Close-up look at unit 4 fuel and walls

4 September 2012

Workers at Fukushima Daiichi have disassembled and examined a fresh (unirradiated) fuel assembly removed from the unit 4 spent fuel pool a few months ago. The BWR fuel assembly's channel box was removed before inspection.

Workers examine unirradiated fuel taken from Fukushima Daiichi unit 4 spent fuel pool
Workers examine unirradiated fuel taken from Fukushima Daiichi unit 4 spent fuel pool

Pictures show no visible damage to fuel pins, although three stone and concrete pebbles, which probably fell in to the pool from the roof or walls of the structure, were removed from it during disassembly. There was some minor surface rust on some fuel pins, although some of it was apparently removed after rubbing it with a cloth.

The examination will help utility TEPCO decide how to remove unirradiated and irradiated fuel from the unit 4 spent fuel pool, which, because the unit 4 reactor was emptied of fuel for maintenance before the 11 March 2011 earthquake and tsunami, contains the greatest amount of fuel and therefore poses the greatest potential risk of all of the reactors and pools.

TEPCO has also completed a detailed survey of the walls of the unit 4 reactor building structure, and concluded that it still remains strong enough to support the spent fuel pool safely, despite evidence of a bulge on the exterior west-facing wall. First, it concluded that the building is not tilting, based on measurements of the distance above the height of the water in the spent fuel pool and the reactor well. Measurements continue to be consistent on all four sides of both, suggesting that the building is not tilting. Second, it measured the bulging of the exterior walls of the reactor building using optical equipment, which are found to be as much as 45mm out of plane (west wall, third floor level). However, these results are consistent with previous measurements, so the situation is not worsening. Second, a detailed visual survey of the walls, inside and out, did not find any large cracks, which could indicate structural weakness. Also, data from measurements of concrete strength using a Schmidt hammer were broadly consistent with previous results in June 2012, and exceeded design margin by about one-third. This inspection will be repeated regularly.

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