Vetter’s detector at Berkeley even catches radioactivity on the wind from treatments received by thyroid cancer patients passing by six stories below.
Taking radioactive fingerprints
Japan's prime minister sounds a resolute note, promising to win the battle against an overheating nuclear plant even as atomic safety officials raised questions about the accuracy of radiation measurements at the complex. (April 1)
Japan’s nuclear emergency
Natural radiation — mostly from airborne radon — “drowns out” the radiation from Fukushima spotted in the United States, said Michael Miller, a University of Washington physicist who, along with Robertson, helped construct a radiation detector in a campus building’s air intake duct.
So a sensor that simply measured the total amount of radiation from airbone particles would be useless in nuclear forensics. Modern detectors do much more. They outline the dust’s distinctive radioactive fingerprint by measuring precise concentrations of five or more radioactive elements, or isotopes.
Each atom of these isotopes is unstable, shedding excess energy — via radiation — in a process called decay. By measuring the form and intensity of this energy, the radiation detectives can identify the isotopes in play and deduce from them what might have happened.
Radioactive iodine-131 and cesium-137 are key to this process. They don’t exist in nature, so their appearance signals a nuclear event — either a bomb or a reactor in trouble. Both can cause health problems in large amounts. But iodine-131 decays relatively rapidly: After eight days, half the original amount is gone. Its presence means that the event that created it occurred just weeks beforehand. Cesium-137 takes much longer to decay, with a half-life of 30 years. Traces of cesium-137 from Chernobyl still waft on Earth’s great jetstreams.
Clues in the air
It was detective work of this kind that alerted the world to the world’s worst nuclear disaster 25 years ago. In April 1986, nuclear power plant workers in Sweden detected a spike in iodine-131 and cesium-137, which — after a check of wind patterns — revealed the unfolding disaster at Chernobyl, which the Soviet Union had not disclosed.
Because both isotopes can come from a bomb or a reactor, nuclear sleuths also search for another isotope that originates only in reactors: cesium-134. It is produced during the slow-boil nuclear fission inside reactors, but not the flash-bang of a nuclear explosion. The ad-hoc sensors built by academics on the West Coast have picked up cesium-134 from Fukushima, as have the permanent CTBTO stations.
Nuclear detectives can dive deeper still, sorting out whether radioactive emissions emanate from a dangerously active and still-fissioning reactor core, from burning fuel rods, or from used fuel sitting in pools.
When the active core of Chernobyl exploded, it sent dozens of different radioactive elements into the atmosphere, including isotopes of strontium, yttrium, and rhodium — all produced only by active reactor cores or burning fuel rods.
The University of Washington team say they have not seen any of these isotopes, indicating that the fuel rods at Fukushima have not caught on fire.
And the lack of other short-lived isotopes — especially iodine-133 — indicates that the primary safety systems at Fukushima kicked in as planned during the earthquake and shut down fission in the reactors. Halting fission starts an atomic clock of sorts in which quickly decaying isotopes vanish within hours or days. Andreas Knecht of the University of Washington team said their Seattle detector can sense iodine-133 up to seven days after a reactor produces it, but has found none.
The detection of still another isotope, tellurium-132, offers a further clue about the source of radioactive emissions from Fukushima. Fuel rods sit in pools for months or years, releasing fewer and fewer different isotopes over time, and — unlike hotter fuel rods — they don’t produce tellurium-132. Spotting that isotope, as the detectors have, points to a damaged reactor core.
By putting these radioactive puzzle pieces together, the Seattle team was able to conclude in a paper published March 28 that fission in Fukushima’s three active cores was halted during the earthquake, and that the reactor cores launched radioactive debris into the air on clouds of steam shortly thereafter.
Loading...
Comments