Tokyo, Oct 15 (IANS/EFE): The level of radioactive contamination detected in the underground water of the damaged Fukushima nuclear plant has increased after torrential rain caused by a typhoon last week, the plant operator said Wednesday.

The level of radioactive cesium water from a well located on the coast and close to the plant amounted to 251,000 becquerels per litre, the highest recorded so far, according to samples taken by Tepco Electric Power Company (TEPCO) Oct 8 and 9, the state television channel NHK reported Wednesday.

The presence of the radioactive isotope is three times higher than what it was four days earlier, an increase that, according to the plant technicians, was due to excessive rainfall caused by Typhoon Phanfone Oct 6.

Rainwater seeped into the sub-soil and mixed with the contaminated fluid coming from the damaged reactors.

Plant managers are expected to take samples of groundwater in the area more often to analyse the evolution of the radiation, according to TEPCO.

For the moment, additional measures have been ruled out since the depth and scope of the contaminated water leaks are unknown, and TEPCO already has in place several measures to control the problem, such as the pumping of groundwater or walls to retain underground water.

Large amounts of contaminated water, much of which flows into the Pacific Ocean, are one of the major challenges that are still unresolved in the nuclear power plant damaged by the earthquake and tsunami in 2011.

The accident at Fukushima, the world's worst since the Chernobyl disaster in 1986, led to thousands of residents from the surrounding area being evacuated and who have yet to return home.

The emissions and discharges have also severely affected agriculture, livestock and fisheries.