Power and temperature transients following a prompt critical pulse applied to an experimental reactor has been studied by prompt jump approximation in two-dimensional phase space. The phenomena are classified into several types characterized by the amount of inserted reactivity and the value of cooling time constant. The temperature is shown to exceed in no case a level twice as high as the equilibrium value.

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The fission gas escape during fissioning in fuel was interpreted with a newly proposed escape mechanism which comprises the three successive processes of release from trapping sites, diffusion of the released fission gas nuclides, and desorption from the fuel surface. The expression for the release rate of fission gas was derived from a simple mathematical model postulating a continuous activation energy spectrum for the release reaction of trapped nuclides. The proposed mechanism satisfactorily explains observed data reported in literature on the dependence of the escape rate on decay constant, the contribution of the precursor, and the change of escape rate with irradiation time. There appears to be only a small effect brought by diffusion of the released nuclides on the escape rate, which can thus be considered to be controlled mainly by the release reaction from the the trapping sites. The present model based on the release from trapping sites can also qualitatively explain the influence of differences in irradiation conditions such as time and neutron flux.

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Scattered γ-rays were measured with 5-in. diameter by 5-in. long NaI(Tl) scintillator for the purpose of obtaining information on the transmission of ⁶⁰Co γ-rays incident to polyethylene and iron slabs, and layered combinations thereof. The primary γ-rays were collimated, and the detector was used without collimator. The spectra of scattered γ-rays above 300 keV were quite similar to each other irrespective of material and thickness when the scattering angle and scatterer thickness in mean free path units were normalized.

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Anode and cathode of identical shape and material were suspended above a ThO₂ powder layer in a beaker in atmospheric pressure at room temperature to obtain active deposits containing ThB etc. on the cathode. The items of experiment were:
(1) Material of electrode plates: Al, Ti, Ni, Zn, Cd and Pt, both bare and with active charcoal powder coating
(2) Applied D.C. potential, 0100 V.
(3) Distance between the electrodes was varied
(4) Height of electrode above ThO₂ layer was varied
(5) Surface area of electrode plate was varied
(6) Surface of spreaded ThO₂ layer was varied
(7) Duration of electro-deposition was varied
(8) Thickness of ThO₂ powder layer was varied
The active deposit was examined radiochemically.

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With the view to establishing a method of directly converting uranium carbide into uranium carbonitride and hydrocarbons, an attempt has been made to induce reaction between UC and ammonia under various temperatures from 25° to 600°C and pressures from 1 to 1, 500 kg/cm². The reaction aimed at was realized to the extent of practical significance under pressures exceeding 500 kg/cm² at 450°C and exceeding 250 kg/cm² at 500°C. The hydrocarbons produced thereby were found to be mainly methane, indicating that the formula of the predominant reaction was
UC+NH₃ → UN_2-x+CH₄+H_2-
Upon heating the powdery fine black product to 1, 800°C in vacuo, unexpectedly marked sintering was found to occur, resulting in dense uranium monocarbonitride without any compaction pretreatment.

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As a contribution to the safety analyses of condition following loss-of-coolant accidents in light water reactor, experiments were performed to obtain data on container spraying and on accompanying heat transfer characteristics, by means of a one-tenth scale model container. The loss of coolant was simulated injecting high temperature saturated water into the model container in which air bad been enclosed at atmospheric and room temperature, and then the steam-air mixture was cooled by sprays of water.
The rate of heat absorption by the spray was obtained from both stationary and transient data, and it was confirmed that the absorption becomes incomplete as the container pressure is lowered.
The evaporation of the water collected at the bottom and of that from sprayed walls of the container was derived from container pressure variation in time, by measuring its rate of change immediately following stoppage of the spray ; the rate of evaporation was found more active as the rate of spray cooling increases.
The evaporation from the sprayed walls was obtained from thermal data taken on a copper cylinder placed in the container, and it was found that the evaporation becomes more active as the pressure increases.

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A quantitative expression of the capacity of a hot laboratory for post-irradiation examinations and experiments should serve most usefully in scheduling hot laboratory operation. The method adopted in the Hot Laboratory of the Japan Atomic Energy Research Institute is what has been termed the "Cave-day method". It has been in trial use for half a year since Feb. 1966. The method has proved to be highly useful for formulating and adjusting the operating schedule of the Hot Laboratory, once the standard times required for the various kinds of cave work have been determined. Another key to its successful application is a good understanding between the utilizers and the hot cave administration concerning the principles applied in determining the yearly or half-yearly amounts of cave-days allotted in advance to utilizers.

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