It's already been more than five years since three reactors melted down at Japan's Fukushima Daiichi Nuclear Power Plant, but the consequences of the disaster will linger for a while yet. Those monitoring the situation have received some good news, however, with scientists reporting that after suffering the largest ever release of radioactive material into the world's oceans, radiation levels across the Pacific are fast returning to normal.

How to stop the spread of radioactive material from the Fukushima site is a problem that continues to plague the containment effort even now. Tons of groundwater still flows into the reactor basements each day, with some of that then spilling into the sea after mixing with the radioactive material. Signs of contamination have been detected as far away as sites off the US West Coast.

Japan has sought to stem the bleeding by activating an underground ice wall around the facility, which is designed to seal away the nuclear waste. But according to researchers at Australia's Edith Cowan University (ECU), the material continues to seep out into the waters off Japan's east coast.

The ECU scientists partnered with international researchers as part of the Scientific Committee on Oceanic Research to conduct a major review assessing the radioactive caesium levels right across the Pacific, between Japan's coast to North America. Caesium lends itself particularly well to the task of monitoring radiation levels as it is a by-product of nuclear power and is highly soluble in water.

In the immediate aftermath of the nuclear disaster, radiation levels off the coast of Japan were tens of millions of times higher than normal. But after assessing data from 20 studies of radioactivity associated with the Fukushima disaster, the researchers are reporting that these levels are now fast returning to normal.

"Oceanic currents have dispersed the radioactive material across the Pacific Ocean as far away as North America,"says ECU Professor of Environmental Radiochemistry, Pere Masqué. "Radiation levels across the ocean are likely to return to levels associated with background nuclear weapon testing over the next four to five years. As an example, in 2011 about half of fish samples in coastal waters off Fukushima contained unsafe levels of radioactive material, however by 2015 that number had dropped to less than one per cent above the limit. However, the seafloor and harbor near the Fukushima plant are still highly contaminated and monitoring of radioactivity levels and sea life in that area must continue."