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 8×8 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 parameters as well as exposure, moderator voids etc.

View full abstract

A two-dimensional (in r-z geometry) space dependent kinetics program coupled with thermo-hydraulics for an accident analysis of a liquid metal fast breeder reactor was developed.
Validation of the code was made by applying the code to the NEA CRP benchmark problem and comparing the result with those calculated at other institutions. Then, the code was applied to study the transient of a homogeneous, a tightly coupled heterogeneous and a loosely coupled heterogeneous core where a control rod was withdrawn with a reactivity insertion rate of 2 $/s and 2 ¢/s. The followings were found : (1) the order of the rapidity of the reactor power increase was the loosely coupled core, the tightly coupled core and the homogeneous core, (2) the difference from the point kinetics calculation was also larger in this order, (3) when the same reactivity insertion rate was applied for the same core, the space dependent kinetics calculation always gave larger transient power than the point kinetics calculation, and the differences were especially large for the withdrawal of the inner rod, which shows that we cannot rely on the point kinetics results even for the integral quantities such as reactor power or the reactivity in these cases.

View full abstract

Based on simple models, magnetic pressure drop and heat transfer coefficient correlations were derived for two-component two-phase liquid metal flow in an annular channel under magnetic field. Magnetic pressure drop and heat transfer experiments of Na-Ar two-phase flow in an annular channel under magnetic field were conducted. It became clear from the experimental results that the magnetic pressure drop of two-component two-phase liquid metal flow was equal to or smaller than that of single-phase liquid metal flow under conditions of the same liquid flow rate and magnetic field strength. The measured magnetic friction factor multiplier was smaller than predictions by the correlations derived in this study. It was also revealed from the experimental results that the heat transfer coefficient of two-component two-phase liquid metal flow was equal to or slightly smaller than that of single-phase liquid metal flow under conditions of the same liquid flow rate and magnetic field strength. The tendency that the measured heat transfer coefficient ratio decreased from 1.0 with increasing the void fraction agreed with trends of predictions by the correlations derived in this study.

View full abstract

In an integral system test on the reflood phenomena of a PWR LOCA with the Cylindrical Core Test Facility (CCTF), a high pressure drop was observed through the broken cold leg of the pressure vessel side. In order to understand the pressure drop and to assess the applicability of the CCTF result to the LOCA analyses for PWRs, the pressure drop characteristics through the broken cold leg is analyzed with CCTF and FLECHT SET data. The high pressure drop is explained quantitatively with the homogeneous flow model of the two-phase flow. The difference of the pressure drop between the FLECHT SET and the CCTF is attributed to the differences of the flow area scaling of the broken cold leg and the ECC water injection method. It is confirmed analytically that the high pressure drop as in the CCTF tests is expectable in a PWR system with a cold leg break due to the pressure losses at the broken cold leg nozzle and the break.

View full abstract

The crushing strength of thorium oxide and thorium-uranium mixed oxide fuel kernels for high-temperature gas-cooled reactors (HTGR) was estimated by application of the Hertz theory of contact to the failure load obtained in a simple crushing test. The strength data were interpreted assuming the Weibull distribution. The crushing strength of the Th0₂ kernels ranged from 1.7 to 1.9 GPa. The strength was found to be closely related to the microstructure of the kernel and to be increased by the kernel properties : small grain size and smooth surface. Fracture would occur in a transgranular manner for the fine-grained kernels and in an intergranular manner for the coarse-grained. Flaws, which are located at or near surface of the boundary of the contact area, act as fracture origins.

View full abstract

Performance tests of a pressure balanced type membrane covered polarographic oxygen detector were carried out in water at 285°C (BWR operating conditions) to determine the effects of environmental factors. Dependencies of detector current on pressure and sample water flow velocity were ascertained and response time of the detector was determined. Effects of H₂ present in the water were also examined. The following conclusions were obtained:
(1) The detector current increased in proportion to the 0.67th power of the flow velocity, but was independent of the pressure.
(2) Response time was about 7 s.
(3) Hydrogen caused a decrease in the current at cathode potentials more positive than -0.75 V against the Ag/AgCl electrode. However, at more negative potentials, the current was independent of _H2 concentration and in proportion to O₂ concentration.
Oxygen concentrations in water containing H₂ at 285°C could be determined by the detector under the measurement conditions of constant flow velocity and cathode potentials more negative than -0.75 V.

View full abstract