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Title: Engineering design and analysis aspects of the beryllia moderated pebble bed reactor reference study.
Authors: Ebeling, DR
Hayes, JE
Issue Date: Oct-1966
Publisher: Australian Nuclear Science and Technology Organisation
Abstract: Design and analysis of a carbon dioxide gas cooled beryllia moderated pebble bed reactor system has been undertaken to assess its technical feasibility and economic potential from the engineering point of view. This report describes the design and shows how subsequent analysis indicated the likely costs of such a system and possible technical improvements. It was found that data vital to a detailed design would not be available for some years and it was decided to proceed initially with a reference design based on optimistic predictions of these data. This was expected to indicate the materials and physics requirements necessary to achieve an attractive working design. The two most important predictions from the initial work on neutron physics and materials properties were that a maximum value of 1 75 for the burnup (in terms of fissions per initial fissile atom (FI.F.A,)) could be obtained with plutonium fuel from natural uranium reactors in which the fuel had been burned to 3000 MWd/tonne, and that fuel elements of .1 — 3/8 inch diameter should survive sufficiently intact in a core in which the coolant flow was upwards at an average core power density of llW/cmJ without an embarrassing fission product release due to an excessive number of failed fuel elements. For this reference design a moderator—fuel ratio of 1650 was chosen. It was assumed that the fuel, cycle for this design was open, that is, spent fuel was discarded. A 200 MWe unit upflow design is presented and the economics of such a system described and extrapolated to cover a range of sizes. Further work using the reference design as a basis was done to investigate a closed fuel cycle and the effect of varying the major design parameters Later nuclear analysis indicated that the burnup would be limited to a value for F.I.F.A, of about 14 for open cycle working. Economic analysis indicated that the best method of operating such a design in a 5000 MWe system would be as a one-pass core at a burnup (F..I.F.A,) of 1-0 - 1-2 with U233 recycle. Since the economics of the cycle tended to remain fairly constant over a wide range of moderator ratio a larger moderator ratio (2000-2500) could be chosen for the recycle system-Calculations indicated that a desirable negative temperature coefficient and a satisfactory control and shutdown system should be achievable. However, to verify this a deep reactor physics investigation would be necessary- including hot critical assembly tests. The analysis of the fuel element thermal stress behaviour was also not conclusive because the material behaviour depends largely on fabrication techniques which are not fully established. There seems a good possibility that the requirements will be met but difficulties with reprocessing are expected and further investigations into thermal stress and fission product retentivity by means of a test bed or in—pile experiment would be essential to establish technical feasibility-Comparison with the 2 x 500 MWe CANDU system shows that even with the presently most optimistic assumptions the reference design upflow P-BR system is economically inferior. It is suggested that further investigation of superposed irradiation damage, contact stresses wear, and adhesion could lead to a feasible downflow design. When combined with investigations of approach to equilibrium, chemical reprocessing, and topping cycles this might lead to significant improvements in unit costs but at this stage it seems unlikely that a system could be devised which would be sufficiently attractive to justify its development as a large power station
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