Neutron cross sections of ²⁴¹Am are evaluated in the energy region of 1 keV15 MeV, by using optical and statistical model calculations. Existing experimental data for this nuclide are very scarce, except for the fission cross section. An empirical formula for the fission cross section is used to reproduce the experimental data. The cross sections on (n, 2n) and (n, 3n) reactions are calculated with a simple formula proposed by Pearlstein. These cross sections are treated as components of competing processes in calculation of the compound elastic, inelastic scattering and capture cross sections. Energy averaged absorption cross sections are compared with those measured by Weston & Todd.

View full abstract

The present work was undertaken to obtain the effect of thermophoretic adherence of nuclear aerosol on the reactor containments in the event of a hypothetical accident of LMFBR. In the accident, sodium oxide aerosol containing PuO₂-UO₂ fuel and fission products may be suspended in the containment and tends to deposit on the cold wall of the containment, so that the aerosol concentration decreases by thermophoresis in addition of settling and diffusional plating. Experiments were made to determine the degree of thermophoretic adherence for sodium oxide aerosol in a chamber during sodium burning. The results obtained by the experiments were compared with those of the calculations and the agreement was satisfactory.

View full abstract

The instabilities occurring in forced-convection upflow passing through a parallelchannel system have been studied theoretically and experimentally, using water as test fluid. A non-linear mathematical model is proposed, that explains the results quite satisfactorily.
Close agreement is obtained between calculations based on this model and measurements of the stable boundary in a system of two channels. The model is further applied to three and four parallel channels, the results of which indicate that the flow instability behavior in such multiple channels are quite similar to that observed in the two-channel system, in such aspects as the stable boundary, as well as the period and the development of flow oscillations in the unstable region.

View full abstract

The spatial distributions of bremsstrahlung in water (1 m thick) and water (60 cm thick)-iron (6.3 cm thick) bombarded by 22-MeV electrons were measured by using a new activation method which we developed. These informations are useful for studying shielding, residual activity and radiation damage of accelerator and target materials.
From the measured activities, the bremsstrahlung spectra in water were evaluated with the LYRA and the SAND-II unfolding codes. The evaluated spectra were in good agreement with the analytical calculation by the DIBRE code, except for the higher energy ends. The longitudinal and the lateral distributions of bremsstrahlung flux in water were obtained by integrating the evaluated spectra above 8 MeV. The agreement of the experimental and the calculated flux distributions was very good except for a large angle to beam direction. The total photon number crossing a plane normal to the beam axis attenuates exponentially along the axial depth. The iso-flux contour of bremsstrahlung flux was given by interpolating the flux distribution curves.
Only the saturated activities of gold detectors were obtained for water-iron in good experimental accuracy. The spatial distribution of gold saturated activities in water-iron clearly shows the attenuating effect due to strong absorption in iron.

View full abstract

Distributions of fission products of low-enriched PuO₂-UO₂ fuel assemblies irradiated in a D₂O-moderated power reactor were non-destructively measured by r-ray spectrometry with a Ge(Li) detector. Proportionality was observed between the ¹³⁴Cs to ¹³⁷Cs activity ratio and the activity of ¹³⁷Cs, which was a measure of the burnup of a fuel element. The proportionality constant depends on the specifications of the fuel element, such as size, density and enrichment as well as on the irradiation history.
The burnup ratio between two fuel assemblies could be determined from the values of the activity ratio between ¹³⁴Cs and ¹³⁷Cs or from the activity of ¹³⁷Cs. The results proved to be in fair agreement with the indications obtained from reactor instrumentation.
The distributions of the ¹⁰⁶Ru to ¹³⁷Cs activity ratio were almost flat, both along the axis and across the fuel assembly. This flat distribution is a distinctive characteristic of irradiated PuO₂-UO₂ fuel rods and could be utilized as an indication of whether or not Pu was originally contained in a fresh fuel rod.

View full abstract

The energy dependent Doppler effect for 238U was calculated using the collision probability method with the resonance parameters of the HRG library, and this calculated result was compared with the experiment carried out at Karlsruhe.
Though the discrepancy between the calculation and the experiment is 14%, the calculated result shows rather good agreement with the experiment in the energy region of 1501, 000 eV, considering the experimental errors of 3.2%, except two experimental data.

View full abstract

The desirable accuracy of nuclear data for minimizing the design margin in fast reactor nuclear designs is evaluated through an optimization method, in which a parameter termed the " degree of difficulty " has been introduced in order to represent the relative degree of difficulty expected to be encountered in improving the accuracy of the data. This parameter serves to determine the desirable accuracy, by which the design margin could be reduced the least onerously, to obviate uselessly severe requirements being imposed on the accuracy demanded of data with high sensitivity coefficients.
Application of this method to the equilibrium core of a prototype fast reactor leads to the conclusion that nuclear data that should to best advantage be improved in accuracy are:
σ⁴¹_f(4000.5 keV), σ⁴⁹_f(1.4 keV) and a=σ²⁸_c(1000.5 keV).
Furthermore, it is indicated that the ratio between the desirable accuracy and the present uncertainty level of nuclear data is roughly inversely proportional to the square root of the product of the sensitivity coefficient and the present uncertainty level of nuclear data.

View full abstract