This year's decision of the Japanese Government to dump radioactive water from Fukushima into the Pacific, starting in 2023, caused widespread alarm and outrage. A new study may revive that by showing tritium (hydrogen-3) from the release will quickly spread across a wide area of the Pacific, and eventually, leak into other oceans. However, the concentrations will be so infinitesimally low this is the least of the threats Pacific ecosystems face – if it even counts as a threat at all.
After the 2011 disaster at the Fukushima Daiichi nuclear power plant an image became widely shared supposedly representing the spread of radiation from leaks into the ocean. In fact, the map was nothing of the sort – instead it represented the height of the waves from the tsunami that killed thousands of people and triggered three reactors to meltdown.
This, however, does raise the question of just how far radioactive particles that make their way into the sea from the planned release really will go, and over what time span. Modeling published in the National Science Review provides the answer.
The study found ocean currents will sweep pollutants from Japan across an enormous area – 30 degrees of latitude by 40 of longitude, or more than a tenth of the Pacific, within 120 days. Initially, most of the spread will be by longitude, creating a strip across the Pacific centered on 35º North.
It will take around 1,200 days, or more than three years, before radiation reaches the shores of distant continents. After 2,400 days some will pass through the Panama Canal to reach the Atlantic, while around the same time material from the release will be pushed through the spaces between South East Asian Islands by the powerful Indonesian Throughflow to enter the Indian Ocean.
All this sounds alarming, made worse by the paper's observation that “more than 60 radionuclides exist in the contaminated water.” Secondary treatment will filter out many of them, however, and the paper notes that “tritium is the main pollutant in the treated water that Japan has planned to discharge.”
This is crucial to why the effects of the discharge shouldn't be exaggerated; tritium is far from the worst radioactive source out there. It releases beta rays when decaying to helium-3, but is so little threat to humans it's used to make toys and watches glow, as well as a medical tracer. Almost any shielding will block tritium's beta radiation – the danger occurs when water molecules containing tritium are drunk or breathed in. The body expels it relatively quickly however, so it does not bio-accumulate up the food chain.
Nevertheless, as by far the largest source of radioactivity in the water that will be dumped, the authors modeled tritium's spread, rather than that of heavier elements. The nature of the Pacific currents means long term there will be more Fukushima tritium off San Diego than Miyazaki, despite the latter being nine times the distance from the source.
A 2016 study found tritium levels in the Fukushima holding tanks to be almost 100 times higher than World Health Organization limits (US limits are considerably lower). Dilute that in the world' s largest ocean however, and the concentration becomes almost homeopathic. The LaHague reprocessing plant dumped 13 times as much tritium as is stored at Fukushima into the English channel in one year.