Key Technological Design Study of a Large LMFBR(2); System Dyrawics Analysis for Mitigating ATWS Consequences of a 1000MWe Loop-Type LMFBR

Yamaguchi, Katsuhisa

A system dynamics analysis was applied to a 1000 MWe loop-type liquid-metal fast breeder reactor (IMFBR) to examine influence of possible innovative reactor designs on mitigating consequences of anticipated transients without scram (ATWS). Theanalysis included all 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 reactor core driven by the expansion of the core support plate (CSP). An upper-core flow chimney was considered to make the CRD expansion feedback effective. The flow coastdown rate of the primary pump and the initial position of the control rod (CR) were treated as parameters. ATWS initiators examined were unprotected loss-of-flow (ULOF), loss-of-heat-sink (ULOHS) and transient overpower (UTOP). The ULOF accident was mitigated and peak sodium temperature was suppressed below boiling point by using the primary pump having a 40 s halving time of flow coastdown. The halving time could be shortened to 10 s by assuming that the CR was initially inserted into the active core by about 250 mm. The CRD expansion feedback controlled the earlier transient, and the CSP expansion feedback became dominant in the latter phase. The ULOHS consequence was eompletely enveloped in that of ULOF accident. The sodium temperatures in the primary system became lower than the ULOF case by about 100 C. The UTOP accident conceivable from the current plant design, i.e., the reactivity insertion of 60 ￠ with the rate of 1 to 3 ￠/s, suppressed sodium temperatures and fuel melt fractions below 650 C and 25 5, respectively.