The fast neutron flux in the central thimble of a TRIGA-II reactor was estimated by using the ²³⁸U(n, 2n)²³⁷U reaction as flux monitor. The thermal neutron flux also was estimated by ²³⁸U(n, γ)→²³⁹Np and ²³²Th(n, γ) →²³³Pa reactions. The latter flux was further measured at the same time by γ-ray spectrometry of the product nuclides and by chemical separation and β counting with α emitting isotopes as tracer. The values of flux thus obtained were compared with those estimated from the amounts of ²⁸Al, ²⁷Mg and ²⁴Na respectively produced by the (n, γ), (n, p) and (n, α) reactions on ²⁷Al. Furthermore, for the first time, the effective cross section for the ²³⁸U(n, 2n)²³⁷U reaction was estimated to be 2.8b from a comparison with that for the ²⁷Al(n, α)²⁴Na reaction.

View full abstract

As a prerequisite to the optimal control of the neutron flux distribution of large reactors, the controllability of the flux distribution is invenstigated. The distributed parameter system is reduced to a lumped parameter system by means of truncated modal expansion. In a one dimensional reactor, any number of modes are controllable by a single control rod. To maintain the amplitude of each mode at a prescribed level however, it is necessary to have as many control rods as the number of the modes. Discussions are also presented on the decomposition of the optimal control of fiux distribution into those of the individual modes, on the controllability of reactors with feedback, and on the controllability when there is constraint on the control rod speed.

View full abstract

Leakage radiation from an infinite plane source reaching the rear surface of a shield provided with an annular duct has been evaluated numerically. The plane of the infinite source is divided into eight zones, for the convenience of integration, and the contribution of each zone is calculated separately by ray analysis. The distribution of the radiation flux and dose rate around the duct have been systematically obtained for different shielding materials and source energies, and the effect of changing duct geometry is also analyzed.
changing duct geometry is also analyzed.Among the contributions from the different zones, it is found that the predominant portion is provided by zones outside the annular area, and that this predominance extends even to the region of shield surface directly above the duct. It is also demonstrated that Rockwell's expression based on Line-of-Sight method is included as a special case of the present formula, which is applicable not only to γ-rays, but also to fast neutrons.

View full abstract

An attempt has been made of using Nal(Tl) crystal as fast neutron detector in conventional Time-of-flight apparatus. Typical pulse hight distributions, in the γ-ray energy range below 1 MeV, produced by the incidence of fast neutrons are presented. The characteristics of this neutron detection system have been studied, using pulsed neutrons from the T(p, n) and D(d, n) reactions. Neutron detection efficiency curves are presented, covering both whole γ-rays and those from a few low excited levels of ¹²⁷I nucleus. The detection bias can be stably set at the first excitation energy (58keV), which is lower than possible for an organic scintillator.

View full abstract

A study was made of the microstructure of swaged UO₂, irradiated to a burnup of 3, 000 MWD/T-U at an estimated center-line temperature of 1, 400°C.
In the peripheral zones of the specimen near the cladding, evidence was observed of sintering of the UO₂ particles by irradiation even at comparatively low temperatures (about 300°400°C). Tracks of fission product fragments were observed at the inner wall surface of cracks in the specimen. The width of the tracks is seen to be larger than observed on thin films and on sintered UO₂. Mixed phases of UO₂ and fission products precipitated in the interior of crystal grains, observed in the form of hillocks left unetched, were surmounted by several sprouts of substances still more chemically stable. Mixed phases of UO₂ and fission products were precipitated on the inner wall surface of the cracks and appeared upon etching as stepped protuberances of irregular contours. A comparison between the microstructures of swaged UO₂ and sintered UO₂ irradiated under the similar conditions is discussed.

View full abstract

Theoretical and experimental studies concerning the effects of Bragg cut-off on the neutron wave propagation in graphite are presented, as analyzed from the two following factors related to the Bragg cut-off.
First, the inordinately long mean free path of cold neutrons of energies below the Bragg cut-off causes in some cases serious problems due to the deep penetration and the strong leakage of the cold neutrons.
Secondly, the extremely low collision frequency of about 2, 500 rad/sec for the cold neutrons produces a "singular frequency" at a point of even lower frequency, wherein the waves do not decay exponentially in the propagating direction.
The peculiar behavior of these cold neutrons severely restricts the conditions of experiment under which one can derive an exact value for the propagation parameter in graphite. In certain cases, however, one can obtain meaningful approximate information about wave propagation from experiments conducted beyond these limiting conditions, provided that certain precautions are taken in carrying out the experiment and in interpreting the results. Analysis to explain these phenomena and experiments to reveal the effects in graphite are described.

View full abstract

Preparation of iron ferrocyanide-anion exchange resin and adsorption of ¹³⁷Cs on the resin are described.
The resin was found to effectively adsorb ¹³⁷Cs from concentrated nitric acid solution, contrast to the negative results previously obtained with copper ferrocyanide-anion exchange resin.
A discussion is given of the possibility of applying the iron resin to the removal of ¹³⁷Cs from fuel reprocessing waste solution.
Fission products were effectively separated from each other with a column of this resin, utilizing the properties possessed by it of both iron ferrocyanide and the parent ion exchange resin.

View full abstract