Hiroshi Mizuta

Fragmentation of uranium dioxide after molten uranium dioxide-sodium interaction

A series of 30 runs were performed to gain information on the fragmentation of molten UO2when dropped into a bath of liquid sodium. In this experiment, hollow cylindrical UO2 pellets were heated with electric current fed to a tungsten rod, passed through to center hole. The resulting molten UO2 dripped down its fall into the sodium bath. High-speed cinematography (500~2,000 frames/sec) was utilized to observe the fragmentation of the UO2 droplets upon its impact with the sodium surface. Records were taken of the changes in pressure and temperature induced in the sodium bath by the impact of the U2 droplets. The physical characteristics of the solid U2 particles resulting from the quench were examined for particle size distribution, particle shape and surface condition, as well as microstructure, to gain an insight into the mechanism of fragmentation. It is concluded from the study that, for the purpose of Fuel Coolant Interaction (FCI) analysis, the U2 particles can be assumed to be spherical in shape and...

Application of Invariant Imbedding to the Reflection and Transmission Problem of Gamma Rays, (I)

Equations were derived for the reflection and transmission functions of γ-rays through slabs of finite thickness, making use of the invariant imbedding principle. Numerical solutions were obtained for the reflection function (albedo) of semi-infinite medium. The results agreed well with the solutions by Monte Carlo method. Comparisons were also made with experimental results. Fair agreement was obtained except for the angular distribution of the reflected photons from light elements.

Few Group Diffusion Calculations of a Light Water Moderated Lattice

An analysis was made of a light water moderated lattice based on few group diffusion calculations. The lattice under investigation consisted of 468 fuel rods in a square lattice arranged in a cylindrical core of 22.46 cm effective radius and 127.2 cm core height, with a water to fuel volume ratio of 2.918. The fuel was 2.02% enriched UO2, clad in 0.8 mm thick aluminum tube. The theoretically calculated values for thermal, epithermal and fast neutron flux distributions, as well as the effective multiplication constant λ of the lattice, were compared with experimental data. After det alled analysis of the problems encountered in the course of the study the value of 0.9935 was determined for λ. Uncertainty in the nuclear data for fast neutrons would appear to constitute the greatest factor of error in λ. The discrepancies between the calculated and experimental activation distributions of the thermal, epithermal and fast neutrons amounted for about 20, 10 and 15%, respectively, in the reflector region adjoin...

STUDY OF RESONANCE ABSORPTION NEAR SODIUM RESONANCE AT 2.85 keV.

A method of calculating the neutron flux in a heterogeneous system, utilizing a combination of two different energy meshes — coarse and fine — is presented. The method has been programmed into a computer code EXRE-2. This method is particularly suitable for computations of fast reactor spectra that carry both broad and fine undulations. Pointwise cross section curves are prepared by superposition of the Breit-Wigner single level formura. In the energy region where the resonance is not resolved, a random sampling procedure is used for the derivation of resonance parameters. Group averaged resonance capture and fission cross sections and the temperature dependence of these quantities are computed for a typical fast reactor. An examination is made of the applicability of the conventional narrow resonance (NR) approximation applied over a rather wide energy group, and the heterogeneity effect on the resonance cross sections, near the sodium resonance at 2.85 keV. The error due to the NR approximation was foun...

Validation of the HELIOS.HX Code for High Conversion Light Water Reactor Lattice Analysis

The HELIOS.HX code has been developed for the design study of high conversion light water reactor (HCLWR) lattices. Analysis of the PROTEUS critical experiments at the Swiss Federal Institute for Reactor Research has been carried out as the first step toward validation of the HELIOS.HX code, and indications are that the accuracy may be at a higher or comparable level compared to that of WIMS-D, EPRI-CPM, and SRAC. In addition, comparisons with Monte Carlo calculations have also been performed for an HCLWR fuel assembly benchmark problem, showing that the accuracy is passable in the prediction of important nuclear characteristics, thereby indicating the validity of various approximations involved in the physics methods. These numerical results indicate that the code has basic potential as a tool for HCLWR lattice analysis, but covers only limited HCLWR lattice conditions.

Improved Intermediate Resonance Approximation in Heterogeneous System

An improvement of the IR (Intermediate Resonance) approximation is proposed. The effective resonance integrals of the infinite cylindrical fuel lattices are calculated by the analytical method, efficiently and with reasonably good accuracy. The fuel escape probability Pf(E) is based on the two-term rational approximation with an accurate Bell factor [Abar]. The IR parameters λ, κ and μ are determined taking into account the interference between the resonance and potential scatterings, and the effect of the fuel temperature. The effective resonance integrals of the main resonance levels of 238U were calculated using the improved IR approximations, and the accuracy of the method was ascertained by comparing the results with those obtained by the RICM code, which solves the neutron slowing-down equations numerically. Simple approximate expressions for J(ξ β) and J(ξ, σ, β) functions, which are useful in the IR approximations, are also given together with a table of J(ξ, β) and the errors of the simple approx...

Method for calculating thermal neutron spectrum in BWR lattice.

A new method for calculating space-dependent thermal neutron spectrum in a BWR lattice has been developed primarily aiming at its application to a lattice design code. Since the method has been constructed from combination of the cell-by-cell THERMOS calculation with leakage boundary condition and the coarse-mesh diffusion calculation for the cell leakage coupling, it is considerably economical as compared with various other transport theory approaches. Validation of the method has been made through comparisons with Monte Carlo calculations for a typical 8x8 BWR lattice. The discrepancies with the reference Monte Carlo calculations are mostly in the ranges of ±1.7 and ±3.0% for the rod-by-rod 235U fission cross section and the absorption rate distribution, respectively. The accuracy is acceptable and ensures the validity of various simplifications and assumptions involved in the present method. The present method is expected to be applicable to the BWR lattice analysis over the practical range of design p...

EFFECTS OF MUTUAL SHIELDING ON RESONANCE INTEGRALS OF

The results of some quantitative studies on resonance interference are presented. The calculations were performed on a FORTRAN IV program RICM2, which solves numerically the slowing down of neutrons over many resonance levels in a two region lattice, and gives reaction rates, average cross sections and effective resonance integrals of the nuclides concerned. Three combinations of resonant nuclides, 235U-238U, 230Pu-238U and 239Pu-210Pu, were considered, in conjunction with three oxide fuel rod radii, 0.2, 0.5 and 2.0 cm, the moderator (light water) to fuel volume ratio being maintained constant at 2.0. An energy range below 150eV has been covered by the present calculations. The effects of resonance interference have been found to be appreciable in this energy range.

sup 235

X-ray cinematography has been used for monitoring fuel motion in the out-of-pile fuel pin joule melting experiments for nuclear, liquid metal cooled fast breeder reactor, safety studies related to fuel pin failure, initial fuel motion and thermal fuel-coolant interaction (FCI) of the hypothetical core distractive accident. In order to visually observe the nuclear fuel motion, the X-ray cinematography system consists of an X-ray source located about 5 cm from the test section and an image intenfifier located at a corresponding position on the opposite side of the test section. The image from the image intensifier has been recorded both with a high speed camera and video recorder.