Sea Water Runoff From Nuclear Accident
Date: April 4, 2011
I am wondering about the sea water that is being sprayed
on the damaged nuclear reactors in Japan. Does the sea water being
sprayed on the damaged Japanese nuclear reactors become toxic and
does it re-enter the ocean and endanger the aquatic life?
This is a good question, and something that seems to confuse a lot of people.
Before answering your question, I want to make sure you understand that this is an ongoing situation. While we understand quite a bit about what is happening, a very close review of the accident will certainly take place after the situation has been resolved. From this review, we will understand what could have been done better, and, just as importantly, what was done right.
All power reactors in the U.S., as well as most reactors throughout the world and in Japan, have many levels of safety. This is known as "Defense in Depth". To understand what is happening with the sea water, you need to understand at least three of these defenses. This first is the fuel pin itself. Nuclear fuel is contained in sealed metal tubes, known as cladding. This is the primary defense against the release of radioactive material. Next is the pressure vessel, which contains the coolant that is used to carry away the heat needed to make electricity. A pressure vessel is a large steel container about 10 inches thick. The last is the reactor containment. The reactor containment is usually made up of two barriers. The first is a thick steel container. The second, surrounding the first, is steel-reinforced concrete that is about four feet thick.
The operating reactors in Japan immediately shut down when the earthquake was detected. When the power plants in Japan lost all their power because of the tsunami, they were able to maintain cooling of the fuel with battery back-up power. However, the destruction caused by the tsunami delayed them in getting backup power and pumping capability to the site before the batteries ran down.
Because they lost the power needed to remove the residual heat (decay heat) that remains after a plant shuts down, the fuel (inside the cladding) began getting very hot. This began to boil the water in the pressure vessel. But the steam had nowhere to go, so pressure builds up inside the pressure vessel. To relieve this pressure, the workers vented the steam into the reactor containment. At this point, nothing has been released to the environment.
As this process continued, pressure in the reactor containment increased. To relieve this pressure, steam was vented through a steam suppression pool to cool it down. Eventually, it will make its way into the reactor building. (This building is just a simple building above the reactor.) If you have seen pictures of this particular design, you have probably noticed the "donut" shaped tank of water below the reactor. This is the steam suppression pool.
As water continued to boil inside the pressure vessel, the water level began decreasing. This exposed part of the fuel cladding to steam. The cladding is made mostly of zirconium, which, when very hot, can react with steam to produce hydrogen. This hydrogen followed the same path as the steam that I described above, and began accumulating in the reactor building. Eventually, the hydrogen caused an explosion the destroyed the reactor building (but not the containment or pressure vessel).
To help cool the fuel and prevent the generation of hydrogen, water had to be added to the pressure vessel. This water was pumped in from the sea. It is like refilling a pot of boiling water. So the sea water did not flow through the reactor and back into the sea. But some amount of it did boil off and was eventually released as steam into the environment. Because it appears that the fuel has been damaged, some radioactive material from the fuel has also been released. As the water boils off, they will keep adding new water. The main problem with seawater is that it is extremely salty. As the water boils off, they salt remains behind and builds up. That is why it is important for them to begin using fresh water. It is one of the reasons they have been working hard to restore power to the reactors.
You may have heard that some workers were exposed to radioactive water. It is not clear at this point how the water leaked, but it is pretty clear it came from the core. However, this water is in the basement of the reactor building. It has not been released to the environment.
The amounts of radioactive material that has been vented could eventually come down with rain or snow. In that way, some radioactive material will make it to the sea.
I know this was a lot to read, but I hope this helps you understand much of what is currently happening. I should also point out that the tsunami itself has created a lot of destruction to coral reefs and sea life. It is unlikely, in my opinion, that the situation at the reactors will have any significant impact on ocean life beyond what was already caused by the tsunami.
Thank you for your excellent question.
Dr. Thomas H. Fanning
Manager, Safety Analysis Section
Nuclear Engineering Division
Argonne National Laboratory
There will probably be some radioactive contamination to the
seawater being used to cool the reactors. If this contaminated water
is released to the ocean, there will be no major or permanent
environmental or health effects because:
1. The ocean will dilute the radioactivity enormously so that the
concentration will be tiny everywhere -- so small that no animals or
plants can be affected. There might, however, be an unhealthy
contamination for a few days during which fishing should be prohibited.
2. The radioactivity itself will die off naturally. After 8 days,
half of the I-131 will be gone. After 30 years, half of the Cs and
Sr will be gone.
Health and environmental effects in the ocean will be very minor,
less than the effects of land contamination. In this case, even the
land contamination will be a limited and short term problem that
will be accommodated by not consuming food products from the
affected area until the contamination is gone.
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Update: June 2012