[Updated 2011/3/16 23:00-JST]

This is an update on the "Q&A on the Nuclear Power Stations" published on

March 13 2011

・3/16 – 23:00 until now the SMCJ staff have updated posts with the latest information once a day.  But given that some topics have now become less relevant, and that it has become harder for journalists to find the information they need, we will stop updating this post.

New information will be published in a new post from now on. 

・3/16 – 17:15 With the large number of differing news reports available now, we have been receiving an equally large number of questions from people who may have misunderstood a news story because they had not checked the date and time the story had been published.  This has mislead some people into thinking the current situation is something it is not.  After discussing the issue with our Tokyo University nuclear sciences team, who have been receiving all questions, we have decided to delete some information (the original information has been saved on our archive).

・Our scientists and the SMCJ are in a complicated situation where we are continuously collecting the latest scientific information.  Because of this, we would like to ask people to refrain from copying and pasting this manuscript.  If you wish to share this information, please keep it to sharing this website link.

・This is to make sure that the answers to questions reflect the current situation as much as possible.

・We have gathered comments about the Nuclear Power Plant from experts in a variety of scientific fields.

・The content below has been put together by researchers specializing in Physics and Engineering (for more details refer to the bottom of this post).

・Responsibility for this published document lies with Science Media Centre of Japan (Corp.).

 

・This document was translated with the support of the Australia-Japan Foundation.

【To Physicists, Nuclear Engineers, and other researchers】

　In regards to revisions based on the latest information, we would greatly appreciate it if you could provide any advice via the SMCJ inquiry form, or Twitter (@smcjapan).  Please include your name, post, and contact details (these details will not be made public).

【To journalists and members of the public】

　Please be aware that the events since Friday have been moving at such a speed that information and data is constantly being updated.  It is for this reason that our experts may have differing opinions depending on what information they had at the time. 

   We will try and keep the information as up to date as possible but ask for your understanding and apologize for the inconvenience this will cause you.  Updated information will be indicated so with the date and time it had come through.

【Contents】

1. Radiation exposure

2. Cooling the reactor

3. About the explosion

1. Radiation exposure

Q1-1. How much longer do we have to be concerned about the danger at the power plants? [3/15-00:00]

A. I cannot give an accurate answer at this time. Once we can confirm that the impact of radioactive materials released are low enough to be ignored, I think that evacuation alerts can be cancelled. However, it’s difficult to estimate how long these materials will have an influence (on the environment) with the information we currently have since it really depends on how much radioactive material has been released. 

Q1-2. What’s the difference between radiation and radioactive materials?  Are they different from radioactivity? [3/15-00:00]
A. The relationship between radiation, radioactive materials, and radioactivity are: radioactive materials emit radiation, whereas the process that emits radiation is called radioactivity.

Let’s take a look at a flashlight for example: The flashlight itself is a radioactive material because light is a form of radiation while the process that emits light is a form of radioactivity.  There is, however, a tendency to refer to radioactive materials as radioactivity but there’s no need to worry too much about their differences.

Q1-3. Will this affect the ocean? I’m worried about eating sea food. [3/14-22:00]
A. At this point I think it would be okay to assume that there's almost no impact on sea food.  Currently, the reactor's core containment vessel at the Fukushima Dai-ichi nuclear power plant is intact and its current radioactive material emission is considered to be low (as of 17:00 March 13).

As for the impact on sea food, the following may happen:

Firstly, small amounts of radioactive material released into the atmosphere would be absorbed into the sea.  Secondly, the material would be diluted by the sea water before finally being absorbed by marine life in the area.  Because only a small amount of radioactive material has been released so far, it would only have a minimal effect on sea food.  Of course, it would be reasonable to assume that Fukushima prefecture and the Government will conduct safety checks in the near future to ensure that things are safe.

Q1-4. If I’m being told to stay indoors but need to go outside, how far out can I go and still remain safe? [3/15-00:00]
A. Please follow the instructions given out by the government or your local government, and not information from friends or the Internet.  There’s also a possibility that the situation will change, so make sure you can get updates from the government and local government constantly.

