Osamu Yokomizo

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.

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.

A new boiling water reactor core concept for a next-generation light water reactor

In this paper a boiling water reactor (BWR) core concept that meets various requirements for a next-generation light water reactor is proposed. This BWR core can be operated as either a high-burnup core or a high-conversion core simply by replacing the fuel assemblies and control rods. The high-burnup core is suitable for a once-through nuclear fuel cycle and has a low fuel cycle cost due to the adoption of advanced spectral shift technology. The high-conversion core is suitable for nuclear fuel recycling and reaches a high-conversion ratio by adopting a tight-lattice arrangement of mixed-oxide fuel rods in the fuel assemblies and using control rods with a zirconium follower. The reactor structures are essentially identical, and they are designed to be as simple as the current BWR to achieve high reliability. The reactor core also has high operability due to the spectral shift water rods that are operated with all control rods withdrawn. At reactor shutdown, the core has a large reactivity control capability due to the cruciform control rods with wider blades and has an ample safety margin.

An equivalent linearization technique for two-phase flow instability analysis

Abstract An equivalent linearization technique has been developed to analyze two-phase flow instability oscillation with finite amplitude in the frequency-domain. A one dimensional slip flow model has been used to formulate equivalent linear equations, and implemented into a computer program. A simple boiling channel has been analyzed by the program. Results for very small amplitude oscillations agreed well with results by a detailed linear frequency-domain program. Analyses for larger amplitude oscillations gave more stable results. Limit cycle amplitudes have been calculated from the amplitudes at which channel characteristics change from unstable to stable as the amplitude is increased.

Man--machine communication system for boiling water reactor core management planning

A man-machine communication system has been developed for boiling water reactor (BWR) core management planning to provide a very flexible tool, which is complementary to automated optimization programs that maximize or minimize one particular performance index under certain constraints. A three-dimensional BWR simulator, which can cover a wide range of BWR operating conditions, has been developed and coupled with a graphic display serving as a main input-output controlling device. The system has been successfully applied to generate a long-term control rod programming of a BWR in which locally poisoned fuel assemblies are loaded. The time required for one cycle analysis is approximately 3 h, out of which the actual computation time is only 4 min with an average of three trials of rod pattern search per exposure step. The quick response (5 sec) and the visualized results on the screen are very helpful in understanding the complicated characteristics of the BWR core, and it is found that this kind of tool has a very great educational effect. A similar approach is expected to be applied in other fields such as core design and safety analysis, as well as in core management.

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 ...

Spectral shift rod for the boiling water reactor

Abstract A Boiling Water Reactor core concept has been proposed using a new fuel component called spectral shift rod (SSR). The SSR is a new type of water rod in which a water level is formed during core operation. The water level can be controlled by the core recirculation flow rate. By using SSRs, the reactor can be operated with all control rods withdrawn through the operation cycle as well as that a much larger natural uranium saving is possible due to spectral shift operation than in current BWRs. The steady state and transient characteristics of the SSRs have been examined by experiments and analyses to certify the feasibility. In a reference design, a four times larger spectral shift width as for the current BWR has been obtained.

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.