Making use of a standard neutron spectrum field with a pure Maxwellian distribution, the thermal neutron cross section for the ²³⁷Np(n, γ)²³⁸Np reaction was measured at a neutron energy of 0.0253 eV by the activation method. The result is 158±3 b, which is obtained relative to the reference value of 98.65 ±0.09 b for the ¹⁹⁷Au(n, γ)¹⁹⁸Au reaction. Although the data in JENDL-3 is larger by about 15% than the present value, the recently revised data in JENDL-3.2 is close to the present. The ENDF/B-V, ENDF/B-VI, JEF-2 and Mughabghab's data are also larger by 715%. Old measurements are larger by 718% than the present data.
The resonance integral for the ²³⁷Np(n, γ)²³⁸Np reaction was also measured relative to the reference value of 1, 550±28 b for the ¹⁹⁷Au(n, γ)¹⁹⁸Au reaction with a 1/E standard neutron spectrum field. By defining the Cd cut-off energy as 0.5 eV for the ²³⁷Np(n, γ)²³⁸Np reaction, the present resonance integral is 652±24 b, which is in good agreement with the JENDL-3, -3.2, ENDF/B-V, -VI, JEF-2 and Mughabghab's data. However, most of the old experimental data are, in general, larger by 2438% than the present measurement.

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The cross sections of ¹⁸¹Ta(n, p)¹⁸¹Hf ¹⁸¹Ta(n, n'p)^180mHf ¹⁸⁴W(n, α)¹⁸¹Hf, ¹⁸⁶W(n, n'p)¹⁸⁵Ta and ¹⁸⁶W(n, α)¹⁸³Hf reactions have been measured in the energy range between 13.4 and 14.9 MeV by the activation method. The intense 14 MeV neutron source facility (OKTAVIAN) at Osaka University was used for irradiation. The γ-rays emitted from the irradiated samples were measured with high-purity germanium (HPGe) detectors. All cross section values were obtained relative to the standard reaction cross section of ⁹³Nb(n, 2n)^92mNb obtained by Netha way. The cross sections of ¹⁸¹Ta(n, n'p)^180mHf reaction were measured for the first time. The present results were compared with previous data and the evaluated data of JENDL-3 and ENDF/B-VI and the comparison showed that some significant discrepancies exist.

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Neutron economy of the transmutation of TRU was examined in well thermalized, thermal and fast neutron fields. Burn-up chains of ²³⁷Np, ²⁴¹Am and ²⁴³Am, which are the main TRU nuclides in the high level waste, were calculated in the flux region from 10¹⁴ to 10¹⁷ n/cm²•s. Numbers of neutrons absorbed and produced of each chain were calculated using JENDL-3. The net number of neutron produced ⁿnet, which was obtained by the difference of the two numbers, largely varied with the neutron fields, the nuclides and the flux levels. The nnet value in the fast neutron field was positive (0.01.0) for ²³⁷Np, ²⁴¹Am, ²⁴³Am, TRU with the nuclide composition in the high-level waste generated by the conventional PWR. The transmutation of TRU by fission can be performed with producing neutrons in the fast neutron field. On the other hand, the ⁿnet value was negative for the well thermalized and thermal neutron fields. For TRU in the high-level waste, the values in those fields were -1.0 at 10¹⁴ n/cm²•s and 0.0 at 10¹⁶ n/cm²•s. In the high flux region of 10¹⁶ n/cm²•s, TRU in the high-level waste can be transmuted by fission without consuming additional neutrons. In the flux region about 10¹⁴ n/ cm²•s, the transmutation of TRU in the high-level waste by fission requires about one neutron.

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An experiment was conducted at the ROSA-IV/Large Scale Test Facility (LSTF) on the performance of a gravity-driven emergency core coolant (ECC) injection system attached to a pressurized water reactor (PWR). Such a gravity-driven injection system, though not used in the current-generation PWRs, is proposed for future reactor designs. The experiment was performed to identify key phenomena peculiar to the operation of a gravity injection system and to provide data base for code assessment against such phenomena. The simulated injection system consisted of a tank which was initially filled with cold water of the same pressure as the primary system. The tank was connected at its top and bottom, respectively, to the cold leg and the vessel downcomer. The injection into the downcomer was driven primarily by the static head difference between the cold water in the tank and the hot water in the pressure balance line (PBL) connecting the cold leg to the tank top. The injection flow was oscillatory after the flow through the PBL became two-phase flow. The experiment was post-test analyzed using a JAERI modified version of the RELAP5/MOD2 code. The code calculation simulated reasonably well the system responses observed in the experiment, and suggested that the oscillations in the injection flow was caused by oscillatory liquid holdup in the PBL connecting the cold leg to tank top.

