Operations at the reactor were suspended after the 1995 accident
Japan's Atomic Energy Agency says it has restarted a controversial nuclear reactor, more than 14 years after its operations were suspended.
A spokesman for the agency said work began at the plutonium-fired reactor in the northern fishing town of Tsuruga after the government gave the go-ahead.
The facility, called Monju, was shut down in 1995 following a fire.
The accident and cover-up created widespread public concern over the safety of nuclear power.
No one was hurt and there was no radiation leak in the accident, which occurred less than two years after it had begun generating power.
But the plant's operators were criticised for concealing extensive damage to the reactor.
The fast-breeder reactor was declared reactivated by Monju's director general, Kazuo Mukai, after one of its control rods, which had been inserted to prevent an atomic reaction, was lifted.
It uses plutonium fuel instead of conventional uranium and produces radioactive substances that can be reused as fuel.
It is expected to reach operational levels by Saturday, and then to reach full output by the spring of 2013.
Only Russia and India, alongside Japan, operate fast-breeder reactors, although China says it wishes to start doing so soon.
Tokyo-based group the Citizens' Nuclear Information Centre said: "We believe that Monju is an accident waiting to happen and that it is, therefore, irresponsible to restart the plant.
"We demand that the government stop playing Russian roulette with our lives and permanently close down Monju."
Japan relies on nuclear energy for about a third of its domestic power supply.
But public concern over the safety of its plants is high, after a string of accidents and malfunctions.