Preliminary studies are made in order to apply an X-ray CT scanner to the measurement of cross-sectional distributions of void fractions in steady state two-phase flows. Using air-water flows in a horizontal U-shaped tube, results of CT scanner measurements are compared with those of the volumetric measurements with quick-closing valves and of hotfilm anemometer measurements. Studied are cross-sectionally averaged void fractions and void fraction profiles. The comparisons give good agreements. The effect of density fluctuations on the reconstruction of void fraction distributions in two-phase flows is also discussed. Consideration of the statistical fluctuation in the projection data for a stationary object gives the sufficient conditions required for the evaluation of the cross-sectional distribution of the scanning time-averaged void fraction in steady state two-phase flows.

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A quantitative evaluation of neutron noise sources in a BWR plant is performed by using a multivariate autoregressive (AR) model. The mechanisms of noise propagation between the principal process signals are first examined, leading to the observations that the conventional AR model requires to be modified by means of an associate matrix in order to identify the source of flow noise. Using such a modified AR model, quantitative evaluation is made of the contribution of flow noise which is distinguished from that of void noise. Local neutron noise sources are also discussed, based on observations covering local power range monitor signals. These results indicate that flow noise, as well as void noise, contributes significantly to neutron fluctuation. The modified AR model is thus shown to be most effective for estimating the noise sources of neutron fluctuations.

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Reports had been made of a spark counter and this has now been used in the studies of detection and counting of fission fragments, incident on an SSNTD at various angles of incidence, from a 252Cf source. The effect of their straggling in air has also been observed with the use of this counter in the determination of the number of fission fragments. The implications of these studies may be useful in the measurements of total fission events occurring from a nuclear reaction of a heavy element which is of some interest to the reactor and applied nuclear physicists.

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The penetration behavior of Cs was studied in the oxide layer growing on SUS-316 stainless steel for a cladding material of fast breeder reactors under the condition of an oxygen potential controlled by a Ni/Ni0 oxygen buffer in the temperature range of 600750°C, using radioactive ¹³⁴Cs as a tracer. Two different kinds of oxide layers grew on the specimen. Outer Fe-rich and inner Cr-rich oxide layers were formed above and below the original surface of the specimen, respectively.
The penetration behavior of Cs in both oxide layers was determined by measuring the residual radioactivity on the surface of the specimen after successive sectioning. The residual radioactivity showed no change with the depth in the outer oxide layer, which means very high permeability of Cs through grain-boundaries or pores in this layer. The residual activity decreases with the depth in the inner oxide layer. Apparent diffusion coefficients were determined from this profile and their magnitude was two or three orders larger than the one of diffusion coefficients of ¹³⁴Cs in the stainless steel. From this fact, it was suggested that the oxide layer is not protective against the penetration of Cs. In addition, some considerations were given to the mechanism of cladding corrosion.

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On the evaluation of the performance of the hydrophobic Pt catalyst packed in the trickle-bed test column, the conventionally defined (K_ya) and the newly defined (K _f ) _G are compared with each other as a measure of the overall D-transfer coefficient. The value of (K_ya) varies in a wide range in accordance with the length of the test column. On the other hand (K _f )G t=l has a finite value in the test column longer than about 0.5 m. By considering the values of k _g and k_l which are the constituents of (K _f ) _G , it is possible to improve the hydrophobic Pt catalyst trickle bed and to design the H₂/H₂O-isotopic exchange trickle-bed column packed with this catalyst.

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A natural language retrieval program NATLANG is developed to assist in the retrieval of information from event-and-cause descriptions in Licensee Event Reports (LER). The characteristics of NATLANG are (1) the use of base forms of words to retrieve related forms altered by the addition of prefixes or suffixes or changes in inflection, (2) direct access and short time retrieval with an alphabet pointer, (3) effective determination of the items and entries for a Hitachi event classification in a two step retrieval scheme, and (4) Japanese character output with the PL-1 language. NATLANG output reduces the effort needed to re-classify licensee events in the Hitachi event classification.

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A time-domain stability analysis program for boiling water nuclear reactors (BWRs) has been developed and applied to analysis of a commercial size BWR. The program takes into account parallel channel effects.
The model incorporates (a) one point neutron kinetics with weighted average reactivity feedback, (b) radial heat conduction and transfer in fuel rods, (c) fuel channel thermal hydraulics with quasi-equilibrium subcooled boiling approximation, and (d) recirculation hydrodynamics.
Nonlinearity and parallel channel effects are examined through analyses of a commercial size BWR. Core behavior has been found virtually linear for small but finite amplitude oscillations, which proved the validity of frequency-domain stability analyses for finite disturbances. It has also been found that single channel analyses with core averaged thermal hydraulic properties give more stable results than parallel channel analyses.

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