Study on Ex-vessel Debris Cooling

爐外熔渣冷卻研究

王德全(Te-Chuan Wang) ； 李敏(Min Lee)

《中國機械工程學刊》 41卷3期 (2020/06) Pp. 289-295

摘要

Uncertainty analyses are important to evaluate the response of severe accident phenomena associated with using computer codes. MAAP Users' Group (MUG) has been divided the accident phenomena into three categories, i.e. dominant, significant, and minor based on their importance to the accident sequence and progression. Ex-vessel debris cooling has been categorized as a dominant uncertainty with respect to severe accident phenomena. MAAP5 analyses have been performed to investigate the phenomena uncertainties of ex-vessel debris cooling. Lungmen nuclear power plant (NPP), an advanced boiling water reactor (ABWR), and a station blackout accident, SRBC-PF-R-N sequence based on Final Safety Analysis Report (FSAR) of Lungmen NPP, are selected as a reference plant and a based case to investigate the uncertainties of ex-vessel debris cooling. This paper presented key parameters associated with ex-vessel debris cooling in MAAP5. For the MAAP5 uncertainty studies on ex-vessel debris cooling, heat transfer coefficients and critical heat flux Kutateladze number (FCHF) are investigated. Increase heat transfer coefficient will increase erosion distances. Maximizing the heat transfer coefficients can increase erosion distances in downward directions (about 0.3 m). But the erosion distances are less than the depth of lower drywell concrete floor (1.6 m). No sideward erosions predict for the different heat transfer coefficients. But corium can be cooled down after passive flooder opens. For FCHF is greater than 0.036, water can ingress into the debris, thus cool down the debris. Little erosions (below 0.6 m) happen in downward directions. No sideward erosions predict for FCHF greater than 0.036. However, corium can't be cool down for the small value of FCHF. Small value of FCHF represents impermeable debris. When FCHF is smaller than 0.01, that results in higher corium temperature and continuous erosion on the lower drywell floor and pedestal. At the end time of calculation (250,000 s), erosion distances in both downward and sideward directions exceed the thickness of lower drywell floor and pedestal when FCHF is equal to 0.0036. Therefore, FCHF is a very important parameter to affect ex-vessel debris cooling. This paper successfully demonstrates the key parameters that effect ex-vessel debris cooling, and analysis results can provide useful information for the MAAP5 users, Level-2 probability risk assessment (PRA), and accident management.

關鍵字

Debris coolability ； Critical Heat transfer ； Kutateladze number

並列摘要

爐外熔渣是否可被冷卻是嚴重事故一個重要的物理現象，對事故演進與後果影響很大，本研究使用美國電力研究所（EPRI）所發展的MAAP5程式，進行爐外熔渣是否可被冷卻的工具，MAAP5程式為核能工業界廣泛使用之嚴重事故分析程式，目前台灣廣泛使用在嚴重事故現象的研究。研究方法是對爐外熔渣冷卻有關的參數進行靈敏度分析，結果發現增加熱傳係數會增加向下侵蝕厚度（約0.3 m），但仍小於下乾井水泥地板的厚度（1.6 m），但不會造成側向的侵蝕，被動式淹灌器（Flooder）開啟後，熔渣即可被水冷卻。臨界熱通率Kutateladze number （FCHF）對爐外熔渣冷卻有顯著的影響，較大的FCHF（大於0.036）可讓冷卻水可滲入熔渣並進而冷卻熔渣，只會造成向下些許的侵蝕（低於0.6 m），並不會造成側向侵蝕。較小之FCHF（小於0.01）無法完全讓冷卻水滲入熔渣進行冷卻，當FCHF值等於0.01，被動式淹灌器開啟後，雖然熔渣溫度緩慢下降，但仍高於1000 K，使得水泥地板與基座持續侵蝕，在計算結束時（250,000秒），會使得熔渣的溫度上升，並持續侵蝕水泥地板與基座（Pedestal），計算結束時（250,000秒），向下侵蝕會超過地板水泥的厚度。當FCHF值等於0.0036，在250,000秒時，側向與向下侵蝕都超過基座（1.7 m）與地板水泥的厚度。因此FCHF對爐外熔渣冷卻是一個很重要的參數。