Q&A- Japanese nuclear crisis

Here are some tweeted responses by Japanese scientists to questions about the crisis at quake-affected nuclear reactors (hat tip - Science Media Centre).

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: 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.

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: Why couldn't the reactor be shut down straight after an earthquake?

A: It has been shut down. 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: 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 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.

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.

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.

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.