USA | Radiation Contamination Risk Growing as Japan Nuclear Crisis Deepens: Q&A

Posted on March 16, 2011


USA | BLOOMBERG | Mar 16, 2011 10:39 AM GMT

The stricken nuclear reactors in Fukushima Dai-Ichi have so far released only small amounts of radioactive material unharmful to the general population outside the 20-kilometer evacuation zone.

That could change in the unlikely event — a worst-case scenario — that engineers are no longer able to cool down either the reactors, or the spent fuel rods outside reactor 4. Tokyo Electric Power Co. officials suspect a breach in reactor 2, which was damaged by a hydrogen explosion this morning.

Those most at risk right now are the 50 or so remaining workers at the plant who are trying to put out fires, avoid further explosions, cool the reactor cores with sea water and re-submerge an array of spent fuel rods into a pool of water.

The highest measurement of radiation so far was taken between two buildings on the plant complex — at 400 millisieverts, it was 20 times the annual limit for nuclear industry employees and uranium miners, according to the World Nuclear Association, a London-based advocate for the industry. A radiation dose of 100 millisieverts a year is the lowest level at which any increase in cancer is evident, the London-based WNA said on its website.

The worst-case scenario, in which engineers are no longer able to cool the rods — both those inside the reactors and those in a spent fuel rod pool that is currently leaking radiation — leading to a buildup of radioactive Cesium and Iodine isotopes, which are then released either through an explosion or a leak in the containment vessels, is highly unlikely, according to scientists. In this scenario, winds blowing inland could spread these radioactive isotopes over a large area, perhaps exceeding the 20-kilometer evacuation zone.

The following questions and answers were created from Bloomberg News interviews with experts on radiation poisoning, nuclear reactor design and past nuclear accidents, and from official websites of the World Nuclear Association, the Science Media Centres of Japan andAustralia, the World Health Organization in Geneva, the U.S. Centers for Disease Control and Prevention in Atlanta, and the U.S. Environmental Protection Agency in Washington.

Q: What sorts of radioactive materials are we talking about, and how harmful are they?

A: In the current situation, radiation can come from at least four sources: the uranium fuel that is placed inside the fuel rods, the spent uranium rods that are now exposed in reactor number 4, the dispersal or venting of gases built up inside the reactors from a chemical reaction that takes place between the rods and their alloy casing if the rods become too hot, and the seawater that is being used to cool the reactors.

Q: What is the potential for release from the spent fuel rods, which are currently uncovered, and no longer submerged in water?

A: The spent fuel rods are kept under water to stop them from burning through their casings, which are made of zirconium. In at least one of the spent fuel ponds, the casings have began to burn, exposing the fuel rods and releasing cesium into the air. Since there are no explosions, or ventilation, the cesium will likely form a layer around the area of the pond, and contaminate it.

Q: How do they stop that?

A: The only way to stop that process is re-submerge the fuel rods — this can be done by helicopters, or by robotic means. If the cesium buildup continues, workers trying to get close to the reactors will be exposed to lethal doses of radiation.

The water will then have to mixed with boric acid to stop the burnt uranium from mixing with water and reaching what scientists call “recriticality.” At that point the fuel rods themselves becomes a sort of open-air reactor, releasing radiation into the sky.

Q: How much of that has been released, and what are the effects of that?

A: The uranium fuel rods inside each reactor are still expected to be largely intact. If the reactors are cooled down over time, the rods will be disposed off as radioactive waste. The spent rods, which were usually stored in a pool near reactor number, are radioactive — or hot — and will be leaking radiation until they are fully submerged with cool, distilled water and return to a normal temperature.

At least one of the pools of spent fuel rods is completely dry and out of control.

Trace amounts of Cesium-137 and Iodine-131 were found in the air around the plant starting March 12, around 1:30 p.m. local time. The two isotopes are produced when the fuel rods inside the reactors overheat and react with their casings. Both are radioactive and can cause health damage. Iodine has a half-life, or reduces in mass and thus radioactivity, every eight days — within two months, it leaves no trace behind. It can be countered with doses of potassium iodide tablets, but only within 24 hours of exposure. It is known to cause thyroid cancer.

Cesium has a longer half-life of about 300 years. External exposure to large amounts of Cs-137 can cause burns, acute radiation sickness and even death. Exposure to Cs-137 can increase the risk for cancer because of exposure to high-energy gamma radiation. Internal exposure to Cs-137, through ingestion or inhalation, allows the radioactive material to be distributed in the soft tissues, especially muscle tissue, exposing these tissues to the beta particles and gamma radiation and increasing cancer risk.

Q: How much of this material is out there? A: So far, very small amounts of cesium and iodine are reported to have been released, mostly in a process when engineers are venting pressurized gases out from near the reactors core, to chambers outside it, where leaks and explosions have spread it out.

How much Cesium or Iodine remains inside the nuclear reactors is the big unknown — the fuel rods have been partially submerged several times, and the temperatures have risen each time. If engineers can get control of the reactors, and finish the cooling process, then this radioactive material should be entombed within the reactor itself.

If the cooling process fails for any reason, and the containment chambers are sufficiently damaged, this material could leak out. This is the worst case scenario.

Q: How far has the radiation leaked so far reached, and should we be worried? A: Radiation levels near Namie, a town 20 kilometers northwest of the Tokyo Electric Power plant, reached as high as 330 microsieverts per hour.

Kitaibaraki city, northeast of Tokyo, had the highest reading so far of radiation at 11:40 PM local time, of 15,800 nanosieverts per hour. That’s about 1/15th of a millisievert. (See below)

Q: What level of radiation is dangerous to human health? A: One hundred millisieverts a year is the lowest level at which any increase in cancer is clearly evident. Above this, the probability of cancer occurrence increases with higher doses.

A cumulative dose of 1,000 millisieverts would increase the incidence of fatal cancer by about 5 percent. A single dose of 1,000 millisieverts causes temporary radiation sickness and decreased white blood cell count, but not death. A single dose of 5,000 millisieverts would kill about half those receiving it within a month.

Air crew on flights over the North Pole between New York and Tokyo are exposed to about 9 millisieverts of radiation a year, and a chest x-ray radiates about 0.1 millisieverts. Humans are exposed to about 2 millisieverts a year from naturally occurring radiation in soil and cosmic rays.

Q: What are the health consequences of radiation?

A: Exposure to high levels of radiation can cause acute radiation syndrome, or radiation poisoning, resulting in substantial damage to human body tissues, premature aging and possibly death. Prolonged exposure to lower levels is also associated with increased risk of ill health.

Q: What are the symptoms of radiation poisoning? A: The first symptoms of acute radiation syndrome are typically nausea, vomiting and diarrhea. These symptoms can start within minutes to days of exposure and can last for days. After that, a person with acute radiation syndrome may look and feel healthy for a short time, then become sick again with loss of appetite, fatigue, fever and possibly seizures and coma. This stage may last a few hours or several months. Radiation poisoning also typically causes skin damage.

Q: What is being done to protect human health? A: Japan has distributed 230,000 units of stable iodine to evacuation centers from the area around the Fukushima Dai-Ichi and Fukushima Daini nuclear power plants. The iodine has not yet been administered to residents; the distribution is a precautionary measure in the event that this is determined to be necessary.

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