Unit 4 of the crippled Fukushima
© Air Photo Service Co. Ltd., Japan
This March 24, 2011 aerial photo shows damaged Unit 4 of the crippled Fukushima Dai-ichi nuclear power plant.
Summary of temperature rise at Reactor No. 4′s spent fuel pool since Thursday morning:
  • Apr. 12 @ 3a = 28°C (Source)
  • Apr. 12 @ 8p -to- Apr. 13 @ 3a = 49.9°C, stable (Source)
  • Apr. 13 @ 10a = 55°C (Source)

    Title: TEPCO Press Conference 4/13/2012 11AM: Reactor 4 SFP Temperature Was 49.9 Degrees Celsius as of 3AM, 4/13/2012
    Source: EXSKF
    Date: April 12, 2012UPDATE: Matsumoto was speaking about the Reactor 4 Spent Fuel Pool, and mentioned the temperature. I took to mean the temperature of the SFP, but it is more likely that Matsumoto suddenly switched back to talking about the temperature of the Reactor 1 dry well. Sorry for the confusion. A lot of viewers apparently thought Matsumoto was talking about the Reactor SFP like I did.

    So, it was the Reactor 1 dry well temperature that started to rise noticeably starting 4AM on April 13, 2012.

    No word yet on exactly what the temperature is in the Reactor 4 SFP.

    - - - - - - END OF UPDATE - - - - -
According to Matsumoto [Tepco spokesman], the temperature of the Reactor 4 Spent Fuel Pool started to rise noticeably starting 4AM April 13, 2012.
From 8PM on April 12, 2012 to 3AM on April 13, 2012, the temperature remained 49.9 degrees Celsius, he said.

Matsumoto didn't say what the temperature was at the time of the press conference (11AM on April 13) [...]
Title: SFP4 over 55°C. Coolant system is still stopped
Source: Fukushima Diary
Date: Apr. 12, 2012
Though Tepco stopped the water leakage, coolant system is still not back on. At the moment, they can't estimate when they can recover it. Also, they estimated the temperature increases at 0.5°C/h, it's increasing at over 1°C/h. It went up by 22°C within 18 hours. Currently the temperature of SFP is 55°C.(10:00 JST) The safety limit is 65°C. At this pace, it will reach 65°C about in 10 hours. [...]
...Developing...

See also: Kyodo: Possible Leak at Unit No. 4 -- Spent fuel pool cooling system halted after alarm sounds