鉛ビスマス冷却中型炉の炉心・燃料設計検討

Design study of medium size lead-bismuth cooled core

山下 巧; 三田 敏男; 水野 朋保

Yamashita, Takumi; Sanda, Toshio; Mizuno, Tomoyasu

FBRサイクル実用化戦略調査研究(FS)においては,様々な冷却材を用いた高速増殖炉が検討されており,その候補の1つとして鉛ビスマス冷却炉心燃料概念について設計研究を行ってきている。ここでは,平成13年度に実施した鉛ビスマス冷却中型炉の炉心・燃料設計に対して,DDI(ダクト(ラッパ管)間相互作用)を回避する観点から燃料仕様を見直し,FSフェーズII中間取り纏め(平成15年度)としての強制循環方式と自然循環方式の代表炉心を設計して,いずれもFSの設計目標を満たす見通しを得た。750MWe強制循環炉心(FC)と550MWe自然循環炉心(NC)の特性を比較した結果,FCを有望概念に選定した。FCの利点を以下にまとめる。(1)全炉心平均燃焼度(炉心+ブランケット)が大きい(FC:105GWd/t,NC:89GWd/t)。(2)装荷時核分裂性Pu重量が少ない(FC:5.8t/GWe,NC:8.0t/GWe)。(3)出力が大きくても炉心等価直径が小さい(FC:441cm,NC:464cm)。(4)NCは自然循環力の確保のため,炉容器軸長が長くなり耐震性の観点からは課題が多い。なお,本検討の代表炉心は炉心取出平均燃焼度の目標値(150GWd/t)を達成したが,被覆管の高速中性子照射量がODS鋼およびPNC-FMS鋼の開発目標の目安値(510 の28乗 n/cm2,E0.1MeV)を上回ることを許容した。これに対し,照射量を目安値以下とした照射量目標対応炉心は,炉心取出平均燃焼度がFCで129GWd/t,NCで146GWd/tとなった。さらに,MOX燃料および金属燃料と軽水炉回収TRU燃料の適用性を確認し,鉛ビスマス冷却炉炉心の柔軟性・多様性を示した。また,最近の鉛ビスマス中の被覆管腐食試験の知見では,現設計で用いた被覆管腐食評価式は非安全側で,かつ現状の制限目安の被覆管最高温度650度Cを570度Cまで下げて設計する必要の可能性が出てきている。現行腐食式を用いて被覆管最高温度を低下した場合の影響について予備解析を実施し,炉心特性への影響は小さいという結果を得た。

In "Feasibility Studies on Commercialized Fast Breeder Reactor Cycle System(FS)", various types of fast breeder reactor using different coolant are studied, in which lead-bismuth is one of the subject. In JFY 2003 study of FS phase-II, design studies of medium scale forced /natural circulation nitride fuel core (FC/NC core) were carried out considering DDI (duct-duct interaction) which was not considered in JFY 2001 studies. The prospect of satisfying a design target of the core performance was acquired by improved design. By comparing properties of FC core (750MWe) and NC core (550MWe), we recommended the FC core as a hopeful one to design. The FC core was superior to the NC core in the following points, (1)Larger average burn-up (core and blanket) (2)Less Pu-fissile loaded weight (3)Smaller core equivalent diameter in spite of larger generating power (4)The NC core has much problems from the viewpoint of earthquake-resistant design. A design target of high burn-up of 150 GWd/t was satisfied, but fast fluence on clad is over the development target of ODS steel and PNC-FMS stee1 (5 10n/cm, E0.1Mev). Cores in which fast fluence was restricted to less than the target were also designed. MOX fuel and metal fuel cores were studied, and the effect of composition of TRU fuel recovered from LWR to core performance was also investigated to show flexibility and diversity in the lead-bismuth cooled core. In recent studies, it is becoming clear that the present evaluation for the clad corrosion in lead-bismuth gives non-conservative result and lowering the highest clad temperature will be required in core design. The lowering of temperature was permissible according to preliminary analysis using the present corrosion correlation.