The operator of the crippled Fukushima nuclear plant has begun injecting more water into one of the reactors after the core temperature rose above Japan's safety limit.
The plant was seriously damaged in last March's earthquake and tsunami.
It's suspected an unstable water flow is causing the rise in temperature, the ABC reports.
A gauge inside Fukushima's reactor No 2 showed the temperature rising to 82 deg Celsius over the weekend, its highest level since the reactor was put into a state of cold shutdown two months ago.
Operator TEPCO insists there has been no nuclear reaction and that the reactor can be controlled.
Responding to the increase, the operator began injecting 15 tonnes of water an hour into the reactor in a bid to get the temperature down.
TEPCO says it has not detected any xenon gas, which is created when a nuclear reaction has been triggered, reports the ABC.
The company suspects that cooling water has not been flowing freely into the the number two reactor, causing the rise in temperature.