Measurements of thermalization parameters in light water have been made by the pulsed neutron source and non 1/v absorber method. In an effectively infinite system of poisoned water, an asymptotic decay constant was measured by detecting capture γ-rays emitted from the whole volume of this system. The thermalization parameters were derived from values of decay constants measured over a wide range of non 1/v absorber concentration. Measured values of these parameters were compared with values calculated from Nelkin, Egelataff-Schofield and McMurry-Russell kernels.

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Fine spectra in large fast reactors of various compositions have been calculated by exactly treating slowing down due to elastic scattering of light elements, and the resulting composition-dependent sets of the effective cross sections have been compared with currently typical standard sets. The following results have been obtained:
(1) Effective cross sections depend largely on composition. The dependence is large for groups where predominant resonances exist. A composition-independent set underestimates sodium void effect compared with a composition-dependent set, when variations of fine spectrum are considered. Therefore a composition-independent set like YOM is not adequate, especially for a large reactor.
(2) In the present standard set, the self-sheilding factors of both elastic removal and elastic scattering cross section are treated under the same principle. This procedure is not valid, especially at the predominant resonances they vary inversely to each other with variations of composition.
(3) The effective cross section in a blanket region, except close to the interface and boundary, are hardly affected by the buckling assumed, and the effects of buckling on effective multiplication factor as well as the sodium void reactivity effect are small enough to be neglected.

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A new definition of subcritical constants is introduced in this paper on the basis of the neutron balance equation. These subcritical constants are obtained by integrating the intensity of the neutron flux over the whole volume of the subcritical assembly with neutron source. Some other constants commonly used in conventional reactor physics can be predicted by analyzing the above mentioned subcritical constants.
As examples of applications of this method, the infinite medium multiplication factor for natural uranium-light water system and the critical buckling for 20% enriched uranium-light water system have been determined experimentally with the use respectively of Ra-Be and spontaneous fission neutrons as the neutron source. The experimental results obtained are in comparatively good agreement with the values from exponential experiment and with theoretical values calculated by a four-factor formula.

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The "few-pole expansion method" gives an approximate solution of the general space-dependent linear kinetics.
The mathematical meaning of this method is first briefly described. A numerical calculation of the space-dependent, two-group, two-region reactor transfer function is made with the use of the fewpole expansion. Even an expansion with only two terms is found to provide good agreement with the transfer function measured by pile-oscillator.

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Few studies have so far been reported on isotopic frequency shifts, which present problems in experimentation. In this paper, the G and F matrix method is applied to theoretical calculation of the frequency shifts of linear molecules XY₂ and XYZ, as well as of octahedral molecules XY₆ Using these values, reduced partition function ratios of lithium, carbon, copper, nitrogen, chlorine and uranium compounds have been calculated.

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A study was undertaken to evaluate and determine "an acceptable defect level" of fuel cladding tubes for light water reactor. The burst strengths of both zircaloy-2 and AISI 304 L stainless steel tube were evaluated by burst testing naturally or artificially defected tube samples.
As a result, (1) the burst strength of zircaloy-2 tube with poorly uniform thickness is affected by the minimum thickness of the tube, (2) a deep natural defect on the wall of the tube generally impairs the burst pressure value more than an artificially-made defect of the same extent as determined by non-destructive test; (3) artificial defects of depths up to 30% of the wall thickness do not cause appreciable deterioration of burst strength.

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