By using the Kyoto University Critical Assembly (KUCA), a series of critical experiments was performed to measure the temperature coefficient of reactivity in a light-water-moderated and heavy-water-reflected cylindrical core loaded with highly-enriched-uranium (HEU) or medium-enriched-uranium (MEU) fuel. The measurement was performed for the approximately 20 to 70°C range to examine the effects of the size of light-water region in a heterogeneous multi-region type core, the reduced ²³⁵U enrichment, and the existence of boron burnable poison (BP) on this quantity by using six types of core configurations. In all the six types of cores, there were large light-water regions at the center of core and between the outer fuel region and the heavy-water reflector region, and it was found that these light-water regions caused a remarkably positive effect on the temperature coefficient of reactivity. In the present study, the temperature coefficients of the MEU core and the core without BP were more positive than those of the HEU core and the core with BP, respectively. The size of light-water region had a larger effect on the temperature coefficient rather than the reduced ²³⁵U enrichment and the existence of BP. The negative temperature coefficient would be realized by reducing the thickness of light-water layer existed in the core.

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The γ-ray emission probabilities for ^116mIn with half-life of 54.15min have been measured by a 4πβ(pc)-γ(HPGe) coincidence apparatus using a live-timed two-dimensional data acquisition system. Eleven γ-ray peaks of 116In were recognised and the uncertainties of the measured emission probabilities for principal γ-rays were found to be less than 1%. On the other hand, the relative intensities for 18 energy γ-rays were determined with improved uncertainties by an ordinary γ-ray spectrometer. The decay parameters were determined with uncertainties of approximately 1% using these values and the evaluated values of weak γ-ray intensities and the internal conversion coefficients.

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In the Nuclear Safety Research Reactor (NSRR) program of Japan Atomic Energy Research Institute (JAERI), the fuel behavior in reactivity initiated accidents (RIAs) has been studied through irradiation tests with simulated power burst using fresh or preirradiated test fuel rods. In order to investigate possible influence of the difference of initial temperature profile in the fuel pellet on the fuel failure behavior, two tests were conducted with fresh fuel rods for RIAs at power operation using the newly developed NSRR operation mode and the results were compared with the results of previous irradiation tests which were for RIAs at zero power.
In the tests for RIAs at power, the reactivity of 2.0 or 2.3 was inserted rapidly after the linear heat rate of the test fuel rod was kept constant at 39kW/m for 5s. It has been shown through this study with fresh fuel rods that the fuel enthalpy of the failure threshold for RIAs at power is the same as that for RIAs at zero power and that the failure mechanism is the same as that of RIA at zero power. It has been clarified that there is no obvious influence of initial temperature distribution on the fuel behavior during RIAs in case of fresh fuels. The evaluation method of fuel enthalpy with which the fuel failure threshold is described was also studied.

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Creep tests of Hastelloy XR (a modified version of the conventional Hastelloy X) were carried out in simulated high-temperature gas-cooled reactor (HTGR) helium at 800, 900 and 1, 000°C. The test results up to about 50, 000h showed no significant degradation in creep properties. The creep-rupture strength obtained through long-term tests was above the level corresponding to the design allowable creep-rupture stress of the High-Temperature Engineering Test Reactor. The values of the stress exponent were 4.5 to 5.7 when the stress dependence of the steady-state creep rate was expressed in terms of the Norton equation. It is judged that dominant creep process is dislocation creep. Rupture lives could be estimated with sufficient accuracy using Larson-Miller parameter. Carburization during creep in simulated HTGR helium did not degrade creep properties of Hastelloy XR. Internally formed cracks were initiated at sites of precipitates at grain boundaries, growing nearly perpendicular to the stress axis. Two phases, Cr-rich carbide and Mo-rich carbide, co-existed in specimens after long-term creep tests.

