Yuichiro Yoshimoto

Space-dependent analysis of BWR core nuclear thermal hydraulic instability and thermal margin

Abstract Nuclear thermal hydraulic oscillations in BWR cores were analyzed by the space-dependent BWR core transient program STANDY. In a simulation of instability in the Lasalle-2 unit, the oscillations that caused a scram were successfully reproduced. The maximum thermal margin decrement was far smaller than the initial margin, and significant margin to thermal limits existed at the time of scram. An analysis of hypothetical control rod insertion suggested that the oscillations could have been suppressed by only a few control rods. Analyses of a core destabilized by various parameters were also carried out to examine thermal margin sensitivity during the oscillations. The results showed that, regardless of which parameters were assumed to make the core unstable, thermal margin changes were substantially smaller than the initial margin expected under operation conditions to cause an instability.

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.

Analysis of BWR core nuclear thermal hydraulic oscillation with three dimensional transient program

Abstract A three-dimensional BWR core dynamics program STANDY has been developed. STANDY takes into account parallel channel effect and evaluates fuel thermal margin. Peach Bottom 2 and Vermont Yankee stability test data have been analyzed by STANDY. Calculated decay ratios and resonance frequencies agreed well with measured data. Limit cycle oscillation at Vermont Yankee test has been also simulated. Oscillation amplitude agreed well with experiment. A hypothetical core condition has been made up to examine unstable oscillations in BWR core. Analyses of the core revealed that oscillation at conditions close to instability initiation reaches small amplitude limit cycle, and change in fuel thermal margin is very small during the limit cycle. Although increase in core power or decrease in flow causes rapid increase in power oscillation amplitude, the ratio of thermal margin change to power amplitude stays almost constant. It has also been found that increase in hot channel power level does not necessarily cause larger thermal margin change because higher power may widen frequency difference between core average and hot channel.

Applicability of a Multivariable Autoregressive Method to Boiling Water Reactor Core Stability Estimation

A multivariable autoregressive (MAR) method is applied to the core stability estimation of a boiling water reactor-5 operation. Noise data measured during steady-state operations at startup tests are used. In this method, the closed loop transfer function from reactor pressure to reactor power is identified from reactor noise data and transformed into an impulse response function. The decay ratio representing stability characteristics is evaluated from this function. The variation range of decay ratio estimates obtained by this method is sufficiently small, if the analyzing conditions are appropriately selected. The value of the decay ratio is 0.23 during natural circulation and decreases with core flow, reaching close to zero at the rated power. A similar power dependence for the decay ratio is seen in results from a core stability analysis code. The MAR method is a useful tool for stability estimation, even if no external disturbance tests are conducted.

Development of a three-dimensional core dynamics analysis program for commercial boiling water reactors

Development and qualification results are described for a three-dimensional, time-domain core dynamics analysis program for commercial boiling water reactors (BWRs). The program allows analysis of ...

CFD Analysis Applications in BWR Reactor System Design

Computational fluid dynamics (CFD) analysis has been used to evaluate phenomena related to the flow since the late twentieth century. Here, through some examples of its applications, the roles of CFD analysis in the actual design process are shown. The first example is an application to the design improvement for flow stabilization at a cross branch pipe in the recirculation loop of the jet pump-type BWR. The second example is an application to evaluations of the ABWR lower plenum flow characteristics and FIV stresses. The third example is an application to the development of a thicker reactor internal pump nozzle for seismic performance improvement. All of these applications were confirmed by tests before being applied to the design of actual reactor structures.

Qualification of a Three-Dimensional Core Dynamics Analysis Program Coupled with a Detailed Mesh Division for Commercial Boiling Water Reactors-I

In the stability licensing analysis and evaluation of boiling water reactors (BWRs), frequency-domain stability analysis programs have been used in Japan. To back up the licensing analysis and evaluation, time-domain, multiregional analysis programs have been used because more detailed analytical results can be obtained by these programs with little more computer time than that used by the frequency-domain stability analysis programs. In the backup calculation by time-domain, multiregional analysis programs, many trial-and-error experiments and much expertise on the reactor core radial regional division and on the initial disturbance input are necessary to analyze properly the stability of the BWR core, particularly its regional nuclear thermal-hydraulic stability. A three-dimensional time-domain core dynamics analysis program called SUPER-STANDY was developed with a detailed mesh division that makes various trial-and-error procedures and experience-based expertise unnecessary and that can treat the stability peculiar to the BWR core accurately. The program was applied to a plant where regional instability was observed, and the results were qualified. They showed that BWR stability can be analyzed using SUPER-STANDY by adding only the core uniform initial disturbance input without considering the reactor core radial regional division. It was determined that core regional mode instability can be properly analyzed by the multiregional analysis program (a) by dividing the core into six or more radial regions, (b) by specifying the hot fuel bundle as one region, and (c) by specifying the surrounding fuel bundles around the hot fuel bundle as one region. A visual display system was also developed for a huge number of stability data and core nuclear thermal-hydraulic characteristics, which are connected to each other in a complex way. These are obtained by the SUPER-STANDY program.

An on-line method to monitor bwr core stability based on an autocorrelation method

Abstract An on-line monitoring method is introduced for BWR core stability. The method utilizes only autocorrelation values for two delay time intervals. Its simplicity makes it suitable for an on-line monitor. Accuracy of the core decay ratio calculated by the method improves as the core condition approaches instability. The error in the decay ratio for regional limit cycle oscillations is 0.2% when calculated from local signals in the most unstable region, and 4% when calculated from core averaged signals.