Unanswered questions of the Fukushima nuclear disaster

Six years after the nuclear disaster in Fukushima, engineers remain vexed by a key question: What damage did the massive earthquake cause at the atomic plant before it was hit by the subsequent tsunami?

The answer matters because of the potential implications for the earthquake safety standards of other nuclear reactors in Japan, which sits on the seismically unstable Ring of Fire around the Pacific. The area accounts for about 90% of the planet’s earthquakes, with Japan being shaken by 10% of them, according to the US Geological Survey.

Just three out of Japan’s 42 usable reactors are running at present, as operators seek to clear regulatory, safety and legal hurdles and overcome community opposition following the Fukushima calamity. Despite the obstacles, Japan still aims to derive between 20% and 22% of its power from nuclear sources by 2030.

Investigations into the Fukushima accident generally accept that the tsunami knocked out backup power to the Tokyo Electric Power Co. Daiichi plant, causing a failure of cooling equipment and then reactor meltdowns.

However, as much of the site is a radioactive no-go zone, it’s not been possible to investigate effects on the plant from the earthquake itself off Japan’s Pacific Ocean coastline in the afternoon of March 11, 2011. The quake registered a magnitude 9, the largest ever recorded in the country.

The impact of the quake is “still actually a question mark,” Mitsuhiko Tanaka, a former nuclear equipment engineer for Hitachi Ltd., said at a press conference in Tokyo.

Tokyo Electric Power (Tepco) has said that the quake at 2.46 p.m. cut off power supply, but operators used emergency diesel generators to keep cooling the reactors. These generators in reactor building basements were subsequently disabled by the tsunami.

No earthquake-related damage to key safety facilities “has been confirmed,” Tepco said in its accounts of the accident. It pointed to the tsunami of “unprecedented scale” that hit the coast 50 minutes later to explain the loss of backup power, which thwarted cooling efforts and ultimately led to explosions and the meltdown of three reactors.

The Fukushima disaster is ranked alongside Chernobyl as the world’s worst civilian nuclear accident, according to the International Nuclear and Radiological Event Scale.

This video shows seismic activity around Japan before, during and after the major earthquake on March 11, 2011. Watch the counter at the top left for the magnitude 9 quake at 2:46 p.m.

Earthquake safety ‘inadequate’

In a briefing at the Foreign Correspondents’ Club of Japan a few days ahead of the disaster’s sixth anniversary this year, Tanaka contended that the cause of the station blackout at unit 1 of the Fukushima plant remained unclear.

He also suggested that the piping system that took in seawater for cooling purposes might have been corroded, adding that such pipes were “generally vulnerable to earthquakes.”

“I’m not saying that the earthquake alone caused damage in lieu of the tsunami – the tsunami no doubt had a significant role,” Tanaka said.

“But I’m also saying that the anti-seismic design of the power stations was inadequate and I’m also saying that without the tsunami the same accident possibly would have occurred. So even excluding the tsunami, just the earthquake alone could possibly cause a major rupture. I’m stressing that one should not neglect or ignore the issue of the earthquake.”

While such comments might appear speculative, Tanaka is in a position to understand a nuclear power station’s vulnerabilities.

He designed reactor pressure vessels for Hitachi, the company that supplied one of the units at the Fukushima Daiichi plant. He conducted stress analysis of the station’s unit 4 reactor pressure vessel and served on the Fukushima accident independent investigation commission set up by the Japanese parliament.

More time

That commission, which had the power to subpoena evidence, differed from other studies by placing a greater emphasis on the potential quake damage. Indeed, its 2012 report said Tepco “was too quick to cite the tsunami as the cause of the nuclear accident and deny that the earthquake caused any damage.”

Naiic Report by Yee Kai Poo on Scribd

The panel, which was also scathing about the lax approach of the then regulators, raised the possibility that the quake damaged equipment necessary for ensuring safety and that a small-scale accident involving a loss of coolant occurred in unit 1.

Looking back at the six-month inquiry, Tanaka said: “It is really quite unfortunate that the investigation committee disbanded without really exposing or explaining much after the accident. Much remains unresolved.”

His view was supported by Masashi Goto, a former designer of reactor containment vessels for Toshiba Corp., who told the same press briefing: “There are many uncertainties still.”

One of the obstacles to finding the truth, investigators cautioned in 2012, was that a lot of the equipment relevant to the accident remained “beyond the reach of inspection or verification”.

That remains a challenge today, as thousands of workers make slow progress on the decommissioning of the plant – a process that is expected to take decades and cost 8 trillion yen ($US70 billion). In addition, 7.9 trillion yen will be spent on compensation from radiation fallout and 5.6 trillion yen on treating and storing contaminated soil, according to latest government estimates.

Push to restart reactors

Meantime the atomic power industry is making slow progress on restarting other reactors in Japan, a situation that calls into question the government’s 2030 target for nuclear power generation.

Takeo Kikkawa, a Tokyo University of Science professor who was a member of the government’s energy mix advisory committee, said achieving the 20% to 22% target would involve “a lot of difficulty.”

In a recent speech to the Foreign Press Center Japan, he noted many of the country’s aging nuclear reactors would need to be decommissioned by 2030 if the government stuck with the rule that such closures occur after 40 years of operation.

Tepco, mindful of the huge costs it is incurring at the devastated Fukushima Daiichi plant, wants to restart two reactors at its Kashiwazaki-Kariwa nuclear power plant, which was the world’s largest such facility but suffered damage from a previous earthquake in 2007.

But in a blow to the plans, voters in Niigata prefecture last year elected a governor who, like his predecessor, opposed a restart at Kashiwazaki due to safety concerns.

Just last month, Tepco was ordered to re-submit documents after revealing that its previous assurances about safety measures at Kashiwazaki-Kariwa had been wrong.

Tepco discovered in 2014 that a key building at the site may not be able to withstand even half of the assumed strongest seismic shaking, but this information was not passed on to the regulator, the Asahi newspaper reported.

Tepco’s managing executive officer, Takafumi Anegawa, apologized for the omission, which was blamed on “insufficient” communication within the company rather than a cover-up. A Nuclear Regulation Authority official was quoted as saying the lessons of Fukushima were “not utilized”.

‘Catastrophic’ implications

Shaun Burnie, a nuclear specialist with Greenpeace Germany, called for a fundamental overhaul of the way the regulator reviews earthquake risks. He praised the engineers who had “spoken out” about the potential pre-tsunami damage at Fukushima Daiichi, saying they were right to demand further investigation.

“That is something the nuclear industry is determined to avoid as the ramifications, if proven, would be catastrophic for the future operation of reactors in Japan – but also have major implications worldwide,” he said in an interview.

Burnie said the International Atomic Energy Agency and regulators worldwide had based their reviews of the Fukushima accident on the basis that without the tsunami, there would have been no multiple reactor meltdowns.

“While this may be the conclusion the nuclear industry want to hear, it may not be correct. It could be many years before this issue is resolved one way or the other. Meanwhile, Japan continues to apply a flawed seismic model for assessing risks at nuclear plants.”

Watch the full press conference here:

(This is the first in a three-part series ahead of the the sixth memorial of the Fukushima disaster. On Thursday we examine progress and setbacks in the nuclear power plant decommissioning that will take decades to complete.)