Diverging conclusions - Questions left unanswered

July 27, 2012

Fukushima probes leave questions

Two blame tsunami for meltdowns but one still points to quake factor

Staff writer

http://www.japantimes.co.jp/text/nn20120727a5.html

The final report handed in Monday by the government panel appointed to investigate the Fukushima nuclear disaster wraps up the work of the three major probes, but many questions remain unanswered.

Key among them is whether the 9.0-magnitude Great East Japan Earthquake may have damaged the Fukushima No. 1 plant's reactors and cooling systems before the tsunami struck.

The three investigative teams' findings diverge sharply over this critical point.

The government panel and an independent team set up by the Rebuild Japan Initiative Foundation and headed by Koichi Kitazawa, a former chemistry professor at the University of Tokyo — largely concluded that the monster tsunami, not the quake itself, critically damaged the plant, leading to the meltdowns.

But the Diet-appointed independent panel, imbued with stronger investigative authority, suspects the offshore quake, which preceded the tsunami by 40 minutes, severely damaged some of the three reactors.

This again raises serious questions about the quake resistance of older reactors across the country, as well as most of the antidisaster measures the government has adopted since the Fukushima crisis started.

Most of the Fukushima-inspired safety measures adopted for reactors focus on the tsunami risk, because Tokyo Electric Power Co. and the government played down the impact of the quake, which was rated a strong 6 on the Japanese intensity scale to seven at the Fukushima plant.

"We concluded that Tepco was too quick to cite the tsunami as the cause of the nuclear accident and to deny that the earthquake caused any damage. We believe there is a possibility the earthquake damaged equipment needed for ensuring safety," the Diet panel headed by Kiyoshi Kurokawa, the University of Tokyo professor emeritus, said July 5.

The Diet panel argued that the quake might have knocked out some of the emergency diesel generators and damaged pipes carrying coolant to the cores.

If a crack as small as even 0.3 sq.-cm developed in a pipe and was left untended for 10 hours, tons of coolant water would escape, causing the core to heat up and eventually melt, the panel said in its final report.

The temblor as recorded at the Fukushima plant was powerful, but still within the scope of jolts that could be expected anywhere in Japan. If the march Great East Japan Earthquake is actually found to have caused severe damage at the Fukushima plant, the government will again likely be forced to rescrutinize the quake resistance at many of the nation's older reactors.

The government's panel, headed by Yotaro Hatamura, another professor emeritus at the University of Tokyo, generally takes the utility's line on quake damage.

"It is unlikely that the reactors lost their ability to contain (the fuel) because of the earthquake," given the data currently available, said Shinji Ogawa, director general of Hatamura's panel, at a press conference on July 23.

The government panel, however, said that computer simulations suggest even a 0.1 sq.-cm crack would cause the pressure in the containment vessel to be significantly different from actually what happened, and thus it is difficult to assume the quake was the main cause of the meltdowns. It is not clear if any scenario was presented for multiple pipe cracks, or where.

Meanwhile, settling this critical debate may take years because the lethal radiation levels in the main reactor buildings will prevent anyone from getting close enough to search for cracks or other damage, including to pipes and other equipment.

Hatamura thus proposed the government set up another investigative body to guide the probe into the foreseeable future, because Tepco's nuclear repairmen are in for a long wait.

The Diet panel said the megaquake might have severely damaged water pipes in reactor 1 because nobody in the control room heard the safety-relief valves engage. Reactor 2's SRVs meanwhile were clearly and repeatedly heard.

The SRVs automatically open to release steam from the pressure vessel to the outer containment vessel if pressure rises dangerously. But if pipes are damaged and leaking water, the internal pressure won't rise and the SRVs won't open, either.

If water was leaking from some of the damaged pipes, the pressure vessel would lose coolant and heat up. This might have been the real cause of the reactor 1 meltdown, the Diet panel said.

Tepco strongly denied that, and the Diet panel has yet to find any evidence to back up its speculation, except for the testimonies of the workers who were there. Oddly, reactor 1 was not equipped to monitor SRV movements, although reactors. 2 and 3 were.

Also, the Diet panel said the earthquake might have knocked out one of the emergency generators at reactor 1 before tsunami flooded the plant.

Tepco has claimed that the initial tsunami smashed into the power station at 3:35 p.m. on March 11 and knocked out all emergency generators and caused a total power outage — a scenario known as a station blackout.

But the Diet panel said that a wave gauge set up about 1.5 km off Fukushima's coast recorded the tsunami sweeping by at exactly 3:35 p.m., and estimated that it would have taken at least two more minutes for it to reach the plant.

This means the generators would have been knocked out sometime after 3:37 p.m. if they were halted by the waves.

Since the generator in question, which was set up in the basement of the turbine building, is believed to have been disabled between 3:35 and 3:36 p.m., this implies the quake and not the waves was responsible, the report said.

Since the tsunami flooded the basement, the generator would have stopped eventually.

The government panel said it is possible that the tsunami might have come after 3:37 p.m., but when analyzing the situation comprehensively, it natural to think that the diesel generator was knocked out by the tsunami.

Another point that the Diet panel differs from others is why engineers turned off an isolation condenser at reactor 1.

The IC is an emergency cooling system.

Shortly after the quake hit the plant, engineers used the IC to cool the reactor core and turned it on and off for three times.

The government panel's report said the engineers turned off the IC because they were trying to control the speed of the temperature drop inside the reactor no faster than 55 degrees per hour, which is stated in the manual for the IC.

But the Diet panel said the engineers told the panel that they wanted to see how the pressure level would change because they were concerned if the pressure was escaping from some cracks caused by the quake.

"It seems Tepco does not admit that the workers turned off the IC to check the leakage because it will bring up the unfavorable matter of the possibility of quake damage," said the Diet panel, adding that it is also conceivable that the pipeline for the IC might have been damaged by the quake.

The Diet panel asked Tepco if it could send investigators inside the reactor 1 building to view the quake damage as much as possible.

But Tepco declined because the utility did not want its employees to be exposed with more radiation more than was necessary.