An Investigation on the Possible Radioactive Contamination of Environment Due to a Large Steam-Line Break, Containment Rupture and Station Blackout Accident in Advanced Boiling Water Reactors
A. Elshahat (Nuclear and Radiation Engineering Department, Faculty of Engineering, Alexandria University), F.G. Thulu (Nuclear and Radiation Engineering Department, Faculty of Engineering, Alexandria University; The Malawi Polytechnic, University of Malawi)
In this work, a possibility of environmental contamination by radioactive elements due to a large steam-line break accident concurrently with containment failure followed by total station black out (SBO) was investigated for advanced boiling water reactors (ABWR) nuclear power plant (NPP). The assumption was that an earthquake exceeding plant design limit of acceleration of 0.3g caused a crack in the containment of 2000 cm2 and a large steam-line break of 800 cm2 inside the dry well, preceded by loss of offsite power. Onsite emergency diesel generators were actuated and started to supply AC power for residual heat removal from the core, but were knocked out i.e by the tsunami. There was limited DC battery power for critical safety systems leading to SBO. In the simulation, it was assumed that off-site AC power supply was interrupted after 30 seconds. Personal Computer Transient Analyzer was used to generate the response data of the plant safety systems numerically for the postulated accidental condition. Simulation was run for time duration of 500 seconds since critical of the passive and active safety features of the plant should respond within 50 seconds after transient. From simulation results, a steam-line break caused a rapid drop in coolant pressure, triggering a SCRAM within 19 seconds. The peak temperatures of fuel and fuel cladding recorded were around 1240oF and 540oF respectively, which were both within the safety limits. The pressure inside reactor boiler had not undergone any significant changes, showing no sign of failure. Again, the pressure inside the reactor containment building was kept within 45 psia by the safety systems. Finally, the readings from radiation monitor showed that there was no noticeable release of radioactive elements to the environment during the accident. Therefore, it could be concluded that the release of radioactive elements in the surrounding environment during a containment break and a large steam-line break preceded by SBO accident was very unlikely provided that the plant safety systems are fully functional. The data obtained from the simulation were satisfactorily consistent with PSAR (Preliminary Safety Assessment Report) data regarding such accident.
改良型沸騰水型原子炉での蒸気管大破断、格納容器破断、及び全交流電源喪失事故により起こりうる環境放射能汚染に関する調査
F.G. Thulu (Nuclear and Radiation Engineering Department, Faculty of Engineering, Alexandria University; The Malawi Polytechnic, University of Malawi), A. Elshahat (Nuclear and Radiation Engineering Department, Faculty of Engineering, Alexandria University)
本研究では、改良型沸騰水型原子炉（ABWR）原子力発電所に対して蒸気管大破断事故と格納容器損傷が同時発生してからの全交流電源喪失（SBO）により放射性元素で環境汚染が生じる可能性を調査した。加速度0.3 gという発電所設計限界を超える地震で2000 cm2の格納容器亀裂とドライウェル内での800 cm2の蒸気管大破断が生じた後、外部電源が喪失すると仮定した。所内非常用ディーゼル発電機が作動し、炉心から残留熱を除去するため交流電源の供給を開始したが、津波により遮断された。重要な安全系用の直流蓄電池電力には限度があり、SBOにつながった。シミュレーションでは、外部交流電源が30秒後に遮断されると仮定した。想定事故状態における発電所安全系の応答データを数値的に作成するため、パーソナルコンピューターの過度現象解析機を使用した。発電所の静的・動的安全機能が過度現象から50秒以内に応答するのが重要なことから、シミュレーションは500秒の期間で実施された。シミュレーションの結果によると、蒸気管破断で冷却材圧力が急激に低下し、19秒以内にスクラムが生じた。記録された燃料と燃料被覆管の最高温度はそれぞれ約1240°Fと540°Fであり、いずれも安全限界の範囲内であった。原子炉ボイラー内の圧力に大きな変化はなく、故障の兆候は見られなかった。また、原子炉格納容器建屋内の圧力は安全系により45 psia内に保たれていた。最後に放射線モニターの示数は、事故中に放射性元素の環境への目立った放出がなかったことを示していた。そのため発電所の安全系が完全に機能していれば、格納容器破断及び蒸気管大破断とその後のSBO事故の際に周辺環境に放射性元素が放出される可能性は極めて低いと結論付けることができた。シミュレーションで得られたデータは、こうした事故に関するPSAR（予備安全評価報告書）データと十分一致していた。
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