TOKYO - Japan's environment and nuclear minister visited the tsunami-crippled Fukushima Dai-ichi nuclear power plant Saturday to inspect a spent fuel pool at the centre of safety concerns.

The visit by Goshi Hosono comes amid renewed concerns about the conditions of Unit 4 after its operator reported partial bulging of the building wall, prompting nuclear regulators to order fresh investigation and seismic tests on the building and its pool.

The pool that sits at the top of the plant's No. 4 reactor, which suffered an explosion and fire early in the crisis triggered by last year's massive earthquake and tsunami, remains one of biggest risks due to its vulnerability to earthquakes.

The power plant's operator, Tokyo Electric Power Co., has reinforced the structure and says it can withstand temblors as strong as the March 11, 2011 quake, which was magnitude 9.0 at its offshore epicenter.

Concerns have persisted because fuel inside storage pools is not enclosed like reactor cores and could cause greater radiation leak in case of a major leak in the pool. Lack of explanation also triggered rumours such as the fuel rods are still exposed to air or the building was falling apart.

Ikko Nakatsuka, vice-minister of reconstruction who saw Unit 4 pool during his plant tour in April, said earlier this week that the building wasn't tilted and the pool had sufficient water inside to keep rods safely cooled, denying the rumours. But he said Unit 4 building and its pool are obviously more vulnerable than the other reactors and uncertainties remain.

TEPCO is preparing to move fuel rods from Unit 4 pool to the joint pool -- for all six reactors -- next to the building, but the process won't start until late next year. The joint pool is located on the ground level and is considered safer, even though it already stores 6,000 fuel rods in it, officials said. It would take 10 years before removal molten fuel from three wrecked reactors would start.

From early on, experts have raised concerns about No. 4 pool, where 1,535 fuel rods -- the largest inventory among Fukushima's six reactors -- as its reactor was undergoing major refurbishing and fuel ordinarily inside the core was also stored in the pool.

A worst case scenario produced by the head of the Japan Atomic Energy Commission two weeks after last year's accident warned that meltdown at Unit 4, along with all Fukushima reactors, would require evacuation of 30 million people from the greater Tokyo area.