It was found that the convergence rate of the alternating direction implicit method for the nodal equation for the diffusion equation in x-y geometry derived by the finite Fourier transformation became slow as the absorption cross section decreases. This difficulty was found to be removed by eliminating the leakage term from the equation. An acceleration method and a method to obtain an optimum acceleration factor were developed for the iteration method to solve this equation.

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A method is described to predict radiation hardness of general purpose CMOS logic ICs.The method uses an impulse response model for radiation-induced threshold voltage shift of an "elemental IC" selected from an IC family. The total dose degradation on ICs can be predicted at any dose rate by simulations using the impulse response model of the threshold voltage shift and their circuit structure data. This method was applied to the prediction of radiation degradation of CMOS logic ICs of HD74HC series, in which an inverter HD74HC04 was selected as the elemental IC. Four inverter samples were irradiated up to 100 Gy (Si) and then annealed to examine the impulse response of the threshold voltage shift of their n- and P-channel MOSFETs. Results predicted by this method represented the static characteristic degradation of the HD74HC04 irradiated up to a large total dose 1 kGy (Si) at two different dose rates 8 and 500 Gy (Si) /h.

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Oxidation velocities of isotropic graphite, pyrolytic carbon, silicon and boron mixed graphites were measured by using a vacuum microbalance of Cahn RG type under an oxygen pressure of 1.33 kPa and temperature range of 650900°C.For isotropic graphites with different ash concentrations, it was observed that the oxidation velocity increased as the ash concentration when the temperature was below 750°C. The pyrolytic carbon had the oxidation velocity approximately an order of magnitude smaller than that of the isotropic graphite. The oxidation velocity of silicon mixed graphite linearly decreased as the increase of silicon concentration. In a case of boron mixed graphite, the oxidation was considerably suppressed by the boron content and the weight gain was observed due to the formation of B2O3 when the boron concentration exceeded approximately 10%.

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Photochemical oxidation and reduction behaviors of neptunium were preliminarily investigated in 3 mol/l nitric acid solution. Nitric acid of 3 mol// simulated the high level waste solution from a spent fuel reprocessing process. Concentrations of Np(V), Np(VI) and nitrous acid were determined with a photospectrometer, and solution potential with an electrode. Without additives, Np(VI) was reduced to Np(V) by nitrous acid which was photolytically generated from nitric acid. With a scavenger for nitrous acid, Np(V) was oxidized to extractable Np(VI) by a photolytically generated oxidizing reagent which were predicted by the solution potential measurement. The reduction rate was higher than the oxidation rate because of the larger quantity and higher reactivity of nitrous acid than an oxidizing reagent. Photo-catalyst was proved to be effective for the oxidation of Np(V) to Np(Vl).

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Horizontally stratified two-phase flow is one of the important phenomena for analysis of small-break loss of coolant accidents (SBLOCAs) in a pressurized water reactor (PWR). Thus separate effect tests have been conducted at the ROSA-N Two-Phase Flow Test Facility (TPTF) to investigate the flow regime transitions in horizontal, co-current, steam-water two-phase flow for high pressure (311.9MPa) in a large diameter (180 mm) pipe. These experiments were analyzed using the TRAC-PFI/MOD1 code to assess the adequacy of the code's flow regime transition criterion and interfacial drag models. The TRAC code predicted slug or transitiona flow regime for tests conducted for relatively high flow rates, whereas, the experimental flow regime was stratified flow. This resulted in a large overprediction of the interfacial drag for these tests. The flow regime transition criterion, the Taitel-Dukler model, was modifiec according to the experimental flow regime transition condition, by replacing the gas velocity in the criterion by the relative velocity. However, the interfacial drag was still overpredicted for the stratified flow regime. A sensitivity study was carried out on this coefficient to find the value which provides the best agreement with respect to the test section void fraction distribution. Based on this sensitivity study, it is recommended to use an interfacial drag coefficient which is a half of the value used in the current version of the TRAC.

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A double-flat-core type high conversion light water reactor (HCLWR) has been developed at JAERI to improve fuel utilization. Experimental and analytical studies on the departure from nucleate boiling ratio (DNBR) under operational and accident conditions for the HCLWR have been performed. It was found by comparing several critical heat flux (CHF) correlations with the CHF data obtained at JAERI and Bettis Atomic Power Laboratory that the KfK correlation has the most promising features for the application to the triangular tight lattice rod array. The minimum allowable DNBR (MDNBR) for the HCLWR was determined to be 1.28 by comparing the Bettis CHF data with the KfK correlation. The best-estimate code J-TRAC was used for system transient calculations under the primary coolant pump trip and locked rotor accident conditions. The subchannel code COBRA-IV-I was then used to obtain local flow conditions and fuel rod surface heat flux. The analytical results indicated that an enough safety margin was assured under the steady-state operational condition. Under the accident conditions, the evaluated minimum DNBR's were also above the MDNBR criterion. Therefore, it was clarified that the present HCLWR design is feasible from a view point of the MDNBR criterion.

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Since chemical regeneration of condensate demineralizers produces a large quantity of radio-active liquid wastes, reducing the volume of such wastes has become a significant concern.In Japanese Improvement and Standardization BWR's, by installing a prefilter upstream from the condensate demineralizer, and improving construction materials and method of controlling water chemistry, the crud and ion load on the condensate demineralizer has been reduced.
Since non-regenerated operation of the condensate demineralizer has been devised and tried on the No. 2 Unit of the Fukushima Daini Nuclear Power Station (2F-2) with variable resin measurement during every scheduled outage and each fuel cycle. The result has been a reduction of the number of drums to be disposed of to 1/5 the previous number.

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The PALLAS (discrete ordinates-integral transport) code was improved to include secondary sources, such as bremsstrahlung and fluorescence, to assure accurate and reliable results. The point buildup factors for high-Z materials were calculated with this code in the energy range of 0.01515 MeV up to 40 mean free paths. The buildup factors for low-Z materials in the low energy range, which are most difficult to calculate, were calculated with PALLAS and were validated by comparison with the results of the EGS4 (point Monte Carlo), and ANISN codes.
A function to calculate coherent scattering was added to the EGS4 code. It was suggested that neglecting coherent scattering and assuming free-electron Compton scattering can result in an error in the buildup factor.
A fitting formula, the geometric progression (G-P) form, was developed to reproduce the data in codes used for shielding design. This formula can reproduce the data over the full range of distance, energy and atomic number within a few percent.
The above cited buildup factor data and the G P formula have been adopted for proposed standard ANS-6. 4.3 "Gamma Ray Attenuation Coefficient and Buildup Factors for Engineering Materials". The G-P fitting function has been implemented in the CCC-493/QAD-CGGP and CCC-494/G33-GP codes available from the Radiation Shielding Information Center (RSIC) at Oak Ridge National Laboratory. These are used for r-ray shielding calculations throughout the world.
Also, the data and the method of evaluation for 1 cm dose equivalent, introduced into the Japanese law according to ICRP recommendations, have been offered by this team.

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