Yukio Takigawa

Caorso limit cycle oscillation analysis with three-dimensional transient code TOSDYN-2

The Caorso limit cycle data observed during the stability tests were analyzed by the three-dimensional transient code TOSDYN-2. The Caorso limit cycle oscillation was spatially out of phase, and both the amplitude and the extent of the large amplitude region were large. For this reason, it is very valuable for the qualification of the TOSDYN-2 code. TOSDYN-2 employs a three-dimensional neutronics model and a multichannel-type thermal-hydraulic model. The channel-type grouping is important for qualification analysis. It was determined by considering the test results and the more detailed three-dimensional steady-state code results. The analytical results imply that many unstable channels or unstable regions might exist separately in the core. To account for this, TOSDYN-2 could accurately simulate both the amplitude of the limit cycle oscillation and the spatial power change profile. Thus, TOSDYN-2 applicability to the spatial power change phenomenon has been well verified.

A Study on Boiling Water Reactor Regional Stability from the Viewpoint of Higher Harmonics

A quantitative study on a mechanism for boiling water reactor regional stability has been carried out from the viewpoint of higher harmonics. In the mechanism, the gain decrease in the void-to-power transfer function can be explained by the higher harmonics mode subcriticality. It is shown that the thermal-hydraulic feedback effect can compensate for the gain decrease, and regional oscillation can be sustained that way. For quantitative evaluations, a three-dimensional higher harmonics analysis model has been developed. The results show that the first azimuthal harmonics subcriticality has a relatively small value under a regionally unstable condition. Comparing the subcriticality and the steady-state power distribution, it is shown that the distribution exists whose first azimuthal harmonics subcriticality takes a small value. A method of decomposition for the oscillated power responses into the harmonics modes is presented. The results show that the corewide oscillation power response consists almost entirely of the fundamental mode, and the regional oscillation power response consists almost entirely of the first azimuthal harmonics mode. This indicates that regional oscillation is a phenomenon in which the first azimuthal harmonics mode oscillates on the basis of the fundamental mode.

TRACG transient analysis code -- Three-dimensional kinetics model implementation and applicability for space-dependent analysis

Space- and time-dependent phenomena, mostly related to neutron flux oscillations, have been observed in several boiling water reactor plants. A time-dependent three-dimensional transient analysis code is indispensable for simulating such phenomena. In a joint effort between the General Electric Company and the Toshiba Corporation, a three-dimensional neutron kinetics model has been implemented into the best-estimate thermal-hydraulics code, TRACG. A neutronics model implementation and the applicability of the modified TRACG code for analyzing space-dependent phenomena are discussed. To verify the code, startup tests with selected rod insertions, where control rods are locally inserted, are simulated. Both corewide, spatially in-phase neutron flux oscillations and regional, spatially out-of-phase oscillations are modeled. The results show that the modified TRACG code has sufficient capability to simulate space-dependent transients and is also a useful tool for investigating the fundamental mechanisms behind such transients.

A Study on Regional Stability Analysis Methodology with a One-Point Neutron Kinetics Model

A methodology for boiling water reactor (BWR) regional stability with a one-point neutron kinetics model is proposed from the higher harmonics viewpoint and is verified with the Ringhals-1 stability benchmark test data. A one-point neutron kinetics model for regional stability analysis is derived from the spatial neutron diffusion equation using the mode decomposition technique. From the derivation, the intermode coupled reactivity coefficient is defined and applied to a frequency-domain BWR stability analysis model. The analysis model traces a unit power perturbation and calculates the open-loop transfer function as the power response to the input perturbation. Combined with the aforementioned reactivity coefficient and the asymmetric shape perturbation that reflects the first azimuthal mode, the first azimuthal mode is excited exclusively without any assumption on the excore model. Therefore, the regional stability can be evaluated with a normal recirculation flow model, which is employed for core-wide stability analysis. The methodology is verified with the Ringhals-1 stability benchmark test data, whose stability conditions were widely distributed and suitable for verification. The results show that the proposed methodology is quite appropriate for BWR regional stability analysis.

An experimental study on rewetting phenomena in transient conditions of bwrs

Abstract It is known that rod temperature rise after boiling transition (BT) is not excursive and that the peak cladding temperature (PCT) is suppressed by rewetting to return to nucleate boiling, even if BT occurs under severe conditions exceeding abnormal operational transients for a BWR. The purpose of this study is to develop and verify the rewetting correlation. The rewetting correlation was developed based on single rod data, as a function of quality, mass flux, pressure and heat flux. The transient thermal-hydraulic code used in the BWR design analysis (SCAT) with this rewetting correlation was compared with transient rod temperature result after the occurence of BT obtianed by the 8×8 and 4×4 rod bundle. It is concluded that the transient code with the developed rewetting correlation predicts the PCT conservatively, and the rewetting time well.

Operational Margin Monitoring System for Boiling Water Reactor Power Plants

This paper reports on an on-line operational margin monitoring system which has been developed for boiling water reactor power plants to improve safety, reliability, and quality of reactor operation. The system consists of a steady-state core status prediction module, a transient analysis module, a stability analysis module, and an evaluation and guidance module. This system quantitatively evaluates the thermal margin during abnormal transients as well as the stability margin, which cannot be evaluated by direct monitoring of the plant parameters, either for the current operational state or for a predicted operating state that may be brought about by the intended operation. This system also gives operator guidance as to appropriate or alternate operations when the operating state has or will become marginless.

Incomplete Discrete Wavelet Transform and Its Application to a Poisson Equation Solver

Recently there have been many developments and new applications of mathematical techniques for describing complex functions and analyzing empirical continuous data derived from many different types of signals. The wavelet theory would be the most important and rapidly developing among these techniques. Wavelets have provided a new method for decomposing a function or signal, just like a Fourier expansion. Several articles on wavelets exhibit the wide variety of applications of wavelet analysis and present many of the key developments of the theory. Daubechies[1] proposed families of compactly supported wavelets with certain degrees of smoothness which are constructed by using multiresolution analysis[2].