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Fukushima Daiichi Nuclear Power Station:Highly-contaminated cooling water

Published on May 18 '2011 : The Denki shinbun(The Electric Daily News)

Dr. Michio Ishikawa
Chief Adviser(Former President & CEO)
Japan Nuclear Technology Institute(JANTI)


My previous column in this series explained my estimation of the current status of the reactor core and the issue of radiation discharge into the environment.Today, I am going to discuss a major headache of the situation, i.e. water contaminated with a high level of radiation.Gas (containing radioactive materials) which continues to be released from the melted reactor core is cooled and becomes mixed in with cooling water.The level of contamination is still rising as we speak.

According to estimation by a group of experts who formerly worked at the nation's pioneering nuclear organization, Japan Atomic Energy Research Institute (JAERI), the three nuclear reactors damaged in the recent disaster contain the amount of radioactivity equivalent to over one billion curies of cobalt-60, if you would excuse me for the use of old unit of measurement.Say just 1% of that gets mixed in and the cooling water would have 10 million curies of radioactivity.This is a horrifically large amount.

Cobalt 60 is widely used as radiation sources.A typical unit containing 10,000 – 50,000 curies of radioactivity is encased in a chamber surrounded with walls measuring about 1meter thick in concrete.In the old days, radiation education used to teach students that the strength of 1 curie of radiation from cobalt 60 is approximately 1 roentgen in 1-meter distance.It was an easy way of having a grasp on the strength of radioactivity.

Exposure to 700 rem of radiation (equivalent to receiving 700 roentgen of radiation for one hour) in a short period of time would almost certainly kill a person.Exposure to approximately one tenth of that or 50 rem of radiation would give a person noticeable effects, whereas exposure to 10 rem or below, I was taught, would have no adverse health effects.That is why the duration of work in a radioactive environment used to be calculated based on the measured amount of radiation indicated by the maximum exposure of about half that figure or 5 rem.

With this perspective, one would realize that ten million curies of radiation is something beyond comprehension.It is like saying to a poor person who is trying to get by with ten yen (ten curies) that he must come up with ten million yen.First-generation experts from JAERI as they are, they are showing hesitation about re-circulating the massive amount of contaminated water to cool the reactors.

Even if we manage to build a core cooling facility using the water, how would we establish radiation shields?Radiation exposure would become a serious problem unless the facility is shielded solidly.Once contaminated water is fed into the system, the radiation dose of all piping will rise, denying human access.Failure is not an option.

Another issue is corrosion.The reactors were doused with seawater for approximately 2 weeks.Judging from the amount of evaporation, around 3,000 to 4,000 tons of seawater must have been injected per reactor unit.The amount of salt contained in the seawater would total around 100 tons per unit.

This means nuclear fuel and an equal amount of salt are mixed together in the reactor core.I have absolutely no idea how such massive amount of salt would react with the reactor core, and what materials such reaction would produce.

The group of former JAERI experts is concerned about salt-aided progression of corrosion on pipes and other facilities.Stainless steel and other quality materials used at nuclear power plants corrode with chlorine, and develop cracks called stress corrosion cracking inside.Magnesium chloride in seawater is said to aggravate such cracks.

There is no need to elaborate what would happen if cracks develop at the reactor and cooling equipment.What's worse, corrosion at existing facilities could leak the highly contaminated water to the external environment.

These points the experts highlight are correct, and we should all take note.Yet, hearing these factors and throwing your hands in the air would only make the operation to cool down the melted cores and its schedule announced the other day un-executable, leaving the discharge of radiation unattended.We must never give up.We must remember that those factors that have been pointed out include elements of speculation that may or may not reflect reality.

In working our way toward the goal, it is important to firstly verify the current state of the reactor cores and contaminated water.We should build and set up an operation base and bridgehead at the reactor buildings and start gathering expert knowledge from in and outside the nation.This step is now being taken on site, although it could have started sooner.

Once we establish the current situation, there is always a way to find a solution for it.The solution should be implemented under international cooperation, which should, in turn, reinforce the world's preparedness against nuclear disasters.That is what the world wants.The international community is carefully observing Japan's next steps.Meeting this expectation is our nation's mission in response to the support the rest of the world has given Fukushima in the face of this tragedy.I believe taking such steps will lead to the future of nuclear energy in Japan.



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