Q1-5. When evacuating, what are some things to keep in mind?  (Covering mouth and nose, change clothes at the evacuation center, etc) [3/15-00:00] 

A. First of all, stay calm and don’t go out by yourself.  The reason for this is that when people are rushing, it can lead to secondary disasters like road accidents, falls, or getting lost.  It terms of radiation, the amount that’s being recorded around the power plant is significantly lower than the amount needed to affect human health, so I don’t think any special measures need to be taken.  To lower the chances of being exposed to radiation, wearing a mask can be effective.

2. Cooling the reactor

Q2-1. What does, "the reactor's cooling system has failed," mean? [3/14-18:00]

A. It means that that the reactor, in particular the fuel, has not been cooled down enough.  In a nuclear power plant, the heat (energy) generated by the nuclear fuel is turned into power by the circulating water.  If the coolant (water) doesn't circulate properly, then the fuel can't be cooled down.  

Q2-2. What'll happen to the water that's being injected into the reactor when it's thrown out?  Will it pollute the environment? [3/14-18:00]

A. Waste water with low levels of radiation will be passed through a filter before being disposed of.  Waste water with high levels of radiation will be boiled.   This will turn the radioactive material into solids which can be picked out before the water is disposed of.  But the amount of water being injected in this case is significantly large so right now I don't know what method they'll ultimately choose to treat the water.

Q2-3.Why did it take one-and-a-half days after the earthquake before they decided to inject in seawater? [3/15-00:00] 

A. Usually fresh water is used as the coolant.  Using seawater would rust the reactor's piping, making it unusable in the future.  However, in the case of the Fukushima Dai-ichi's Unit 1 reactor, either there wasn't enough time to inject in sufficient fresh water or all of the fresh water supplies in the tank had already been used up.  I can imagine that this would've lead them to give up on the idea of using the reactor in the future so they went ahead with injecting seawater.

Q2-4. What happens when a nuclear reactor is completely shut down?  Is it necessary to keep cooling down the fuel rods as long as they’re there? [3/15-00:00] 

A. It’s hard to define “completely shut down”.  The meaning can change depending on what is completely shut down.

For example, when the news had reported an automatic shutdown, it probably referred to the situation in which nuclear chain reactions of uranium fuel had been stopped.  This happened right after the earthquake.

But even after such an automatic shutdown, you cannot say that the nuclear reactors have been completely shutdown.  The reason for this is due to decay heat.  When the nuclear reactors are automatically shut down, it stops the nuclear fission chain reactions, but the fission products (material produced by nuclear fission) already present generate heat as it decays.

Decay heat also depends on the fuel’s state, but if we compared the level of decay heat with the reactor’s output temperature, it would decrease at a rate of 1/12th within one minute of stopping, 1/200th a day later, and 1/800th a week later (according to ANS-5, a subcommittee of American Nuclear Society: 1968).   This is why a large scale cooling system is required when there’s a large amount of decay heat being produced straight after operation has been stopped.   But even long after the cooling system has done its job, the reactor will still need some kind of cooling mechanism to keep, for example, used fuel.

Q2-5. Why did the cooling system break down all at once?  Why did the pumps break and the diesel generator become unusable?  Weren't there any backups?  I've also heard that the cooling system in all of the reactors at Fukushima Dai-ichi had broken down at the same time, why is that? [3/16-23:00]
A. Some people have suggested that the earthquake and tsunami had affected the reactors more than it could withstand, but because there's no information detailing what exactly has been damaged in the power plant yet, it's difficult to give a precise answer.

But here are some general points about what could happen in such an incident.

Nuclear power plants have a number of safety measures put place in case of an accident, including several backup cooling systems and diesel generators (usually 2, 3).  This to make sure that even if one backup system fails, there's another backup system which can take over and ensure that the reactor can be safely shut down.
An accurate answer to what happened at the Fukushima Dai-ichi Nuclear Power Plant can't be given right now without knowing what precisely happened at the time and what the situation is like now, but given that all of the backup cooling systems and diesel generators have stopped working, it could be assumed that Friday's earthquake and subsequent tsunami had a much greater impact than anyone could have imagined.

3. About the explosion

Q3-1. Is a hydrogen gas explosion the same as a hydrogen bomb explosion? [3/14-14:43] 

A. A hydrogen explosion is not the same thing as a hydrogen bomb explosion.  It's just a chemical reaction where hydrogen and oxygen gas had mixed together, ignited, and exploded to make water.  A reaction like this wouldn't emit radiation.  It's only if the hydrogen was radioactive to begin with that such an explosion would create detectable radiation.

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