A new model of the transition mechanism from stratified gas-liquid two-phase flow into a slug pattern in horizontal circular tubes was developed. This model incorporated the contribution of liquid kinematic energy to wave growth on the interphase surface. The transition limit was numerically determined so as to allow a wave crest to reach the top wall of the horizontal tubes.
Air-water experiments were performed to obtain data for comparison with model predictions. Three test sections were used. They had different inner diameters and the largest test section had a rod bundle inside it. Predictions of liquid flow rate at the flow pattern transition boundary, with given gas flow rates, were within data scatter of the experiments. This suggested that the present model satisfactorily described effects of tube size and internal structure on slugging onset in horizontal circular tubes.

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For gaining basic data of post-accident heat removal, coolability of degraded core debris bed was studied with emphases on the effects of particle mixing and coolant flow, as a sequel to the effects of system pressure and particle size in Part ( I ). A fuel debris bed with decay heat was simulated by steel ball particles induction-heated in a water-filled 50 mm I. D. pyrex glass cylinder. The system pressure was fixed at 0.1 MPa and the bed height at 8 cm. The particle diameters were 3.0, 2.0, 1.0, 0.7 and 0.3 mm. The mixture of 1.0 and 3.0 mm or 1.0 and 2.0 mm particles was used as the simulate debris bed.
The dryout heat flux for a mixture bed agreed fairly well with the Lipinski's model when a simple average diameter weighted by number was taken as the equivalent diameter, the same as for homogeneous beds. This means that small size particles with large number govern the dryout. The dryout heat flux increases with mass flux of coolant flow, asymptotically approaching to the complete vaporization of coolant. Coolability of debris bed can be greatly improved by a small amount of flow, which is produced by natural convection due to boiling, when coolant can flow into the bed from the bottom.

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A computer code SEFDAN is developed for one-dimensional thermal-hydraulics in a partially uncovered core of a light water reactor during a severe core damage accident. The developed models include :
(1) Froth level (or dry-out level) calculation (2) Transition and mixing between convection flow regimes in convective heat transfer
(3) Radiant heat transfer between solid walls and flowing gas
(4) Heat generation by zirconium-water reaction
(5) Crucibilization effect of zirconium-oxide layer
(6) Steam starvation effect on zirconium-water reaction.
This code does not calculate motion of fuel rod material but predicts the beginning of relocation. The major affecting models, froth level calculation model, heat transfer model and crucibilization model, are verified through analyses of experiments. This code can be used for thermal hydraulic analysis of a severe accident and fuel damage experiment until significant material relocation occurs.

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Microspheres of (Th, U)O₂ with 040% U and with 0.31.3 mm diameter are produced through gelling source-sol drops in CC1₄-ammonia media, and also their characteristics are measured.
Their sphericity becomes worse with increasing U content or with decreasing colloid fraction of source sols ; this is due to greater shrinkage which occurs anisotropically on the gelation. So, acceptable microspheres are obtained for U content up to 35% but not for 40% U. The shrinkage also makes microsphere surface rougher for higher U content ; this is clarified by comparing their BET and geometrical surface areas and by observing them with a microscope. Their apparent and bulk densities are measured and compared. Microspheres are easily sintered up to 98% T. D. for mixed oxides but not for ThO2; densities of the latter are scattered from 87 to 99% T. D. For all including ThO₂ with such low densities, open porosity is less than 1% T. D.

