The physics characteristics of large axially heterogeneous liquid-metal fast breeder reactors (LMFBRs), particularly the parameters for use in design and safety assessment, were examined using the JAERI fast critical assembly facility, arranged in Assembly XII-1, a partial mock-up of axially heterogeneous LMFBR. The properties measured were (1) criticality, (2) reaction rates and reaction rate ratios, (3) material sample worths, (4) sodium-void worths and (5) B₄C control rod worths.
The results were compared with those of prior experiments with assemblies representing conventional homogeneous core. Confirmation was obtained of the typical nuclear characteristics attributed to axially heterogeneous LMFBRs, including flattening of the axial distribution of power and of the differential worth of control rod, as also lower sodium void worth.
Theoretical analyses paralleling the experiments, using JENDL-2 cross section library and JAERI standard calculation code system for fast reactor neutronics, resulted in some discrepancies, particularly for the internal blanket, in respect of plutonium sample worth, fission rate and fission rate ratio.

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In order to examine the correlation of fusion and fission neutron damage in silicon semiconductor devices, silicon bipolar transistors were irradiated with fusion neutrons from a D-T neutron source OKTAVIAN and fission neutrons from a ²⁵²Cf neutron source. The degradation of common emitter current gain (h_FE) of the transistors was measured in-situ during neutron irradiation. The change in reciprocal current gain increased proportionally with neutron fluence, which determined neutron damage constants for the transistors. The correlation factor of fusion and fission neutron damage in the transistors was determined from the ratio of fusion and fission neutron damage constants and was found to be 1.61.7. We also calculated the rate of fusion and fission neutron displacement damage for silicon by using the damage energy formula based on the Lindhard theory and the ENDF/B-IV neutron cross section data. The correlation factor of fusion and fission neutron damage from the calculation agreed well with that from the transistor irradiation experiments.

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Isothermal oxidation tests of Zircaloy-4 have been performed in steam/argon mixtures at the temperatures of 1, 0001, 490°C to study the oxidation behavior of fuel cladding under severe accident condition in LWRs. The change in mass increase of oxidized specimens was related to the concentration of steam in the atmosphere, f(H₂O) (^mol/_O). Thereby the critical value of f(H₂O) above which the mass increase settles on a constant level was estimated at each test temperature. It was of the order of several percent, increasing with the temperature. This showed that the steam starvation for the Zircaloy-steam reaction took place only in the extremely diluted condition. Under very low steam fraction condition below the critical value, the rate determining step of the reaction was the surface processes which allowed oxygen to reach Zr0₂ surface. High concentration hydrogen of several hundred wt. ppm could be absorbed in Zircaloy during oxidation in the steam/argon mixture of very low steam fraction.

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Formation of NiFe₂O₄ from iron hydroxides. and an oxide has been studied in high temperature water by in situ measurements. The five kinds of Fe compounds used in the experiments are known as components of Fe crud in BWR condensate water. Among the reactants and products, only NiFe₂O₄ shows ferromagnetism. An in situ method, using a magnetic balance, was developed to measure the mass of NiFe₂O₄ in high temperature water. The lower detection limit of this method is 0.01 mg of NiFe₂O₄ with a maximum detection error of ±2%. The iron hydroxides react with Ni(OH)₂ to form NiFe₂O₄ and α-Fe₂O₃ through a dehydration reaction in 553 K water. Amorphous iron hydroxide and γ-FeOOH react rapidly, reaching saturated values of the conversion to NiFe₂O₄ within a few hours at 553 K. The dissolved oxygen has no influence on the NiFe₂O₄ formation. The amount of generated NiFe₂O₄ decreases with the reduction of the pH value, due to dissolution of the reactant. The rate of NiFe₂O₄ formation increases with reaction temperature, while the saturated value of formed NiFe₂O₄ decreases.

