Tracking the Currents of Fukushima

Ken Buesseler of the Woods Hole Oceanographic Institute describes methods for tracking oceanic Cesium released by the Fukushima disaster and misperceptions about radioactive danger in the Northwest Pacific Ocean.

We live in a radiative world. Some radioactive elements occur naturally, while the decay of others has been facilitated by human interference. Because of the known detrimental impacts of radiation on human health, escaped radioactive isotopes are tracked as closely as malefactors on the lam. The most recent disaster-status spill occurred three years ago this month in Fukushima, Japan and Dr. Ken Buesseler of the Woods Hole Oceanographic Institute has been tracking the fallout ever since.

Periodic symbol for Cesium. Image Credit: ECO.

Periodic symbol for Cesium. Image Credit: ECO.

On March 11,2011, the Tōhoku Earthquake off the coast of Japan triggered a tsunami. The tsunami wreaked havoc on coastal towns and caused a loss of power and subsequent overheating, hydrogen explosions and meltdown of the Fukushima Dai-ichi Nuclear Power Plants. Water used to cool the reactors leaked, carrying contaminants to the ocean.

Three months after the initial spill, the Woods Hole Oceanographic Institute led a cruise to measure ocean-borne radionuclides from Fukushima. In an interview with Earthzine, Buesseler explained why they selected 137Csand 134 Cs for study and how research on radionuclides is conducted.

Cesium (Cs) is a radioactive element that is formed during the decay of uranium that takes place in nuclear power production. 137 and 134 indicate different isotopes, or forms, of the element.

“Cesium is of concern because it is relatively abundant as one of the isotopes released during Fukushima, and it has health concerns because of its decay properties: whether it’s external exposure [or] …uptake into the seafood and…internal exposure,” Buesseler said.

nuclear power plant

Ken Buesseler and the nuclear power plant, Sept 2013. Image credit: University of Tokyo.

In 2011, Buesseler and his team collected water and biota samples from 30-400 kilometers off the Fukushima coast. The samples were shipped to a lab, where they were processed chemically to isolate Cs from other, naturally occurring radioactive isotopes in the water, and the samples were reduced from almost 19 liters down to nearly 5 milliliters. Because of its radioactive properties, the concentration of Cs can be inferred using a high-purity germanium well detector—essentially a gamma detector.

“The origin of Cesium is as a fission product—that’s when uranium atoms splits apart in the nuclear reactive core and breaks down into different molecules, and one of those is Cesium,” Buessler explained.

The Cs continues to decay, and when a water sample containing Cs is placed on a gamma detector, each decay event is registered as an electrical signal, creating a spectrum of peaks on a graph. Which isotopes are present can be derived from the energy of the peak, and the amount of each isotope is measured in Becquerels (Bq), a unit of decay events per second. Isotopes decay at a predictable rate known as half-life. So the likelihood of a measurable event occurring within a given amount of time directly correlates with the amount of an isotope present: the more of it there is, the higher the number of decay events you will see.

Half-life also gives different nuclear events their own distinct signature.  Nuclear events release two different isotopes of Cs: 134Cs and 137Cs.

Water sampling off the coast near the Fukushima nuclear power plant, Sept 2013 on a University of Tokyo ship. Image Credit: Ken Buesseler.

Water sampling off the coast near the Fukushima nuclear power plant, Sept 2013 on a University of Tokyo ship. Image Credit: Ken Buesseler.

134 Cs has a half-life of 2 years, while 137 Cs has a 30 year half life. By looking at the ratio of 134Cs to 137Cs, scientists can determine the most likely source of the radionuclide and whether Cs present in the ocean likely resulted from the Fukushima meltdown or another event.

This relationship is important, Buesseler says.

“We actually take advantage of [it]….ever since we started atmospheric nuclear weapons testing there’s been Cesium all over the planet and in the ocean….I can say if it’s from Fukushima or something that was already there.”

