Nuclear regulators on Friday called on Tokyo Electric Power Co. to take action to address rising radiation levels at the border of the crippled Fukushima No. 1 plant's premises, which have jumped to levels eight times higher than the regulatory limit.

According to plant operator Tepco, an evaluation of radiation exposure caused by toxic water, rubble and debris, and other waste kept at the plant was below the limit of 1 millisievert per year as of March, but surged to 7.8 millisieverts as of August.

The rise is attributed to radiation emitted from tanks storing contaminated water generated in the process of cooling the damaged reactors.

In April, Tepco found some of its underground storage pools leaking water and had to transfer the contaminated water to tanks located near the site boundary.

The water stored in the tanks mainly contains strontium-90 and other beta ray-emitting radioactive materials. Beta rays can be easily blocked by a thin sheet of metal, but X-rays, with greater ability to penetrate materials, are generated when beta rays hit the interior walls of the tanks, contributing to the rise in the radiation level at the border.

During a meeting of a Nuclear Regulation Authority panel, officials and experts agreed that Tepco should set a clear timeline for bringing the radiation level below the 1 millisievert limit.

They also pointed to the need for yearly evaluations of the situation and whether or not it is improving.

Currently, areas close to the Fukushima plant are designated by the government as a zone where former residents will not easily be able to return.