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Levels of the radioactive element tritium remain high even after more than two years removed from the devastating meltdown at Japan’s Fukushima Dai-ichi nuclear plant after an earthquake and ensuing tsunami damaged the facility.

At issue is an area on the coastal side of the plant’s heavily damaged No. 2 reactor, one of the locations Tokyo Electric Power Co. (Tepco) regularly monitors groundwater to check for radioactive elements that may have leeched from the plant’s partly melted fuel cores and into the environment.

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In May, Tepco found tritium levels in the groundwater there suddenly rose to 17 times their December levels. Since then, Tepco drilled more monitoring holes and stepped up measurements. They’ve found tritium levels continuing to rise, with the latest readings, taken on July 5, some 20 percent higher than they were in May.

Tritium is a radioactive form of hydrogen with a half life of around 12 years, which occurs naturally and also forms in the process of nuclear fission. It tends to flow along with water — both in and out of human bodies, for instance — and could be less harmful than other radioactive elements, such as cesium, associated with nuclear plants. The U.S. Environmental Protection Agency calls it “one of the least dangerous of radionuclides.”

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The tritium level Tepco measured July 5 in one of its coastal monitoring wells near reactor No. 2 is ten times above Japan’s safety standard of 60,000 becquerels per liter, and rising. (A becquerel is a measure of how much radioactive energy releases per second.) That’s the highest such level the company has recorded since the accident.

As a point of reference, a person who drank two liters of water at the 60,000 becquerel safety limit every day for a year would get a bit more radioactive exposure than one stomach X-ray, according to a 2000 report from the United Nations Scientific Committee on the Effects of Atomic Radiation.

Tepco’s engineers think the sudden rise in tritium levels is a factor of the slow seepage of groundwater following the March 2011 accident, said spokesman Yoshikazu Nagai. Right after the accident, Tepco blocked the main routes of contaminated water to the sea — trenches and ducts, many of which housed cables — by pouring in hardening chemicals backed by concrete. But radioactive water that still managed to leak out into the ground and has been making its way toward the sea ever since and the tritium spike may show it finally hit the Unit 2 monitoring wells in May, Tepco officials said.

Radioactive cesium, on the other hand, tends to bind to dirt rather than flow with water, and Tepco has not seen significantly elevated levels in those wells.

Now Tepco said it is working to prevent the contaminated water from getting to the sea. The current plan is to drill a line of holes to the sea side of the reactor, and fill them with a chemical that hardens the surrounding earth, forming a barrier.

Tepco’s seawater samples show tritium levels, though rising slightly, are still significantly lower than the safety standard, at 2,300 becquerels per liter on July 3.

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