The operator of the Fukushima Daiichi nuclear plant now says it has detected a huge spike in radioactive cesium levels at a series of wells that recently drew attention for elevated levels of the radioactive element tritium.

Tokyo Electric Power Co. said Tuesday that the radioactive cesium levels were the highest it has recorded in groundwater at the plant, following the March 2011 nuclear accident.

The latest finding is potentially more troubling than a rise in tritium levels since radioactive cesium, particularly cesium 137, has a higher energy level and lingers longer than tritium, making it a more serious contaminant and health hazard.

Specifically, Tepco found at one well that the level of cesium 134, a radioactive element with a half life of around 2 years commonly associated with nuclear accidents, was 9,000 becquerels per liter on July 8--90 times higher than it was just three days earlier. (A becquerel is a measure of how much radioactive energy is released per second.) That's 150 times higher than Japan's safety standard of 60 becquerels per liter for the element.

Similarly, the level of cesium 137, another radioactive form of cesium with a half life of 30 years, was 18,000 becquerels per liter, around 85 times higher than three days earlier, and 200 times higher than the safety standard.