The operator of Japan's crippled nuclear plant says melted fuel at the bottom of the No.1 reactor is being kept cool. Tokyo Electric Power Company says its latest probe reveals the water level in the reactor's containment vessel is at 2.8 meters - enough to stop the fuel from overheating.

TEPCO used endoscopes to look inside the vessel at the Fukushima Daiichi nuclear power plant.

Footage from the probe shows steam in the vessel but no dripping water. That means damaging humidity is lower than in an earlier test on the No.2 reactor.

But the footage highlights problems. The surface of scaffolding inside the container vessel is corroded. The probe revealed a 30-centimeter long metal rod lying on the scaffold and polluted water underneath. TEPCO says it detected radiation of over 11 sieverts per hour in some parts of the container. That's high enough to kill in 40 minutes of exposure.

But it adds that melted fuel rods lying at the bottom of the reactor are stable, thanks to the water level.

The find means that TEPCO's schedule for decommissioning the reactor is still on course.