Fukushima Update #78, February 7, 2012
TEPCO increased the volume of water being injected into Unit 2 by 35% in the early hours of February 7.
This attempt to lower the temperature has stabilized it around 70 degrees C. But it has not lowered it significantly.
This large increase in volumes of cooling water injection also means that TEPCO is outside the limits of operation for this reactor as I noted yesterday. They acknowledge this at the very bottom of in their morning press release for Feb 7.
The best article on the net explaining what is going on at Unit 2 is the one below from Asahi news. It has the fewest errors and omissions and also has an excellent drawing and an excellent chart which you can see at the link to the article.
TEPCO struggles to cool Fukushima plant's No. 2 reactor
"Tokyo Electric Power Co. is taking steps to prevent a possible self-sustaining nuclear chain reaction at the crippled Fukushima No. 1 nuclear power plant.
"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….
"TEPCO also plans to increase the amount of cooling water by 3 tons per hour….
"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….
"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."
source: http://ajw.asahi.com/article/0311disaster/fukushima/AJ201202070027
It clearly shows the stabilization, but no reduction, in the temperature. (Note the 0, 135, and 270 are NOT degrees of temperature, but refer to the location of the 3 thermometers on the circular RPV in the accompanying picture.)
NHK also carries a story today which makes clear TEPCO is not sure what is causing the temperature rise in Unit 2. Many press stories comment on the "unknown cause".
Tuesday, February 07, 2012 13:05 +0900 (JST)
"The utility said the flow of water in the reactor may have changed after plumbing work in late January, causing difficulties in cooling part of the melted nuclear fuel.
"It added that no temperature rise has been observed at 2 other thermometers in the same reactor and that it will continue to carefully monitor the reactor."