Tokyo: A Japanese utility sought government approval on Monday to restart two nuclear reactors even though some key upgrades to prevent another nuclear crisis will take three years.

All but one of Japan's 54 reactors is offline for regular safety checks, and the last will be shut down in May.

Residents fear another disaster like the Fukushima crisis, but Japan faces a severe power shortage if reactors are not restarted.

The government issued new safety guidelines to address residents' worries, but it gave no deadline for when the improvements must be finished. Utility officials say the full upgrades will take three years.

Kansai Electric Power Co. submitted its safety plans for two reactors at the Ohi plant in Fukui prefecture, and the government's final decision on whether to restart the reactors is reportedly expected later this week.

"We'll aim to achieve the world's top-class safety at our plants," said Kansai Electric President Makoto Yagi as he handed the safety improvement roadmap to Economy and Industry Minister Yukio Edano.

However, more than one third of the necessary upgrades on the list are still incomplete, utility officials said.

Filtered vents that could substantially reduce radiation leaks in case of an accident threatening an explosion, a radiation-free crisis management building, and fences to block debris washed up by a tsunami won't be ready until 2015.

This means the plant, as well as plant workers and residents, won't be fully protected from radiation leaks if a Fukushima-level accident occurs while the measures are being taken.

Currently, the crisis management headquarters at the Ohi plant is in the basement, which would be flooded in case of a major tsunami, Kansai Electric officials said. The plant is relocating the function to a room next to the control room for the two reactors.

The Fukushima Dai-ichi completed a similar building at a slightly elevated area on the complex just a year before the disasters -- though it was meant for quakes. It was the key crisis management center after surviving the March 11, 2011, tsunami that washed into the plant, destroying the plant's power and cooling systems, causing three reactor cores to melt. Plant officials have said the building was key to their survival.

None of Japan's 54 nuclear reactors are equipped with filtered vents, although their operators are moving to install them in coming years.