Integral blowdown refill/reflood tests have been made at the Two Bundle Loop (TBL) to clarify the BWR safety margin for the Emergency Core Cooling System (ECCS) against the recirculation line break. The tests were based on the scaling considerations of the blowdown process as affected by stored heat release from the vessel structures. The dis-cussion led to a scaling compromise of the nozzle diameter increment of the automatic de-pressurization system to compensate for the excess stored heat release in the TBL. Test results, taking the break size as a parameter, were given of a single-ended recirculation line break under a single failure of the high pressure core spray system. The comparison of the results with the previous study indicated that the uncovery period of the bundle during blowdown had a peak in the small break test of AB/V=-1.0×10^-5 m^-1 (AB: Break area, V : Vessel volume). Based on the scaling principle developed, the uncovery period of heater bundles was corrected. From the estimated uncovery period and the regression line which correlated the Peak Clad Temperature (PCT) with the integrated decay heat energy during uncovery, corrected PCT was found to be more than 500°C lower than the PCT criterion of 1, 200°C.

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When subcooled water in accumulator tanks is injected during a loss-of-coolant accident of a pressurized water reactor, violent condensation takes place in cold legs because of direct contact of flowing steam with the injected water. An experiment is performed to determine the condensation and mixing processes of steam and water under oscillatory conditions. The observed flow patterns are classified into three groups and boundaries of these flow patterns are correlated in terms of the condensability of the injected water. The mutual relation of the condensation phenomena to the mixing behavior of steam with water are studied by applying synchronous measurements of pressures to flow observation with high speed movie. The test results confirm that the condensation is a driving force to produce the restoring force to the plugging water motion. Good consistency is observed between the condensation efficiency and the mixing pattern of steam with water. A correlation for the oscillation period is developed which agrees with test data within an error of ±20%. The correlation shows that the oscillation period is proportional to the square root of the plugging water mass.

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The selective removal and fixation of Cs and Sr have been studied in zeolite A and chabazite. Cesium ion was preferentially distributed into chabazite with a high distribution coefficient (K_Cs>10³ cm³•g^-1) in the presence of NaCl (10^-1 mol•dm^-3). The Ksr values for zeolite A attained about 10³ cm³•g^-1 in the pH range of 810, and they gradually decreased with an decrease in pH.
The initial rate of Cs adsorption was fairly fast in chabazite, and the adsorption ratio reached almost 100% within a few hours. The adsorption ratio of Sr in binderless A zeolite reached almost 100% after 15 h. The adsorption of Cs and Sr on these zeolites was followed by Langmuir-type isotherm. Cesium forms of these zeolites recrystallized to pollucite (CsAlSi₂O₆) above 900°C for zeolite A and above 1, 200°C for chabazite. As for Sr forms, these zeolites changed to SrAl₂Si₂O₈ above 900°C.
These recrystallized phases were suitable hosts for the immobilization of Cs and Sr in the nuclear waste solutions.

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Data obtained from continuous on-line monitoring system using NaI (Tl) scintillation counter for a period of 7 years at a number of sites in Fukui Prefecture in Japan have been used to study the seasonal variations of the specific activity of the short-lived radon daughters (RaB and RaC) in rainwater. The levels of the specific activity of rainwater tend to show the seasonal variations, in which the levels in winter are higher than in summer. The specific activity in each rainwater is a decreasing function of the rainfall rate, and in the case of the same rate of rainfall the specific activity of rainwater in winter is higher than in summer. From these observations, the seasonal variations of the specific activity of Ra(B+C) in rainwater have been treated as the phenomena which depends upon the seasonal variations of both rainfall rate and the concentration of radon (²²²Rn) in cloud. Namely, we have assumed the two factors on which the specific activity depends. One of them is the rainfall rate, the other is the concentration of radon in cloud.
It is concluded that the seasonal variations of the specific activity of rainwater depends almost upon the variations of the concentration of radon in cloud, and the rainfall rate de-pendency is 2540% of the total variations.

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Simulation tests of a BWR loss-of-coolant accident (LOCA) by a postulated full size, double-ended break of a recirculation suction line were conducted using the Two Bundle Loop (TBL) with two full size, electrically heated bundles. In the tests, single failure of emer-gency core cooling systems (ECCSs) was assumed. The following results were obtained from the tests :
(1) The rods heated up early in one of the two bundles, while heatup initiations were delayed in the other. The difference was observed for any ECCS activation mode.
(2) The peak cladding temperature (PCT) of each bundle was expressed as a linear func-tion of integrated power during the heatup period.
(3) The highest PCT in the tests was 861 K under the assumption of an HPCS (high pressure core spray) failure, which was well below the licensing limitation of 1, 473 K.

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Bending and compressive strengths, and Young's modulus were measured for Pechiney nuclear grade graphite irradiated in the temperature range 220400°C in the environment of CO₂ in a commercial reactor, up to the neutron fluence 6.2×10¹⁹ and 2.2×10²⁰ n/cm² (E>0.85 MeV), respectively.
All of them increased owing to neutron irradiation, and the changes in both strengths were almost similar in the whole range of irradiation temperature, however the changes in Young's modulus depended on irradiation temperature.
It was clarified in the present experiment that both strengths were related with Young's modulus and the relation could be expressed by the formula σ=kEⁿ, where σ and E are strength and Young's modulus, respectively, and n is constant which has different value for bending or compressive strength and also for their measured direction.

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An effective homogenization of control rods is studied by considering neutron leakage as a fixed boundary source. Good numerical convergence is seen in the iterative calculation of effective homogenized cross sections that preserve integrated reaction rates. Usefulness of the present method is demonstrated by treating off-center control rods in a one-dimensional slab model of an FBR.

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