Venting Was A Major Source Of Radioactivity Near Fukushima Plant

The two reactors that workers vented during the Fukushima Daiichi nuclear disaster in Japan were likely the source of most of the radioactive contamination later found near the plant, according to a new analysis (Environ. Sci. Technol., DOI: 10.1021/es300556m).

After tsunamis knocked out electricity at the plant in March 2011, cooling systems failed and fuel inside the reactors heated up rapidly, generating hydrogen gas. Fearing that pressure buildup would lead to catastrophic explosions, workers vented the hydrogen at reactor units 1 and 3. Meanwhile, an explosion occurred at another reactor unit’s spent-fuel pool, likely as a result of hydrogen buildup in pipes, according to a post-accident report by the Institute of Nuclear Power Operations, an organization funded by the U.S. nuclear industry.

Researchers reconstructing the accident have debated whether radioactive elements escaped primarily from the venting at reactors 1 and 3 or from the explosion at the spent fuel pool. Jon M. Schwantes, a senior scientist at Pacific Northwest National Laboratory, suspected that clues lay in soil and water samples from around the plant.

A standard modeling tool called ORIGEN-ARP allows researchers to input data about nuclear fuel, such as the type of fuel used, the length of cooling time, and the specifications of a reactor. The software then calculates possible ratios of radioactive isotopes found in the fuel.

Using this tool, Schwantes and his coauthors generated ratios of radioactive isotopes, such as ¹³⁴Cs/¹³⁷Cs, ¹³⁶Cs/¹³⁷Cs, and ¹³¹I/¹³⁷Cs, that were likely present in reactor units 1 and 3 just before venting and in the spent fuel pool at the time of the explosion. The researchers compared those ratios to soil and water measurements taken soon after the accident. The ratios of radioactive isotopes measured near the plant closely matched the simulated ratios for reactors 1 and 3, indicating to the researchers that most of the radioactive contaminants originated there.

Schwantes says that this modeling technique could help people who respond to future nuclear accidents by enabling them to determine the source of escaping radioactive elements.