The mangled wreckage of Unit 3 is seen at the tsunami-crippled Fukushima Daiichi nuclear power plant in Fukushima prefecture. Photo: Reuters/ Kyodo

It has been seven years since an accident at the Fukushima Daichi nuclear plant on March 11, 2011 precipitated three meltdowns. Yet, despite three independent investigations, there is still no consensus on what actually caused the meltdowns.

These are not idle questions.

“Without solving the ‘mysteries’ of Fukushima, it is impossible to say we have learned ‘the lessons’ of Fukushima,” says Caitlin Stronell, a researcher for the Citizens Nuclear Information Center.

Those lessons are critical to create new regulations and designs to make reactors safer. The governor of Niigata, site of a massive nuclear power complex, says he will not green light any restart until all lessons are learned. But it could be years before there is any kind of certainty.

Below are five of the most significant unresolved “mysteries” of Fukushima:

  1. What actually caused the disaster?

It is widely agreed that when the Great East Japan Earthquake struck Fukushima, cooling systems for the emergency core, powered by diesel generators, came online – as they were designed to do when outside power sources are damaged or destroyed by an earthquake. But about 40 minutes later, two waves of seawater inundated the basements of the plants, destroying 12 of the 13 diesel generators and other electrical equipment, leading ultimately to simultaneous meltdowns.

That is the generally accepted scenario, but not everyone is convinced that the tsunami is the only culprit. Some workers say that they witnessed water leaking from some interior safety gear right after the earthquake before the tsunami struck. The question can’t be answered, they say, until it is safe to enter the plants.

Four previous investigations, one by the Ministry of Economy and Industry, one appointed by the Diet, one private investigation led by Yoichi Funabashi, former editor of the Asahi newspaper, and a probe by the operator of the plant, Tepco, failed to settle whether the plants were fatally damaged by the tsunami alone. The utility’s official position is that the tsunami, not the earthquake, set off the sequence of events that led to the three meltdowns.

The Diet Commission – probably the investigation most critical of Tepco’s response – claimed that the utility “was too quick to cite the tsunami as the cause of the nuclear accident and deny that the earthquake caused any damage.” It said: “It is impossible to limit the direct cause of the accident to the tsunami without substantive evidence.”

The issue is a politically loaded question, which could affect the future of nuclear power in Japan. If it can be shown conclusively that the plants survived the initial earthquake, registered at 9 points on the Richter scale – one of the largest earthquakes ever recorded – it would indicate that nuclear power can coexist with earthquakes, assuming measures are taken to mitigate against tsunamis.

Adding weight to the tsunami argument is the fact that the Onagawa nuclear plants owned by the Tohoku Electric Power Co., which were actually closer to the quake epicenter but protected by a sea wall, were largely undamaged. So too were Fukushima Units 5-6, which did not melt down, as they were fortunately connected to the one surviving emergency generator which kept the spent fuel pool cool.

  1. Where did Unit 1’s hydrogen explosion occur?

There is a dispute about whether the hydrogen explosion at Unit 1 occurred on the 4th level or the 5th level. This may seem trivial, but the matter is closely linked to the question of what actually caused the accident. The quake set events in motion that led to the hydrogen explosion and the three resulting meltdowns. The quake severed offsite power to run safety systems. It also caused the huge tsunami that completed the job by knocking out all remaining safety systems.

The isolation condenser, a critical piece of safety equipment, is located on the 5th level, so it is important to determine if it was damaged and made nonfunctional during the earthquake. The isolation condenser is a safety device found on older reactors that draws in steam and converts it to water. Critically, the water falls back into the core by gravity and is not dependent on emergency generators.

The isolation condenser is Exhibit A for those who maintain that the plants were severely weakened by the quake even before the tsunami hit and the explosion occurred. A team actually entered Unit-1 in October 2011 and found no damage. But they were only allowed to spend 10 minutes in the reactor building due to the high levels of radiation.

  1. What happened to the melted cores?

Until very recently it has been impossible to say exactly where the molten cores are located and in what kind of condition they are. Through trial and error, the decommissioning team is beginning to develop the kinds of robots to withstand severe radiation and navigate through extremely small passageways to photograph the debris. Although robots can absorb vastly more radiation than humans, there are limits; strong radiation short-circuits the bots.

In July, a robot christened the Manbo survived its journey into the belly of the beast to bring back the first genuine images of the Unit-1 reactor’s melted core. It found what could be some of the control rods that collapsed on to the bottom of the reactor pressure vessel.

“It is important to know the exact locations and the physical, radiological and chemical forms of the [molten core] to develop the necessary defueling technologies,” said Lake Barrett, an official with the U.S. Nuclear Regulatory Commission. Tepco has said it will make its first plans for defueling by 2019.

  1. Was there a hydrogen explosion at Unit 2?

On the morning of March 15, a loud noise was reported near the base of the Unit 2 nuclear reactor, leading many to suppose that the reactor had sustained a hydrogen explosion. The power company also reported that pressure in the steam suppression pool located near the bottom of the reactor’s containment dropped significantly after the sound was heard.

For months, the government insisted that Unit 2 had sustained a hydrogen explosion, or that explosions occurred simultaneously at Unit 2 and another plant. A later review of the seismic data led to a different conclusion. It was determined that a hydrogen explosion in Unit 3 blew out the windows, which allowed hydrogen to escape before it could ignite. Certainly, there was less damage to the Unit 2 building, so clearing of debris there is not so difficult.

  1. How did contaminated water get in the turbine building?

For the past seven years, Tepco has had to deal with tons of contaminated water cooling the reactor core that enters the turbine building and mixes with groundwater. Tepco has tried numerous ways to decontaminate the water and to keep it from seeping into the ocean. Most have involved putting up barriers, such as the “ice wall”, but have only been a partial success. Contaminated water is also being stored on site, in hundreds of storage tanks.

In fact, the company and regulators still don’t know exactly how cooling water gets into the turbine building – whether through pipes or other passageways. This is one of the mysteries of Fukushima that can only be solved once workmen are able to safely enter and spend time inside each reactor.

Why the mysteries must be solved

For now, the overall consensus is that the tsunami caused the meltdowns, although there are high-profile dissenters, such as the Niigata governor.

But it seems unlikely that workmen will be unable to enter the stricken plants for years. Until they do, and the answers to the questions above are provided to the satisfaction of all, the future of nuclear power – which still accounts for 20% of Japan’s electric power – will remain uncertain.

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