Spallation reaction calculation based on the intranuclear-cascade-evaporation model is modified to take into account the pre-equilibrium process by using a closed form exciton model. For the double differential (p, xn) reaction cross section, the calculation of the exciton model is devised to produce smooth connection of that the cascade process. The result of the exciton model calculation is compared with the neutron energy spectra decomposed from the experimental data by the moving source model analysis. Two parameters for the transition probability of the excitons and the termination conditions of the pre-equilibrium process are adjusted to reproduce the experimental data. The addition of pre-equilibrium process into the intranuclear-cascade- evaporation model improves the accuracy of the calculation code. The improvement is signifi- cant in the backward emission at incident energy between 20 and 100MeV for target nuclei more massive than aluminum.

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Motion of off-centered beams in an RFQ for basic researches on nuclear transmutation was analyzed by using a beam optics computer code. In this analysis not only higher-order harmonics in the intervane potential but also 3D beam-generated forces including image effects were taken into account. Beam loss property and transmission efficiency were investigated for different input beam currents as well as for different injection angles. Results are compared with predictions by the PARMTEQ code. Enhancement of beam displacement at the exit of the cavity as well as increase in transverse beam loss was observed as effects of the image charge force. In any case beam loss due to the image effects was less than 3% of the injected particles. For beam injection angles larger than 1°, the beam transmission efficiency decreased owing to non-linear terms in the intervane potential. Tolerance for the error in beam injection angle into the RFQ cavity is also discussed.

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An integral test of γ-ray production data of iron in the latest version of Japanese Evaluated Nuclear Data Library (JENDL-3.2) has been performed by means of a shielding benchmark analysis of KfK leakage neutron and γ-ray spectrum measurements from iron spheres with a ²⁶²Cf source in the center. Two comprehensive systems which consist of a continuous-energy Monte Carlo method and a multi-group Sn transport method have been adopted in this benchmark analysis. For comparison, analyses with JENDL-3.1, FENDL-1 and ENDF/B-IV have been also carried out. The calculation using JENDL-3.2 showed a good agreement with the experiment. It has been concluded that the γ-ray production data of iron in JENDL-3.2 were applicable for use of shielding designs and analyses of the fission neutron source problem.

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The moderator temperature coefficient (MTC) of a light water reactor is a very important parameter from the reactor safety point of view, because it plays a role in providing negative feedback during power excursions. From this point of view, the MTC, in Japan, is designed to be negative for all operating conditions. In pressurized water reactors (PWRs), the MTC is known to be close to a positive value at beginning of cycle and hot zero power, and it is measured and confirmed to be negative under these conditions at the startup test after each refueling. It takes a few hours to obtain this data, and if possible it would be better to reduce this time in order to shorten the refueling outage time. In this paper we propose a new procedure for measuring the MTC in about a quarter of an hour using Fourier transform of both the reactivity and temperature signals.

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Void fraction measurement tests for BWR fuel assemblies have been conducted as part of a Japanese national project. The aim was to verify the current BWR void fraction prediction method. Void fraction was measured using an X-ray CT scanner. This paper describes typical results of void fraction distribution measurements and compares subchannel-averaged void frac- tion data with current subchannel analysis codes. The subchannel analysis codes COBRA/BWR and THERMIT-2 were used in this comparison. The agreement between data for an actual BWR fuel assembly with two unheated rods was good, but in the case of many unheated rods, the codes were unable to predict well large void fraction gradient in the radial direction observed in the measured data over the unheated rod region. The prediction errors of COBRA/BWR and THERMIT-2 for the subchannel-averaged void fraction were <A> (average of difference between measurement and calculation)=-1.1%, σ (standard deviation)=5.3% and <A>=-2.2%, σ= 6.3%, respectively.

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A wavy-dispersed flow regime was observed between slug and annular-dispersed flow regimes in TPTF high-pressure steam/water horizontal pipe experiments, employing the video probe visual observation. The onset of entrainment was identified to cause slug to wavy-dispersed flow transition. The wavy-dispersed flow regime extended towards lower gas flow rates as pressure was increased. Furthermore, it was found that the gas-liquid relative velocity for the onset of entrainment decreases significantly, resulting in decrease in the minimum void fraction. Consequently, the slug flow regime was found to disappear for pressures above 8.6 MPa, as observed in the previous TPTF experiments. Applicability of available models and correlations on the onset of entrainment was assessed against the TPTF data. Steen-Wallis parameter correlated the data well when the superficial gas velocity term in this parameter is replaced by the gas-liquid relative velocity.

