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There is are also doubts about whether the levy will be sustainable when Japan's electricity market opens to competition from April 1. In a recent interview, Tepco chief executive Naomi Hirose insisted the company would make enough money to clean up the Fukushima Daiichi plant.
"We have to preserve that earning power," Mr Hirose said. "Victory for us means having the money to meet our responsibilities in Fukushima. If we can't, that's failure."
But one way to judge Tepco's contribution is its share price, which should reflect past losses, as well as any levies the market expects in the future. Compared with March 10 2011, the day before the disaster, Tepco's equity has lost Y2.6tn in value. Debtholders have not suffered losses.
That implies Tepco has borne slightly less than 20 per cent of the total cost, with taxpayers picking up the other Y10.7tn. The figure is rough, and ignores the cost of shutting down all Japan's nuclear reactors, so it is likely to understate both the total cost and the proportion paid by the public.
Tepco, the finance ministry and the economy ministry declined to comment on the estimate. A government official insisted all costs would ultimately be recouped from Tepco and said it could not pass the burden on to electricity customers. "As a whole, Tepco is paying its own costs," said the official.
Evacuees are now being allowed to return to some villages near the Fukushima Daiichi plant but decommissioning will take decades, with radiation levels still too high even to evaluate the stricken reactors. The final cost is unknown and Mr Oshima expects his estimate to rise.
"The government's approach has worked in that Tokyo Electric has not shut down," said Mr Oshima. "But with the costs increasing to this extent it's hard to see the purpose of having kept Tepco alive."