Numerical Simulation of Inundation at Fukushima Nuclear Power Plant due to 2011 East Japan Tsunami

Abstract

A megathrust earthquake of Mw=9.0 rocked off the east coast of Honshu Japan (38.3°N, 142.4°E) at 14:46 JST on March 11, 2011. This was the strongest earthquake for the past 1200 years in Japanese history. The tsunami generated by this earthquake caused thousands of casualties and destroyed coastal settlements and infrastructures, including the nuclear accidents in the Fukushima Daiichi Nuclear Power Plant complex. After the tsunami ravaged the coast of Japan, it rapidly spread to the central Pacific islands and coastal countries. The tsunami has propagated across the entire Pacific Ocean in less than 23 hours. In this study, first, I have performed numerical simulations of the generation, propagation, and inundation at Fukushima Daiichi Nuclear Power Plant for 2011 East Japan Tsunami, and then the simulated results were compared with the measured results. To do this, the time dependent tsunami source model proposed by Japan Nuclear Energy Safety Organization was employed. This model is based on the source models proposed by the research groups including University of Tokyo and Tohoku University was employed. The finite difference far field model based on the linear Boussinesq wave equation was used to simulate the propagation of tsunami from the source region to the coastal area of the nuclear power plant site. For the simulation of inundation at the site the near field numerical model using nonlinear shallow water equation with moving boundary scheme was employed. The results of numerical simulation show that the main buildings and the most of the area of Fukushima Daiichi Nuclear Power Plant site are flooded. The maximum inundation depth is 12 m. Comparison of observed data with simulation results, the simulated inundation pattern agrees well with the observations, even though the simulated inundation depths are overestimated for almost of the site area. The overall performance of the numerical simulation is satisfactory. Through this study, it is shown that the numerical simulation can serve as a useful tool for the risk assessment of subsequent tsunamis and the design of nuclear power plant.|2011년 3월 11일 14시 46분(일본 표준시) 일본 동북부 혼슈 (38.3°N,142.4°E) 해안에서 규모 9.0의 지진이 지진해일을 발생 시켰다. 이 지진은 과거 일본 역사 1,200년 동안 일어난 지진 중 가장 강력한 지진이다. 이 지진에 의해 발생된 지진해일은 수 천명의 사상자를 발생시키고 연안의 기반시설 등을 파괴시켰다. 또한 후쿠시마 제1원자력발전소의 냉각수 시설도 지진해일로 인해 파괴되었다. 지진해일은 일본 연안에서 피해를 발생시킨 후 지진 발생 23시간 이전에 태평양을 통해 섬 및 태평양 연안에 위치한 국가로 전파되었다. 본 연구에서는 먼저 2011년 동일본 지진 해일의 생성, 전파 및 후쿠시마 제1원자력발전소에서의 침수를 수치모의 하였다. 수치모의를 통해 계산된 결과와 후쿠시마 제1원자력발전소에서 관측된 값을 비교하였다. 수치모의를 위해 사용된 지진해일 초기파형은 일본 원자력 안전기구에 의해 제안된 시간 변화를 고려한 모델을 사용하였으며 이 모델은 도쿄 대학과 도호쿠 대학의 연구 그룹에서 제안된 모델을 기초로 만들어졌다. 원지에서의 지진해일 전파 계산을 위한 지배방정식은 선형 Boussinesq 파동 방정식에 기초한 식을 이용하였으며, 범위는 초기파원에서부터 근해 외부 영역까지로 설정하였다. 근해에서는 비선형 천수방정식과 이동경계를 적용하여 계산하였다. 수치모의를 통해 도출된 침수심은 후쿠시마 제1원자력발전소 부지 대부분이 침수되어 있음을 보여주며, 최대 침수심은 12m로 계산되었다. 계산된 침수심은 관측된 값보다 과대평가되었지만 전반적인 경향은 잘 일치한다. 본 연구를 통해 지진해일 수치모델은 지진해일에 의한 침수 위험 평가 및 원자력 발전소 설계에 유용하게 활용될 수 있음을 보였다.; A megathrust earthquake of Mw=9.0 rocked off the east coast of Honshu Japan (38.3°N, 142.4°E) at 14:46 JST on March 11, 2011. This was the strongest earthquake for the past 1200 years in Japanese history. The tsunami generated by this earthquake caused thousands of casualties and destroyed coastal settlements and infrastructures, including the nuclear accidents in the Fukushima Daiichi Nuclear Power Plant complex. After the tsunami ravaged the coast of Japan, it rapidly spread to the central Pacific islands and coastal countries. The tsunami has propagated across the entire Pacific Ocean in less than 23 hours. In this study, first, I have performed numerical simulations of the generation, propagation, and inundation at Fukushima Daiichi Nuclear Power Plant for 2011 East Japan Tsunami, and then the simulated results were compared with the measured results. To do this, the time dependent tsunami source model proposed by Japan Nuclear Energy Safety Organization was employed. This model is based on the source models proposed by the research groups including University of Tokyo and Tohoku University was employed. The finite difference far field model based on the linear Boussinesq wave equation was used to simulate the propagation of tsunami from the source region to the coastal area of the nuclear power plant site. For the simulation of inundation at the site the near field numerical model using nonlinear shallow water equation with moving boundary scheme was employed. The results of numerical simulation show that the main buildings and the most of the area of Fukushima Daiichi Nuclear Power Plant site are flooded. The maximum inundation depth is 12 m. Comparison of observed data with simulation results, the simulated inundation pattern agrees well with the observations, even though the simulated inundation depths are overestimated for almost of the site area. The overall performance of the numerical simulation is satisfactory. Through this study, it is shown that the numerical simulation can serve as a useful tool for the risk assessment of subsequent tsunamis and the design of nuclear power plant.