Still, the incident was an uncomfortable reminder of the many mishaps that have plagued the containment and cleanup efforts at the plant, as well as the hundreds of tons of contaminated groundwater that still flows unchecked into the Pacific every day.
The company, known as Tepco, said it had traced the latest leak to a pair of valves that were left open by mistake.
The leaked water was among the most severely contaminated that Tepco has reported in the aftermath of the March 2011 disaster at the Fukushima Daiichi plant, when damage caused by an earthquake and a tsunami led to meltdowns in three of the plant's reactors.
(Read more: Japan ponders Fukushima options, but Tepco too big to fail)
Each liter of the water contained, on average, 230 million becquerels of particles giving off beta radiation, the company said. About half of the particles were likely to be strontium-90, which is readily taken up by the human body in the same way as calcium, and can cause bone cancer and leukemia.
That means the water was about 3.8 million times as contaminated with strontium-90 as the maximum allowed under Japan's safety standards for drinking water. It also showed levels much more radioactive than a worrisome groundwater reading that Tepco announced earlier this month.