A whiteboard in the central control room for the No. 1 and No. 2 reactors at the Fukushima No. 1 nuclear power plant was quickly filled with short memos in the hours after the Great East Japan Earthquake on March 11 that shows how the situation spiraled out of control.
A whiteboard in the central control room for the No. 1 and No. 2 reactors at the Fukushima No. 1 nuclear power plant was quickly filled with short memos in the hours after the Great East Japan Earthquake on March 11 that shows how the situation spiraled out of control.
Tokyo Electric Power Co., the operator of the Fukushima plant, released a large volume of data on May 16 taken from instruments at the crippled plant as well as from records kept by plant workers.
One item on the whiteboard said, "3:37 p.m. SBO (station blackout)."
That memo meant all AC power sources had been lost.
The tsunami that struck the plant after the quake also knocked out all emergency power sources. That meant the main cooling systems that maintain the plant's safety systems could not operate. The memos show an increasing sense of urgency in the central control room.
Another series of memos said:
"3:50 p.m. Gauge power source cut off; water level unclear."
"3:53 p.m. Phone call to department head."
"4:36 p.m. Unable to pump water into ECCS (emergency core cooling system)."
"5:16 p.m. No lights in No. 2 reactor central control room."
"6:42 p.m. Unable to use PHS (mobile phones)."
Workers on duty at the plant were forced to try to restore the cooling system while lights and communications systems were cut off.
Seawater also made work difficult. The basements of the turbine buildings were completely submerged in the wake of the tsunami.
Memos on the whiteboard record the times when major tsunami warnings were issued and for high tide.
However, the efforts to restore power sources were unsuccessful.
Another memo said, "5:47 p.m. Switching station for No. 1 and No. 2 reactors inoperable (from office)."
Workers were informed that the switching station that connects the power plant to transmission lines could no longer be used.
A subsequent memo records being contacted about the increased difficulties in restoring power sources.
Another memo said, "6:12 p.m. Unable to monitor water level at No. 2 reactor."
Without a power source, workers in the central control room were unable to monitor the gauges that could tell them about the situation in the reactor core.
As night fell on March 11, radiation levels within the reactor building began to rise.
One memo records the fact that a high radiation level of "0.8 millisieverts per 10 seconds" was detected.
A subsequent memo said, "9:51 p.m. No. 1 reactor building off limits."
The memo recorded a radiation figure of "1,000 CPS" and indicated that entry into the No. 1 reactor building was prohibited.
CPS is a unit of radiation recorded per second.
At 11 p.m., a radiation level of 0.5 millisieverts was recorded at the south side of the hallway in the first floor of the turbine building and of 1.2 millisieverts at the north side of the hallway. The measurement sites were close to the double doors that lead to the reactor building.
As March 11 turned into March 12, memos on the whiteboard record a venting of gas in the containment vessel to reduce pressure as well as the pumping of water into the core.
Operating records of those on duty normally include handwritten figures for hourly data such as heat output from the reactor. However, there were no such entries after 3 p.m. on March 11.
Other data released by TEPCO also points to the possibility that workers manually stopped the emergency condenser at the No. 1 reactor. The emergency condenser pumps in cooling water to the reactor core during emergencies and had automatically started in the wake of the quake.
TEPCO officials have until now explained that the condenser stopped working due to the tsunami.
If workers actually stopped the condenser that would call into question the judgement made by those working at the plant as well as the appropriateness of the work procedures established by TEPCO.
According to data for core pressure at the No. 1 reactor, after operations were automatically stopped following the quake, the valve to the pipe to release steam closed and pressure within the core increased.
At 2:52 p.m. on March 11, the emergency condenser automatically began operating because of the higher pressure.
With the start of cooling, the core pressure fell, but the pressure began rising again after the condenser was stopped at around 3 p.m.
At about 3:30 p.m., the reactor was struck by a tsunami and core pressure data also comes to an end at that point.
Although the condenser began operating again, it stopped at 1:48 a.m. on March 12 due to pump problems.
With inadequate cooling of the fuel rods in the reactor core, the fuel rods were in a condition of a dry boil and hydrogen gas was produced. That gas somehow leaked into the reactor building and caused a hydrogen explosion.
TEPCO officials said there was the possibility that the initial stopping of the condenser was done manually in line with work procedures which call for such action when core pressure drops.
On May 15, TEPCO officials analyzed the situation under conditions in which the condenser became inoperable after the tsunami. Under those conditions, the fuel rods began melting about five-and-a-half hours after the tsunami hit and by the morning of March 12 a meltdown had occurred at the core.
Power generator vehicles were not stationed at the Fukushima No. 1 power plant. Officials at TEPCO headquarters gave instructions to obtain generator vehicles at about 5 p.m. on March 11, about 80 minutes after power sources were lost.
However, those vehicles could not reach the nuclear plant because of the traffic jam after the quake as well as the damage to roads.
At 6:20 p.m., TEPCO officials asked Tohoku Electric Power Co. to send a high-voltage generator vehicle.
The first generator vehicle from Tohoku Electric arrived at the Fukushima No. 1 plant at about 11 p.m. However, workers experienced difficulties connecting a power transmission cable from the vehicle to the power plant because of rubble and pools of water left behind by the tsunami.
The connecting of the cable was completed at about 3 p.m. March 12. About 30 minutes later, a hydrogen explosion occurred at the No. 1 reactor building. The cable was damaged and the generator vehicle also automatically stopped.
(This article was written by Tatsuyuki Kobori, Susumu Yoshida and Takashi Sugimoto.)