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Science Alert: Experts Respond

Japan’s nuclear crisis – Q&A with experts

Posted in Science Alert: Experts Respond on March 14th, 2011.

Japanese nuclear authorities continue their efforts to avert nuclear meltdown at the Fukushima earthquake-damaged power plant.

Testing for presence of radioactive material

The following Q&A backgrounder is based on ‘tweets’ from scientists in response to questions from the journalists and the public on the nuclear situation. It was collated by Professor Ryugo Hayano (@hayano) at the School of Science, Tokyo University and edited by researchers at the Japanese Society for Science and Technology Studies. Note, not all answers are written by Prof. Hayano.

The SMC has local experts who can talk in general terms about nuclear material and radiation. Contact the NZ SMC for details.

Questions covered:

1. Radiation Exposure

2. Cooling with Fresh water and Seawater

3. Why use Boric Acid?

4. Regarding the Explosion

5. The Operational Status of the Nuclear Power Stations

Radiation exposure:

Q: Are the amounts of radiation that are being released at the plant enough to cause radiation exposure?

A: Geiger counters are sensitive enough to detect radiation if you have as many as fifty to several hundred radioactive atoms on your skin. I do not know what the present level of exposure is, but I can say that even very low levels will be detected.

Q. Do we need to worry about secondary exposure to radiation? For example, is seafood from this area safe to eat?

A: If reports of how much radiation has been released are accurate, there should be no reason for concern. There is radioactive matter present in the natural environment as well.

Q. Is it a good idea to stay indoors?

A. The most important thing is to stay away from the nuclear power stations. As the government has announced, an evacuation perimeter of 20 kilometers is a good estimate. Also avoid exposure to the open air.

Q: You’ve said it’s safe as long as people stay 20km away, but should people living in Tokyo also stay indoors and avoid open air?

A: Tokyo is more than 250km away from Fukushima so there’s no need to do so.

Q: Can you explain why the evacuation perimeter was extended to 20km within the plant?

A: I’m not sure of what the Government’s based their decision on, but during the US Three Mile Island nuclear power plant accident in 1979, no damage was recorded past 16km. The Government may have adjusted the evacuation distance based on this observation.

Q: Should people living in Tokyo avoid going outside with their skin exposed?

A: That unnecessary. The government’s 20km evacuation requirement is reasonable. Radioactive matter carried by the wind can lead to people being exposed to radiation, but the further the distance the radioactive matter has to travel, the lower the radioactivity, so those living in Tokyo have nothing to worry about.

Q: If radioactive matter leaks out of the power station, how far could it go?

A: That’s extremely difficult to predict without knowing the amount of radioactive isotope being leaked and weather conditions (especially wind). This situation is similar to the Three-mile Island accident where the storage container remained intact and has been assumed to have not affected areas beyond a ten-mile (about 16km) radius.

Q: At one point the radiation levels started fluctuating, why was that?

A: It may have been caused by xenon and iodine that had leaked out when they released air from inside the container to lower the internal pressure.

Q. What is the worst-case scenario?

A. That will all depend on how much radiation leaks and the weather conditions at that time. Iodine 131 is heavier than air so if winds are mild, then it shouldn’t spread too far. It also has a short half-life of eight days.

Q: I thought half-lives of radioactive substances was generally much longer.

A: Xenon 137 has a 3.8-minute half-life but there’s a risk it’ll change into cesium 137, which has a 30-year half-life.

Q: I don’t understand how the radiation level could fall to 1/100th of what it was in just an hour. Does it have something to do with wind direction?

A: Most of what gets released are rare gases such as Xenon and Krypton with short-lived radioactivity. The fact that levels dropped so quickly suggests they were only released for a short period of time.

Q: It looks like patients and staff at Futaba Hospital, located close to the power station, have been exposed to radiation.

A: The hospital is located about 4 kilometers north-northwest from the power plant. They were waiting outside in the grounds for Self-Defense Force helicopters to evacuate them. The fact that they needed decontamination (first they needed to be washed) appears to indicate that they were exposed to radioisotopes carried by the wind.

2. Cooling with Fresh water and Seawater

Q: Is it safe to cool down the reactors with seawater instead of fresh water?

A: Usually fresh water is used in this situation in order to extend maintenance periods and reduce costs. However, right now it’s far more important to cool down the reactor in any way possible. I doubt anyone would consider using this reactor again.

Q: What’s the problem with seawater?

A: Simply because the salt would rust the pipes and create holes for radiation to leak out from. That’s why fresh water is usually used as coolant. But now isn’t the time to be worrying over that so seawater’s being used as the coolant.

Q. Rusting and pipe holes seems like something that occurs over a longer period of time, so will we be okay as long as this reactor is no longer used?

A: Yes, holes will start appearing after a longer time period. Right now the priority is to cool the reactor down by any means possible.

