Computational method to design the efficient divertor configuration in tokamak reactor is presented. The two-dimensional code has been developed to analyze the distributions of the plasma and neutral particles for realistic configurations. Using this code, a method to design the efficient divertor configuration is developed. An example of new divertor, which consists of the baffle and fin plates, is analyzed.

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Various SRI (Selected Rod Insertion) tests performed during the start-up of a 1, 100 MW-class BWR/5 plant and many useful data were obtained. Those tests were simulated by the three-dimensional (3-D) transient code "TOSDYN-2" and the results were compared with the test data. The purpose of this study is to see the spatial effects which appeared during the transient phenomena with SRI, to confirm that SRI does not distort the power distribution locally and to verify the TOSDYN-2 code as the 3-D transient code.
Analytical results show good agreement, both qualitatively and quantitatively, with the 3-D effect test results, such as the transient response of spatial power distribution. From the test data and the analytical results, it is shown that the spatial effect by SRI is very smooth. Also, TOSDYN-2 is well qualified through this analysis as a 3-D BWR transient code.

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The accuracy to which the friction factor is correlated is important in counter-current flow (CCF) analysis using a two-fluid model. Current two-fluid model codes use correlations of friction factors developed for co-current flow, or correlations developed on the assumption of zero wall shear stress. An assessment of two-fluid model codes with these existing correlations shows that the downward flow rate of water is overestimated. An analytical model is developed to calculate the shear stress profile in a liquid film of the CCF in order to obtain information on the shear stress between the interface and the wall. Both this analysis and Bharathan's CCF data show that the wall shear stress acting on the falling water film is approximately of same order as the interfacial shear stress, and the correlation for co-current flow cannot be applied to CCF. Tentative correlations of interfacial and wall friction factors are developed based on the results of the present study.

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A computer code REACT incorporating 30 rate equations of reactions, i.e. radiolytic formation and decomposition of HNO₂, redox and disproportionation reactions, was developed to simulate behavior of actinide elements in the aqueous nitric acid solution. Main aspects of REACT code were explained briefly and then calculated results were compared with reported data to evaluate the model in the systems of radiolytic accumulation of HNO₂, stabilization process of Pu solution. The study showed that some radiolytic products other than HNO₂ would play a significant role and should be taken into account for precise simulation of very slow valency change of Pu in the neat Pu solution particularly with high radiation power density. Some examples of calculation were also shown for systems of reduction of Pu and Np by uranous or HAN and oxidation of Np(V) to Np(VI).

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Thermal decomposition behavior for ion-exchange resins in air flow was examined by differential thermobalance, The resins were decomposed in three steps : first, by water evaporation; second, by functional group decomposition, and finally by base polymers combustion. Following this work, the incineration of resins was examined, using a fluidized bed incinerator with copper oxide catalyst, which had been installed at the freeboard. The resins were burned at about 750°C in fluidized bed. Any combustibles remaining in the off-gas were incinerated by catalytic oxidation at the surface of the copper oxide catalyst at 650°C. From this experiment, it was confirmed that the incineration with copper oxide catalyst is feasible for treatment of ion-exchange resins.
In-situ desulfurization of nascent sulfur dioxide in fluidized bed by Ca(OH)₂ was also examined, and was found effective. However, the excess Ca(OH)₂ would be adsorbed on the surface of the catalyst or accumulated in pipelines, and further block the gas flow. Therefore, an advanced method for desulfurization should be developed.

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In an aerated soil layer under natural condition, water that infiltrates into it flows discontinuously because of repeated cycles of rainfall and drying. Therefore, column tests, in which 2, 000 ml of deionized water was fed into sandy soil layer dried for 390 days after it is conaminated by ⁶⁰Co, ⁸⁵Sr and ¹³⁷Cs aqueous solution, were carried out to examine influence of the drying period on migration behavior of those nuclides.
All the radionuclides showed both a steeply decreasing part corresponding to their cationic form and a gradually decreasing part corresponding to their particulate form in the concentration distribution curve along the soil column depth. In the vicinity of the top surface of soil layer, no influence of drying on ⁶⁰Co and ¹³⁷Cs concentrations was observed, but ⁸⁶Sr moved a little into a deeper part from the top surface with longer drying period. Such moving tendency of ⁸⁵Sr was found to be connected with the Ca²⁺ concentration in the interstitial water. In the deeper soil layer, no influence of drying on ⁸⁵Sr concentration was observed, but the ⁶⁰Co and ¹³⁷Cs concentrations were increased with the drying period. This is considered to be caused from that each product of the movable ⁶⁰Co(OH)₂ and the ¹³⁷Cs adsorbed on the fine particle increased with the rise of pH and the fine particle concentration, respectively, during the drying period. Thus, this study has suggested possible causes affecting on the radionuclide migration under discontinuous flow condition.

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In order to evaluate migration of ¹³⁷Cs adsorbed on fine soil particles in natural aerated soil layers, filtration constants of fine soil particles between soil and pore water were determined by the inverse analysis of the results of a large scale indoor migration test (STEM) using the soil layer samples collected from the natural aerated soil layers of Tokai and Rokkasho-mura without any disturbance. The amounts of ¹³⁷Cs which migrated through the soil layer samples with the movement of fine soil particles were also determined by the inverse analysis. The filtration constant for a loamy soil layer was larger than that for a sandy soil layer and the dependence of the filtration constant on mean diameter was the same as that obtained in small scale column experiments. On the other hand, the amount of ¹³⁷Cs which migrated through the soil layer sample with the movement of fine soil particles was 0.15-6.5% of introduced ¹³⁷ Cs and dependent on size distribution and solidification degree of the soil layer sample. Therefore, it seems that the amount of ¹³⁷Cs which migrates to a long distance with the movement of fine soil particles is small in the case of a fine and solidified soil layer. A trial evaluation of the migration of ¹³⁷Cs adsorbed on fine soil particles using the result of the analysis showed that the concentration decreasing effect of Rokkasho's soil layer including the loamy soil layer was larger than that of Tokai's sandy soil layer.

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In this study, to evaluate the core cooling characteristics and water level transient more realistically during refill-reflood phase for a BWR LOCA, several separate effects tests have been carried out. Several correlations and physical models, based on this study, were utilized to develop a computer code SAFER03. The verification of SAFER03 by ROSA-III and FIST-ABWR tests showed that the SAFER03 predicted well the water level transient and the peak cladding temperature with some margin. This study enabled a realistic prediction of BWR emergency core cooling system performance during LOCA. This paper is a summary report on the study results.

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