* Plant operator says 7 tonnes of radioactive water leaked
* Tepco says leak contained, six workers exposed to radiation
* Japan's nuclear regulator says exposure level "not serious" (Adds Japan regulator comments, details)
By Mari Saito
TOKYO, Oct 9 (Reuters) - Six workers at Japan's crippled Fukushima nuclear plant were exposed to a leak of highly radioactive water on Wednesday, the latest in a string of mishaps the country's nuclear watchdog has attributed to carelessness, saying they could have been avoided.
Tokyo Electric Power Co, also known as Tepco, has been battling to contain radioactive water at the plant, which suffered triple meltdowns and hydrogen explosions following a devastating earthquake in March 2011.
In the latest incident, a worker mistakenly detached a pipe connected to a treatment system to remove salt from the hundreds of tonnes of water Tepco pumps over the melted fuel in wrecked reactors at Fukushima to keep them cool.
"It is serious in that it was another problem caused by carelessness, but I do not believe it is a seriously troubling dosage," Shunichi Tanaka, chairman of Japan's Nuclear Regulation Authority, said on Wednesday.
"But the fact that there has been a string of incidents occurring on a daily basis that could have been avoided - I think that is the large problem."
Tanaka urged Tepco to improve its handling of contaminated water, but stopped short of saying if it faced any penalties.
The accidents at the Fukushima Daiichi plant, 220 km (130 miles) north of Tokyo, are adding to a crisis no one seems to know how to contain, and stirring doubt over Tepco's abilities to carry out a complex cleanup widely expected to take decades.
Just last week, the regulator ordered Tepco to draft in additional workers and report within a week on its measures to tackle the hazardous clean-up.
Tepco said seven tonnes of water were spilled in Wednesday's incident at the treatment facility but were contained within the site, adding that the leaked water had an all-beta radiation level of 34 million becquerels per litre.
Tanaka said the leaked water had already been treated to remove cesium, which emits strong gamma radiation harmful to humans.
On Monday, Tepco said a plant worker accidentally halted power to pumps used to cool the damaged reactors. A backup system kicked in immediately, but the event was another reminder of the precarious situation at the plant.
Last week, Tepco said 430 litres (113 gallons) of contaminated water had spilled out of a storage tank at Fukushima and probably flowed to the ocean.
Japan's nuclear regulator said on Wednesday that incident was equivalent to "Level 0" on the International Nuclear and Radiological Events Scale (INES), but gave no official rating.
In August, a leak of 300 tonnes of highly radioactive water from a hastily built site tank was given a "Level 3" or "serious incident" rating on the INES scale.
Also in August, Tepco said two workers were contaminated with radioactive particles, the second such incident in a week involving staff outside the site's main operations centre.
Tepco is trying to restart its only remaining viable plant - Kashiwazaki Kariwa, the world's largest nuclear power station, to cut high fuel costs and restore its finances.
(Additional reporting by Yuka Obayashi; Editing by Aaron Sheldrick and Clarence Fernandez)