(Adds quotes, reading measurements)
TOKYO, Sept 1 (Reuters) - Radiation near a tank holding highly contaminated water at Japan's crippled Fukushima nuclear plant has spiked 18-fold, the plant's operator said on Sunday, highlighting the struggle to bring the crisis under control after more than two years.
Radiation of 1,800 millisieverts per hour - enough to kill an exposed person in four hours - was detected near the bottom of one storage tank on Saturday, Tokyo Electric Power Co , also known as Tepco, said.
An Aug. 22 readings measured radiation of 100 millisieverts per hour at the same tank. Japanese law has set an annual radiation exposure safety threshold of 50 millisieverts for nuclear plant workers during normal hours.
Last month, Tepco revealed that water from the tank was leaking. Japan's nuclear regulator later raised the severity of the leak from a level 1 "anomaly" to a level 3 "serious incident" on an international scale for radiation releases.
The Fukushima Daiichi power plant north of Tokyo was devastated by a tsunami on March 11, 2011 that resulted in fuel-rod meltdowns at three reactors, radioactive contamination of the air, sea and food and the evacuation of 160,000 people.
It sparked the worst nuclear crisis since Chernobyl a quarter of a century earlier.
While there were no new leaks found at the tank, a Tepco spokesman said another leak had been detected from a pipe connecting two other tanks nearby.
"We have not confirmed fresh leakage from the tank and water levels inside the tank has not changed," the Tepco spokesman said. "We are investigating the cause."
Tepco said the radiation measured was beta rays, which would be easier to protect against than gamma rays.
The Tepco spokesman also said the higher level of radiation from the latest reading was partly because investigators had used a measuring instrument capable of registering greater amounts of radiation.
Instruments used previously had only been capable of measuring radiation up to 100 millisieverts, but the new instruments were able to measure up to 10,000 millisieverts.
Radiation of 220 millisieverts was also recorded near an adjacent storage tank, where a reading of 70 had been registered last month.
Radiation of 230 millisieverts was detected from the new leak from the pipe connecting two nearby tanks, a new measurement of 70 was taken from another, separate storage tank.
Those tanks are built of steel plates stuck together by bolts - the same structure as the tank that was found last month to have leaked 300 tonnes of highly toxic water.
With no one seeming to know how to bring the crisis to an end, Tepco said last week it would invite foreign decommissioning experts to advise it on how to deal with the highly radioactive water leaking from the site.
Japan has also signalled it might dip into a $3.6 billion emergency reserve fund to help pay for the clean-up of a situation the chief cabinet secretary has described as "deplorable".
Its nuclear regulator has also expressed fear that the disaster was beyond Tepco's ability to cope in some respects.
Foreign Minister Fumio Kishida visited Chernobyl in Ukraine, the site of the 1986 disaster, hoping to apply lessons learned there to Fukushima.
(Editing by Paul Tait)