Characteristics of small reactor core for transportable reactor

Otani, Nobuo*; not registered; not registered; Haga, Kazuo*

Core physics of small reactor was examined as a part of conceptual design study of space reactor which is an application of transportable reactor. The design requirements were a fast spectrum reactor using nitride fuel and lithium coolant. Firstly, characteristics of typical uranium core and plutonium core was compared by means of one-dimensional calculation using simple sphere model. Followings were revealed from the comparison of both calculation results. (1)Reactivity loss of uranium core in ten years is smaller than that of plutonium core. (2)Shorter lifetime of plutonium core is due to decay of Pu-241. Hence, plutonium core must be designed to compensate higher fuel degradation. (3)Installation of thermal neutron absorber between core and reflector region is effective to extend lifetime of plutonium core. Secondly, study to optimize the design of enriched uranium core was performed as a parameter of core configuration, fuel composition and core size from the point of reactivity adjustment. Attention was focused on criticality of fresh core, reactivity degradation, sub-criticality and reactor shut down margin. The results showed that some safety margin can be obtained although it was less than that objected. A 3-dimensional Monte Carlo code was partly used in the analysis. It was revealed that a more superior code in simulating core configuration is necessary.