Radiation Risk to Japan Public May Be Minimal
U.S. nuclear experts say it is usually three days before investigators begin to understand what happened in a nuclear accident such as that which occurred in a Japanese uranium processing plant Thursday, 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 sustained large 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 it to fission, 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 Japanese accident occurred, workers reported seeing a blue light. That 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 that the workers saw blue light suggests that many neutrons were present and that 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, said Thomas Cochran of the Natural Resources Defense Council.
At the top end, historically, the amount of energy would be enough to produce an explosion equivalent to about 44 pounds of TNT, he said. That would damage a room or plant such as that in Japan but is orders of magnitude smaller 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 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.
Water acts as a moderator in nuclear reactions. Neutrons released during fission are of such high energy that they need to be slowed down in order to be absorbed by nearby uranium nuclei and cause them to split. Passing the neutrons through water slows them down, but removing the water makes the chain reaction less likely to continue.
The second way is to dump the chemical boric acid onto the uranium. The boric acid, Cochran said, “gobbles up neutrons just like Pac-Man gobbles up energy pellets” and quickly halts the chain reaction. Japanese authorities, who brought in nearly half a ton of boric acid to the site, reported using both techniques.
Historically, Cochran said, “in a criticality accident, you get high radiation exposure to people in the room or nearby, but you wouldn’t expect much in the way of consequences off-site.”
Although few details have been released, the picture that emerges to date is as follows:
The building involved was using British-made equipment to convert enriched uranium purchased in France into fuel pellets for use in Japanese reactors. Most commercial power reactors use uranium that is enriched to contain 3% to 5% uranium-235, compared to the normal content of about 0.7%.
The Tokaimura plant, however, was processing fuel containing 19% to 20% uranium-235, suggesting that the fuel was destined for either a test reactor or Japan’s Joyo breeder reactor.
The enriched uranium is usually shipped as uranium hexafluoride, which is solid at room temperature, but becomes a gas above about 260 degrees Fahrenheit. In order to be used in a reactor, the fuel must be converted into uranium oxide pellets, which remain solid at virtually all operating temperatures.
At one stage of the process, the uranium is dissolved in concentrated nitric acid so that it can be converted into uranium oxide. The chamber where this takes place is designed to hold about 5.2 pounds of uranium, but plant workers apparently put in 35.2 pounds, more than enough for a critical mass.
All that remained was for something to trigger a reaction. That, said David Albright of the Institute for Science and International Security in Washington, could have been something as routine as a neutron from outside the room (a cosmic ray, for example) or the fission resulting from a uranium neutron striking a uranium atom.
(BEGIN TEXT OF INFOBOX / INFOGRAPHIC)
The Anatomy of an Accident
The plant where Thursday’s accident occurred turns uranium hexaflouride into uranimum oxide pellet nuclear power plants. Normally, nuclear fission does not occur at this location. When it did, workers unable to control it and radiation was released.
The town of Tokaimura, in Ibaraki Prefecture, has a population of about 34,000. Authorities ordered 313,000 people living within a six-mile radius of the accident to stay indoors.
The Fuel Cycle
Uranium passes through these steps on the way to a nuclear power plant.
Mining, Milling, Enrichment, (The accident occurred at this stage-Fabrication), Reactor, Spent fuel reprocessing
Where the Process Went Wrong
1. Uranium powder is added to nitric acid inside the chamber in a process to produce nucelar pellets.
2. The mixutre reaches a critical mass, causing a runaway chain reaction and spewing out radiation. Nuclear experts suspect that too much uranium powder was added.
3. Uranium then dips back down from the top of the tank and the reaction occurs again every few hours.
Nuclear Fission
Nuclear fission happens when a neutron hits uranium, causing the atom to split and release huge amounts of energy and more neutrons.
Researched by NONA YATES and JOHN JACKSON / Los Angeles Times
Sources: Nuclear Energy Institute, The Energy Handbook, Union of Concerned Scientists. The Nuclear Information and Resource Service, World Books Encyclopedia. Times wire and staff reports.
