More than two years after the devastating accident at Japan's Fukushima Daiichi nuclear plant, operator Tokyo Electric Power Co. is seeing levels soar of a radioactive element called tritium.

The problem spot is on the coastal side of the plant's heavily damaged No. 2 reactor, one of the areas where 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. In May, Tepco found that tritium levels in the groundwater there had...