The tunnels have been inundated with water from the plant's heavily contaminated reactor buildings.
Tokyo Electric Power Company, or TEPCO, this year began work to freeze water at the ends of the tunnels to block the inflow. The firm finished the work early this month.
But TEPCO officials found that water levels in the tunnels were still changing in sync with volumes in the reactor buildings.
The officials admitted to the Nuclear Regulation Authority on Friday that the tunnels hadn't been plugged.
They said they're giving up on the plan, and proposed pouring cement into the flooded tunnels while removing water from them. They said they want this done from late November.
The authority's commissioners asked whether the new method can really halt the inflow. They also spoke of the risk of cracks forming in cement.
The authority approved TEPCO's plan in the end, on condition that the procedure be halted in late December to see whether it's working.
Commenting on the change, one commissioner asked what all the trouble over the past months was for.