(UPDATE) TEPCO managed to bypass the UPS and restarted the cooling system at 3:07PM on July 1. At the time of the restart, the temperature of the water inside the Spent Fuel Pool was 42.9 degrees Celsius. TEPCO plans to replace the faulty UPS this week. (From the tweets of another independent journalist who receives TEPCO email.)
Independent journalists who cover TEPCO's press conference posted TEPCO's email updating them about the problem in the Reactor 4 SFP cooling system that automatically shut down on June 30, as follows (I got the text from Yasumi Iwakami's tweets):
Today's investigation of the automatic shutdown of the Fukushima I Nuclear Power Plant Reactor 4 Spent Fuel Pool cooling system, as of 7:40PM on June 30, 2012:
We conducted the investigation at the site, and there seemed to be a problem with the UPS (Uninterruptible Power Supply). We are considering bypassing the UPS, and the work to restore the system will start tomorrow as soon as we are ready. The temperature of the pool as of 6PM [on June 30] was 36.6 degrees Celsius. We estimate that it will take about 60 hours for the water temperature of the Spent Fuel Pool to reach the upper limit of the safety regulations at 65 degrees Celsius.
That gives TEPCO until 6AM on July 3 to fix the problem. TEPCO already said they didn't find any leak.
Sankei Shinbun has additional details (6/30/2012; part):
TEPCO investigated the site, and said that the UPS (Uninterruptible Power Supply) may have broken. The UPS is connected to the main power supply and the cooling system, and provides electricity in an emergency.
In order to cool the fuel stored in the Spent Fuel Pool, the cooling system draws water from the pool, cools the water and circulates it back into the pool. There are two lines, and TEPCO tried to resume the cooling operation by starting the backup line. However, since both lines use the same UPS, TEPCO couldn't restart the system. TEPCO is planning to bypass the UPS and restart the system.
There are rumors (like the one from Professor Takeda) that the Reactor 4 SFP is mechanically damaged because of shoddy construction. I have no idea what the basis is for Professor Takeda's assertion that shoddy construction work damaged the pipe of the cooling system. They seem as baseless as the rumors that everything is dandy or that the building is listing like the Tower of Pisa.
The worker who tweets from Fukushima I Nuke Plant tweeted this about the problem:
Hello. I was surprised by DM [twitter's direct message] from one of my followers saying "Right now, the rumor of Reactor 4..." At least mechanical engineers like me haven't been called in. It looks like a problem with the control system. Repair will probably be tomorrow, as they need to arrange for the repair parts. For the time being, looking at the temperature trend, I don't think there is anything to worry.
If it is really a serious situation, we would receive an emergency call. Then the crane will be used [Putzmeister concrete crane that pours water directly into the Spent Fuel Pool]. If that should happen, I will tweet right away. But I'm enjoying my day off, eating sweets. See ya.