Q: There’s a support ship heading this way from America. Would it be possible to get fresh water?

A: We need to cool this down immediately. The reactor container will be filled with seawater containing boric acid to cool it down. That’s what’s important.

Q: Why couldn’t the reactor be shutdown straight after an earthquake?

A: It has been shutdown. Control rods have been inserted and the fission chain reaction has been stopped. However, the reactor needs to be cooled down to prevent the radioisotopes in the fuel rods and decay-heat from heating up the container.

Q: Was using seawater a last resort? What if there’s a mistake?

A: Since they made the decision, the container must be cooled down regardless of what method is used. I have confidence in the ground staff.

Q: Have we prevented a possible containment vessel rupture?

A: Yes, it should be safe now, but it is imperative that they get the container cooled down.

Q: If they succeed in cooling the reactor, is it safe to say they’ve avoided a major disaster?

A: Yes. I have faith in the ground staff.

Q. If the coolant has been leaking out from somewhere, doesn’t that mean that it will all drain away from the hole? Is that why they’re going to fill the container itself with seawater?

A: I don’t have enough information to be able to answer that question. It was assumed there was a leak because the water level kept going down.

Q. Is there a possibility that the coolant simply boiled and evaporated away because it had stopped circulating properly?

A: Even if water is compressed, the total volume doesn’t decrease so the water level shouldn’t go down significantly even with a rise in water vapor pressure. It must be considered that water was being lost somewhere.

3. Why use Boric Acid?

A: Boron isotopes, boron-10, effectively capture thermal neutrons. Boric acid is a water solution containing boron. This is being mixed with seawater and injected into the reactor as a coolant.

Q: What does boric acid do?

A: It effectively absorbs neutrons in the reactor. In case fuel ends up outside scope of the control rods inside, the boric acid will reduce the risk of a fission chain reaction starting up.

Q: By using the seawater and boric acid mixture now, does that mean the reactor will be unusable in the future when things have settled down?

A: It’s clear that the fuel rods have been damaged so any immediate re-start would be out of the question.

4. Regarding the Explosion

Q. How did hydrogen explode at the Fukushima 1 nuclear power plant? How did hydrogen get into the containing structure and vessel?

A: The material used in fuel rods, called zirconium, reacts with water at high temperatures and releases hydrogen gas. Hydrogen gas passed through the pipes, leaking out of the reactor vessel and built up in the housing around the reactor. It looks like the hydrogen gas reacted with oxygen and exploded.

Q. Despite the explosion, the reactor was not destroyed. Does this mean a hydrogen gas explosion isn’t enough to damage a reactor?

A: Yes. The housing may break but the reactor containment is built on strength. This is a nuclear reactor’s last defense in order to prevent a serious accident. It looks like the reactor wasn’t damaged so for the time being everything’s okay. As long as the reactor is safe, disaster can be avoided.

Q. Assuming that hydrogen was leaking directly from the reactor, wouldn’t a hydrogen explosion lead to an explosion inside the reactor itself?

A: The reactor is filled with nitrogen so it’s unlikely that an explosion would occur inside the reactor.

Q. So the reactor hasn’t been damaged?

A: As long as the radiation level reports released by the Fukushima 1 Nuclear Power Plant are true, I think you can be certain that the reactor is safe.

5. The Operational Status of the Nuclear Power Stations

Q: What is a “runaway nuclear explosion”?

A: Like the Chernobyl disaster, it is when nuclear fission gets out of control and the reactor power output increases to 10 times more than its standard output. However, this case is different because the nuclear reactor’s operations have been stopped. Still, it’s important to ensure that water levels do not go down.

Q: Is it true that Tokyo Electric Power Company’s eight radiation-monitoring posts aren’t working right now?

A: Plant 1’s (Daiichi) monitoring post isn’t working, but the posts at Plant 2 (Daini) are.

Q: Aren’t there any real-time monitors?

A: While the monitors on Plant 2 (Daini) are working, the monitors on Plant 1 (Daiichi) haven’t been working since March 11. The Fukushima Prefecture radiation monitors also aren’t working.

(As of 13:00 JST March 13) the Tokyo Electric Power Company is using monitoring cars to continue measurements.

Q: I’m worried about Fukushima Plant 2 (Daini). I’m guessing that it’s not getting enough coolant as it should, but would it be necessary to carry out the same measures as what’s being done at Plant 1 (Daiichi)?

A: While the situation is worrying, as of 13:00 JST March 13, radiation levels in the area are normal. No radioactive material is coming out from exhaust vents at Fukushima Plant 2 (Daini).

This article is based on ‘tweets’ from scientists in response to questions from the public. It was collated by Professor Ryugo Hayano (@hayano) at the School of Science, Tokyo University and edited by researchers at the Japanese Society for Science and Technology Studies,

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