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Report No.
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Design Studies on Small Fast Reactor Cores (II)

Takaki, Naoyuki; not registered ; Mizuno, Tomoyasu

Core design studies have been performed to make a comparative evaluation on the effects of different reactivity control mechanisms and different coolant circulation methods of small fast reactors as follows, (a) 150MWe reflector-controlled forced-circulation core, (b) 150MWe forced-circulation core with control rods, (c) 150Me natural-circulation core with control rods and (d) 50MWe forced-circulation core with control rods. In the present study, the fuel reloading interval of 10 years is targeted under following conditions as fuel pin diameter $$<$$ 8.5mm, pressure drop for forced-circulation core $$<$$ 0.75kg/cm2 and sodium void reactivity for forced-circulation core $$<$$ 2$.The reactivity control mechanism of the reflector-controlled core is composed of movable radial reflector and a few additional control rods. This core attains 10 years long life and burnup of about 48GWd/t. The ATWS analyses indicate the possible passive safety feature which does not rely on the effects of radial expansion of core support plate and self-actuated shutdown system.The core with control rods shows slightly superior criticality than the reflector-controlled core due to the closer arrangement of radial reflectors. The shorter core column length and reduced fuel inventory result in improvements on the sodium void reactivity and burnup of fuels. This core also shows possible passive safety features in case of ATWS events. Ten years of control rod life time is calculated to be achievable from a viewpoint of absorber cladding mechanical interactions. Large core diameter of about 3.3m and small pin gap less than 1mm are common problems of the reflector-controlled core and core with control rods. Those are ascribed to the sodium void reactivity limitation and further works considering the economy and fabrication feasibility are necessary. The natural-circulation core with low pressure drop has large core diameter of about 3.8m and low fuel burnup of around 36GWd/t. A prominent characteristic

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