Optimization of Start-up Fuel Mana gement for OTTO Cycle Pebble Bed Reactor

Abstract
Fuel management of once-through-then-out (OTTO) cycle Pebble Bed Reactors (PBR) was studied, in particular the
start-up condition of the core before it achieves the equilibrium condition. Optimum and simple fuel management
performance in the start-up condition is important for the practical deployment of PBR. There is no option for fuel
re-insertion in an OTTO cycle PBR, hence nuclear fuel utili
zation is an important factor not only in the equilibrium
condition but also in the start-up condition. The purpose of the study was to find an optimum procedure to improve
burnup performance of fuel management in startup of the OTTO cycle PBR. Initial Heavy Metal (HM) loading in
the core, power density, and multiplication factors were the main parameters investigated. The target of the analysis
was a small sized 10MW PBR. A newly developed code system for OTTO cycle PBR was used. The code system is
capable of performing neutron transport and depletion calculations of the OTTO cycle PBR covering whole of its
fuel management scheme from initial loading to equilibrium condition. In this study, fuel composition in the start-up
condition was limited to the same composition (a single enrichment) as the fuel in equilibrium condition for
simplicity of the whole fuel management. The equilibrium condition of the PBR was investigated first. Based on the
equilibrium condition two start-up fuel management schemes, a mixed and top-bottom scheme, were investigated. It
was found that mixed scheme is better compare to top bottom scheme in achieving efficient HM-loading. Mixed
scheme also gave a lower maximum power density. For the chosen target equilibrium design with 10wt%
enrichment and 12g-HM/pebble the minimum initial HM-loading using mixed and top-bottom scheme was 97.1 kg
and 161.9 kg, respectively. While the maximum power density at that minimum initial loading was 4.5 W/cm
3
and
4.9 W/cm
3
, respectively.