Cs isotopes from the Fukushima incident initially had a ratio of approximately 1-to-1; in other words, equal amounts of 134Cs and 137Cs. However, since 134Cs breaks down much more rapidly than 137Cs, as time goes by 134Cs will disappear from the ocean more rapidly than 137Cs. So while Cs from Chernobyl initially had a Cs ratio to that of Fukushima, 134 Cs is no longer present in radiative plumes from these events.

When Buesseler and his team joined Japanese scientists to collect water and biota samples near the coast of Fukushima in 2011, they found that Cs isotopes were 10-1,000 times higher than levels prior to the meltdown (the concentration diminished with distance from shore). These levels were of enough concern to shut down some fisheries in the area, particularly those associated with bottom-dwelling fish. For external exposure only, however, radioactivity levels generally fell below the thresholds harmful to marine animals and humans.

Buesseler believes efforts by the Japanese government have helped to contain the spill and that leaks are now smaller than they were in the initial months after the accident. He also believes, however, that smaller leaks continue, with those to groundwater being especially difficult to remediate. Cs continues to be released into the ocean, although concentrations of the leaks have diminished considerably, and high concentrations from any of the leaks are being diluted as they are mixed by ocean currents.

While radioactivity off the coast of Japan has diluted, however, concern has grown in the United States. Carried across the Northwest Pacific by the Kuroshio Current, radionuclides are projected to begin reaching the western coast of the U.S. and Canada this year.

As a CBS report described, anxiety about the potential health effects of ocean-borne radiation were amplified when California beach-goers carrying Geiger counters reported that their machines detected radioactivity on the sand. Geiger counters, however, cannot effectively test for the isotopes that were released from Fukushima: Cs and Strontium (Sr). As Buesseler explained, Geiger counters simply measure the presence of any type of radioactivity, and the ocean contains multiple radioactive isotopes.

“In [Geiger counters] you don’t know what’s in the sample,” he said. “You never know. You need something like these $75,000 gamma detectors to tell that.”

Buesseler clarified that Geiger counters are likely detecting radiation from a naturally-occurring Thorium isotope that is released into the ocean during weathering.  Recognizing the public’s desire to be able to monitor beaches themselves, the Woods Hole Institute responded by creating an informational website and implementing a crowdsourcing approach to data gathering.

The website is called “How Radioactive is Our Ocean?” It provides information about natural and manmade sources of radiation in the ocean, radiation and human health, and current results from existing data. It also provides an opportunity for donations—an important source of funding for continuing research.

The website additionally provides opportunity for public involvement in data collection. Citizen scientists or groups can purchase a test kit through the site. The kits cost $550-$600 and come with instructions for sample collection. The samples are shipped back to the Woods Hole Institute where they are tested, and the results are added to the website. So far, the experimental project has been a success. Researchers received 120 contributions toward 20 different sites within the first three weeks of posting the kits on the website. The crowdsourcing approach will help to gather information from a variety of locations along the coast.

Buesseler does not anticipate that the Cs levels on U.S. beaches will be higher than they were near the coast of Japan—if anything they should be considerably lower. As a salt, Cs is excreted over time by fish that ingest it and does not move up the food chain.  Furthermore, aqueous Cs will dilute as it is mixed into the larger ocean, unlike particulate Cs that settled on to soils after its released at the time of the accident.

Buesseler suggests that in terms of radionuclides in the ocean, Strontium (Sr) also may be worth tracing in the future. Sr was released in much smaller amounts than Cs during the nuclear meltdown but has been much harder to remove from water during the continuing leak. Furthermore, Sr behaves like Calcium, storing itself in bones.

Buesseler and his team returned to Japan, joining a Japanese led expedition from the University of Tokyo, for another round of sampling in September 2013. This time around, they were able to work as close as 1 kilometer off shore. They plan to continue to track Cs in its distribution.

“There’s a whole range of radionuclides out there that have different chemical properties,” he said. “Some of them follow the ocean currents; some of them stick to the sea floor; some of them accumulate in fish and other organisms. So by selectively studying different isotopes, in different parts of the ocean, you can learn about how the ocean works.”