In order to investigate the nuclear medium effects in the intranuclear cascade (INC) model for the nuclear reaction analysis at incident energies between 20 and 200MeV, the ISOBAR code which treats the reflection and refraction was modified to employ the parametrized in-medium nucleon-nucleon (NN) cross sections and thethreshold energy for (p, n) and (n, p) reactions. It was found through the calculations of the proton inducedreactions on both thin and thick targets of Fe to Pb that ISOBAR with the in-medium NN cross sections gave successful agreement with the experiments for both the neutron and the proton double differential cross sections.In particular, the calculation reproduced the quasi-elastic peak in the neutron energy spectra better than the Bertini-model calculation. Moreover, the accuracy of NMTC/JAERI-MCNP-4A code system for the thick targetcalculation was also improved significantly by including the ISOBAR code using the in-medium NN cross sections.

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To obtain fundamental data for research on the transmutation of nuclear wastes, the thermal neutron cross section and the resonance integral of the ¹²⁹I(n, γ)¹³⁰I reaction have been measured using an activation method. The neutron cross sections for the formation of the ground (5⁺) state and the isomeric (2⁺) state of 130I were measured separately.
Six ¹²⁹I targets were irradiated for 10min with thermal reactor neutrons; three of them containing 2.55-2.61 kBq of ¹²⁹I were irradiated within a Cd capsule, and the other three targets containing 259-261Bq of ¹²⁹I were irradiated without it. The Co/Al and Au/Al alloy wires were used to monitor the neutron flux and the fraction of the epithermal part (Westcott's epithermal index). The gamma-ray spectra from the irradiated samples were measured with a Ge detector.
The thermal neutron capture cross section (the 2, 200m/s neutron cross section) and the resonance integral of the ¹²⁹I(n, γ)¹³⁰I reaction were determined to be 12.5±0.5b and 15.6±0.7b for the formation of the groundstate ^130gI(5+), 17.8±0.7b and 18.2±0.8b for the formation of the isomeric state ^130mI(2+), and 30.3±1.2b and 33.8±1.4b for the formation of ¹³⁰I (the sum of the 2⁺ and the 5⁺ states), respectively. The sum of the thermal neutron capture cross sections forming the 2⁺ and the 5⁺ states was 12% larger than the evaluated values of JENDL-3.2 and ENDF/B-VI and that reported by Roy et al. This discrepancy is explained by the population of the isomeric level.

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In the system analyses of a large-break loss-of-coolant accident (LBLOCA) of pressurized water reactors (PWRs), the TRAC-PF1 code predicted an unrealistic depressurization and required much computational time due to the problem of the condensation model. To eliminate the unrealistic depressurization, the TRAC-PF1 code was improved using a simplified condensation model that determined the total condensation rate at cold leg. Through the assessment calculations for CCTF, UPTF and LOFT tests, it was confirmed that the simplified model could eliminate the unrealistic depressurization and reduce the computational time. It was also confirmed that the model could improve the accuracy of the system calculation for the core inlet flow rate and clad temperature as the result of the elimination of the unrealistic depressurization. It has been concluded that the simplified condensation model is useful for the system calculation of the. PWR LBLOCA.

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The self-triggering mechanism of vapor explosions was investigated analytically and experimentally using molten tin and water. First, we modeled a simple droplet system consisting of a hot liquid droplet in a pool of cold liquid. Then, to model the self-triggering mechanism, we assumed that an instability (i.e. perturbedoscillation) in the vapor/cold-liquid interface produces a collapse of the vapor film, which in turn would produce a vapor explosion. To investigate the stability of perturbed oscillations in a vapor film, we did a linear stabilityanalysis of a vapor film surrounding a hot liquid. We found that there was a region of film stability in the cold-liquid temperature where spontaneous vapor explosions did not occur.
To validate our model, we experimentally determined the thermal interaction zone (TIZ) in which spontaneous vapor explosions occur. The occurrence conditions for spontaneous vapor explosions were investigated for moltentin, as the hot liquid, dropped into a water pool, as the cold liquid. We varied the tin temperature and the water temperature, and then monitored the occurrence and location of spontaneous vapor explosions. We found that the upper limit for the water temperature of the TIZ can be explained by our model.

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This paper deals with the safety design of a light-water-cooled fast breeder reactor operating at supercritical pressure (SCFBR) which has the advantages of high thermal efficiency and simple reactor system. A new computer code was developed and five important flow-induced events were analyzed for the 1, 245MWe SCFBR with two coolant loops.
A core flow rate must be maintained for the safety of once-through type plant. Thus this reactor has two high-pressure auxiliary feedwater, four low pressure coolant injection and automatic depressurization systems.They are activated by the emergency signals of the decrease in coolant flow rate. The feedwater pumps should have fly-wheels for prolonging the coast-down time.
It has been found that the peak cladding surface temperature of fuel rods does not exceed the allowable temperature under the accidents of the total loss of feedwater flow and feedwater pump shaft seizure. The minimum deterioration heat flux ratio satisfies the criterion under the transients of the loss of feedwater heating, inadvertent start of the auxiliary feedwater pump and partial loss of feedwater flow. The loss of feedwater heating causes a flow-induced transient in this reactor, because the flow rate in the downstream decreases with a decrease in coolant temperature.

