Plutonium that may have come from a reactor core at the Fukushima No. 1 nuclear plant was detected in soil on the premises, indicating fuel rods suffered heavy damage, Tokyo Electric Power Co. has revealed.

Although Tepco said late Monday the amount detected, based on soil samples taken a week ago, is extremely small and not enough to harm human health, the latest finding indicated not just heavy damage to fuel rods but also that strong radioactive materials may be leaking from at least one reactor containment vessel.

A Tepco spokesman told reporters Tuesday the utility will continue to monitor plutonium levels in the soil in the plant but has no plans to expand its checks to beyond the premises for the time being.

“Plutonium is emitted only when the temperature (inside a reactor’s core) is really high. That shows how badly (the fuel rods) were damaged and how serious this accident is,” Nuclear and Industrial Safety Agency official Hidehiko Nishiyama said Tuesday. “Still, the amount is not an immediate threat to human health.”

According to Tepco, the plutonium levels were around the same as those detected in Japan after the United States and the Soviet Union conducted nuclear tests in the past.

Tepco collected soil samples in five locations within the plant’s premises on March 21 and 22. At one of the five sites, plutonium-238 with a density of 0.54 becquerel per kilogram of soil was detected, while another site had 0.18 becquerel.

An average soil sample in Japan can contain up to 0.15 becquerel, Tepco said.

Reactor No. 3 uses and stores MOX fuel — a mix of uranium and plutonium oxide. But reactors 1 and 2 also generated plutonium, thus the source of the leak at present is uncertain, Tepco said.

Plutonium doesn’t vaporize and thus is believed not to have carried very far from the compound, unlike other radioactive substances, particularly iodine and cesium.

Another radioactive pollution threat looms in the reactors’ subterranean “trenches,” which could overflow with contaminated water — a dilemma because Tepco must continue pouring water to cool the No. 1, 2 and 3 reactors and their spent fuel cooling pools. It is believed the cooling water is reaching the trenches.

Chief Cabinet Secretary Yukio Edano said the priority is to keep pouring water to prevent further damage to the fuel rods, despite the leak and trench overflow threat. “We need to prevent overheating and boil-drying of fuel rods. That should be the priority,” he said.

On Monday, the underground trenches outside the turbine buildings of reactors 1, 2 and 3 were filling with radioactive water, prompting Tepco to pile sandbags near the surface port of reactor No. 1 to prevent the water from flowing into the sea.

Tepco on Tuesday didn’t pile sandbags at the other two reactor trench openings because they weren’t in imminent danger of overflowing.

The levels of water in the trenches, whose exits are some 55 to 70 meters from the shore, have been stable, and radioactive water has not been confirmed to have overflowed into the Pacific, the Nuclear and Industrial Safety Agency said.

The water level in the reactor No. 1 trench was only 10 cm below the ground-level port Monday night, while that of No. 2 was 1 meter from overflowing and that of No. 3 was 1.5 meters below ground level.

The turbine building basements of reactors 1 through 4 are also flooded with highly radioactive water, slowing efforts to restore critical safety functions, particularly the cooling pumps.

Information from Kyodo added

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