TOKYO - Two high-tech machines intended to help workers at Japan's tsunami-hit nuclear plant malfunctioned Friday, including a long-awaited Japanese robot making its first attempt to take important measurements in areas too dangerous for humans.

A handout picture taken and released June 24, 2011 shows a T-Hawk survey vehicle (C) on the roof of unit No. 2 reactor after it ditched at Fukushima Daiichi Nuclear Power Station in Fukushima prefecture. The small unmanned vehicle, used to help survey the damage and monitor the level of radiation at the plant, crashed onto the roof of reactor after losing control, TEPCO said on Friday. [Photo/Agencies]

The other machine that failed was a drone helicopter that made an emergency landing on a reactor roof at the Fukushima Dai-ichi nuclear power plant.

Operator Tokyo Electric Power Co. is trying to cool down three molten reactor cores and stop radiation leaks to end a crisis set off when the March 11 earthquake and tsunami crippled the plant. The job is expected to take several more months, and is complicated by massive amounts of radioactive water that could soon leak into the sea.

The Quince robot, developed by Chiba Institute of Technology for nuclear and biological disaster relief activity, had ventured out into the Unit 2 reactor building to set up a gauge to measure the contaminated water pooling in the basement. Radioactivity inside the reactor buildings is too high for workers to take measurements there.

The machine got stuck at a staircase landing and failed to go downstairs, TEPCO spokesman Junichi Matsumoto said. A cable that was supposed to drop a gauge into the basement also malfunctioned.

The workers retrieved the robot and were going to make adjustments before sending it back in for another try, Matsumoto said. He did not elaborate.

The other machine that malfunctioned Friday was a T-Hawk drone helicopter, made in the US by Honeywell, that is used to inspect hard-to-access areas of the plant.

The drone developed engine trouble during a radiation sampling flight and made a remote-controlled emergency landing on the roof of Unit 2 _ the only one of the four damaged reactor buildings that still has a roof, Matsumoto said.

Matsumoto said photos taken by a camera installed on a water pumping vehicle showed the drone was lying on its side, but neither the aircraft nor the roof suffered major damage.

The cause of the engine failure was under investigation. Matsumoto said it was not immediately known when or how the drone may be retrieved, but a backup drone can take over the mission.

TEPCO and the government have said they hope to achieve a cold shutdown of the reactors by January by bringing the core temperatures to below 100 Celsius (212 Fahrenheit.)

Workers have cooled the reactors and spent fuel by pumping in fresh water, which becomes contaminated with radiation. About 110,000 tons of tainted water have accumulated, and it could start overflowing in early July unless workers get a trouble-plagued water treatment system working properly.

The system became fully operational a week ago but shut down after a few hours when one of the radiation absorbing cartridges reached its limit much more quickly than expected. Matsumoto said the system, which has since been on test run, has been working better after corrections were made on some valves.

Goshi Hosono, director of the government's nuclear crisis task force, said the water treatment system, which eventually becomes part of a cooling system, is key to resolving the crisis.