Workers miscalculated pressure levels inside a reactor during the early stages of the Fukushima nuclear crisis, leading to a reduction in cooling water and a possible increase in the volume of radioactive materials released.
Tokyo Electric Power Co. estimated the pressure inside the No. 2 reactor containment vessel at 400 kilopascals on March 16, 2011, five days after the Fukushima No. 1 nuclear plant was crippled by the Great East Japan Earthquake and tsunami.
The actual pressure was 40 kilopascals, far below the 101 kilopascals of the surrounding atmosphere, suggesting that a large amount of radioactive materials escaped from the reactor.
TEPCO later discovered the mistake but did not announce it. Instead, the correct pressure figures were included in a deluge of information released by the utility.
TEPCO concluded that pressure was rising on the afternoon of March 16 and halved the amount of water being injected into the No. 2 reactor on the morning of March 17.
Tadayuki Yokomura, chief of the company’s Kashiwazaki-Kariwa nuclear plant in Niigata Prefecture, warned TEPCO officials against lowering the water level later that morning.
“I think the airtightness (of the containment vessel) has not been maintained,” Yokomura said, according to a video footage of a TEPCO teleconference.
However, TEPCO further reduced the amount of water by midday, apparently fearing that flooding the reactor could lead to a rise in pressure that might cause an explosion.
TEPCO increased the amount of water in the late afternoon of March 17 because some officials suspected that much of the injected water was leaking out.
TEPCO noticed the mistake in the pressure level when it reconfirmed data more than a month later. The company said it cannot say whether the mistake affected the situation.
“Radiation levels were unchanged before and after the amount of water injected was changed,” an official said.
The No. 2 reactor was considered the most dangerous at the Fukushima No. 1 nuclear plant early in the crisis. Cooling functions were lost on March 14, and the reactor melted down following the No. 1 and No. 3 reactors.
The difficulty in venting fueled concerns that mounting pressure could rupture the containment vessel and release lethal levels of radioactive materials.
Early on March 15, TEPCO temporarily evacuated all but the minimum required 70 or so workers from the plant compound.
Officials were closely monitoring pressure levels inside the containment vessel as an indicator of whether radioactive materials were contained within the reactor.
At TEPCO’s news conference on the afternoon of March 16, reporters asked about the possibility that the containment vessel had already lost airtightness due to structural damage, and that the pressure inside had fallen to the level of the atmosphere.
Late on the night of March 16, a TEPCO official told reporters that the pressure was rising.
The company presented data that showed the pressure had increased from 220-240 kilopascals earlier in the day to 400 kilopascals or more between 7 a.m. and 2 p.m.
However, the actual figure was 40 kilopascals from early morning through noon.
According to TEPCO, workers thought the reading of a pressure gauge at the central control room was either 40 or 400 kilopascals past noon. However, they were unable to reconfirm the reading for fear of being exposed to high radiation levels.
Instead, workers calculated the pressure based on data from a system that suspends reactor operations if it detects an abnormal pressure rise. But they used a wrong conversion formula and erroneously concluded that it was 400 kilopascals.
TEPCO noticed the mistake in late April at the earliest. The company included corrected pressure figures when it distributed a large volume of electronic data--including figures for other reactors--at a news conference on May 16, 2011.
TEPCO officials did not clearly explain that the March 16 pressure data for the No. 2 reactor had been corrected.
The corrections went largely unnoticed. And the panels set up by the government, the Diet and TEPCO to investigate the nuclear accident failed to address the issue.
The Asahi Shimbun independently analyzed the data for the No. 2 reactor and has asked TEPCO to provide an explanation since April 2012.
“Workers applied a wrong conversion formula while they were preoccupied with dealing with the accident,” a TEPCO public relations official said.
The official also indicated that there was no problem with the way the corrected figures were disclosed.
“We compiled and provided as much information as possible while giving priority to recovery operations at the plant,” the official said.