Key Design Parameter Study(1) for Large Scale-up Fast Breeder Reactor

Yamaguchi, Katsuhisa

A system dynamics analysis was applied to a 1000 MWe loop-type liquid-metal fast breeder reactor (LMFBR) with mixed oxide fuel to examine inherently safe, passive shutdown performance of the reactor under anticipated transients without scram (ATWS) conditions. The analysis included all of the reactivity feedbacks having been employed in current analyses of hypothetical core disruptive accidents (HCDAs). In addition, the present analysis stressed inherent responses of the reactor system by including structural reactivity feedbacks due to axial expansion of control rod driveline (CRD) and radial expansion of the reactor core driven by the expansion of the core support plate (CSP). The ATWS initiator examined was an unprotected loss-of-flow (ULOF) accident caused by a site power blackout. Some design options were considered: An upper-core flow chimney was introduced to make the CRD expansion feedback effective. The flow coastdown rate of primary pumps was selected to have a halving time of 10 seconds. The initial position of the control rods (CRs), the pony-motor flow level, and so on, were treated as parameters. By assuming that the CRs were initially inserted into the active core by about 0.2 m and that a pony-motor has a capacity of driving more than 20 % of the rated flow, the ULOF consequence was mitigated and the peak sodium temperature was suppressed below boiling point. The CRD expansion feedback controlled the short-term transient, and the CSP expansion feedback became dominant in the later phase. The asymptotic temperature of the primary heat transport system against the upset was successfully lowered below 650 C by optimizing the design parameters.