by the subchannel analysis. Using these distributions, film flow rates along fuel rods are then calculated with the film flow model. Dryout is assumed to occur where one of the film flows disappears. This method is expected to give much better adaptability to variations in geometry, heat flux, flow rate and quality distributions than the conventional methods.
In order to verify the method, critical power data under BWR conditions were analyzed. Measured and calculated critical powers agreed to within ±7%. Furthermore critical power data for a tight-latticed bundle obtained by LeTourneau et al. were compared with critical powers calculated by the present method and two conventional methods, CISE correlation and subchannel analysis coupled with the CISE correlation. It was confirmed that the present method can predict critical powers more accurately than the conventional methods.

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The sorption behavior of Cs, Sr and Am ions on loarns, which is widely distributed in Tohoku district of Japan, was studied under static and flowing conditions.
The measurements of ion distribution ratios, which span a range of nearly three orders of magnitude, were fitted to Freundlich or Langmuir isotherms and the sorption parameters of isotherm equations were determined. It was found that the sorption of these ions on loams is due to the ionic exchange with Na, Ca and Mg ions of the loams. Analyzing the breakthrough curve of Cs ion sorption on a loam sorbent yielded 0.2 s^-1 and 0.32 cm for values of the overall mass transfer coefficient and of the length of adsorption zone, respectively.

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Distribution coefficients K_ds of several radionuclides on suspended particulate matter (SPM) have been determined with laboratory batch experiments using samples collected in the Kinki district, Japan. Most SPM in coastal water was retained on the initial filter of four sequential filtering stages (20, 5, 1 and 0.45μm pore size), maximum K_d also being found for the SPM retained in this first stage. When the filtrations were not performed in series, however, but were conducted from the same tracer solution in parallel, the K_d values were identical for the smaller filter sizes (5, 1 and 0.45 am) and were greater, by ca. 150% than those derived from the 20μm filter. The degree of association of the radionuclides with SPM was ⁸⁵Sr<¹³⁷Cs<⁶⁵Zn <⁵⁴Mn<⁶⁰Co. The particulate radioactivity fraction of the SPM in coastal water was estimated at most 0.1 for radionuclides with K_d of ca. 5×10⁴ ml/g, i.e. which are highly associated with the SPM (2 mg/l). The scavenging effect by SPM seemed to be, however, considerable, the settling velocity being estimated at ca. 25 cm/day for particles of the Stoke's diameter of 30μm.

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Cell calculations of a Th-fueled PWR are carried out to discuss the burnup characteristics, coolant void reactivity coefficients, and the effectiveness of the mechanical spectral shift control method by fertile rod insertion. It is shown that the Th fuel can achieve a high discharge burnup with less increase of the fissile concentration than in the U fuel. It is also shown, particularly in the Th-fueled cores, that the fertile rods are effective for the spectral shift control and for improving the conversion ratio.

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