Cross sections for the ¹⁹⁹Hg (n, n')^199mHg reaction have been measured at 10 energy points from 0.78 to 6.3 MeV by the activation method. Monoenergetic neutrons below 2 MeV were produced by the ⁷Li(p, n)⁷Be reaction and those above 2 MeV were produced by the D (d, n)³He reaction using a 5.5 MV Van de Graaff accelerator. The neutron flux was determined with a proton recoil telescope counter and In-foils. The measured cross sections were expressed by an empirical formula. The fission spectrum averaged cross section calculated with this formula is 238.3 mb for the Watt-type fission spectrum, and is about 14% smaller than that recently measured by three of the authors.

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The neutron activation cross sections of Mo isotopes have been measured for the 14.8 MeV neutron. The cross sections have been determined with reference to the known ²⁷Al (a, a) ²⁴Na and the ²⁷Al (n, p) ²⁷Mg reactions. The cyclic activation method was employed for the T-ray measurement of short-lived nuclei. A 55 cm³ Ge (Li) detector was used for the measurement of r-ray spectra. Cross section data are presented for (n, 2n), (n, p) and (n, a) reactions on Mo isotopes. The cross sections of (n, np) reactions on ⁹⁸Mo are also presented. The exponential dependence on (N-Z)/A of the (n, p) reaction cross sections are discussed.

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In the "JOYO" initial power ascension tests to 75 MW, unexpected large negative power coefficients were observed during ascension to new power levels beyond 50 MW, which results in approximately a 400 permanent loss in reactivity.
This anomaly was studied from the viewpoint of radial dimensional change in the core using a three-dimensional structural analysis code along with a whole-core thermal-hydraulic analysis code. Conclusions drawn from the study can be summarized as follows :(1) The absolute value of the power coefficient increases sharply beyond the 50 MW power level due to the outward leaning and thermal bowing of core elements, but the calculated increase in the coefficient is less than 20% of the observed value.
(2) At most, an 80 permanent loss in reactivity is caused by this outward leaning, which is 20% of the observed value.
(3) These results confirm that the radial dimensional change can not be the main cause for the power coefficient anomaly.

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For providing background information on the possible vapor explosion in the event of a core disruptive accident of LMFBRs, an experiment was conducted on the film boiling characteristics of liquid metal potassium in association with the Leidenfrost phenomenon. In a steel container filled with Ar gas, K droplets were put on a joule-heated plate of 316-SS or Ta. The behaviors of droplet were observed by a camera and a color VTR through viewports. The experimental conditions were the Ar pressure 1 bar, the initial K temperature 350-760°C, and the plate temperature 9001, 250°C for 316-SS and 1, 1001, 600°C for Ta.
Stable film boiling known as Leidenfrost phenomenon was observed for a high temperature condition of the plate, whereas an instantaneous break-up of droplet with extensive vaporization occurred for a low temperature. The heat transfer characteristics of film and transition boiling regions were obtained by estimating the heat flux from the volumetric reducing rate of droplet due to vaporization. The results in the film boiling region showed an appreciably good agreement with the prediction based on Bromley's expression combined with the theory of Baumeister et al. The minimum film boiling temperature and heat flux were found to be about 1, 300°C and 15 W/cm², respectively, for a droplet size of 0.15 cm³.

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Natural circulation in a PWR has received considerable attention since the TMI-2 accident as an alternative cooling method or recovery technique from certain kinds of accidents or transients involving a loss of pumped circulation. Among the three modes of natural circulation (i.e. single-phase, two-phase and reflux cooling), reflux cooling has not been well investigated in a PWR configuration. The present study was thus focused on reflux cooling of natural circulation and analytical method was developed to estimate the liquid velocity of the condensed liquid in a hot leg of a PWR.
The results of the present study showed that the liquid velocity and the liquid thickness are estimated as 2.7 m/s and 3.0 cm, respectively, at the hot leg inlet from the upper plenum for the typical PWR reflux condition (2% core power at 6.9 MPa). Therefore it was concluded that a flow-blockage of the steam flow from the core by the condensed liquid flow is unlikely to occur in a hot leg. The results are also useful for designing a special instrumentation for measuring the condensed liquid flow rate and the liquid thickness in an experimental test facility for reflux cooling test.

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Photo-neutron emission in H₂ and He discharges was observed in the initial ohmic heating experiments of the Heliotron-E. Typical total neutron yield was 10⁹ neutrons per pulse under high level of runaway electrons ( ?? 10 MeV). Neutron flux was localized near the limiters. Energy spectrum of neutrons was continuous up to about 2 MeV. The radioactive nuclides in the limiters and the vacuum chamber irradiated by runaway electrons showed that ⁵⁸Ni (r, n) ⁵⁷Ni and ⁵³Cr (r, n) ⁵¹Cr reactions had occurred, proving that the photodisintegration process was the source of neutron flux.

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Measurements were simultaneously made of the concentrations of short-lived ²²²Rn daughter radionuclides present in the atmosphere at elevation of 1, 10 and 100 m above ground level, of the count rate of unscattered r-rays, and of the exposure rate, the latter two measurements limited to readings at 1 m above ground. The resulting data revealed close correlation of the short-lived ²²²Rn daughter concentrations in the atmosphere with the values at 1 m elevation of the peak count rate for 609 keV ²¹⁴Bi r-rays and of the exposure rate. The exposure rate at 1 m elevation attributable to the short-lived ²²²Rn daughters in the atmosphere was found to have been limited to at most 10% of the total exposure rate during the period of observation covered in the present study.

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Tests were made to evaluate the effect of two barriers (a convection barrier and a drip receiver) against sodium mist deposition on LMFBR conponents in the cover gas space. Two models of LMFBR rotating shield plugs (1, 800 mm in height, 680 and 850 mm in diameter) and a sodium test tank were manufactured for this purpose, and the mist deposition rate on the walls of these models was measured both in the cases with and without a barrier. The sodium pool temperature during the test was 580°C. In parallel with these tests, the relation between the form of deposit and the wall temperature was examined to determine the critical wall temperature above which deposits do not accumulate. This was conducted by exposing a vertical stainless steel cylinder to cover gas entrained with mist over 500d°C sodium for 30 to 1, 000 h.
The test results revealed that the barriers functioned effectively, and they reduced the local deposition rate near top of the annulus by three orders of magnitude relative to the cases without a barrier. The critical wall temperature to avoid deposit accumulation was found to be 100 to 150°C.

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A new computer program SPICA developed for evaluating fluid-structure interaction (FSI) due to pressure wave propagation in straight pipe is presented. In SPICA, equations of one, instead of two, dimensional fluid dynamics are coupled with those of two-dimensional structural dynamics. This simplification is expected to improve computing time significantly without reducing the accuracy of calculation.
The validity and the effectiveness of SPICA is shown by good agreement of the pressure time histories and deformations predicted by SPICA with the experimental results of FSI in straight pipe, and by the fact that the results are obtained with the computing time a factor of 100 smaller than that for conventional fully two-dimensional program. Finally, the limitation of the simplification used in SPICA is discussed briefly.

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