Supporters and skeptics alike will soon learn if that gambit will succeed. After two years of work, Kajima finished installing the pipes and refrigerator units to create the ice wall in February. At the end of March, it switched on part of the ice wall for the first time — roughly half a mile that runs between the reactor buildings and the Pacific. Most of the other, uphill side of the wall was activated in mid-June.

Kajima is freezing the wall in stages under orders from the Nuclear Regulation Authority, Japan’s nuclear watchdog. The authority is concerned that cutting off the groundwater too suddenly might lead to a reversal of flows, causing the radioactive water accumulated inside the reactor buildings to start pouring out into the surrounding soil, possibly reaching the Pacific. It has told Kajima to leave a half-dozen “gateways” in the uphill side that will not be closed until much of the contaminated water is drained from the buildings.

This month, Tepco told the nuclear agency that the seaside segment of the ice wall had frozen about 99 percent solid. It says a few spots have failed to solidify because they contain buried rubble or sand left from the plant’s construction a half-century ago, which now allow groundwater to flow through so quickly that it will not freeze.

Tatsuhiro Yamagishi, a spokesman for Tepco, said the company was trying to plug these holes in the ice wall with quick-drying cement. “We have started to see some progress in temperature decrease,” he said.

Even if the cement helps make the ice wall watertight, skeptics question how long it can last. They point out that such frozen barriers are usually temporary against groundwater at construction sites. They say the brine solution used to chill the pipes is highly corrosive, which could make them break or leak. It is also unclear whether the system could break down under the stresses of operating in a high-radiation environment where another earthquake could lead to another power loss.

“Why build such an elaborate and fragile wall when there is a more permanent solution available?” said Sumio Mabuchi, a former construction minister who has called for building a slurry wall, a trench filled with liquid concrete that is commonly used to block water.

Isao Abe, a Kajima engineer overseeing the ice wall, said his company had made the wall more durable by installing underground pipes that are easy to replace if they corrode. He also said the ice wall was self-sealing, meaning that if another earthquake caused cracks, any incoming water would freeze right away, restoring the wall. He also said it would take months for the wall to thaw, giving engineers ample time to restore power even if the plant has another outage.

Mr. Abe said the wall was intended to operate until 2021, giving Tepco five more years to find and plug the holes in the reactor buildings, though skeptics say this difficult task will require more time. Mr. Abe also pointed out that the ice wall was part of a broader strategy for containing the radioactive water. Before installing the ice wall, Kajima also built a conventional steel wall underground along the plant’s border with the Pacific last year.

Tepco says that wall has already stopped all measurable leaks of radioactive materials into the sea. However, some scientists say that radioactive water may still be seeping through layers of permeable rock that lie deep below the plant, emptying into the Pacific far offshore. They say the only way to eliminate all leaks would be to repair the buildings once and for all.

Even if the ice wall works, Tepco will face the herculean task of dealing with the huge amounts of contaminated water that have accumulated. The company has installed filtering systems that can remove all nuclear particles but one, a radioactive form of hydrogen known as tritium. The central government and Tepco have yet to figure out what to do with the tritium-laced water; proposals to dilute and dump it into the Pacific have met with resistance from local fishermen, and risk an international backlash.

For now, the only visible sign that the freezing has begun are silver-dollar-size patches of ice that have formed on top of the aboveground, silver pipes. At one spot, the No. 4 reactor building loomed, an enormous cube six stories tall with concrete sides that showed large gashes left by the tsunami.

“The water is here, just three meters beneath our feet,” said Mr. Okamura, the Tepco general manager, who stood near the pipes wearing a white protective suit, goggles and a surgical mask. “It still flows into the building, unseen, without stopping.”