Radiation accident

As the crew brought the reactor up to 1.4% of capacity at 5pm on 1 September 1974,^[7] there was a minor shielding inadequacy that permitted the escape of neutrons and gamma rays^[5] from the reactor shielding enclosure.^[7] Westinghouse Electric Corporation had reviewed the design and warned of this possibility, but no changes were made to the design.^[7] There was no significant radiation exposure, but it became a political issue, with local fisherman blocking her return to port for more than 50 days.^[5] The government finally came to an agreement with the local government and fishermen; the Mutsu was allowed back to port on condition that it was to find a new home port, and the ship returned to Ōminato on 15 October.^[7]

In Sasebo, between 1978 and 1982, various modifications were made to the reactor shield of the Mutsu, and its home port was moved to Sekinehama in 1983.^[7] Following an overhaul, the Mutsu was completed in February 1991.^[7] She then completed her original design objective of travelling 82,000 kilometres (51,000 mi) in testing, and was decommissioned in 1992.^[7] Over 25 years the programme had cost more than 120 billion yen (about US$1.2 billion).^[7]