Hiroyuki Kadotani

Axial dependence of partial void reactivity in a light water-cooled, heavy water-moderated, pressure-tube reactor

In this paper the effect on reactivity of changes in the coolant levels in the pressure tubes of a pressure-tube heavy water reactor is experimentally studied to clarify the effect of an axial coolant void fraction distribution. The coolant void fraction distribution is simulated by stepwise changes in the coolant levels in the Deuterium Critical Assembly (DCA). The reactivity is measured for a 25.0-cm pitch square-lattice core with a positive cool void reactivity. The reactivity changes resulting from changes in the coolant levels are measured as changes in the critical heavy water levels. The axial distribution of the thermal neutron flux is also measured by the copper activation method. In these measurements, the critical heavy water levels show a sinuous curve having a maximum and a minimum, and a positive reactivity larger than those of both the 9% and the 100%, void uniform cores is introduced at certain coolant levels by stepwise changes in the coolant levels. An experimental analysis is performed with the coupled WIMS-ATR/CITATION code system, whose analytical method was established through DCA critical experiments. Agreement between experiment and analysis is fairly good.

An eigenvalue and eigenfunction study of space-dependent fast neutron spectra in /sup 232/Th

The multigroup space eigenvalues and eigenfunctions of a one-dimensional steady-state diffusion theory operator have been used to study the spatial behavior of a fast neutron field in certain thorium systems. The nuclear data used are from the 26-group ABBN data set. It has been shown that for a fast thorium system, unlike a fast uranium system, all the space eigenvalues lie in the continuum and no discrete space eigenvalue exists. A fast thorium system behaves more like a fast nonmultiplying system. The spectra shifts continuously to lower energies as one moves away from the source; however, pseudoasymptotic conditions are established in certain distance ranges. In order to test the validity of the diffusion theory and eigenfunction expansion method, results have also been obtained using transport theory. In all cases the two sets of results are in reasonably good agreement. To see the effect of geometry, the spectra at certain distances inside a 1-m-thick thorium slab are compared with the corresponding spectra inside a thorium sphere of 1-m radius. At all distances the normalized slab and sphere spectra are nearly the same.

Temperature dependence of total neutron cross sections in thermal and keV regions

Abstract In the thermal neutron energy region, the inelastic scattering cross sections of Si have been measured at room temperature and at 600° with a time-of-flight spectrometer using a single-crystal Si sample. The results are in good agreement with the calculated ones based on Debye model. A filtered neutron beam with an energy band of 23.3 to 25.3 keV has been used to measure the temperature dependence of the total cross section of Nb in the keV region. The cross section is averaged over the neutron energy band which covers a number of resonances. This average cross section shows the effect of Doppler broadening of the resonances. The sample thickness dependence of Nb has also been measured at 54 and 144 keV by using a Si-filtered beam.

Calculation of Gamma Ray Buildup Factors for a Very Thick Slab with Angular Eigenvalue Method

The buildup factors of gamma rays are still widely used in the practical shielding calculations. Although the early compilation of buildup factors was published back to 50’s, it is still a tedious, though straightforward, task even for state-of-the-art computers. The most recent database of buildup factors covers the distance up to 40 mean free paths. The angular eigenvalue method, which was proposed by one of the author (A. S.) to calculate the transmission of gamma rays through a homogeneous slab, is based fundamentally on the invariant embedding method. This method has been applied to obtain the gamma ray buildup factors in homogeneous medium. It was found that the buildup factors were easily obtained for the distance over 40 mean free paths.

Calculations of the spectra of fast neutrons in iron spheres using the vitamin-C file

Steady-state space-dependent fast neutron angular and scalar spectra and total flux in various iron spheres have been calculated using the one-dimensional discrete ordinate transport code ANISN and Vitamin-C nuclear data file. The results have been used to study the question of establishment of equilibrium and of an associated fast neutron diffusion length in iron. The authors find that true equilibrium conditions are not established even inside a 3-m-radius iron sphere. However, from the study of spatial decay of total flux, one can obtain the value of the fast neutron diffusion length in iron, which in the present case is found to be 24.4 cm.

Measurements of Spatial Distribution of Cadmium Ratio in Heterogeneous Lattices

Spatial distribution of the Cd-ratio of Au foils were measured for various lattice pitches in a natural uranium-light water system. Comparison of the experimental results with theoretical calculation using the THERMOS-code shows some divergence of the theoretical Cd-ratio toward larger than experimental values in the fuel rod.