TOKYO — As Japan moves to cut back on nuclear power after last year’s disaster in Fukushima, it is running into a harsh economic reality: the cost of immediately abandoning its nuclear reactors may be too high for some big utilities to shoulder.

If the country’s 50 nuclear reactors were permanently closed this year, power companies would be hit with losses totaling 4.4 trillion yen ($55.9 billion), rendering at least four of them insolvent, according to calculations this summer by the government’s Agency for Natural Resources and Energy.

The extraordinary costs of an immediate shutdown have emerged as a major concern for the Japanese government, which has struggled to balance the desire for improved nuclear safety with the bottom-line realities of the big utilities. Nuclear plants generated about one-third of Japan’s electricity before the Fukushima accident, but most remain at least temporarily offline.

“People talk easily about shutting down Japan’s nuclear power plants, but the economic and financial consequences are severe,” said Reiji Takeishi, professor in environmental economics at Tokyo International University.