To stem the advance of radioactive water to the sea, the operator of the stricken Fukushima Daiichi nuclear-power plant has tried plugs, walls, pumps and chemicals that harden the ground into a solid barrier.

But as Tokyo Electric Power Co. prepares this week to start work on a new set of measures designed to ring off and cap the area where the most highly contaminated water has been found, some experts and regulators are saying that the battle to completely contain radioactivity to the site of one of the world's worst nuclear...