TOKYO — Even before the troubled Fukushima Daiichi nuclear plant has been brought under control, two conglomerates vying for contracts in an eventual cleanup are estimating that the effort could take 10 years — or 30.

The widely divergent outlooks underscore the basic uncertainties clouding any forecast for Fukushima. It is far from clear when the cooling system will be restored and radiation emission halted; how soon workers can access some parts of the plant; and how bad the damage to the reactors, their fuel and nearby stored fuel turns out to be. The United States Nuclear Regulatory Commission has warned that at least one reactor’s fuel may even have leaked out of the reactor pressure vessel.

A global team led by Hitachi said Thursday that it would take at least three decades to return the site to what engineers refer to as a “green field” state, meaning within legal limits of radiation for any residents. Toshiba, Japan’s biggest supplier of nuclear reactors, said it could take as little as 10 years.

Both companies have large nuclear-related businesses and appear to be eager to speak about endgame possibilities for a crisis that has heightened global public mistrust of nuclear power. Billions of dollars are likely to be at stake in the cleanup, which could help Hitachi and Toshiba improve their bottom lines. The two said last week that annual profits would fall short of their forecasts because of the widespread disruptions in production and supply chains caused by the disaster.