RELAP5 post-test analyses of OECD/NEA ROSA project experiments on steam generator depressurization with or without non-condensable gas inflow

非凝縮性ガス流入の有無を条件とした蒸気発生器減圧に関するOECD/NEA ROSAプロジェクト実験のRELAP5実験後解析

竹田 武司 ; 浅香 英明*; 渡辺 正; 中村 秀夫  

Takeda, Takeshi; Asaka, Hideaki*; Watanabe, Tadashi; Nakamura, Hideo

PWRでの0.5%低温側配管小破断冷却材喪失事故(LOCA)を模擬したOECD/NEA ROSAプロジェクト実験をLSTFを用いて行った。高圧注入系全故障と蓄圧注入タンクからの非凝縮性ガス(空気)流入の有無を仮定し、安全注入設備作動信号発信後10分に逃し弁全開による蒸気発生器(SG)二次側減圧を開始した。また、低圧条件下での自然循環(NC)現象をよく観察するため、一次系圧力が2MPaに低下時にSG安全弁を全開して減圧を促進させるとともに、低圧注入系不作動を仮定した。非凝縮性ガスの一次系ループへの流入により、SG U字管内凝縮熱伝達が阻害され、一次系減圧率は低下したが、非凝縮性ガスの流入後でさえも二相NCは継続した。非凝縮性ガスの影響下で、順流となるSG U字管本数がループ間で異なり、非対称なNC挙動となった。原子力機構改良版RELAP5/MOD3.2.1.2コードは、特に非凝縮性ガス流入後の一次系ループ流量,SG U字管水位挙動の予測に課題が有り、正確な予測には非凝縮性ガス混合条件下での凝縮熱伝達モデルやSG U字管モデルの改良が必要と考えられる。

Two LSTF experiments were conducted for OECD/NEA ROSA Project simulating PWR 0.5% cold leg small break LOCA. Steam generator (SG) secondary-side depressurization was performed by fully opening the relief valves at 10 minutes after safety injection signal with or without non-condensable gas (air) inflow from accumulator tanks with total failure of high pressure injection system. Further assumptions were made to conduct enhanced SG depressurization by fully opening the safety valves when the primary pressure decreased to 2 MPa and no actuation of low pressure injection system, both to well observe natural circulation (NC) phenomena at low pressures. The primary depressurization rate decreased when non-condensable gas started to enter primary loops because of degradation in the condensation heat transfer in SG U-tubes, while two-phase flow NC has continued even after non-condensable gas inflow. Asymmetric NC behaviors appeared between two loops due probably to different number of forward flow SG U-tubes which would have been under influences of non-condensable gas. Post-test analyses by using JAEA-modified RELAP5/MOD3.2.1.2 code indicated that the code has remaining problems in proper prediction of primary loop flow rate and SG U-tube liquid level behaviors especially after non-condensable gas inflow. The improvement of the condensation heat transfer model under non-condensable gas mixture condition and the SG U-tube model may be necessary for correct analysis of the LSTF SG depressurization transients.