Unit 3 reactor building stable, TEPCO concludes

15 July 2011

An analysis by utility TEPCO has concluded that the damaged Fukushima Daiichi unit 3 reactor building has sufficient seismic safety margin to not require additional supports. The reactor building roof above the fifth floor was destroyed by what is believed to have been a hydrogen explosion on 14 March; steel and concrete beams collapsed on the fifth floor; most of the fourth floor walls were also damaged.

TEPCO has performed a three-dimensional FEM analysis of the building, including the spent fuel pool, comparing recorded loads against the seismic design basis motion. It found that the safety margin of the remaining walls were all within reasonable limits: the remaining seismic wall below the fifth floor was 28 times below the maximum; the maximum plastic limit strain of spent fuel pond rebar was 3.8 times below the maximum; the out-of-plane shear force at the point of minimum margin was 1.85 times below the maximum. In similar analyses of the primary containment shell wall, it found similar safety margins: maximum rebar plastic limit strain was 10.66 times below the maximum; out-of-plane shear force at the point of minimum margin was 1.32 times less than the maximum.

Following preparation works over the last several weeks, nitrogen injection into the unit 3 primary containment vessel has begun. The injection will reduce the possibility of a hydrogen explosion.

TEPCO has submitted its weekly report to the regulator about the water treatment system. It has reported that from 6-12 July the system had an availability of 72.9% and treated 6130 m3 of contaminated water (and in total just under 20,000 m3 of water has been treated as of 12 July). During the week, about half of the treated water (2688 m3) was recycled by injecting back into reactor vessels 1, 2 and 3. Despite this activity, accumulated water only dropped by about 500m3 during the week, partly because water levels increased by 700 m3 in unit 2. TEPCO stopped pumping water out of unit 2 on 7 July to preserve capacity in the Central Radiation Waste Treatment Facility, a spokesman said.

In other news, work has begun on installing a line of pipe-based sheet piles at the southern end of the inner Fukushima Daiichi water intake canal, which was damaged by the tsunami. Although the existing dyke has already been reinforced with sandbags and a silt screen, pipe-based sheet piles will further reinforce the barrier. The water in the intake canal has been contaminated by highly radioactive water from two leaks, although a circulating water treatment system is now operating to reduce the radioactivity of that water.

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