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The effects of equal ∑Ai for different combinations of the annealing temperature and annealing time on corrosion resistance and evolution of precipitates of Zircaloy-2 were investigated.
Nodular corrosion resistance in the out-of-pile corrosion test was degraded with increasing ∑Ai only when it was increased by extending the annealing time at 894K but did not depend on ∑Ai which was increased by raising the annealing temperature for a constant annealing time of 2.5h. Extensive observation and micro-analysis of precipitates by analytical electron microscope (AEM) suggested the cause of degradation of nodular corrosion resistance to be the remarkable increase in volume fraction of Si-containing precipitates such as Zr₃Si and Zr₂Si, which were observed more frequently in large ∑Ai only when it was increased by extending the annealing time at 894K.
On the other hand, uniform corrosion resistance was improved with increasing ∑Ai irrespective of the annealing conditions, but was slightly better when ∑Ai was increased by extending the annealing time under constant annealing temperature at 894K. This behavior can be understood from the reduction in content of alloying elements such as Fe, Cr and Ni which remained as solutes in the matrix at large ∑Ai.

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Numerical simulation of the electrotransport of fuel actinides, minor actinides and rare earths in an electrorefiner for pyrochemical reprocessing of a typical spent metallic fuel has been attempted based on an improved thermochemical model developed for application to a multicomponent system in the electrorefiner. Variation of the decontamination factors (DFs) for minor actinides and rare earths and minor actinides fraction in the cathode product (fMA_c) as a function of some important parameters such as percentage transport, catholyte concentration, fuel composition (with respect to U and Pu) and batch number (batch processing) are presented and discussed.

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Conversion reactions of metal chlorides into oxides with boric acid (H₃BO₃) were studied to develop the method for vitrification of radioactive molten salt wastes generated in the pyroreprocessing process. Mixtures of metal chlorides and H₃BO₃ with appropriate compositions in Pt crucible were heated at 1, 000°C for 1h in an electric furnace, followed by rapid cooling to room temperature. The chloride compounds used were LiCl, NaCl, KCl, CsCl, SrCl₂, BaCl₂•2H₂O, and CeCl₃•6H2O. All chlorides other than CsCl were found to be transformed into oxides (borate glass) in the molar ratios of H₃BO₃/chlorides ?? 2. Furthermore, four-component systems, i.e., the mixtures of LiCl, KCl, NaCl (or SrCl₂, BaCl₂•2H₂O, CeCl₃•6H₂O), and H₃BO₃ were also examined in a similar manner. The borate glasses were found to be formed in the molar ratio of H₃BO₃/chlorides ?? 2.

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The exchange reactions of n-Octyl(phenyl)-N, N-diisobutylcarbamoylmethylphosphine oxide (CMPO) in La(III), Nd(III), and U(VI) nitrate complexes with CMPO (La(III)-, Nd(III)-, and U(VI)-CMPO complexes) have been studied in CD₃COCD₃ by means of ³¹P NMR method. The number of CMPO coordinated to the first coordination sphere of La(III) ion was directly determined to be 3 by the area integrations of ³¹P NMR signals of free and coordinated CMPO molecules. The same coordination number of 3 was also obtained for the U(VI)-CMPO complex. The coordination number was not determined for the Nd(III)-CMPO complex, because of its paramagnetic behavior. The exchange rate constants of CMPO in La(III)- and U(VI)-CMPO complexes were obtained by the two-site exchange model. Paramagnetic line broadening was observed in the Nd(III)-CMPO complex and the rate constant for the exchange of CMPO was determined by the line-broadening method. The exchange rates of CMPO in La(III)- and Nd(III)-CMPO complexes depend on the free CMPO concentration ([CMPO]), while that in U(VI)-CMPO complex is independent of [CMPO]. The dissociative (D) and dissociative interchange (Id) mechanisms were proposed for the exchange reactions in the La(III)- and Nd(III)-CMPO complexes, and dissociative (D) or Id mechanism was proposed for the U(VI)-CMPO complex. The dissociative rate constants (s^-1) at 25°C and activation parameters ΔH^≠(kJ•mol^-1) and ΔS^≠(J•K^-1•mol^-1) are 4.76×103, 28.7±0.1, -78.4±0.2 for La(III)-CMPO complex, 4.72×10³, 42.6±0.4, -31.7±1.3 for Nd(III)-CMPO complex, and 3.20×10³, 46.9±0.6, -20.5±2.2 for U(VI)-CMPO complex, respectively.

