2011年3月日本福岛核电站核泄漏在海洋中的传输

使用全球版本的迈阿密等密度海洋环流模式对2011年3月日本福岛核电站泄漏在海洋中的传输以及扩散进行了数值模拟。数值模式中核废料(示踪物)排放情景采取等通量连续排放,排放时间从3月25日开始,分别持续20 d以及1 a,两种情形分别积分20 a。为了减少海洋环流年际变化带来的数值模拟的的不确定性,20 a的模式积分分别用2010年、1991-2011年、1971-1991年以及1951-1971年4个不同时段的NCEP/NCAR逐日再分析资料作为大气强迫场,因此每种排放情形包含4个数值试验。模拟结果的分析表明,不同核废料排放情景及其在不同时段大气资料对海洋模式的驱动下,模拟的示踪物总体的传输扩散路径(包括表层以及次表层)、传输速率以及垂直扩展的范围没有显著的差异。集合平均数值模拟的结果显示:在两种排放情景下,日本福岛核泄漏在海洋的传输路径受北太平洋副热带涡旋洋流系统主导,其传输路径首先主要向东,到达东太平洋后,再向南向西扩散至西太平洋,可能在10~15 a左右影响到我国东部沿海海域,且海洋次表层的传输信号比表层信号早5 a左右。通过进一步分析模式积分过程中最大示踪物浓度随时间变化发现,在积分第20 a(2031年3月),海洋表层和次表层浓度的最高值分别只有模式积分第一年浓度的0.1%和1%。在积分的20 a里,排放的核废料主要滞留在北太平洋海域(超过86%±1.5%的核废料在积分结束时,滞留在北太平洋),而在积分的前10 a(2021年之前),几乎所有的核废料滞留在北太平洋;在核废料的垂直分布上,主要集中在海洋表层至600 m的深度,在积分的20 a时间里,没有核废料信号扩散至1 000 m以下的深度。数值模拟的结果也表明核废料浓度减弱的强度以及演变的时间特征主要受洋流系统的影响,与排放源的排放时间长短关系不大。值得指出的是,更加准确地评估一个真实的核泄漏事故对海洋环境所造成的可能影响,还需要考虑大气中的放射性物质的沉降以及海洋生态对核物质的响应。

Transport of nuclear leakage from Fukushima Nuclear Power Plant in the North Pacific

The massive nuclear leakage into ocean from Fukushima Nuclear Power Plant was observed on March 25^th,2011. The transport of leaked radioactive pollutant from the Fukushima Nuclear Power Plant was simulated using a global version of Miami Isopycnic Coordinate Ocean Model (MICOM). The simulation was performed by assuming constant and continuous leakage for 20 days (scenario 1) and for one year (scenario 2) starting from March 25th,2011 and was integrated for 20 years. Taking into account of the annual and inter-annul variability of the ocean circulation,the model for each scenario was forced with four different time periods of NCEP/NCAR daily atmospheric forcing,namely,repeated 2010,1991-2011,1971-1991 and 1951-1971. There is no remarkable difference of transport pathways (both on surface and subsurface layers),transport timescales and vertical penetration for the nuclear waste among different ensemble members. An ensemble mean for each scenario was obtained in order to reduce the uncertainty in the simulations. The results of the ensembles indicate that the nuclear pollutant for both scenarios transports eastward to eastern Pacific and thereafter follows a southwest pathway towards the western Pacific. It takes about 10 to 15 years to reach the coast of East Asia. The transport pathway is governed by the ocean current system of the North Pacific Subtropical Gyre. The transport at the subsurface (100-400 m depth) is about 5 years faster than that at the surface when the nuclear signal reaches the coast of the East Asia. The time evolution of maximum tracer concentration shows that the surface and subsurface maximum concentrations of leaked nuclear pollutant at the end of the simulation are only 0.1% and 1% of the values at the first year. Most of nuclear pollutant stays in the North Pacific throughout the integration with almost 100% of the released nuclear pollutants remaining in the North Pacific for first 10 years and 86±1.5% at the end of the integration. The penetration depth of the nuclear pollutant is less than 1 000 m with relative high concentration between the surface and 600 m depth. The simulations also suggest that the ocean circulation and mixing instead of the source function dominate the variability of the maximum concentration. It should be emphasized that a realistic source function is required and atmospheric fallout and role of ocean ecology should also be taken into account,in order to get a more reliable assessment of possible impact of the radioactive leakage on the ocean environment.