Drainage water containing high levels of radiation may have leaked into the Pacific Ocean near the Fukushima Dai-ichi nuclear plant in Japan, the site of a meltdown in 2011, reports NBC. Despite knowing about the issue since April, the company that operates the plant, Tokyo Electric Power Co. (Tepco), failed to report it. Citing a need to report "everything all at once," an unnamed company official told NBC that Tepco wanted to find the source of the contamination first — something it only did recently.

The contamination may have occurred when rainwater came into contact with radioactive materials, possibly gravel and blocks, that Tepco had laid out on the roof of the No.2 reactor building. The water then flowed into drainage ditches. Tepco detected 23,000 becquerels per liter of cesium-137, a radioactive substance, from the rainwater found on the roof — that's about 256 times above the legal limit.

No workers were exposed to the radiation, Tepco says. Exposure to radiation emanating from cesium-137 can cause radiation sickness, cancer, and death. The company doesn’t know how large the leak was, but it says that it hasn’t caused a significant increase in radiation in the sea water adjacent to the plant. As a result, the leak doesn't violate regulations, a Nuclear Regulation Agency official told Reuters.

This isn’t the first time a leak has been reported at the site. A year ago, 100 metric tons of highly radioactive water have leaked from a storage tank. That is actually less than the 300 metric tons of radioactive water that leaked in August 2013. Now that the leak has been detected, Tepco says it will conduct daily ocean water sampling tests, instead of weekly ones. The company also plans to remove the blocks and gravel from the roof by the end of March.