A monoenergetic disadvantage factor is introduced by extending the method of Amouyal & Benoist^(1)∼(3) so as to permit the treatment of a cell with thin moderator. To take into account the flux depression in a fuel rod, the blackness of the fuel rod is introduced into the calculation as in the paper by Amouyal et al. The Wigner-Seitz equivalent cylindrical cell method is incorrect for closely packed lattices. To cover this shortcoming, an extended Dancoff factor is introduced to take into account the interference among fuel rods. This factor can be easily calculated in a homogeneous system, and so no consideration is necessary to account for the shadowing effect by other fuel rods. The result obtained is similar to that from the collisionprobability method but can be calculated more easily. And improvement over the Amouyal-Benoist method is thus obtained.

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Transmission electron microscopic observations were made on hot-rolled Be sheet prepared from cast ingots irradiated to about 5×10¹⁹ nvt (fast) in JRR-2.
The aim of the study was to elucidate the effect of irradiation on changes in microstructure and the effect of pre-irradiation heat treatment on the formation of He gas bubbles during post-irradiation annealing.
The formation and growth of He bubbles during post-irradiation annealing was distinctly less inhibited in the Be irradiated as hot-rolled than in the metal irradiated after recrystallization treatment. In the former case, He bubbles could be observed upon post-irradiation treatment of only 3hr at 750°C, while in the latter case, a similar formation of bubbles could only be seen after a final treatment at 900°C for 50hr.
The difference thus observed in the tendency of bubble formation depends upon the grain boundary mobility during post-irradiation heat treatment.
The nucleation and growth of bubbles occurred preferentially at the grain boundaries, inclusions and dislocation sites. The recrystallization of irradiated Be was retarded by the interaction between dislocations and He atoms.
The inclusion which has the largest affinity to the bubbles in Be is Be₁₁Fe.

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A study was made on UP preparation by two-step reactions starting with UF₄, Si and red phosphorus.
The first step was to produce an intermediate uranium phosphide U₃P₄ from the reaction of UF₄ with Si under phosphorus vapor at temperatures above 900°C.
In the second step, the intermediate phosphide was heat treated under vacuum. To obtain single phase UP from the intermediate phosphide, the treatment required a temperature above 1, 050°C, at which temperature the minimum holding time was 60min.

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This paper describes a new type of digital reactor period meter utilizing integrated circuits. The period is calculated from the number of counts N₂ during t₂ at t=τ, where t₂ is equal to t₁ which is the interval required for N₁=10₄ at t=0. Its output is the digital inverse period signal. The statistical error is delimited by fixing the number of counts instead of the counting interval. The absolute error is about 0.015sec_-1 in the case of a delay time of τ=1sec. For τ ?? 2sec, it becomes smaller than 0.008sec_-1. The delay time τ is set to a value chosen among 1, 2, 4, 8, 16 and 32sec, depending upon the count rate. The period meter contains about 250 integrated circuits, of which approximately 80 circuits are used for display and auxiliary functions not directly related to the primary purpose of period calculation.

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A pair of adiabatic calorimeters were designed, constructed and applied to the estimation of the dose absorbed by epoxy resin, in a vertical experimental pipe of JRR-4 (swimming pool). One of the calorimeters was made of epoxy absorber, and the other of graphite (carbon). With this pair of calorimeters, the contributions of γ-rays and neutrons to the dose can be separately determined. The dependency of the dose upon the vertical location of the material in the experimental pipe was investigated with the reactor power set at 2.5MW, and the dependency upon the reactor power was measured in the reactor power range of 0.05∼2.5MW. The calibration of the calorimeter output (V) to absorbed dose (rad) was made with a Fricke dosimeter in a γ-ray field.

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Measurements on the effects of multiple control rods arranged in ring geometry were undertaken with application of the pulsed neutron technique in SHE, a heavily refiected graphite-moderated 20% enriched uranium critical assembly. The aggregate number of experimental cases exceeded 100, which systematically covered the four fundamental factors to determine the control rod effects:
(1) The atomic ratio of ²³⁵U to C in the core region (C/²³⁵U=2, 226 and 5, 378)
(2) The neutron absorbing substances in the control rods and their densities (B₄C powder and Cd plate. Density of B₄C: 0.1 and 1.0g/cm³)
(3) Geometrical factors of the control rods (Cylindrical shape, 20 or 44mm in diameter)
(4) Number of control rods (One to six control rods inserted in circular geometry)
The experimental results are compared with calculations based essentially on a system of simple two group Nordheim-Scalettar method used as tool to solve the three-dimensional diffusion equation. Agreement within about 10% was obtained between measurement and calculation utilizing extrapolation distances for the fast neutron group, determined semi-experimentally.

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In the liquid metal MHD power generator, the electric conductivity of the working fluid is so large as to be comparable with that of the electrode. Under such circumstances the perfect conductor approximation no longer holds for the electrode, because the electric potential loses its constancy and acquires a certain pattern of distribution in the electrode as well as in the fluid. The extent of distortion in the electric equipotential lines and lines of force depends on the ratio of conductivity between electrode and fluid, and on the load connection. The power density becomes non-uniform, and furthermore the average power density becomes smaller than in the case of a generator with the perfect conductor electrode.
The effect due to this finite conductivity of the electrode is contrary in trend to that due to end loss, as analyzed by Sutton with both aspect ratio and load factor as parameters.
The results obtained in our present experiments with a mercury blow-down facility have agreed very well with the analysis. A numerical analysis has also been made, with end loss taken into account.

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