|Abstract: ||本研究利用根據核研所提供之核二廠MAAP5輸入檔及計算書，修正核研所提供之MELCOR輸入檔，模擬核二廠電廠全黑及提早洩壓案例，並與MAAP5模擬結果做比較。比較重點包含了重要事件發生時間、熱水流參數、Debris移動、 In-vessel、Ex-vessel放射性核種釋出及環境外釋量、氫氣產生量，並比較高壓SBO及提早洩壓案例之間壓力槽底部失校模式的差異。MELCOR修改重點部分包括控制體積 (Control Volume, CV)、流徑 (Flow Path, FL)、熱結構體 (Heat Structure, HS)、控制函數 (Control Function, CF)、放射性核種 (Radionuclide, RN) 等參數。|
兩程式對於Debris的模擬及判定上有不小的差異，MELCOR會區分Debris及完好的爐心材料，MAAP5則無此區分；造成結果顯示的Debris質量不同。由於兩程式對Candling現象的模擬機制不同，MELCOR有大量金屬與水的反應發生。氫氣在RPV內產生的量，為921 kg；而MAAP5因燃料被Debris覆蓋，無法產生反應，只生成76 kg；外部產生的量則是MELCOR 1,382 kg，MAAP5 2,016 kg，因為MAAP5有許多在RPV內未反應的金屬，進入爐穴後與混凝土分解氣體所產生的水反應，以及熔融爐心與混凝土反應較劇烈，故產生氫氣量較多。
In this study, the MELCOR input deck of Kuosheng Nuclear Power Plant is developed based the the plant data as specified in the MAAP5 input deck and calculation sheets of the plant, which are provided by Institute of Nuclear Energy Research. The plant is deployed with two Boiling Water Reactors (BWR VI) designed by General Electric and enclosed in Mark III containment. A high pressure station blackout (SBO) sequence of the plant is simulated using MELCOR and MAAP5. The results of the simulations are compared to assess the differences of these two codes. The comparisions are concentrated on the timing of major events, thermal hydraulic response of reactor coolant system and containment, debris relocations from one region to another, hydrogen production, in-vessel and ex- vessel release and environmental releases of radionuclides.
The differences of the simulation results are very significant due to the differences in the severe accident phenomenological models adopted by these two codes. The amount of hydrogen generation within the reactor pressure vessel as predicted by MELCOR is 921 kg and that as predicted by MAAP5 code is 76 kg. Nevertheless, the amount of hydrogen production during molten core concrete interaction as predicted by MELCOR and MAAP5 code is 1,382 kg and 2,016 kg, respectively. The extent of in-vessel and ex-vessel releases of radionuclides as predicted by these two codes is also very different. In environment release, there are several fission products that MELCOR is bigger than MAAP5, including Cs, I, Te, Ru, Mo, Nb, U, Sn; And other fission products such as Xe, Ba, Zr, La, Ce, Cd, MAAP5 is larger than MELCOR.
In the study, sensitivity study is performed to assess the impact of depressurization on the failure mode of reactor vessel bottom head. In a high pressure SBO sequence, the vessel failure is caused by the stress and strain produced in a high pressure environment. In a low pressure SBO sequence, the failure of vessel is caused by the melting of instrumentation tubes.