Two reactors at a crippled Japanese nuclear plant are safely under control after their fuel storage pools cooled down, officials said, as the estimated toll from the devastating earthquake and tsunami rose to more than 18,000 deaths.

Tokyo Electric Power Company said Sunday that Units 5 and 6 at the Fukushima Dai-ichi plant had been declared safe, after pumping water into the reactors pools for days and bringing temperatures under control.

The units were the least problematic of the facility's six reactors, but marks a minor advance in efforts the stop the nuclear plant from leaking radiation. The Fukushima plant still has four reactors overheating, after an earthquake-triggered tsunami shut down its cooling systems nine days ago.

The northeast coastal region has been struggling to recover from fallout from the disaster, including concerns that leaks at the nuclear plant had contaminated food and water.

Earlier on Sunday, Japanese nuclear authorities stopped venting radioactive gas from the leaking reactor, a move that could have caused as-yet minor cases of food and water contamination to worsen.

Traces of radiation began appearing in food and water sources near the Fukushima Dai-ichi plant over the past several days, after the cooling systems to its six reactors were knocked out by a massive March 11 earthquake and tsunami.

Milk and spinach tainted by radiation were found as far as 120 kilometres away from the troubled facility, while trace amounts of radioactive iodine was found in tap water as far away Tokyo, some 200 kilometres to the south.

Taiwan reported on Sunday receiving a shipment of fava beans imported from Japan contaminated with a slight, and legal, amount of iodine.

While radiation levels in food and water have frequently exceeded government-imposed safety limits, officials said they so far pose no immediate health risks.

The plant's operator, Tokyo Electric Power Co., has so far suspended venting plans, but the measure could still be considered if the reactor's pressure rises further. Internal temperatures reached 300 degrees Celsius early Sunday.

Nuclear safety officials said venting would release a cloud dense with iodine, krypton and xenon.

A company spokesman said the high pressure may have been caused by seawater pumped into the vessel, an extreme measure used to reduce temperatures. The decision to douse the troubled reactor with sea water means the facility will never operate again.

Japanese officials acknowledged Sunday that the entire complex would be scrapped once the emergency was resolved.

"It is obviously clear that Fukushima Dai-ichi in no way will be in a condition to be restarted," Chief Cabinet Secretary Yukio Edano told reporters.

The growing concern about radiation is the latest in a series of troubles raised since the 9.0-magnitude earthquake and massive tsunami crushed Japan's east coast.

One official acknowledged on Sunday that the government was late in realizing they needed to provide potassium iodide pills to those living near the nuclear complex. The pills help reduce chances of getting thyroid cancer from exposure to the radiation.

But officials continued to reassure residents on Sunday that radiation in food and water did not pose immediate health risks.

A spokesperson for the Miyagi prefecture's disaster response team said drinking one litre of affected water was the equivalent of receiving 188th of the radiation from a chest X-ray.

With files from The Associated Press