Tokyo (CNN) -- Airborne radiation levels continue to fall outside the stricken Fukushima Daiichi nuclear plant, though concerns remain about potentially ominous breaches in reactor cores after water showed alarming radiation levels in tests at two locales.
An official with the Tokyo Electric Power Co. -- which operates the facility -- told reporters Saturday that water samples from the turbine buildings for the Nos. 1 and 2 reactors similarly found high levels of radiation.
The readings, while high, were about half that detected earlier in the basement of the No. 3 turbine building, which officials said were 10,000 times above normal.
An analysis of the tainted water suggests "some sort of leakage" from the No. 3 reactor's core -- signalling a possible break of containment vessel that houses the core, said Hidehiko Nishiyama, an official with Japan's Nuclear and Industrial Safety Agency.
As authorities work to determine the cause and extent of water contamination in the turbine buildings themselves, Graham Andrew of the International Atomic Agency reported Friday that "there has not been much change ... over the last 24 hours."
In fact, the world nuclear agency noted on its website that on-site monitoring shows radiation in and around the plant, which is 240 kilometers (150 miles) north of Tokyo, "continues to decrease."
Tokyo Electric reported on its website that at 7 a.m. Saturday, radiation at the plant's main gate were 0.219 millisieverts per hour -- a fraction of the 400 millisieverts per hour measured between Units 3 and 4 on March 15.
The measurements are a significant drop from readings taken at the same gate over the past week.
By comparison, people in industrialized countries are typically exposed to 3 millisieverts of radiation per year.
Japan's health ministry has said those trying to stem the crisis at the Daiichi plant can be exposed to 250 millisieverts total before they must be taken off the job.
Concerns have grown over radiation leaks into the water
Work near the No. 3 reactor has been halted since three men laying cable stepped in highly contaminated water, according to the nuclear safety agency. Water is still being being pumped into the contaminated vessel.
The workers were exposed to between 173 and 181 millisieverts of radiation, in two cases when the tainted water rubbed against their skin. They have been admitted for a four-day stay at a research hospital in Chiba. Experts said their injuries may not be more serious than a bad sunburn.
Evidence of radioactivity in the water around the plant is not surprising given the amount of water sprayed onto and pumped into the reactors, said Ian Hutchinson, professor of nuclear science and engineering at the Massachusetts institute of Technology.
"I am not particularly alarmed," he said.
Still, the issue gained urgency Saturday after Japan's nuclear and industrial safety agency reported the amount of radiation iodine in seawater just over 300 meters offshore from the plant was 1,250 times above normal. A Tokyo Electric official speculated that water runoff or leakage from the turbine buildings may have caused the sudden increase, though he said other factors might have contributed as well.
Moreover, the fact the three workers were directly exposed to contaminated water has raised questions about safety at the plant as nearly 450 people -- including government authorities and firefighters -- continued working there Saturday.
The high measure prompted a top official with Japan's nuclear safety agency to urge Tokyo Electric to "improve its radiation management measures."
The No. 3 reactor is of particular concern, experts have said, because it is the only one at the plant to use a combination of uranium and plutonium fuel, called MOX, considered more dangerous than the pure uranium fuel used in other reactors.
Plant workers also had already been watching the plant's No. 1 reactor, prior to the announcement of similarly contaminated water in its own turbine building. They had been concerned that an increase in pressure noted inside that reactor could be a troublesome sign. Earlier, buildups of hydrogen gas had driven up pressure that led to explosions at three of the nuclear plant's reactors, including the No. 1 unit.
Nishiyama conceded that "controlling the temperature and pressure has been difficult" for that reactor, which on Friday had been declared stable.
By Saturday, authorities had begun to pump fresh water -- rather than the corrosive seawater they have been using -- into the external pumps being used to cool nuclear fuel at the Nos. 1, 2 and 3 reactors. Those units' own cooling systems are not functioning.
Efforts also continue at the Nos. 4, 5 and 6 reactors -- each of which have less pronounced concerns because the units were on scheduled outages when the quake struck.
None of these three units had nuclear fuel inside their reactors, though efforts are ongoing to control temperatures inside the spent fuel pools.
CNN's Jennifer Rizzo contributed to this report.
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