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

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山口 勝久

Yamaguchi, Katsuhisa

酸化物燃料を用いた1000MWe級ループ型高速増殖炉を対象に、FBR固有の安全性に係る性能について解析し、熱流動一構造一核のカップリングによる負の反応度効果で、所内全動力電源喪失に伴う流量喪失・炉停止失敗事故を想定した場合でも、炉が核的停止に導けるようにするための設計への要求事項を求めた。チムニー型炉上部構成の採用、流量半減時間の延伸、制御棒の部分挿入状態での待機等の条件を組み合わせ、以下の結論を導いた。(1)主循環ポンプ流量半減時間=10秒、制御棒待機位置=0.2m炉心へ挿入、ポニーモータ流量=30%とすれば、初期の温度オーバーシュートを840(非沸騰)に、数時間後の整定温度を600630に抑えることができる。(2)炉心を偏平化し、燃料の線出力、Doppler係数、ナトリウムボイド係数を各々20%低下させれば、ポニーモータ流量=20%で金属燃料を使わずとも、より裕度の高い結果にできる。

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.