Masanao Moriwaki

A New Direct Calculation Method of Response Matrices Using a Monte Carlo Calculation

A novel direct calculation method of response matrices on heterogeneous lattices by using the Monte Carlo method is proposed. These direct response matrices (DRMs) can be used in core calculations in place of the conventional homogenized lattice constants. The DRMs are formalized by four sub response matrices (sub-RMs) in order to respond to a core eigenvalue, k; thus the DRMs can be re-evaluated on each outer iteration in the core calculations. The sub-RMs can be evaluated by analyzing each neutrons trajectory from ordinary lattice calculations with the Monte Carlo code. Since these sub-RMs are calculated directly under an actual complex assembly geometry, i.e., without a homogenization process, intra-assembly heterogeneous effects can be reflected on global partial current balance calculations. With using two of the sub-RMs, which deal with neutron production probabilities for each fuel pin, and the obtained partial current balance, pin-wise neutron production distributions can also be reconstructed. T...

ABWR-II Core Design with Spectral Shift Rods for Operation with All Control Rods Withdrawn

Abstract An innovative reactor core concept applying spectral shift rods (SSRs) is proposed to improve the plant economy and the operability of the 1700-MW(electric) Advanced Boiling Water Reactor II (ABWR-II). The SSR is a new type of water rod in which a water level is naturally developed during operation and changed according to the coolant flow rate through the channel. By taking advantage of the large size of the ABWR-II bundle, the enhanced spectral shift operation by eight SSRs allows operation of the ABWR-II with all control rods withdrawn. In addition, the uranium-saving factor of 6 to 7% relative to the reference ABWR-II core with conventional water rods can be expected due to the greater effect of spectral shift. The combination of these advantages means the ABWR-II with SSRs should be an attractive alternative for the next-generation nuclear reactor.

Improvement of Monte Carlo Lattice Burnup Calculation Performance with the Correlated Sampling Method

For next generation reactor designs, which are attempting wide variations of assembly configurations, the flexibility Monte Carlo method holds is attractive, but still costly for repetitive design study works. This paper presents an advanced correlated sampling (ACS) method which was developed to speed up Monte Carlo lattice burnup calculations. The ACS method is the combination of the correlated sampling method and a pseudo-scattering technique. All burnup steps are considered as consecutive perturbed problems using the same neutron collision history, which is pre-calculated based on a selected unperturbed problem. Since neutron weights can be adjusted on every collision point, rather than along paths between them, the perturbed calculation is very fast and the neutron collision history is light enough to be stored in memory or physical storage, which is an indispensable feature for consecutive perturbed calculations. The presented theory shows that the ACS method has good potential to work for a wide range of neutron absorption variations, the dominant perturbation in the lattice burnup. In an example calculation on a BWR lattice, the ACS calculation results of 600,000 neutrons/step agree well with the independent Monte Carlo runs of 20,000,000 neutrons/step within 0.1%dk/k in terms of kœ throughout 95 steps (~50GWd/t). Average calculation time of neutron tracking with the former method is 3.4 s/step with 600,000 neutron histories on a single processor of an Alpha21164-600 MHz, and the speed-up factor against the Monte Carlo calculation turns out to be about 100.

Multi-Assembly Analysis with an Advanced Correlated Sampling Method

The Advanced Correlated Sampling (ACS) method, which can greatly enhance the calculation efficiency of Monte Carlo (MC) methods using a perturbation technique and a pseudo-scattering technique, is improved so as to be able to handle multi-assembly analysis. Two boundary connection methods—the neutron path connection and neutron current connection methods—are developed, and their accuracies for a 2×2 multi-assembly system are confirmed. The speedup factor against the corresponding direct MC calculation is a maximum of about 100. These boundary connection methods are also applied in pin-wise 2-D quarter core calculations as an example of a large-scale system. A newly developed perturbed neutron source iteration technique allows for very smooth source convergence. Furthermore, by incorporating the Direct Response Matrix (DRM) method as the acceleration means for the source convergence, the number of source iteration cycles is significantly shortened. The results confirm that this improved ACS method has the ability to perform pin-wise whole core calculations very accurately and efficiently.