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The radical reaction mechanisms in the presence of F-atom scavenger gases were investigated in the p-H₂ Raman laser-induced infrared multiphoton dissociation (IRMPD) of gaseous ²³⁶UF₆/²³⁸UF₆ cooled to -35°C in a static gas cell. When CH₄ was added as a scavenger of F-atoms produced via IRMPD of UF₆, the dissociation rate of UF₆ became several tens of times larger than when no scavenger gas was added. Gas-chromatographic analysis revealed that as low as 7% of the nascent CH₃ radicals were involved in the radical reaction with UF₆. On the other hand, H₂ and C₂H₆ were found to increase both the dissociation rate of UF₆ and the contribution of this non-selective reaction. These results agreed with those obtained in the UV photolysis of UF₆ with scavengers.

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Existence of the structure water in the hydrophilic porous substrate can give various effects on tritium behavior in a tritium recovery system consisted of precious metal catalyst bed and adsorption bed because the catalytic isotope exchange reaction or the isotope exchange reaction proceeds besides oxidation reaction or adsorption reaction.
Properties of Pt catalysts loaded on various hydrophilic porous substrate are compared in this study. It is known from experiment that Pt-Alumina catalyst has the largest exchange capacity and that Pt-MS 5A catalyst has the largest total capacity because it has the effective adsorption capacity. For silica gel substrate, certain transport reaction of tritium in the substrate whose rate is rather slower than the rate of isotope exchange reaction is observed and it gives peculiar undesirable quality for silica gel to use in a tritium recovery system.

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A solvent wash procedure employing tetramethylammonium hydroxide (TMAH) as an alternative to sodium carbonate was investigated in order to minimize the volume of radioactive waste resulting from solvent regeneration in the Purex process. The wash efficiency of tetramethylammonium base solutions was measured in comparison with sodium carbonate solution on a 30% TBP-70% dodecane mixture containing zirconium-dibutyl phosphate complex. The experimental evidence showed tetrametylammonium base solutions to be as effective as sodium carbonate solution. In particular, addition of oxalic acid which complexes strongly with zirconium enhanced the wash efficiency. The ³¹P NMR spectra of dibutyl phosphate (DBP) indicated that the cleanup of DBP depended mainly on ionization of DBP. As tetrametylammonium hydroxide can be decomposed by heating at about 200°C, it is considered to have good applicability as a salt-free wash reagent.

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A new chair type whole-body counter has been developed for rapid mass monitoring of workers for internal exposure, to be used in projects involving work risking radiation exposure by a large force of workers, such as the reconstruction and decommissioning of reactors at the Japan Atomic Energy Research Institute (JAERI). The new counter is capable of discerning from 2-min count a body burden corresponding to a recording level of internal doseequivalent to 0.2 mSv committed dose for nuclides commonly encountered in projects such as cited above. The counter incorporates two Nal (Tl) detectors (20 cm dia. × 10 cm) housed in a collimator. The upper detector covers in its field the lower half of the head and chest, and the lower detector the trunk. The chair is installed in a shielding cubicle of 5 cm thick lead in the main part and open toward ceiling and at entrance. Data processing is completely computer-automated Calibration experiments using plastic phantom containing homogeneous radioactive solution have proved the counter to be capable of descerning in 2-min counting time a ⁶⁰Co activity of 80 Bq contained in the trunk, distinguishing it above 3 times standard deviation of background count rate.

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One of the safety aspects of innovative reactor concepts is the integration of steam generators (SGs) into the reactor vessel in the case of the pressurized water reactor (PWR). All of the reactor system components including the pressurizer are within the reactor vessel in the SG integrated PWR.
The simple heat transfer code was developed for the parametric study of the integrated SG. The code was compared to the once-through 19-tube SG experiment and the good agreement between the experimental results and the code predictions was obtained. The assessed code was used for the parametric study of the integrated once-through 16 m-straight-tube SG installed in the annular downcomer. The proposed integrated SG as a first attempt has approximately the same tube size and pitch as the present PWR and the SG primary and secondary sides in the present PWR is inverted in the integrated PWR. Based on the study, the reactor vessel size of the SG integrated PWR was calculated.

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