An approach has been tried to provide more realistic estimates of pressure history and mechanical work caused by fuel-sodium interaction (FSI) in postulated fast reactor accident. The work reported here is specifically focussed on the evaluation of the effects due to fragmentation process or finite mixing time and presence of non-condensable gas on FSI.
As one of the methods to describe the fragmentation process in FSI, the mass of fuel which interacts with sodium has been introduced as a function of time. The presence of non-condensable gas in the unheated sodium regions above and below the interaction region as well as in the interaction region itself has been taken into account in the model, which affects value of sonic velocity in the former, and volume change in the latter.
An extensive parametric study has led the conclusion that the total mechanical work as well as a value of peak pressure at the initial period in which sodium remains liquid strongly decrease with the mixing time constants and/or non-condensable gas content in the system, and in the most plausible case, a possible maximum value of peak pressure through the entire period in the fuel-sodium interaction appears to be at most a few hundred atmospheres.

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Intensity and power spectral density of acoustic emission in subcooled nucleate pool boiling of water from boiling inception to the heater burnout was investigated experi-mentally. Platinum and Ni-Cr wires with dia. 1.0 mm or smaller were used as the heater. The measurements were performed in containers at atmospheric pressure and in a dam at pressures up to 3 atm. In the former case photographic observations of boiling bubbles were also made.
The acoustic emission in boiling had frequency components up to and sometimes beyond 50 kHz ; the shape of power spectral densities behaved differently for heat fluxes above and below a certain value at which the overall acoustic intensity assumed a maxi-mum value. With the increase in heat flux the acoustic emission increased in high fre-quencies below this heat flux, while it became eminent in low frequencies above the same heat flux. This phenomenon is related to the transition from the region with single bubbles to the region with coalesced bubbles. Resonance-like peaks found in the measurements with containers that may depend on the container geometry were not observed in the measurements in the dam and smoother power spectral densities were obtained.

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The energy dependent Doppler factor of resonance absorption in UO₂ was measured in the region of neutron energy between 1 eV and 10 keV using a UO₂ cylindrical sample and a lead slowing-down time spectrometer.
The Doppler factor is herein defined as the ratio of neutron capture rate in the sample at the high temperatures T = 450 and 300°C to that at the low temperature T = 20°C. The Doppler factor measured at T = 450 and 300°C showed the maximum values of 1.225 ± 0.044 and 1.186 ± 0.040, respectively around the resonance of 66.3 eV. In the energy region above about 200 eV, the Doppler factor decreased with increasing neutron energy. In the energy region below about 70 eV except for region around the resonance of 6.68 eV, the Doppler factor decreased with increasing neutron energy.
The energy dependent Doppler factors were calculated by the collision probability method and the resonance parameters of ENDF/B-III. In the energy region of 1454 eV, the calculation agreed with the experiment within the error of 5.4% which was not unreasonable when one considered the experimental errors of 3.14.7%. In the energy region of 454 eV3 keV, the calculation was systematically 16.5% larger than the experiment, though the experimental errors were 23.6% in this energy region.

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The energy spectrum of neutrons in a pile of thoria was measured from several keV to several MeV by the time-of-flight method with an electron linear accelerator and the results were compared with the theoretical ones predicted with three group constants for thorium, (1) constants of Abagyan et al.'s (ABBN), (2) newly produced constants of JAERI-FAST type (NNM) and (3) constants produced from ENDF/B-III in ORNL (DLC-2D).
General agreement between measurements and calculations was seen for all cases.However, (1) the predicted spectra with ABBN and DLC-2D gave less flux by about 30% than the measured in the energy below about 100 keV, (2) similar disagreement was observed only above 10 keV for the spectrum with NNM and (3) the measured spectrum became about 20% lower than all of the predicted above 1 MeV.
In parallel with the spectrum measurement, neutron spatial distribution was meas-ured by the activation method and the results showed spherical symmetry around a photoneutron target at the center of the pile. From this fact adequancy of using one-dimensional transport code (ANISN or DTF-IV) with spherical symmetry for analysis could be verified.

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Iodine stress corrosion cracking of Zircaloy-2 cladding under static tensile stress has been studied over the temperature range of 250450°C. The lowest iodine concen-tration required to cause cracking was 0.15 mg iodine per cm³ free volume in test ampoule at 300°C, and 1.1 mg/cm³ at 350°C. The minimum circumferential plastic strain to cause failure was about 0.4% for specimen tubes possessing high cracking susceptibility. Softer tubes tolerate more circumferential deformation before they failed. This is attributed to greater margin retained for work hardening, rather than to smaller residual tensile hoop stress. Cracking was also inhibited by the presence of oxygen in the iodine atmosphere, the threshold partial pressures being 28 torr at 300°C and 9 torr at 350°C. Scanning electron micrographs of the fractured surfaces revealed brittle transgranular fracture carrying river patterns on part of the cleavage facets.

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In order to improve the reliability of operation and to confirm how the operating conditions, especially the sodium flow rate, affect the measuring accuracy, an automatic plugging indicator was designed and tested. This indicator has specially shaped orifices by which the perfect choking of impurity is prevented from the orifice passage.
By the experiment on its characteristics, the followings have been confirmed.
(1) A stable automatic operation is obtained successfully by the plugging characteristic of the orifices.
(2) The optimum operating region of the sodium flow rate to improve the measuring accuracy is obtained.

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