Six workers at the crippled Fukushima nuclear plant have been exposed to leaking radioactive water.
Plant operators, Tepco, say one of them mistakenly removed a pipe connected to a water treatment system.
Several tonnes of water are reported to have spilled out over the treatment centre.
Tepco say the tainted water was contained within the facility.
The Tokyo Electric Company has been battling radioactive water leaks at the damaged site.
The plant suffered triple meltdowns and hydrogen explosions following the twin earthquake/tsunami that struck Japan in March 2011.