(UPDATE) One of the local papers in Fukushima Prefecture shares my suspicion that someone (possibly plural) tried to hide the mistake. Fukushima Minyu (2/22/2014; part):
[TEPCO] disclosed [on February 21] that the valve in question was in fact open for 9 hours at the maximum. There is a possiblity that someone made a mistake in operating the valve. Further, there is a possibility that they [those in the section in charge of the water transfer operation] tried to hide the mistake. In the early hours of February 20 after the leak was discovered, the valve in question was confirmed to be closed. However, in the photo taken around 11AM on the previous day (February 19) the valve was open, and the lever to open and close the valve was attached to the valve, which is normally removed.
From TEPCO's photos and videos library, 2/21/2014:
The valve in question (V347) on February 19, 2014 morning, "open" position, as it shouldn't:
The same valve on February 20, 2014, "closed" position, as it should, but after the leak:
That's what increasingly the government public relations broadcaster aka NHK says. NHK also says TEPCO has dispatched investigators from the TEPCO headquarters (in Tokyo) to Fukushima I Nuclear Power Plant and started interviewing the plant workers.
It is rather difficult to grasp what NHK is trying to say (they do talk like TEPCO's PR), but I think they are saying:
1. The RO waste water was to be transferred to a storage tank that was NOT the tank it subsequently went and overflowed.
2. Someone opened the wrong valve, and the water went to the wrong tank which was almost full. The water leaked from the top of the tank.
3. AFTER the water leaked from the wrong tank, he realized his mistake; he closed that valve, and opened the valve that should have been opened to transfer the contaminated water to the correct tank.
A gross human error (I hope), which caused (at least) 100 tonnes of extremely contaminated (230 million Bq/L of all-beta, or 23 trillion Bq (terabecquerel) in 100 tonnes) water to leak (see my post on 2/20/2014).
According to Mainichi Shinbun, the Nuclear Regulation Authority has said it is speaking with IAEA on whether to have an INES event scale assigned to this particular incident. The last year's leak of 300 tonnes of the same RO waste water from the storage tanks into the surrounding soil was INES Level 3 "incident". (Level 4 and above are "accidents".)
From NHK News (2/22/2014; emphasis is mine):
Contaminated water leak: Did someone open the valve?
Regarding the 100 tonnes of highly contaminated water that leaked from a tank at Fukushima I Nuclear Power Plant, TEPCO has dispatched investigators from the TEPCO headquarters to the plant who are interviewing the workers. TEPCO suspects someone opened the valve that should remain closed, causing the leak.
At Fukushima I Nuclear Power Plant, about 100 tonnes of contaminated water leaked into the surrounding ground in the plant compound in February 19 and 20 when too much water was poured into a tank on the mountain-side (west) of Reactor 4.
Initially, TEPCO explained that one of the valves [there are three] on the pipe that goes to this tank may have malfunctioned, as it let the water flow even if it was closed.
However, a photograph was found that captured the same valve on a different job; the photo showed that the valve, suspected of malfunctioning, was open as of the morning of February 19 when the water was being transferred intermittently.
On the other hand, the valve on the pipe that goes to the tank where the contaminated water was supposed to go remained closed as of the morning of February 19, blocking the water flow. However, after the leak was found, this valve was open.
TEPCO thinks someone opened and closed these two valves, causing the leak of the contaminated water.
In order to find out the details of how the incident happened and why the valves were operated, TEPCO has sent investigators from the headquarters to the plant who are interviewing the workers.
TEPCO is considering a review of the maintenance/operation procedure, as the water level monitoring procedure when valves were operated was not strictly followed and the tools to open and close the valves were readily available to anyone.
(Hmmm. So TEPCO does suspect someone may have done it on purpose?)
For now, it looks like some worker made a mistake, and after the leak corrected the mistake without saying anything to anyone, hoping no one would notice.
Too bad TEPCO found the photos...