Seismic analysis of the unit 1 and unit 4 Fukushima Daiichi reactor buildings, both of which suffered hydrogen explosions during the aftermath of the March earthquake and tsunami, has found that both retain an adequate safety margin in the event of another earthquake. The analysis of unit 3, which was also damaged by an explosion, has not been completed. The seismic response analysis took building damage into account in determining whether design basis vibration would cause seismic walls to collapse.
The unit 1 structure above the refuelling (fifth) floor was destroyed by an explosion on 12 March. The maximum shear strain occurring on the remaining wall below the fifth floor was estimated at 0.12 x10^-3 (N-S direction, 1st floor), which is 33 times lower than the criteria, 4.0 x10^-3. So operator Tepco has concluded that the reactor building has adequate structural integrity.
The unit 4 structural frame above the fifth floor, most of the roof slabs, most of the fourth floor walls, and part of the third floor walls were destroyed on 15 March. The maximum shear strain on the remaining seismic wall below the fifth floor was estimated at 0.17 x10^-3 (E-W direction, 1st floor), which is 23 times lower than the criteria. So again Tepco concluded that the reactor building retains adequate structural integrity.
In addition, Tepco implemented a three-dimensional finite element analysis of the unit 4 spent fuel pool. The estimated maximum earthquake and other loads leads to a maximum rebar strain of 1230×10^-6, four times lower than the 5000×10^-6 plastic limit strain. At the weakest point, the out-of-plane shear force was 800 N/mm, 1.44 times lower than the criteria, 1150 N/mm. Based on this margin, Tepco concludes the spent fuel pool retains adequate structural integrity.
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