RV Mirai

The reactor was completed on 25 August 1972, and fuel was loaded on 4 September.[7] When officials announced that the first test run was to be run at the pier in Ōminato, local protests forced them to reconsider.[7] Eventually it was decided to test the ship in the open ocean, 800 kilometres (430 nmi) east of Cape Shiriya.[7] The ship departed Ōminato on 26 August 1974, and the reactor attained criticality on 28 August.[7]

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]

After removing the reactor in 1995 and decontaminating the vessel, Mutsu was rebuilt as the ocean observation vessel Mirai.[5][8]