Q&A: Japan's nuclear crisis

(CNN) -- Despite the heroic efforts of technicians and engineers battling to prevent a full nuclear meltdown at the quake-damaged Fukushima Daiichi plant, radioactive material is still seeping into the surroundings of the power station.

Plutonium has been detected in soil samples near the plant, the Tokyo Electric Power Company (Tepco) said Monday.

And highly radioactive water has been found for the first time outside one of the reactor buildings on Monday, albeit in a tunnel. Radioactive iodine and cesium have also been detected in sea water near the plant, Japan's nuclear and industrial safety agency said last week.

These latest reports of radioactive leaks -- and the possibility that there may have been a partial meltdown in three reactors -- raise questions about the immediate dangers poised to human health and the environment.

How did radioactive water leak out of reactor No. 2?

It is still not entirely clear how radioactive water made its way into a maintenance tunnel leading to reactor No. 2's turbine building.

However, Japanese Chief Cabinet Secretary Yukio Edano says he has received a report that the No. 2 unit's containment vessel "is damaged and water is leaking."

Professor Robin Grimes, Director of the Center for Nuclear Engineering at Imperial College London, says it is possible that some contaminated water could also have leaked from the "external steam suppression unit," which may have been damaged during a hydrogen explosion in the building surrounding the reactor on March 14.

"This is a guess, it has to be emphasized, " says Grimes, "There is a complex cacophony of different sources that could have contributed to the leaking water."

What about the contaminated seawater?

Levels of radioactive iodine in seawater just offshore of the nuclear plant spiked to more than 1,250 times higher than normal, Japan's nuclear and industrial safety agency said Saturday. Those levels have been dropping in recent days.

The measurements also showed high levels of cesium and were taken outside the discharge canal for the plant's Nos. 1, 2, 3 and 4 reactors.

Journalist Martin Savidge, who is reporting on the incident for CNN from Tokyo, says one theory is that the tunnel may have contributed: "The fact that water has been detected in the tunnels could explain how it (contaminated water) has got into the ocean."

However, Japan's nuclear safety agency says there's no evidence yet of overflow. It said workers are using sandbags and concrete panels to keep the water inside the tunnel, which is located about 55 meters (180 feet) from the shore. The radioactive iodine and cesium could have also been in the atmosphere and then been washed down by precipitation.



And the radioactive soil?

Plutonium was detected in soil on the grounds of the nuclear power plant. The element was found in soil samples taken March 21-22 from five locations around the plant, says Tepco.

The No. 3 reactor has been of particular concern because it is the only one to use mixed-oxide fuel that contains a small percentage of plutonium, which is also a byproduct in other reactors.

Chief Cabinet Secretary Yukio Edano told reporters Tuesday that it was "likely" at least some of the plutonium came from the plant.

Grimes said it is not clear where the plutonium in the soil had come from, also adding that it is possible that it was from the Fukushima plant.

"These are incredibly small amounts at the moment but we need to keep watching," says Grimes.

What is radiation?

In the context of nuclear energy it is ionizing radiation that is referred to, which passes through matter causing it to become electrically charged or ionized. In living tissues, the electrical ions produced by radiation can affect normal biological processes.

Are these leaks dangerous?

"The sea is a phenomenally large dilution vehicle so when you get radioactive material in the ocean most of it just gets diluted. Some of the radioactive particles could collect in material near the reactor and this will have to be monitored and cleaned," says Grimes.

"If it got into local sea

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