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

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山口 勝久

Yamaguchi, Katsuhisa

1000MWe級ループ型高速増殖炉を対象に、炉停止失敗事故を想定した場合の事象推移を解析し、高速増殖炉の有する固有の事故緩和能力を評価した。解析では、従来から仮想炉心崩壊事故の解析に用いられてきた反応度フィールドバック以外に、制御棒延長管の軸方向膨張による炉心への制御棒挿入効果、炉心支持板の径方向熱膨張による炉心全体の膨張効果を考慮できるようにした。炉設計では、チムニー型炉上部機構を考え、主循環ポンプ流量半減時間、制御棒待機位置をパラメータと扱った。流量喪失事故は、流量半減時間が40秒以上となるように設計対応するか、10秒以上で制御棒待機位置を250mm炉心に挿入した位置になるようにすることにより、事象推移を緩和し、冷却材最高温度を飽和温度以下に抑えることができる。除熱系機能喪失事故は、約100下廻る温度レベルで流量喪失事故と類似した事象推移をたどる。反応度挿入事故は、60￠の反応度を持つ制御棒が13￠/Sで挿入される設計条件では、最高ナトリウム温度が650以下、燃料溶融割合が25%以下の現行基準以内に納まる。

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.