Pulsed neutron method and dynamic measurements analysis allow reactivity assessment in subcritical systems. However, in ADSR, detectors responses dependency on their positions is amplified by core heterogeneities like zoning and the presence of the beam neutron source itself. Neutron decay constant measurements by optical fiber have been conducted in KUCA to evaluate the variation on deduced reactivities, and compared with calculations.
A study of PBMR core has been carried out applying once-through refueling scheme. In this scheme only fresh fuel pebbles are inserted into the top of reactor core and will be removed to spent-fuel tank after 1-pass. All of pebbles will reach their target burnup before removed from reactor core. Therefore power plant could be impressively simpler due to the lack of fuel checking system. Lattice and full core calculations with continuous fuel discharged are performed by MVP code via PERL scripts. The large excess reactivity and high power peaking factor are controlled by adding burnable poison particles containing B4C and Gd2O3 into fuel pebble.