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A probabilistic safety assessment (PSA) technique was applied to the design of JAERI Passive Safety Reactor (JPSR). A PSA was performed to clarify safety features and identify vulnerabilities of the original design. Based on the PSA results and considering thermal-hydraulic analyses and experiments, the JPSR design was improved to enhance plant safety. The improved design was re-evaluated with the PSA. Initiating events selected in this study were: large-break LOCA, medium- and small-break LOCAs, SGTR, main steam line break, loss of offsite power, loss of feed water, and other transients. Fault tree analyses were used to evaluate the system unavailabilities. The total core damage frequency due to internal events was estimated to be less than 10^-7/RY.The contribution of high frequency non-LOCA events could be significantly reduced by the design modification.The dominant initiating event was the small break LOCA and the dominant sequence was the failure of residual heat removal system. The present study indicated that the improved JPSR design has sufficient safety margin and the PSA methodology is very effective to improve reactor safety systems in a conceptual design phase.

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Selective extraction of uranium from a phosphate are was studied by using the mixed gas of Cl₂ and O₂. On heating the ore and carbon mixture in Cl₂, the volatilized chloride of uranium is accompanied by iron, aluminum, phosphorus and silicon chlorides. The addition of O₂ gas effectively lowered the volatilization ratios of aluminum, phosphorus and silicon. The ratio decreased with increasing oxygen flow rate up to 50ml/min at 1, 223K (Cl₂:100ml/min, O₂+N₂: 400ml/min). The volatilization ratio of uranium was almost unchanged at 90% up to 50ml/min O₂ (carbon amount: 15wt%). The effect of the other parameters, i.e. Cl₂ flow rate, carbon amount, reaction temperature and time was examined.

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Synthetic zeolites AR-1 (a mordenite), 4A and 13X, all in their Na-forms, have been found to be suitable for the uptake of Cs, Sr and Th ions, respectively. Samples of Cs-loaded AR-1, Sr-loaded 4A and Th-loaded 13X were prepared and heated in a furnace to various temperatures (up to 1, 200°C), for fixing the exchanged ions in the host zeolite matrix. X-ray diffraction patterns of the products indicated that the crystallinity of Cs-AR-1 and Sr-4A was intact till 500°C, partially lost between 500°C, and 1, 000°C, and improved again at 1, 200°C. For Th-13X, the crystallinity was gradually lost after 500°C.
Thermogravimetric-differential scanning calorimetric (TG-DSC) studies indicated the occurrence of phase transformations at around 890-1, 020°C, for Na-4A, Sr-4A, Na-13X and Th-13X. For Na-AR-1 and Cs-AR-1, no clear inference of change of phase could be drawn. Above 1, 300°C, all the zeolites converted to a glassy mass.Desorption studies in vigorously stirred liquid media, viz., demineralized (DM) water, 10mM NaNO₃, 1M NaNO, and 2.8M HNO₃, and Static leach tests in DM water and tap water media, with fresh replacement of the water after preset time durations, were carried out with the zeolite samples heated to 1, 000°C. The leach rates at equilibrium were found to be low, of the order of 10^-7g/cm²•d for Cs and Sr and still smaller for Th.

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The isotopic enrichment of ²³²U and ²³⁴U against ²³⁸U in the U(IV)-U(VI) exchange process has been studied by anion exchange chromatography. The enrichment of ²³²U in the uranium adsorption band was monitored by the activity ratios measured by α spectrometry and the isotopic abundances of ²³⁴U were determined by mass spectrometry. The results have shown that the isotopic enrichment is proportional to the mass difference among isotopes of ²³²U, ²³⁴U and ²³⁸U. The anomalous isotope effect previously observed for ²³⁵U was not observed for ²³²U. The ideal separation coefficient, εo (=S-1), between two isotopes of ²³²U and ²³⁸U at the ideal distribution state in the U(IV)-U(VI) exchange system has been inferred as 2.2×10^-3 at 308K.

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Applicability of two kinds of optical model potentials to evaluation of neutron nuclear data for the 1p-shell mass region (⁶Li to ¹⁶O) has been studied in the intermediate energy region (20 to 100MeV). A global optical potential proposed by Walter and Guss, and a microscopic one by Jeukenne-Lejeune-Mahaux (JLM)have been chosen as representatives of the phenomenological and microscopic approaches, respectively. The total and reaction cross sections, and elastic and inelastic scattering angular distributions calculated by using these potentials have been compared with experimental data. It was found that both the potentials give good explanation of the experimental values in spite of the fact that these potentials were not adjusted in this mass region. The present results have indicated that the JLM potential is suitable to the intermediate-energy nuclear data evaluation for the 1p-shell mass region. A qualitative interpretation of the sensitivity of the total cross section to the parameters in the JLM approach has been carried out; it was found that the oscillatory and phasecorrelated pattern of the sensitivity coefficients to the real and imaginary potential strengths obeys a simple sineand cosine-rule with the amplitude decreasing as E^-1/2 as derived by a semi-classical model.

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