Cascade reactor concept for neutron multiplication of subcritical core

A 'cascade reactor concept' represents a kind of system with an accelerator driven subcritical core consisting of two sections with different neutron spectra. It can generate more neutrons than one-section fast core with the same subcriticality and the same neutron source strength. This study deals with the effect of the cascade reactor concept by analogy of a coupling core and direct simulation using a Monte Carlo code. The proton energy is assumed to be 1.0 GeV and a Pb-Bi target is at the center of the core. The cascade reactor has two sections. The inner section is a fast flux core and the outer section is a thermal flux core. There is a neutron absorber between two sections. As the result of direct simulation, the cascade reactor concept is confirmed to generate twice as many neutrons as a one-section reactor with the same neutron source. It is found that the thickness of fast core should be small to enhance the cascade effect, the thickness of Sm absorber is optimized to be 5 cm and that cascade effect increases when the subcriticality becomes small. These results are consistent with the basic theory of coupling core. (authors)