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. Last Updated: 07/27/2016

Japan Lifts Ban After Accident

TOKAIMURA, Japan -- Japan lifted an order on Friday for thousands of people living near the site of its worst nuclear accident to stay inside, and experts said there was little chance the impact on health or the environment would spread.

Authorities were probing into how workers at the uranium processing plant in Tokaimura, 140 kilometers northeast of Tokyo, caused the accident by ignoring proper procedures.

Top government spokesman Hiromu Nonaka said at a midafternoon news conference that people within a 10 kilometer radius of the uranium processing plant were now free to go outside.

The order excluded people living within 350 meters of the plant, most of whom were in any case evacuated soon after the accident Thursday morning.

Authorities said the 80 or so people staying in a community center were unlikely to go home before Saturday morning, when they would be given health checks again.

Residents welcomed the lifting of the order, which had turned Tokaimura, which has a population of 34,000 and 15 nuclear facilities, into an eerie ghost town, with police in white protective suits roaming the streets.

The government declared the incident to be Level 4 on the scale of nuclear accidents, making it Japan's worst ever. The worst previous accident, in 1997, had been Level 3.

The United Nations' atomic watchdog said later in Vienna that the accident could be given a higher rating after a comprehensive investigation.

It is already the world's third-worst nuclear accident, after the 1979 U.S. Three Mile Island incident, which was Level 5, and the 1986 Chernobyl accident in Ukraine, which was Level 7.

Level 4 means that there has been a leak of a small amount of radioactive material outside a nuclear facility.

U.S. nuclear experts say it is usually three days before investigators begin to understand what happened in such a nuclear accident, but they are beginning to piece together a picture of what may have occurred.

The accident at the Tokaimura reprocessing plant was almost certainly a "criticality" event: a runaway nuclear chain reaction. If so, that is bad news for the plant and its workers, but probably good news for residents of the surrounding countryside. Such accidents do not normally release large sustained amounts of radiation.

A chain reaction is relatively simple: When the nucleus of a uranium atom absorbs a neutron, the nucleus breaks in half. That is fission, and it releases large amounts of energy. The process also releases some toxic elements, such as radioactive cesium and iodine, which can be absorbed by the human body, and neutrons. Each of the new neutrons, in turn, can strike another uranium atom, causing fission again, and producing more neutrons.

The amount of uranium that must be present for the reaction to sustain itself is called critical mass. When a chain reaction occurs where one was not intended, as was the case in Thursday's accident, researchers call it a criticality accident. (In an atomic bomb, enough uranium is present in a small space for an explosion to occur.)

When the accident occurred, workers reported seeing a blue light. The light is called Cerenkov radiation and is produced when neutrons strike water molecules. Used uranium pellets soaking in a water pool at a nuclear power plant emit a continuous blue light of this sort. The fact the workers saw blue light suggests that many neutrons were present and criticality occurred, experts said.

The amount of energy released in a criticality accident can vary widely. At the bottom end, the energy would be "enough to raise the temperature of a small lump of uranium a few tens of degrees" - not really a lot, Tom Cochran of the Natural Resources Defense Council said. At the top end, historically, the amount of energy would be enough to produce an explosion equivalent to about 20 kilograms of TNT, he said. That would damage a room or plant, such as that in Japan, but is many magnitudes less than an atomic bomb.

An explosion would scatter the uranium widely, stopping the chain reaction immediately. The fact that the chain reaction at Tokaimura persisted for several hours makes it clear that such a large explosion did not occur.

Two ways are known to stop a runaway chain reaction. The first is to remove as much water as possible from the scene, such as the cooling water surrounding the container.

The second way is to dump the chemical sodium borate onto the uranium. Sodium borate, Cochran said, "gobbles up neutrons just like PacMan gobbles up energy pellets," and quickly halts the chain reaction. Japanese authorities, who brought in nearly half a ton of borate to the site, reported using both techniques.

As authorities scrambled to assure residents they were safe, an official in the nuclear safety division of the government's Science and Technology Agency said they had detected no further spread of radiation that would adversely affect humans.

Officials said it was safe to drink tap water, but warned against drinking water from local wells, which was being tested for possible contamination, a process that would take days.

Masahiro Saito, a professor at Kyoto University, also said the environmental impact would be minimal.

At least 55 people were known to have been exposed to radiation as of Friday, including 45 workers at the plant and three firemen.

Among the most critically injured were two of the workmen present when the accident happened.

They were still in serious condition Friday evening despite a slight improvement, but some experts said the two might not survive given the high level of radiation suffered.