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The efficient use of nuclear fuel is one of the important issues in the current development of nuclear reactors due to the limitation of natural uranium resources and the need for overall economy. Simplicity in the reactor design could further increase its economy while making it easier to operate. A pebble bed reactor is one of the most promising reactor systems to fulfill these criteria. The purpose of this study was to design a simplified pebble bed reactor by removing the unloading devices from the system and then optimizing the fuel composition and reactor configuration so that the system could achieve better burnup and use scarce uranium resources more effectively. A computer code based on the Monte Carlo method was developed and used in this study in order to obtain precise calculation results due to the weakness of the diffusion method in treating the large cavity region in the core during most of the reactor operation. With this code, analysis and optimization were performed for a 110 MW simplified pebble bed reactor using peu à peu fuel loading scheme. An optimized design using 12% uranium enrichment and 7% packing fraction was the result, calculated to achieve high burnup of 135 GWD/T for more than 20 years' operation time. Neutronic analysis, steady-state thermal hydraulic analysis, and fuel economic analysis for this optimized design are discussed in this study.

The capture cross sections and capture-ray spectra of 209 Bi were measured in a neutron energy region from 5 to 80 keV and at 520 keV, using pulsed keV neutrons from the 7 Li(p; n) 7 Be reaction and a time-of-flight method. The capture rays from a bismuth or standard gold sample were detected with a large anti-Compton NaI(Tl) spectrometer. The capture yield of the bismuth or gold sample was obtained by applying a pulse-height weighting technique to the corresponding capture-ray pulse-height spectrum. The derived capture cross sections from 5 to 80 keV were in good agreement with recent measurements, but that at 520 keV was about half of previous measurements. This large discrepancy at 520 keV was ascribed to the incorrect background-subtraction in the previous measurements from a comparison between the present and previous capture-ray spectra. Strong transitions from the capture states to low lying states of 210 Bi were observed in the present-ray spectra. The multiplicities of observed rays were obtained from the-ray spectra.