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Irradiation temperature dependence of void swelling, and the effects of aging before irradiation on void swelling and on precipitation were examined in three Ti-modified austenitic steels called No. 1 (Fe-14Ni-16Cr-1.8Mn-2.4Mo-0.1Nb-0.1Ti-0.9Si-0.07C), No. 2 (Fe-14Ni-15Cr-1.6Mn-2.6Mo-0.24Ti-1.0Si-0.06C) and No. 3 (Fe-25Ni-15Cr-1.6Mn-2.4Mo-0.4Ti-1.0Si-0.06C ). After irradiation to 10 dpa with 150200 keV proton, the cold worked No. 1 showed swelling peak of 0.4% at 823 K, the cold worked No. 2 0.15% at 773 K and the cold worked No. 3 0.08% at 723 K. Sample Nos. 2 and 3 which were aged at 923 K for 5.4 Ms (1, 500 h) showed larger void swelling because of higher void number density than the unaged ones after the irradiation to 2040 dpa at 923 K. The amount of intragranular TiC precipitates in the irradiated specimens did not vary significantly with preirradiation treatments. Voids were mostly attached to eta (M₆C) phase which was produced during irradiation. The number density of eta phase produced during the irradiation in aged specimens was much higher than that in unaged ones. This is thought to give a main reason why void swelling in aged specimens was larger than that in unaged ones.

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The catalytic effect of six metallic impurities on the oxidation reaction of cation exchange resin was investigated. The impurities, which were originally adsorbed onto the resin by an ion exchange method, combined with S present in the functional sulfonic acid group during heat-treatment in a nitrogen atmosphere to give the metal sulfides. The sulfides were subsequently oxidized to metal oxides.
In the case of Pd²⁺, Cu²⁺, Fe²⁺ and Fe³⁺ impurities, their sulfides were easily converted into oxides, which catalyzed the resin oxidation. Their catalytic activities depended on the heat of formation of the catalyst oxides ΔH'_o; the lower was ΔH'_o, the more active the catalyst was. However, Co²⁺ and Ni²⁺ impurities had relatively low catalytic activities. This was because Co and Ni sulfides slowly changed into oxides, and the main chemical compositions of the impurities were not oxides but sulfides, which had no catalytic effect on the resin oxidation.

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A centrifuge essentially consists of a rotor rotating at very high speed. Gas tube assembly, located at the center of the rotor, is used to introduce feed gas into the rotor and remove product and waste streams from it. The gas tube assembly is thus a static component, the product and waste scoops of which are lying in the high pressure region of a fluid rotating at very high speed. This can cause flow induced vibrations in the gas tube assembly. Such vibrations affect not only the mechanical stability of the gas tube assembly but may also reduce the separative power of the centrifuge. In a cascade, if some of the centrifuges have gas tube vibration, then cascade performance will be affected.
A theoretical analysis of the effect of waste tube vibrations on product and waste flow rates and pressures in the centrifuge is presented. A simple stage consisting of two centrifuges, in which one has tube vibration, is considered for this purpose. The results are compared with experiment. It is shown that waste tube vibration generates oscillations in waste and product flow rates that are observable outside the centrifuge.

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Cellulose nitrate films were examined for thermal neutron monitoring in contact with several kinds of (n, α) converter materials such as a plastic sheet doped with 1% boron, a solidified boron oxide plate, a single crystal plate of boron oxide and that of lithium fluoride. After thermal neutron irradiation in a research reactor, the films were etched in alkaline solution. The etch-pits on the films were counted with a spark counter. The single crystal converter of lithium fluoride was found to be the most sensitive for thermal neutron. A method for producing the single crystal converter was described. Linear relation between thermal neutron fluence and spark counts was found to be in the fluence range of 2 ×10⁶ 3 × 10⁷ n/cm² in case of using a lithium fluoride single crystal converter.

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General NXTEVT's (next event surface crossing estimators), which have no restriction for detector location and orientation setting, have been developed for the Monte Carlo method of radiation transport calculation. The detectors are of ring, spherical band, cylinder and rhomboid geometry. The detailed formulas of flux scoring are described for each detector geometry. Two sample problems are analyzed in order to study the applicability of NXTEVT's to radiation shielding analysis. First, through the analysis of a simple one-velocity problem for an infinite isotropic scattering medium with an isotropic radiation source, using the spherical band type NXTEVT, the accuracy of the results is verified. Second, a problem of neutron transport in the system with an axial symmetric geometry is solved by using the cylinder and ring type NXTEVT's. The results shows NXTEVT is more efficient to obtain the solution in such a system than the point detector and boundary crossing estimators.

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