高速炉の過渡自然循環崩壊熱除去時における炉心高温点評価手法の開発

摘要

ナトリウム冷却高速炉は，炉心出入口温度差が大きrnいことから，浮力による自然循環を利用して炉心の崩rn壊熱を除去させるシステム設計が可能である．このたrnめ大型ナトリウム冷却炉の実用化に向けては，経済rn性と信頼性を高い次元で確保する1つの方策として，rnポンプなど動的機器に頼らず自然循環のみで炉心崩壊rn熱除去を行う「完全自然循環方式」の採用が検討されrnている．%Toward the commercialization of fast reactors, a design study of Japan Sodium-cooled Fast Reactor (JSFR) is being performed. In this design study, the adoption of decay heat removal system operated by fully natural circulation is being examined from viewpoints of economic competitiveness and passive safety. This paper describes a new evaluation method of core hot spot under transient conditions from forced to natural circulation operations that is necessary for confirming feasibility of the fully natural circulation decay heat removal system. The new method consists of three analysis steps in order to include effects of thermal hydraulic phenomena particular to the natural circulation decay heat removal, e.g., flow redistribution in fuel assemblies caused by buoyancy force, and therefore it enables more rational hot spot evaluation rather than conventional ones. This method was applied to a hot spot evaluation of loss-of-external-power event and the result was compared with those by conventional 1D and detailed 3D simulations. It was confirmed that the proposed method can estimate the hot spot with reasonable degree of conservativeness.