IWAKI, FUKUSHIMA PREF. – Four and a half years after the Fukushima disaster started, and as the country tentatively restarts nuclear power elsewhere, legal challenges against the crippled plant’s operator are mounting.

They include a judge’s forced disclosure of a 2008 internal document prepared for managers at Tokyo Electric Power Co. warning of a need for precautions against an unprecedented nuclear catastrophe.

Also, class actions against Tepco and the government now have more plaintiffs than any previous Japanese contamination suit and, overruling reluctant prosecutors, criminal charges have been leveled against former Tepco executives for failing to take measures to prevent the 2011 meltdowns and explosions.

Radiation from the worst nuclear plant disaster since Chernobyl in 1986 forced 160,000 people from their homes, many never to return, and destroyed businesses, fisheries and agriculture.

The criminal and civil legal cases do not threaten financial ruin for Tepco, which is now backstopped by Japanese taxpayers and faces far bigger costs to decommission the Fukushima plant and clean up the surrounding areas.

Rather, the cases could bolster opposition to reactor restarts — which consistently beats support by about 2-to-1 — as Prime Minister Shinzo Abe’s government pushes to restore nuclear to Japan’s energy mix to reduce reliance on imported fossil fuel.

“The nuclear plant disaster has upended our way of life,” evacuee and former beekeeper Takahisa Ogawa, 45, testified recently in a court in Iwaki, near the Fukushima power station. “We’ve lost the support we counted on.”

Ogawa and other plaintiffs are seeking ¥20 million each in damages from Tepco. More than 10,000 evacuees and nearby residents have brought at least 20 lawsuits against the utility and the government over the handling of the disaster at the Fukushima No. 1 nuclear plant 220 km north of Tokyo.

The biggest class action, with 4,000 plaintiffs, seeks to dramatically increase Tepco’s liability by proving negligence under civil law, rather than simply proving harm and seeking compensation, said lead attorney Izutaro Managi.

The government recently approved increasing the amount of compensation payments through a government-run fund to ¥7 trillion.

Prosecutors twice declined to charge former Tepco bosses over their handling of the disaster, citing a lack of evidence. But a citizens’ panel overruled them last month. It’s unlikely the three former executives, who will be asked to give evidence in court, will be convicted as it is hard to prove criminal acts in this type of case, said Nicholes Benes of The Board Director Training Institute of Japan.

The legal actions against Tepco are “serious for the industry” as it seeks to gradually bring some of the country’s 43 idled nuclear reactors back online, said Tom O’Sullivan, an independent energy consultant and former investment banker.

“With potentially up to 25 reactors coming online, board members of other electric power companies must be quite nervous about what could happen if something goes wrong,” he said. “Most reactors have been switched off for four years so switching them back on is going to be potentially problematic, not to mention the risk of natural disasters.”

It’s unclear what bearing the various lawsuits against Tepco might have on one another, but a common thread is that it should have anticipated the possibility of a devastating quake and tsunami and taken steps to reduce the impact.

The firm maintains that the severity of the 9.0-magnitude quake and 13-meter tsunami could not have been predicted.

But the document introduced as evidence in the shareholders’ suit after a judge forced Tepco to produce it, appears to challenge that. The “Tsunami Measures Unavoidable” report, dated September 2008, was filed with the Tokyo District Court in June, but has not been widely reported.

The unnamed authors prepared the report for a meeting attended by the head of the power station and marked the document “to be collected after discussion.” It’s not clear whether senior executives in Tokyo saw the report at the time.

The report called for Tepco to prepare for worse tsunami than previously assumed, based on the views of experts.

“Considering that it is difficult to completely reject the opinions given thus far of academic experts on earthquakes and tsunami, as well as the expertise of the (government’s) Headquarters for Earthquake Research Promotion, it is unavoidable to have tsunami countermeasures that assume a higher tsunami than at present,” the report says.

“This is prime evidence that Tepco recognized the need for tsunami measures,” said Hiroyuki Kawai, lead attorney in the shareholders’ suit. “This will have an important impact on the lawsuit.”

Tepco, in a court filing, counters that the document “does not mean there was a risk that tsunami would strike and did not assume any specific tsunami countermeasures.”