The steam void effect on core reactivity and flux distribution was experimentally studied on cores loaded with fuel assemblies of various void fractions. The voids were simulated by hollow polyethylene tubes. The local flux distributions within the assembly cells of various void frac-tions have already been discussed in Part (I) of the present series of papers.
The study proved that the reactivity calculation can be improved by considering the double heterogeneity that characterizes an assembly lattice simulating the BWR core, by means of the following treatments: (a) Consideration of the spatial variation of the thermal neutron spectrum around water gaps. This gave good results for the analyses of local flux distributions, (b) Adoption of the effective Dancoff factor, which evaluates the difference in the resonance neutron shielding effect between fuel rods lining the water gap and those others closed in by similar rods.

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A chlorination-distillation process for irradiated UO₂ fuel treatment is presented, which utilizes the difference in vapor pressure between the chlorides of uranium and those of F.P. Good recovery of uranium and decontamination of F.P. were not obtained with single distilla-tion. Improvement was seen with the adoption of a process in which most of the F.P. were removed from the irradiated UO₂ fuel through a sorption-desorption process using a BaCl₂ bed. The present report describes the behavior of uranium chlorides in this process.
The vapor of uranium chlorides formed by chlorinating UO₂ powder with a mixed gas of Ar and CCl₄ vapor was passed through the bed whose temperature varied along its length from 500° to 100°C, under which conditions the chloride vapor was completely sorbed on the bed. The uranium chlorides thus sorbed did not desorb in a mixed gas stream of Ar and CCl₄ vapor at bed temperatures below 480°C, but when the bed was heated to 480°560°C, and Cl₂ gas added to the gas stream, the chlorides were completely desorbed in the form of vapor of higher uranium chlorides.

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The thermal conductivity of THO₂-UO₂ and THO₂-CeO₂ systems were measured by laser flash method, and the results are discussed with the aid of the relaxation-time theory. Two param-eters characteristic to systems of the same base - the ratio α of scattering rates between N- and U-scatterings and the strain parameter ε - were determined experimentally from the measured thermal conductivity of the THO₂-UO₂ system at 25°C. Using these parameters, the thermal conductivities of THO₂-UO₂ system at 100° and 500°C and that of THO₂-CeO₂ at 25°C were calculated. The results obtained agree very well with the experimental values.
In the THO₂-UO₂ system, the predominant contribution to the scattering cross section of phonons is the strain factor, while in the THO2-CeO2 system, the contribution from the mass factor increases, which makes the thermal resistivity of the THO₂-CeO₂ system higher than that of the THO₂-UO_2-

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Numerical calculations on neutron age and neutron spectra for a fission source and a D-T source in light water have been performed by means of a stochastic method of calculation for the slowing down process. The formulas for calculation have been derived through direct operational treatment. In dealing with neutron degradation, account has been taken of both inelastic scattering and anisotropy of elastic scattering in the center of mass system (c.m.s.). To elucidate the effects brought by these factors, and to compare the present exact results with those by G.G. approximation for isotropic scattering, numerical calculations have been performed and discussions on the results are given. The results obtained on neutron age are in good agree-ment with values from experiment by other authors. The neutron ages from a fission source and a D-T source to In resonance have been found to be 26.34 and 156.4 cm2, respectively.

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The transport equation relevant to the heavier of two elements forming a gaseous mixture was derived from the following equations.
Heat balance: d²T/dr²+(1/r)•(dT/dr)=0
Material balance: d²υ_z/dr²+(1/r)•(dυ_z/dr)=α•(T-T_c)
Diffusion equation concerning the heavier element:
∂(rJ_c)∂z+∂(rJ_DZ)/∂z+∂(rJ_DR)+∂(rJ_TD)/∂r=0
Concentration of the heavier element in the radial direction:
C₁(r, z)=C₁(R_i, z)+(r-R_i)K₁.
The transport equation thus derived became that of Jones & Furry when the smaller terms were neglected.
The influence on the separation factor brought by the axial gas velocity υi along the hot wire surface was then analyzed. The results were as follows:
The maximum separation factor displaces toward lower operating pressures when the gas velocity υ_i is increased to a certain degree. It would appear from the present calculation that to obtain maximum separation, lower operating pressures should be combined with higher gas velocity υ_i, while with increasing operating pressure, υt, should have to be gradually reduced, finally to reach zero at a certain pressure.

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The slowing down of neutrons to very low energy has been examined with particular refer-ence to cold neutron production. The neutron spectrum formed in very cold light water ice has been measured with time-of-flight technique. It is observed that at extremely low tempera-ture the neutron temperature is much higher than the moderator temperature, whereas in the intermediate range of temperature the neutron temperature does not differ much from the moderator temperature.
It is suggested that there is a limiting neutron temperature below which it does not fall, even when the moderator temperature drops further. This observation can lead to information on the nature of the mechanism for removing small amounts of energy from slow neutrons in cold solid hydrogenous moderators. By using a simplified model for the cold solid hydrogenous moderators, it is shown that low frequency lattice vibrations play an important role in produc-ing cold neutrons. Also, quantitative analysis indicates that while the cold neutron temperature depends only slightly on the neutron absorption, the gain in cold neutron flux is strongly affected by the neutron absorption.

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The majority of radioactive self-luminous paint produced in Japan uses ¹⁴⁷Pm.
The characteristics of the radiation from a ¹⁴⁷Pm-activated self-luminous plate which had a luminous area of 12×8 cm² and radioactivity of 12.5 mCi were examined. Bremsstrahlung was produced from the luminous plate absorbing the 220 keV β-particles of ¹⁴⁷Pm. The energy of the bremsstrahlung was analyzed with use of both NaI scintillation and Ge solid state detectors: the spectrum obtained showed a peak at around 50 keV and also a small peak due to γ-rays emitted from the ¹⁴⁷Pm at 121keV. Measurements of the bremsstrahlung made with Lauritsen electroscope, film badges, fluoroglass dosimeters and thermoluminescent dosimeters gave estimated exposure rates of at most only 0.5 and 0.3 mR/hr respectively at distances from the plate of 3 and 7 cm.

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