Load Follow Simulation of Three-Dimensional Boiling Water Reactor Core by PACS-32 Parallel Microprocessor System

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Abstract
The three-dimensional boiling water reactor (BWR) core following the daily load was simulated by the use of the processor array for continuum simulation (PACS-32), a newly developed parallel microprocessor system. The PACS system consists of 32 processing units (PUs) (microprocessors) and has a multiinstruction, multidata type architecture, being optimum to the numerical simulation of the partial differential equations. The BWR core model includes the modified two-group finite difference, coarse-mesh model for neu-tronics, steady-state model for thermohydraulics, criticality control by core coolant flow, and the time-dependent solution of iodine-xenon dynamics with constant flux level. The analysis of the parallel processing program revealed that the overhead is independent from the number of PUs and that the efficiency of PUs, i.e., the ratio of effective calculation over total, amounts to 75%, even up to 90% if it is limited to the core part. Simulation was made on the daily load follow for 144 h including the weekend, which took 1.3 central processing unit hours by the PACS system. The PACS system demonstrated a computation speed nearly one-tenth that of the large-scale high-speed general purpose computer, with a 25 times better cost-performance ratio and showed that the system could be used as the practical BWR core simulator with more complicated core models.

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