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The oxidation behaviors of IG-110 and PGX graphites were experimentally investigated in the temperature range from 500 to 1, 500-C for the oxygen concentration of 10 and 30 wt% (1.37 and 5.09 mol%) and the reacting gas velocities of 6.63 and 19.9 m/s (estimated at 900-C). The graphite specimens were prepared to simulate the annular coolant channel in the core of the HTGR. It has been found that the upstream surface is more oxidized for both IG-110 and PGX graphites, and that the oxidized surface of PGX is rougher than that of IG-110. The oxidation of IG-110 starts at about 700-C and reaches the maximum above 800 to 900-C accompanied by the maximum production of CO₂ around 850-C and a monotonic increase in CO with increasing temperature. The oxidation behavior of PGX is similar to that for IG-110, except that the oxidation starts at lower temperature and produces more CO₂ over 750 to 950-C while the concentration of CO becomes lower in this temperature range. The present result for the product ratio of CO to CO₂ deviates from the linear line in the Arrhenius plot in the region where the production of CO₂ increases. However, it agrees best with the Rossberg correlation among the previous empirical correlations. New empirical correlations for the product ratio of CO to CO₂ have been obtained from the present data which exclude those in the region of CO₂ increase.

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A computing system for evaluating the micro- and macro-behaviors of zirconium-base alloys has been developed. The system is composed of three independent computing stages ; electron micro-structure analysis, molecular dynamical analysis, and rate-theory-based analysis of defects. The stages are connected with each other by providing input information for the calculations of the next stage. Here, the calculated results are presented on the electronic structure of zirconium oxides, characteristic feature of defect clustering and irradiation growth behaviors.

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An implantation method based on the glow-discharge of hydrogen gas has been applied to the measurement of diffusion coefficients of protium and tritium in zirconium, and the following results have been obtained in a temperature region from 673K to 873K :
The diffusion coefficient of tritium in zirconium has been also measured by making use of the usual implantation method based on the nuclear reaction ⁶Li(n, α)T and the following result has been obtained in a temperature region from 573K to 923K :
The validity of the present discharge implantation method has been confirmed by comparing the obtained results with each other and with literature values.

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The energy dependence of MeV He⁺ ion-induced re-emission of hydrogen isotopes (H and D) implanted into graphite has been measured by means of the elastic recoil detection (ERD) technique in order to clarify the collision process for the ion-induced detrapping. The experimental re-emission profiles have been analyzed by solving the mass balance equations, in which the ion-induced detrapping cross section σ_d and the rate constants of the retrapping Σ_T and local molecular recombination K between an activated hydrogen atom and a trapped one are taken into account. The values of σ_d and K/Σ_T have been determined from the best-fit analytical solution to the experimental re-emission profiles. It has been found that the average values of σ_d and K/Σ_T for H are twice as large as those for D, which is the so-called isotope effect.
It has been shown that the experimental values of σ_d and their energy dependence agree well with the theoretical ones, which are calculated using the power-law approximations for Thomas-Fermi potential, on the assumption that the ion-induced detrapping of hydrogen isotopes takes place due to elastic displacement collisions with energetic carbon recoils produced by incident MeV He⁺ ions.

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The soil-to-plant transfer factors (T_fs) of Sr and Cs summarized in the IUR report were statistically analyzed for three leafy plants : cabbage, spinach and grass. According to their statistical characteristics, suitabilities of default T_fs reported by IAEA were examined. The T_fs in the IUR report showed log-normal distributions for the three leafy plants. The geometric mean was found to be significantly different between plant species and between two soil types : loam and sand. In comparison to the IUR values, the default T_fs of Sr were considered to be overestimated for cabbage and underestimated for spinach, while the default T_fs of Cs were likely to be overestimated for cabbage and underestimated for spinach and grass.

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A distributed database system named "Data-Free-Way (DFW)" is under development by a cooperation among three Japanese national research organizations to support the creation of advanced nuclear materials. The development of DFW started in 1990 as a five-year program with a support from Science and Technology Agency of Japan. Before starting the program, a preliminary survey of both domestic and foreign databases of nuclear materials had been made for two years. Then, subjects for the construction of DFW were extracted. To meet the subjects, the development and construction programs were established.
The DFW is constructed on the computer network which connects engineering workstations in the separate organizations. A relational database management system is used, a distributed material database is equipped on the hardware with specially designed common data structure. Data storage has been carried out continuously in each organization. The equipment of useful user-interface systems, such as retrieval, data entry and process supporting and image data handling systems, have also been constructed to make it friendly for users.
The collection of nuclear material data from three research organizations and its mutual usage have become possible by the construction of DFW.

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