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The effects of polarization in the electrorefining of spent nuclear fuel were analytically studied through electrotransport experiments with uranium. When the uranium concentration in molten salt was set at 0.5wt%, polarization caused the measured cell resistance to increase from 0.16 to 0.33Ω as the cell voltage was raised from 0.1 to 0.7V. At 2.0wt% uranium concentration in salt, on the other hand, the resistance was almost independent of cell voltage. A code named DEVON has been developed to estimate the effects of polarization on the electro- refiner operating condition. The Laplace equation is solved in the bulk salt region by finite element method. In the calculation, the effects of polarization are taken into account by adopting the diffusion layer model on defining the boundary conditions. The result of calculations agreed well with the measured resistance data at two sample concentrations for a diffusion layer thickness of 0.025cm on the solid cathode. The calculations indicated that significant polari- zation at the cathode could be avoided by maintaining the uranium concentration in salt above 1 to 2wt%. When it was held at 5wt%, which is a typical level for normal operation, polarization proved to exert little influence on the cell resistance, but it was indicated to contribute appreciably toward flattering the current distribution along the cathode surface.

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The interaction of iodine species with ⁶⁰Co-irradiated extractants of 30% TBP/70% n-dodecane was examined to infer the behavior of iodine downstream of the dissolution step in reprocessing. When a simulated spent-fuel solution (in which the main iodine species was colloidal iodine) was contacted with the extractant with irradiation dose of 2.8×10²C/kg or less at 25°C, 70 to 80% of iodine was transferred to the organic phase together with 100% of uranium. About 50% of the iodine was retained therein during back-extraction of uranium by water at 60°C and transferred to aqueous phase in the solvent cleaning by alkaline solution. Only 4 to 5% of iodine was finally fixed in the organic phase. When the extractant was irradiated to 10³C/kg or more, a "third phase" appeared in extraction step and the majority of iodine (about 65%) was sorbed in this phase. Over 90% of elemental iodine (I₂) in a UO₂(NO₃)₂-HNO₃ solution was extracted to the organic phase regardless of irradiation dose of the extractant and half of it remained therein after solvent cleaning. Iodate (IO₃^-) in the same solution was extracted increasingly with increasing irradiation dose of the extractant.

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The mesoscale atmospheric dynamic model, a submodel of the numerical atmospheric dispersion model named PHYSIC, was improved and its performance was examined in a coastal area with a complex terrain. To introduce temporally changing synoptic meteorological conditions into the model, the initial and boundary conditions were improved. Moreover, land surface temperature calculations were modified to apply the model to snow-covered areas. These improvements worked effectively in the model simulation of four series of the observations during winter and summer in 1992. The model successfully simulated the wind fields and its temporal variations under the condition of strong westerlies and a land and sea breeze. Limitation of model's performance caused by the temporal and spatial resolutions of input data was also discussed.

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The continuous energy Monte-Carlo/collision probability hybrid method has been developed for efficient burnup calculations of light water reactor fuel assemblies. This hydrid method was applied to the NEACRP LWR fuel burnup benchmark, and the numerical results were in good agreement to those of the reference Monte-Carlo calculations in about 1/5 CPU time compared to the reference one, though there is a large difference between the results of RESPLA⁽¹⁾ (collision probability method) and VIM⁽²⁾ (Monte-Carlo method). Thus this hybrid method is found to be effective for burnup calculations of light water reactor fuel assemblies.

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COSTA (A Computerized Support System for the Emergency Technical Advisory Body in Japan) has been developed by the Japan Atomic Energy Research Institute to provide national response capability during radiological accident emergencies. COSTA can be used to identify plant status and predict accident progression based on emergency data reported by the utility of an affected plant. To assess the capability of COSTA, it was applied to the TMI-2 accident progression in the reactor cooling system during the period that neither the emergency core cooling system nor the auxiliary feed water system was available. Although there are some differences in the steam generator between the TMI-2 plantand Japanese PWR plants, these differences would not significantly affect the accident progression of interest. A case study was also made by altering the amount and content of emergency data, as well as the timing of reporting. This study was performed to investigate the dependency of the COSTA predicted results upon the emergency data. This study showed that COSTA predicted results were reasonable when accurate emergency data were reported a few times an hour.

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Dynamic characteristics of a passive radon monitor which is designed based on the diffusive deposition, was investigated by the use of a silicon semiconductor detector (silicon photo diode) settled in the monitor's interior. For the purpose, time variations of alpha particles emitted from gaseous radon in the monitor and deposited radon daughters on the internal surface were measured under the following conditions: (1) there is no aerosol in the monitor, (2) there is abundant aerosol in the monitor, (3) there is no aerosol but there is electrostatic field due to electric charges, i.e., the internal surface is positively charged and the detector (solid state track detector) is negatively charged. From these experimental results, it was clarified that the influence of aerosol on the behavior of radon daughters depends on the dimension of a monitor, while there is little influence of the electrostatic field in the monitor. Furthermore, the ventilation rate of the monitor, which is an important factor for passive monitors, was also determined directly from the time variation of alpha particles emitted from radon.

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