More than 1,000 tanks brimming with irradiated water stand inland from the Fukushima nuclear plant. Each day 300 tonnes of water are pumped through Fukushima’s ruined reactors to keep them cool. As the water washes through the plant it collects a slew of radioactive particles.

The company that owns the plant – The Tokyo Electric Power Company (Tepco) - has deployed filtration devices that have stripped very dangerous isotopes of strontium and caesium from the flow.

But the water being stored in the tanks still contains tritium, an isotope of hydrogen with two neutrons. Tritium is a major by-product of nuclear reactions and is difficult and expensive to remove from water.

Now, Japan’s Nuclear Regulation Authority (NRA) has launched a campaign to convince a sceptical world that dumping up to 800,000 tonnes of contaminated water into the Pacific Ocean is a safe and responsible thing to do.

NRA chairman Shunichi Tanaka has officially called on Tepco to work towards a release. The International Atomic Energy Agency (IAEA) last year also issued a call for a release to be considered and for Tepco to perform an assessment of the potential impacts. For its part, Tepco has said there are no current plans to release the water. But the Associated Press (AP) reported that company officials are saying in private that they may have no choice.

According to Tanaka, Tritium is “so weak in its radioactivity it won’t penetrate plastic wrapping”. The substance can be harmful if ingested. According to AP, Tanaka had demonstrated the relatively tiny amount of tritium present in the combined Fukushima standing tanks – 57ml in total – by holding a small bottle half full of blue liquid in front of reporters.

A more useful measure of the amount of tritium is its radioactivity, which is measured in becquerels. According to the NRA, the tanks at Fukushima contain 3.4 peta becquerels (PBq) of tritium.

Despite the number of zeros in this measurement (there are 14), this is not a big number, said Ken Buesseler, a senior scientist at the Woods Hole Oceanographic Institution.

To put it in context, the natural global accumulation of tritium is a relatively tiny 2,200 PBq. The isotope has a half life of 12.3 years and is only created naturally on Earth by a rare reaction between cosmic rays and the atmosphere. By far the largest source of tritium in our environment is the nuclear weapons testing programme of last century, which dumped a total of 186,000 PBq into the world’s oceans. Over time this has decayed to roughly 8,000 PBq. Another significant source of tritium are nuclear power stations, which have long dumped tritium-contaminated water into the ocean.

“I would think more has been put into the Irish Sea [from the UK’s Sellafield plant] than would ever be released off Japan,” said Buesseler. So far, the Fukushima disaster has seen 0.1-0.5 PBq leaked or released into the Pacific.

Even if all of the contaminated water were released into the ocean, it would not contain enough tritium to be detectable by the time it dispersed and reached the US west coast about four years later, said Simon Boxall, an oceanographer at the University of Southampton.

“In the broad scale of things, if they do end up putting the material in the Pacific, it will have minimal effect on an ocean basin scale,” said Boxall. “In an ideal world, we wouldn’t be in this situation. But the question is, what is the safest way forward? In many ways this is a pragmatic solution.”

But Boxall said there may be local effects – especially on the already heavily impacted fishing industry – as the contaminated water would take time to disperse.

International maritime law prohibits the building of a pipeline to send the waste offshore. Therefore any release would need to be slow. Tepco did not respond to questions regarding the environmental impact study called for by the IAEA.

Despite harbouring few prima facie fears about the 3.4PBq of tritium stored at Fukushima, Buesseler said the lack of transparency surrounding much of the post-tsunami decommissioning process made it impossible to be definitive about the safety of any course of action.

“Until you get the hard data, it’s hard to say if it’s a good idea or not. I want to have independent confirmation of what’s in every tank, which isotopes, how much they want to release per day. You get more of ‘don’t worry, trust us’,” said Buesseler

He notes that there have been minor differences between the official Tepco line that all leaks have stopped and Buesseler’s own measurements of very low levels of caesium and strontium still entering the ocean from the plant.

“It’s easy to have conspiracy theories when no-one is independently assessing what is going on,” he said.

The push for release will also be a blow to the hopes of US start-up Kurion, and their new parent company Veolia, which was awarded a $10m (£7m) grant from the Japanese government in 2014 to demonstrate that its tritium scrubbing technology could be scaled to meet the challenge of the Fukushima problem. The plan would create 90,000 tonnes of hydrogen gas, which Kurion said could be used to power vehicles.

Neither Tepco, nor Kurion, responded to requests for cost estimates of implementing this technology at the site. Kurion’s website calls it “cost-effective” and has said it could have its demonstration plant running within 18 months.

These costs are fundamental to the question of whether to release the material, because whatever they are, it is the price Japan seems unwilling to pay to fully clean up the lingering mess at Fukushima.