Tue, 07 Feb 2012 22:28 UTC
Readings from a thermometer at the bottom of the No. 2 reactor's pressure vessel rose from 50.8 degrees at 5 a.m. on Feb. 1 to 73.3 degrees at 7 a.m. on Feb. 6.
Melted fuel is believed to have accumulated at the bottom of the reactor, but high radiation levels have prevented workers from checking the exact situation within the reactor.
After the flow of cooling water was increased to 10.6 tons per hour on Feb. 6, up from 8.6 tons two days earlier, the temperature fell to 69.2 degrees at 5 p.m. on Feb. 6. That night, TEPCO injected boric acid into the reactor to prevent criticality, the point at which a nuclear fission reaction becomes self-sustaining. Boric acid absorbs neutrons, which induce nuclear fission.
The Nuclear and Industrial Safety Agency (NISA) instructed the company to consider injecting boric acid earlier in the day.
TEPCO also plans to increase the amount of cooling water by 3 tons per hour.
At a news conference on Feb. 6, Haruki Madarame, chairman of the Nuclear Safety Commission of Japan, told TEPCO and NISA to keep the public informed.
"We expect them to address the public's concerns by methodically explaining what could happen and how they plan to deal with it," Madarame said.
A TEPCO official said there were no signs that the melted fuel had reached criticality. The official said the level of radioactive xenon, an element with a short half-life, remained below a measurable detection limit, and that monitoring devices around the nuclear power plant have not detected a rise in radiation levels.
TEPCO officials said the rise in temperatures was not steep enough to indicate that criticality had been reached.
However, a temperature of 80 degrees or more at the bottom of the pressure vessel would ring alarm bells. TEPCO has assumed a margin of error of up to 20 degrees for the thermometers in the reactor because it is not clear what damage was done to them by the Great East Japan Earthquake.
Therefore, a reading exceeding 80 degrees could mean an actual temperature of more than 100 degrees, compromising the reactor's status as being in cold shutdown.
The rise in temperatures appears to coincide with changes in the flow of water through two separate systems for cooling the No. 2 reactor: the feed water system and the core spray system. TEPCO temporarily increased the amount of water being pumped through the feed water system and reduced the amount of water going through the core spray system as it strengthened outdoor piping in late January.
After that work was completed, it gradually decreased the amount of water flowing through the feed water system and increased the flow through the core spray system in an effort to restore flows to the setup before the strengthening.
The thermometer that has produced the high readings is located just under the feed water system. Its temperature readings rose when the water passing through the feed water system was reduced and water going through the core spray system was increased. Readings from two other thermometers at the same height in the reactor have been stable at 44-45 degrees.
"The temperature may have risen because water has not reached part of the fuel since the amount of water through the feed water system decreased and the flow of water changed," said an official at TEPCO's Nuclear Power and Plant Siting Division.