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We describe an applications-oriented method for determining continuum photonuclear angular distributions for emission of light particles (n, p, d, t, ³He, and α) in the quasideuteron regime (40 ?? E_inc ?? 140MeV). Based on theoretical considerations by Chadwick and Oblozinsky for the angular forward-peaking in preequilibrium reactions, Kalbach's 1988 angular distribution systematics for a neutron projectile can be straightforwardly modified for use in photon-induced reactions. This results in photonuclear angular distributions which are less forward-peaked than their nucleon-induced counterparts, due to the small momentum carried by a photon. Our predictions are compared against double-differential monochromatic ¹²C (γ, xp) data at 60 and 80 MeV and are seen to describe the measurements fairly well.

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A new method for controlling axial spatial xenon oscillations in large PWRs has been proved to be effective. The method is based on two additional newly defined axial offsets, AOi and AOx together with the conventional axial offset of power distribution AOp. AOi and AOx are the axial offsets of power distributions which would give the current iodine and xenon distributions under equilibrium conditions, respectively. Further efforts have been continued to establish a new control concept which can give continuous and clear guidance on a display to the operator for xenon oscillation control. The new concept uses information from AOi, AOx and AOp to display the trajectory of (AOp-AOx, AOi-AOx) in the X-Y plane. The trajectory shows a very characteristic behavior. With these simple characteristics in mind the xenon oscillation can be controlled quite easily. In other words, to eliminate the xenon oscillation, we watch the trajectory and use the control rods to lead the plot to the origin where three AOs are identical.
Verification of the concept was carried out based on the full record of the axial offset during a xenon oscillation in an actual reactor and good results were obtained.

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Sensitivity analysis of model output is relevant to a number of practices, including verification of models and computer code quality assurance. It deals with the identification of influential model parameters, especially in complex models implemented in computer programs with many uncertain input variables. In a recent article a new method for sensitivity analysis, named HIM^* based on a rank transformation of the uncertainty importance measure suggested by Hora and Iman was proved very powerful for performing automated sensitivity analysis of model output, even in presence of model non-monotonicity. The same was not true of other widely used nonparametric techniques such as standardized rank regression coefficients. A drawback of the HIM^* method was the large dimension of the stochastic sample needed for its estimation, which made HIM^* impracticable for systems with large number of uncertain parameters. In the present note a more effective sampling algorithm, based on Sobol's quasirandom generator is coupled with HIM^*, thereby greatly reducing the sample size needed for an effective identification of influential variables. The performances of the new technique are investigated for two different benchmarks.

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A Separation factor was measured in isotopically selective infrared multiphoton dissociation (IRMPD) of SUBercooled UF₆ in a SUBersonic expansion by multiphoton ionization (MPI) and time-of-flight mass spectrometry (TOFMS). A pulsed free-jet nozzle was used to generate a UF₆-molecular beam seeded in Ar (-10^-7 Torr in UF₆ partial pressure). Two-frequency p-H₂ Raman laser beams around 16μm were used for the dissociation of UF₆ under collisionless conditions in the molecular beam where the flow velocity for UF₆ is about 500m/s. The ²³⁵U/²³⁸U isotopic ratios in nascent UF₅ photoproducts were determined by selective MPI of UF₅ at 532nm followed by TOFMS with a mass resolution as high as 1200. A separation factor of about 2 was observed under the experimental conditions chosen for the demonstration of this method.

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A database of research publication was constructed for the purpose of grasping all of the research activities at the Research Reactor Institute, Kyoto University. The database named KURRIP collects all of the research publications of the Institute by not only its own staff but also visiting scientists. The publications are in the form of original papers, review papers, papers in proceedings, short notes and letters, synopses over 3 pages presented orally at scientific meeting, books and doctoral theses. At present, the KURRIP database contains the information on 6, 210 items which have been published for 30 years since the Institute was established as an interuniversity research institute for joint use of a research reactor and other related large facilities in 1963. By utilizing the KURRIP database, the analyses have been done: (1) affiliation of the authors, (2) kind of publications, (3) classification of publishers, (4) research fields, and (5) experimental facilities. The KURRIP database is now stored in the Data Processing Center of Kyoto University and can be utilized through a computer center at one of the main national universities in Japan.

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