Japan is planning to release tons of water from the Fukushima nuclear site into the Pacific Ocean – water contaminated with tritium, a radioactive form of hydrogen. This has led to protests by China, South Korea, Russia, and other nations; an outcry from a variety of environmental groups … not to mention seafood lovers concerned about the safety of their sushi. Given these concerns, it seems reasonable to dig into the subject a tad to see how serious a concern this is.
Let’s start with tritium and its source
Tritium is hydrogen that happens to be radioactive. Like all hydrogen atoms, tritium has a single proton and a single electron. But when we pass the normal hydrogen atoms through a nuclear reactor core (which has a lot of neutrons flying around), some of these hydrogen atoms will capture two neutrons, forming hydrogen’s radioactive counterpart, tritium. This happens in all nuclear reactors – when the reactor coolant from the broken Fukushima reactors leaked into the environment, the groundwater became contaminated. Millions of gallons of the groundwater, along with water that had collected in the basements of some of the buildings on the site, were collected in tanks and are sitting, awaiting final disposition.
What would happen if this water is released into the Pacific Ocean?
We need to start with the amount of radioactivity that’s to be released. Radioactivity is measured by the rate at which it decays. One Becquerel or Bq is that amount of any radioactive material that will undergo one radioactive decay every second. According to a report developed in 2016 looking specifically at tritiated water from Fukushima, there are about 820,000 cubic meters of water (328 Olympic-sized swimming pools ) containing about 760 trillion Becquerels (760 TBq) of tritium.
The Pacific Ocean is fairly large – the North Pacific, the location of this discharge, holds 331 million cubic km of water. This is 400 billion times as voluminous as the water being held at Fukushima. If we mix the 760 TBq of tritium into 331 million billion cubic meters  of water, we end up with a tritium concentration of 0.0023 Bq of tritium per cubic meter of water. This is not enough to hurt anybody – or any creature living in the water.
It’s natural to wonder how I can state that so confidently.
Let’s start with the fact that our planet has far more tritium on it than most people think, and the vast majority of it is natural, formed when cosmic rays slam into atoms in the atmosphere. Sometimes the cosmic rays break into pieces (or they can knock protons and neutrons off of atoms in the air), and some of those pieces just happen to have a proton and two neutrons. Other times the cosmic ray collisions will form neutrons that are captured by hydrogen atoms to form tritium.
However the mechanism, nature accounts for the formation of about 1000 times as much tritium as is in the tanks at Fukushima. Natural tritium in the waters of the Earth is present at concentrations of about 185-925 Bq per cubic meter of water. This is thousands of times more tritium per cubic meter than the 0.0023 Bq per cubic meter that we calculated above. To put it another way, discharging the water at Fukushima into the ocean is like adding a few grains of sugar to a pitcher of the sugary fruit punch I used to drink as a kid.
There’s a lot more radioactivity in seawater than just tritium – uranium, potassium, and rubidium are also dissolved in seawater, and some of these contain natural radioactivity as well. Both potassium and uranium emit radiation that is more damaging than the low-energy beta particle given off by tritium.
When we put all of this together:
- The oceans already contain natural radioactivity
- There is far more natural radioactivity than there is tritium in the tanks at Fukushima
- Tritium is among the most innocuous radionuclides to which we can be exposed
We can only conclude that discharging this water into the Pacific poses no significant risk to the sea life or those of us who enjoy dining on it.
A deeper dive into tritium and your health - calculating radiation dose from tritium
When it comes to determining the health effects of radiation, it all comes down to the radiation dose – the energy deposited by ionizing radiation per unit of mass. To begin, we measure the amount of energy radiation creates, in thousands, keV, or millions, MeV of electron volts. The energy released is measured by the radioactivity “counts.” No matter how large or small, a radioactive material that undergoes one radioactive decay every second will have an activity of 1 Bq; 1000 decays per second gives us 1 kBq, and a million decays each second represents 1 MBq.
We need to then measure the amount of energy absorbed per gram or kg of material, the radiation dose. One Gray or Gy represents the deposition of 6.242x109 MeV per gram of material – the material can be air, water, or me. A mGy is 0.001 Gy
Start with the energy of the radiation given off by tritium. Say we have one liter (1 kg) of water laced with 1 MBq of tritium. This means that every second there will be one million tritium atoms emitting beta particles, with an average energy of about 6 keV each. 
6 keV x 1,000,000 decays/second x 3600 seconds/hour = 21,600,000 Mev = 3.46 mGy. That is the radiation dose contained in that liter of water
Applying these same principles and calculations to the 0.0023 Bq/m3 of tritium we calculated would result from the discharge of water from Fukushima, the water (and anything living within it) will receive a radiation exposure of just under 0.0000007 mGy every year.
To put this in perspective, when I turn on my radiation detector in my apartment in Brooklyn, it reads about 0.0000050 – 0.000010 mGy/hr – I receive ten times as much radiation exposure in one hour sitting in my living room than I’d receive in an entire year floating in the waters of the Pacific Ocean after the waters of Fukushima have been discharged.
It’s just not worth worrying about - the stress from worrying about that level of radiation exposure is more damaging to one’s health than is the radiation.
 One cubic km contains one billion cubic meters.
 A tritium beta particle has an average energy of 5.7 keV and maximum energy of about 18 keV.