Improvement of CHF correlations for research reactors using plate-type fuels

板状燃料を用いた研究炉用の限界熱流束相関式の改良

神永 雅紀  ; 山本 和喜 ; 数土 幸夫

Kaminaga, Masanori; Yamamoto, Kazuyoshi; Sudo, Yukio

通常運転時において、下向流により炉心を冷却している研究用原子炉では、1次冷却材の流量が喪失した場合、炉心内で強制循環による下向流から自然循環による上昇流へと流れの向きの逆転が生ずる。このため、設計では原子炉停止後の補助ポンプ等による炉心冷却の必要性、流れが逆転する際の燃料の安全性を評価する上で流速零を含むCCFL条件下の限界熱流束(CHF)の検討が重要となる。著者らがこれまでに提案したCCFL条件下のCHF相関式は、保守的な評価をするために冷却材のサブクール度を考慮しなかった。本研究では、垂直矩形流路におけるCCFL条件下のCHFについて、CHFに及ぼす流路入口サブクール度及び軸方向出力分布の影響を既存の実験データに基づき定量的に評価し、新たな相関式を提案した。さらに、提案したCHF相関式を安全評価に適用した場合の具体例を解析結果と共に示し、これまでの評価の保守性を定量的に示した。

In research reactors, plate-type fuel elements are generally adopted so as to produce high power densities and are cooled by a downward flow. A core flow reversal from a steady-state forced downward flow to an upward flow due to natural convection should occur during operational transients such as "Loss of the primary coolant flow". Therefore, in the thermal hydraulic design of research reactors, critical heat flux (CHF) under a counter-current flow limitation (CCFL) or a flooding condition are important to determine safety margins of fuel against CHF during a core flow reversal. The authors have proposed a CHF correlation scheme for the thermal hydraulic design of research reactors, based on CHF experiments for both upward and downward flows including CCFL condition. When the CHF correlation scheme was proposed, a subcooling effect for CHF correlation under CCFL condition had not been considered because of a conservative evaluation and a lack of enough CHF data to determine the subcooling effect on CHF. A too conservative evaluation is not appropriate for the design of research reactors because of construction costs etc. Also, conservativeness of the design must be determined precisely. In this study, therefore, the subcooling effect on CHF under the CCFL conditions in vertical rectangular channels heated from both sides were investigated quantitatively based on CHF experimental results obtained under uniform and nonuniform heat flux condition. As a result, it was made clear that CHF in this region increase linearly with an increase of the channel inlet subcooling and a new CHF correlation including the effect of channel inlet subcooling was proposed.