on 9 june, staff entered the reactor building to conduct radiation surveys. on 25 june and the following day boric acid dissolved in 90 tons of water was pumped into the spent fuel pool of reactor 3. concrete debris from the march hydrogen explosion of the reactor building has been detected in the spent fuel pool. in june tepco discovered that the water in the pool was strongly alkaline: the ph had reached a value of 11.2. leaching of calcium hydroxide (portlandite) or calcium silicate hydrate (csh) from the concrete could have caused this. the alkaline water could accelerate the corrosion of the aluminium racks holding the spent fuel rods. if the fuel assemblies would fall, this could lead to re-criticality. in the meantime preparative works began to install a recirculation cooling system at the fuel pool, that should be operational in the first weeks of july.