The war in Libya is just one of the stories we're following during another breathtaking week of news.

We go next to Japan. Technicians there are still working to gain control of the troubled nuclear reactors, but we do have one piece of positive news. Some scientists have been examining the radioactive substances that have spewed from the plant, and they have concluded that most of the substances will turn harmless in the next few months.

As NPR's Richard Harris reports, the material came from the reactors themselves, not from the pools of used fuel that had been a major source of concern.

RICHARD HARRIS: Officials in Japan have been measuring increased levels of radioactivity in the region around the plant, and they've seen three spikes, major releases, on Tuesday and Wednesday of last week. Now they're looking at what specific radioactive substances were released. It turns out most of it is an isotope called iodine-131.

Per Peterson, chairman of the nuclear engineering department at the University of California Berkeley, says this tells him a lot about the nature of the accident that led to those releases.

Dr. PER PETERSON (Chairman, Nuclear Engineering Department, University of California Berkeley): Because there's iodine, this certainly was material that had come out of the reactors, not the spent fuel pools, because in those cases, the iodine-131 is pretty much completely gone.

HARRIS: Iodine-131 decays quickly. It has a half-life of just eight days. That means that over the course of two or three months, virtually all of it will be gone. Since the used fuel has been sitting in pools for months or even years, it clearly can't be the source of this material. That's reassuring news for engineers concerned about the condition of those pools.

Peterson says closer study of the isotopes will also help engineers figure out exactly what went wrong inside the reactors - for example, the temperature there at the time of the releases.

Dr. PETERSON: Of course, we can't go in and look at it inside the reactors, and so we need to gather as much evidence as possible, because this will aid in the subsequent cleanup effort as we move into that phase of this accident.

HARRIS: It's also good news for people who live within 100 miles of the plant. They have been exposed to higher doses of radiation since the accident. The amounts vary a lot, depending upon location. But Gerhard Proehl at the International Atomic Energy Agency says a wide area is now experiencing radiation that's many times higher than natural background levels.

Mr. GERHARD PROEHL (International Atomic Energy Agency): So, you may worry about that, and these levels are really are high. However, they will not remain such high.

HARRIS: Proehl says that's because most of the radiation is coming from iodine-131, which will be gone in a matter of months. In fact, radiation readings in the area are already declining noticeably. And, barring more releases, for most of the region, it appears it won't be long before the levels are equivalent to those in Denver.

It's worth noting that Denver has unusually high levels of background radiation because of the natural uranium in the rocks and because the thin atmosphere at the Mile High City lets in more radiation from space. And studies of Denver residents find no ill effects from those high background levels.

But when iodine-131 is gone altogether, that's not the end of the story. The reactors also emitted another radioactive isotope: cesium-137. The amount of that material, even right at the nuclear plant, is low enough that it's below safety standards for the workers there. But cesium-137 has a half-life of 30 years, so it will remain radioactive for a long time to come.

Per Peterson at Berkeley says, fortunately, that's contributing just a small fraction of the radioactivity in the region.

Dr. PETERSON: We still need to see how things might change going forward. But as long as things continue on the current trajectory, we should see these radiation levels dropping off steadily over time, because they're still being dominated by the shorter-lived radioactive species.

HARRIS: Iodine-131 does pose a health hazard for the short term, because it concentrates in the thyroid gland and increases the risk of thyroid cancer. However, that cancer can be prevented through the use of potassium iodide, and Japanese health authorities have been distributing that to people who live near the plant.

Richard Harris, NPR News, Tokyo.

INSKEEP: Keeping all the news in perspective, it's MORNING EDITION, which you hear right here on NPR News.

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