BY EISUKE SASAKI STAFF WRITER
Steam rises as the water temperature rises in the fuel storage pool of the No. 4 reactor of the Fukushima No. 1 nuclear power plant on June 29. (Provided by Tokyo Electric Power Co.)
The temperature in the nuclear fuel storage pool at the No. 4 reactor of the Fukushima No. 1 nuclear power plant fell 2 to 4 degrees on July 31 in the first seven hours that a circulating cooling system began operating, Tokyo Electric Power Co. said July 31.
Construction to reinforce the No. 4 reactor building was completed July 30, TEPCO said. The building was severely damaged by a March 15 explosion whose cause has yet to be fully explained.
The utility ran a test trial of the circular cooling system early July 31 by pumping water out of the pool and returning the water to the pool after cooling it.
In the afternoon, full operations commenced, bringing the water temperature--which stood at 86 degrees before the full operation--down to between 82 to 84 degrees in seven hours.
During earlier repair work, pipes ruptured in the explosion were replaced.
The storage pool, which cools the intense heat that constantly arises from used fuel rods, became incapable of cooling after the Great East Japan Earthquake and tsunami knocked out its water circulating system.
Initially, it was suspected that the stored fuel rods had melted due to the evaporation of coolant from the pool.
TEPCO plans to start up a similar circulating cooling system at the No. 1 reactor in early August. Cooling systems at the No. 2 and No. 3 reactors started operating on May 31 and July 1, respectively.
Prior to the March 11 quake, the No. 4 reactor had been shut down for a regular inspection, and its large fuel rods placed in the cooling pool. The heat output from the cooling pool was about 10 times greater than the pool at the No. 3 reactor and 30 times that of the No. 1 reactor.
On March 15, the No. 4 reactor building that houses the storage pool exploded, raising suspicions that the fuel rods had been damaged and produced hydrogen that triggered the explosion.
However, it was later found by water analysis, after water was pumped into the reactor, that the cooling pool's radiation levels were relatively low. No significant damage to the fuel rods was detected by remote observation using cameras.